This paper deals with an improved decoupling control of the current components applied to three-phase modular multilevel converter (MMC) via rotating d-q frame theory based on the mathematical model of the system. In order to apply independent controllers for the two axes in d-q coordinates, the impact of the q-axis on the d-axis component and vice versa has to be removed. This is done by creating a decoupling block consisting of modified coupling terms. The feed-forward ac-side voltages compensation can effectively reduce any harmonic disturbance of the load side. Furthermore, we employed the strategy to reduce the capacitor voltages variation in each arm. A comparison of simulation results for both existing and improved decoupling current control algorithms using a (6.6 kV, 6 MW) induction motor drive verifies the superior performance of the proposed controller especially for good current reference tracking and disturbance rejection properties despite the variations in the system parameters. Copyright © 2013 John Wiley & Sons, Ltd.

This paper is concerned with determining the compensation capability of an interline dynamic voltage restorer (IDVR) under various balanced and unbalanced voltage sag scenarios. An IDVR consists of two or more DVRs with a common dc link which enables active power exchange among DVRs. This feature increases the compensation range of an IDVR as compared with separate but otherwise similar DVRs with no energy storage. The steady-state modeling and analysis of an IDVR is briefly explained in this paper. The expressions governing power requirement for compensating various voltage sags are derived. Consequently, the constraints associated with an IDVR operation are obtained and an algorithm is developed to calculate the compensation range of an IDVR and two similar storage-less DVRs. Illustrative graphs demonstrate the operating characteristics of an IDVR and compare these characteristics with those of two similar storage-less DVRs. Copyright © 2013 John Wiley & Sons, Ltd.

Current differential protection is widely used in transmission line due to its inherent simplicity and good performance. In order to strive to increase sensitivity of protection without compromising stability on external faults, phase correlation current differential protection is presented in this paper. Current phase difference is introduced to the differential current principle so as to improve the sensitivity and reliability of differential protection simultaneously. Comprehensive analysis and comparisons of performance characteristics in Alpha Plane demonstrate that phase correlation current differential protection has preferable performance than that of conventional current differential protection. As far as CT saturation is concerned, the proposed protection scheme has flexible adaptability to ensure significant improvement in sensitivity and dependability for internal faults while still improving stability for external faults. The performance of the proposed protection is investigated for various faults of transmission line simulated by Electro-Magnetic Transients Program including DC and record fault data from real power system. The test results show that the proposed protection has satisfactory operation performance. Copyright © 2013 John Wiley & Sons, Ltd.

This paper identifies a new mode called “critical voltage mode" in emerging distribution systems. The small-signal stability analysis indicates that load voltage dynamics significantly influence the damping of the newly identified mode. This mode has frequency of oscillation between the electromechanical and subsynchronous oscillation of power systems. A novel voltage controller is designed to damp this voltage mode of the system. The controller is designed using the linear quadratic Gaussian (LQG) method with norm-bounded uncertainty. The approach considered in this paper is to find the smallest upper bound on the H_∞ norm of the uncertain system and to design an optimal controller based on this bound. The design method requires the solution of a linear matrix inequality. The performance of the designed controller is demonstrated on a distribution test system for different types of induction motors. Simulation results show that the proposed controller with a careful uncertainty modeling has significant performance to improve the voltage profile of the distributed generation system compared to the conventional excitation controller and standard LQG controller. Copyright © 2013 John Wiley & Sons, Ltd.

This paper proposes an index, called Undetectability Index (UI), to classify the measurements according to their characteristics of not reflecting their errors into the residuals of the weighted least squares state estimator (WLS) from a geometric analysis of this estimator. Gross errors in measurements with high UIs are very difficult to be detected by methods based on residual analysis, because the errors in those measurements are “masked”, i.e. they are not reflected in their residuals. The paper also presents a very simple algorithm to compute the UI of all available measurements in a metering system. This algorithm is very simple and uses routines already available in the existing WLS Estimator software. Two UI applications are investigated (i) to design more reliable metering systems for bad data processing, i.e. metering systems formed by measurements with UI lower than a pre-specified value and (ii) to estimate the measurement errors. Several simulation results (with IEEE-14 and 30 bus systems) have validated the propositions. Copyright © 2013 John Wiley & Sons, Ltd.

This paper proposes a new approach to distribution system named Three-wire Network (3-NET). Considering the integration of different distributed generation (DG) technologies as well as the diversity of loads, the idea of 3-NET is splitting a single power network into three qualitatively different networks: (i) a high quality direct current (DC) network; (ii) a conventional alternating current (AC) network; and (iii) a low quality DC network. By this configuration, the multi-stage conversion of energy from DG sources can be reduced, whereas loads can be supplied with different power quality and reliability levels as desired. In order to evaluate the potential benefits of 3-NET, the operational strategy is formulated in a market model; the reliability performance is investigated using Monte Carlo simulation and quantified in terms of interruption costs. In the case study, the proposed 3-NET is considered for integrating a certain amount of DG into a specific distribution system and then compared with another system approach: hybrid AC–DC microgrid. The simulation shows that although the operation costs can be slightly higher, the 3-NET shows a great improvement in reliability performance in comparison with the hybrid microgrid. Copyright © 2013 John Wiley & Sons, Ltd.

Power transformers of high voltage play a key role in the power transmission system. Statistics show that acquiring an accurate assessment of transformer insulation can avoid financial losses of electrical companies, because it can detect a potential failure and therefore decrease the risk of transformer failure. Condition assessment of transformers is full of uncertain, fuzzy and randomness information and can be considered as a multiple-attribute decision-making problem. Aiming at this problem, this study presents a cloud and evidential reasoning integrated approach for assessing the condition of transformers. Data from the main body, maintenance record and accessory were chosen to form the assessment index system. Analysis results show that the integrated approach is effective and provides some valuable information to the maintenance of high voltage transformer. Copyright © 2013 John Wiley & Sons, Ltd.

A group of energy resources (including energy storage units) physically distributed in the network which are placed under a unified and integrated management with a central control system is called a Virtual Power Plant (VPP). The bidding strategy of a VPP, as a participant in energy and reserve markets, has significant role in maximizing its profit. This paper proposes a new mathematical approach based on a comprehensive model for bidding of a VPP in energy and reserve markets. In our proposed model network topology, VPP security constraints, constraints of distributed energy resources (DER) composing the VPP, power loss in the VPP and the balance between supply and demand are considered. The method determines the amount of energy and reserve that should be bought or sold in day-ahead markets, commitment of DER units, charge or discharge status of storage units and the amount of load curtailments. We have used Benders decomposition (BD) method for solving the problem. The results obtained using BD technique is compared with those obtained using genetic algorithm (GA). Simulation results confirm that using BD is more advantageous for solving the problem. It is also shown that BD can be applied to bidding problem of large-scale VPPs considering all the constraints. It is also observed that computational time for solving the problem using GA and the optimality of the solution are major obstacles for applying GA to solve large-scale VPPs. Copyright © 2013 John Wiley & Sons, Ltd.

This paper focuses on voltage fluctuation mitigation and fault-tolerant control methods for modular multilevel converters (MMC). The reason for MMC output fluctuation caused by the floating capacitor voltage fluctuation at low frequency is carefully analyzed. To reduce voltage fluctuation, two control freedom degrees of high-frequency zero-sequence voltage and phase-to-phase circulating current are proposed. In fully implementing MMC functionalities, we comprehensively investigated its fault-tolerant operation without redundant submodules for fail-safe energy transfer. A fundamental-frequency zero-sequence-based neutral-shift method is adopted to enable continuous operation and avoid complete shutdown of the MMC system. These methods enhance both the reliability and availability of MMC while avoiding the undesired unbalanced stresses among the healthy submodules. Rigorous simulation studies are performed to demonstrate the validity and effectiveness of the proposed control methods. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents a comprehensive formulation for the security constrained optimal power flow (SCOPF) problem considering valve loading effect, multiple fuel option, and prohibited operating zones of units as well as alternating current network modeling and contingency constraints. Also, the SCOPF formulation includes the integer variables, such as discrete transformer tap settings, in addition to continuous variables, such as generation of units. Thus, the suggested SCOPF model is a mixed integer, nonlinear, non-convex, and non-smooth optimization problem. To solve this problem, a new solution method composed of Benders decomposition and special ordered sets is presented. The proposed formulation decomposes the problem into a master problem and a sub-problem. The master problem relaxes the nonlinear constraints of the model using a convex linear outer approximation based on the concept of special ordered sets, whereas the sub-problem contains the nonlinear and non-convex SCOPF formulation with fixed integer and binary variables. To show the effectiveness of the proposed solution method, it is tested on the well-known test systems and compared with several other recently published solution methods. These comparisons confirm the validity of the developed approach. Copyright © 2013 John Wiley & Sons, Ltd.

In this paper, the most important problem in the transformer relaying which is based on differential protection has been taken into consideration, and an effective solution has been proposed. When an internal fault occurs, as the differential current increases, relays should trip, but in the case of magnetizing inrush current which has almost the same amplitude as internal fault, it should stop wrong action. Hence, a method which can discriminate magnetizing inrush current from internal fault is required. Sometimes, when an external fault occurs, the magnitude of differential current can increase and leads to incorrect performance of differential relays. Therefore in the proposed method, discrimination of external fault from two former signal types is considered as well. The method proposed is based on the combination of S-transform and hidden Markov model (HMM). S-transform is used to extract some useful time-frequency information of the input signal, and then HMM is applied to classify signals by means of features extracted using S-transform. Classification process is performed in two different and independent routes (MAC and SDC routes). In the overall classification, results of both previous routes are considered to ensure this classification is very much reliable in different testing situations. Simulation process is performed by means of the combination of PSCAD/EMTDC and MATLAB software, and the effectiveness of the proposed technique has been verified with test cases. Copyright © 2013 John Wiley & Sons, Ltd.

A novel scheme is proposed for secondary arc extinction, preferably applicable to extra high voltage (EHV) and ultra high voltage (UHV) transmission lines. The topology and the operational principle of the scheme are elucidated in details. Mathematical expressions for topology parameters design and optimization under different conditions have been established based on equivalent circuit transform and are verified by simulations. Results suggest that the proposed methodology can largely reduce the forced component of the secondary arc current, one part of the complete response of a circuit under excitation. The recovery voltage is not affected by fault location, transmission power and the shunt inductors. The decay factor and oscillation frequency of the free components of the secondary arc current and the recovery voltage are also computed based on the equivalent impedance network and Laplace Transform. In addition to causing possible resonant overvoltage, the proposed theme may prolong the arcing time due to cancelling zero-crossing of the secondary arc current while shunt inductors are installed at the transmission line terminals. Simplified formula for calculating the critical compensation point are given as to be used as a reference for the re-optimization of the novel theme. The proposed methodology presents an alternative to suppress the secondary arcs for EHV and UHV transmission lines. Copyright © 2013 John Wiley & Sons, Ltd.

The aim of wave energy converters is to extract energy from ocean waves and turn it into electricity. Since the earliest developments, optimal control strategy has been considered the one that optimizes the power extracted from the oscillating system. This technique requires energy exchanges between the oscillating system and an auxiliary storage energy system, and has been shown to be potentially capable of substantially increasing the amount of energy absorbed. Nevertheless, it requires very efficient devices due to the huge losses, which occur when operating conditions are far from the natural frequency of the oscillating system. Moreover, this optimization criteria cause large heave excursions in the oscillating system and high peak-to-average power ratios of the system. In this paper, a point absorber converter that uses a linear direct-drive generator and power electronics converters jointly is considered.

This article presents a novel method of control, which optimizes the power transferred from the generator to the power electronic converter considering the copper losses in the electric generator. This approach of the optimization method allows a significant increase in the wave energy converter's capacity of energy conversion. Applying the proposed control strategy, power exchanges between the oscillating system and the generator can be reduced, and in consequence, system efficiency significantly increases. In addition, the oscillating system's heave excursions and the peak-to-average power ratio decrease. The formulation of the proposed method is presented as well as numerical simulations in irregular waves. Copyright © 2013 John Wiley & Sons, Ltd.

As one of the important security criteria, power system operators closely monitor reactive power reserve in voltage control areas (VCAs) of their system to ensure voltage security and to prevent voltage collapse. Therefore, from a system/market operator perspective, proper management of reactive power reserve must be considered in reactive power procurement stage. This paper proposes a long-term procurement model to meet reactive power reserve requirements in competitive electricity markets considering both economical and technical issues. The model would be of interest to system/market operators since it is based on a new indicator referred to as normalized effective reactive power reserve (NERPR) as a more meaningful signal compared to other physical/economical indicators. The proposed framework for reactive power procurement determines accepted bids by minimizing the associated procurement cost while maximizing NERPR in each VCA of the system. Physical constraints such as generator capability curves and voltage security are taken into account in the formulation. Also, a population-based algorithm is introduced here to solve the mixed integer non-linear programming (MINLP) problem resulted from the procurement model. An algorithm is proposed to change the MINLP problem to series of NLP sub-problems. The CIGRE 32-bus test system is used to test the feasibility of the proposed procurement model. Copyright © 2013 John Wiley & Sons, Ltd.

Multi-objective evolutionary algorithm (MOEA) based on Pareto optimality has emerged as an important approach for optimal reactive power dispatch (ORPD) problem. However, because of the deficiency of adopting evolution operators with single search characteristics, existing MOEA for multi-objective ORPD (MORPD) often fails to maintain universal and robust performance in different phases of optimization process. On the basis of running multiple algorithms simultaneously and adaptive selection strategy, this paper proposes a new optimal method for MORPD. Firstly, an algorithm candidate pool containing four different algorithms is constructed through the analysis of characteristics of state-of-the-art MOEA while considering the rules of consistency and complementarity. Then, during the optimizing search, the quantity of offspring individuals generated by each candidate algorithm at different stages of optimization process is determined adaptively by learning from its previous experience in generating promising solutions. The proposed method is tested on the IEEE 30-bus system; its computing performance is compared with existing popular MOEAs from the point of view of Pareto fronts, outer solutions and C measure. Experimental results show that the new method can obtain better performance of convergence during the entire optimization process. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents an implementation of double-frequency oscillation cancellation (DFOC) technique in phase locked loop less synchronous reference frame theory for control of a distribution static compensator (DSTATCOM). This control algorithm on DSTATCOM is used for improvement of power quality under linear and nonlinear loads. The DFOC technique is used for cancellation of double-frequency component from the load fundamental active and reactive power components of currents which are used for deriving the reference supply currents without time delay. A DSTATCOM is developed, and its performance is tested using a digital signal processor. The performance of DSTATCOM is found satisfactory for different types of loads in transient and steady state conditions. Copyright © 2013 John Wiley & Sons, Ltd.

Characteristic study of cavity discharge is important in detecting and diagnosing internal failure of power transformers. Based on the kernel principal component analysis method, eight new feature parameters are extracted from 29 statistical parameters, which are commonly used in the characteristic study of partial discharges. Redundancy and computational complexity for applications are greatly reduced. Using cluster analysis method, new parameters are proved to be effective in the application of stage classification in the cavity discharge development. Four stages are classified: incipient discharge, unstable discharge, consistent discharge, and disruptive discharge. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents a platform for integrating a voltage stability assessment and enhancement (VSA&E) application into wide-area situational awareness (WASA) based on international standards. VSA&E is a key tool for maintaining and enhancing voltage security of bulk power systems. As power systems become more complicated and the penetration of renewable energies increases, system operators require powerful tools to analyze voltage security of the system based on actual online system operating conditions. We have proposed a concept of the international standard-based WASA that can provide such a next generation VSA&E application platform with no vendor lock-in. The effectiveness and practicability of the international standard-based WASA are demonstrated with VSA&E application integrated into a WASA evaluation system. Copyright © 2013 John Wiley & Sons, Ltd.

In this paper, a new double-end fault locator is presented for transmission lines. The fault location is achieved using synchronized measurement data and applying the least error squares (LSQ) principles disregarding the line parameters. The LSQ is applied on the positive sequence equivalent circuit to locate the fault distance. The proposed algorithm is non-iterative and simple as it is derived from the lumped parameter fault locator model. Due to using double-end data, the algorithm provides promising performance against the fault resistance and untransposed transmission lines. Furthermore, as it is derived using the positive sequence network only, it is suitable for locating any fault type. When the algorithm is evaluated for long overhead transmission system, its accuracy is found acceptable up to 220 miles because of double-end measurements. The ATP-EMTP program is used to perform the fault cases and therefore evaluating the locator performance. The test results provide evidence the efficacy of the proposed algorithm. Copyright © 2013 John Wiley & Sons, Ltd.

With the implementation of West-to-East Power Transmission Project in China, transmission lines will inevitably pass through high altitude regions with complex atmospheric conditions. The effects of such atmospheric parameters as pressure and humidity on the corona discharge performances of conductors are complicated. Especially as to the effect of air humidity on corona inception voltage, researchers have not reached an agreement. Therefore, by adjusting air pressure and air humidity in a corona cage, the positive direct current corona discharge performances of a smooth steel tube are tested. Results indicate that corona inception voltage increases significantly with the increase of air pressure, and the calculated results by discharge model are in accordance with measured results. Yet, with the increase of absolute humidity, corona inception voltage increases slowly. Under high humidity condition, corona inception voltage decreases because of the condensation on conductor surface, which distorts the surface electric field. In view of the condensation, the calculating model is improved, whose calculated results are in line with measured results. Moreover, it is found that inception discharge is easier to turn into glow discharge under low pressure and low humidity condition. The average pulse amplitude decreases with the increase of humidity, but with the increase of pressure, it decreases first and then increases. The rising time of the discharge pulse presents increasing trend with the increase of both air pressure and air humidity. All the test results are analyzed from microscopic mechanism. Copyright © 2013 John Wiley & Sons, Ltd.

Due to wind power fluctuation, it is difficult to make generating schedule of power system with wind power. This paper presents a new approach to make generating schedule using electrical relationships between generators and loads. A two-layer structure is used in the approach, i.e. generating schedule is made and revised in on-line time scale in upper layer, and automatic generating control (AGC) is carried out in real-time time scale in lower layer. Both generating schedule-making and AGC are implemented using power transmission factors among wind farms, related traditional generators and loads. And generating schedule is revised taking the operation limit of generators and transmission congestion into consideration. At last, the technical and economic benefits are analyzed in case study. The results show that the approach can effectively track fluctuation and forecasting error of wind power and enhance the operation quality of entire power system. Copyright © 2013 John Wiley & Sons, Ltd.

Multilevel converters have been widely used in medium-voltage high-power applications. One of the typical breeds of multicell converters is stacked multicell converter (SMC). This paper presents an improved configuration for SMCs, accompanied by its analysis, modeling, and implementation. The main advantages of the proposed converter, in comparison with the conventional one, are that the numbers of required dc voltage sources are halved in the improved topology and also the voltage rating of half of the power switches are decreased by 50% which results in reducing the cost, size, and weight of the SMCs effectively. This progress is achieved by connecting the upper stacks to the lower stacks back to back while the number of high-frequency power switches and flying capacitors (FCs), voltage ratings of FCs, as well as the number of high-frequency switchings during a full cycle are kept constant. The control methodology is based on the phase-shifted carrier sinusoidal pulse width modulation technique; therefore, the well-known natural balancing phenomenon of FC voltages, one of the critical advantages of SMCs, is maintained in the proposed converter. This paper also presents an analytic model to study and analyze the dynamic of FC voltages in the proposed SMC. Numerical analysis results of the derived analytic model, simulation results, and, moreover, measurements taken from the laboratory prototypes are presented in order to validate the effectiveness and advantages of the proposed configuration as well as its control strategy and analytic model. In addition, the comparison of the proposed converter with other types of multilevel converters is carried out to highlight the advantages of the proposed structure. Copyright © 2013 John Wiley & Sons, Ltd.

In this paper, a new method for online voltage instability control suitable for smart grid application is proposed. The method uses the Maximum Loadability Index to determine the amount of load relief necessary to restore stability. This measure can be determined based on local measurements. The load relief is obtained by: increase of local generation, virtual load shedding via adjusting the Load Tap Changers (LTCs) set-point, intelligent load control and the increase of reactive power compensation from Static Var Compensators. For the local generation, the control of two types is described: for micro Combined Heat and Power (micro-CHP) units and for large synchronous generators. In order to obtain constant power supply from a group of micro-CHPs, a Virtual Power Plant coordination method is proposed. The control of the actuators is outlined, and a method is described to coordinate their actions. Based on simulations it is demonstrated that the proposed method works properly: voltage collapse can be avoided, and voltage stability is restored. A sensitivity analysis is performed to determine the optimal settings of the coordinated controller. Furthermore, the merit of including reactive power control in the proposed control strategy is investigated. Compared to classical controls, such as Under-Voltage Load Shedding and LTCs tap blocking, the method performs better. Copyright © 2013 John Wiley & Sons, Ltd.

A hybrid electricity price forecasting model for the Finnish electricity spot market is proposed. The daily electricity price time series is analyzed in two layers – normal behavior and spiky behavior. Two different data preprocessing techniques are applied to handle trend and seasonality in the time series. An ARMA-based model is used to catch the linear relationship between the normal range price series and the explanatory variable, a GARCH model is used to unveil the heteroscedastic character of residuals and a neural network is applied to present the nonlinear impact of the explanatory variable on electricity prices and improve predictions based on time series techniques. The probability of a price spike occurrence and the value of a price spike are produced by a Gaussian Mixture model and K-nearest neighboring model, respectively. Forecasts of normal range prices and price spikes are generated to form an overall price forecast up to one week ahead. The results show that hybridization of the normal range price and price spikes forecasts may provide comprehensive and valuable information for electricity market participants. Copyright © 2013 John Wiley & Sons, Ltd.

In this paper, we propose an optimal bidding strategy for industrial combined heat and power (CHP) installations selling their power on multiple markets. This corresponds to the typical situation where power is traded on a day-ahead market (DAM) and a continuous intraday market (CIM). Each market has its own trading rules. The considered installations consist of a CHP, a conventional heating installation and a heat buffer. Each device has its own constraints, such as maximum and minimum deliverable heat and electrical power, and minimum and maximum buffer capacity. The objective is to determine the bidding strategy that will maximise the expected profit, while the future time evolution of both heat demand and market prices are unknown. To tackle this problem, we assume that the probability density functions (PDFs) of these variables are known or can be extracted from historical data. Then, by applying a tailored stochastic programming algorithm, the optimal bidding strategy can be constructed based on these PDFs and includes the different market rules and constraints on the installation. For a DAM, the bidding functions must be estimated in advance, which is a typical open-loop problem. On the other hand, the bidding functions for a CIM may be estimated almost in real time. This new scheme is exemplified for the Belgian market. Combining both markets can increase the expected profits significantly because risks due to uncertainties in heat demand are better controlled. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents a model for the analysis of internal faults of synchronous generators with multiple parallel connection paths for phase. Several kinds of faults between different portions of the stator windings (and ground, if the case) can be described in the subtransient time frame. Data necessary for the model derive from the electromagnetic design of the machine, as well as from details on equivalent parameters of the external grid and from the neutral connection characteristics. Such modeling procedure allows accurate insight of currents in windings and terminals, exploring each possible fault configuration. In the second part of this work, the developed model will be applied to assess the validity and to define possible improvements of conventional protection configurations against internal faults. Copyright © 2013 John Wiley & Sons, Ltd.

Two-stage isolated converters for photovoltaic (PV) applications commonly employ a high-frequency transformer (HFT) on the DC–DC side, submitting the DC–AC inverter switches to high voltages and forcing the use of insulated-gate bipolar transistors instead of low-voltage and low-loss metal-oxide-semiconductor field-effect transistors. This paper shows the modeling, control and simulation of a single-phase full-bridge inverter with HFT that can be used as part of a two-stage converter with transformerless DC–DC side or as a single-stage converter (simple DC–AC inverter) for grid-connected PV applications. The inverter is modeled to obtain a small-signal transfer function used to design the P + Resonant current control regulator, whose main advantages are the simplicity for implementation in single-phase systems and zero steady state error with sinusoidal current. A high-frequency step-up transformer results in low-voltage switches and better efficiency compared with converters in which the transformer is used on the DC–DC side. Simulations and experimental results with a 200-W prototype are shown. Copyright © 2013 John Wiley & Sons, Ltd.

The well-established under-frequency load shedding (UFLS) is deemed to be the last of effective remedial measures against a severe frequency decline of a power system. With the ever-increasing size of power systems and the extensive penetration of distributed generators (DGs) in power systems, the problem of developing an optimal UFLS strategy is facing some new challenges. Given this background, an optimal UFLS strategy for a distribution system with DGs and load static characteristics taken into consideration is developed. Based on the frequency and the rate of change of frequency, the presented strategy consists of several basic rounds and a special round. In the basic round, the frequency emergency can be alleviated by quickly shedding some loads. In the special round, the frequency security can be maintained, and the operating parameters of the distribution system can be optimized by adjusting the output powers of DGs and some loads. The modified IEEE 37-node test feeder is employed to demonstrate the essential features of the developed optimal UFLS strategy in the MATLAB/SIMULINK environment. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents a novel approach to quantify the effect of customer-owned distributed generation (DG) units on the life expectancy of power cables. Numerical results obtained from a realistic distribution system are presented and discussed under various scenarios, including different penetration levels of DG technologies. The results show that despite differences among various DG technologies, significant benefits can be realized in the area of power cable life extension, when DG units are installed by low-voltage customers. However, the extent of the benefits largely depends on the penetration level of DG units, the production pattern of DG technologies and the utilization factor of the analyzed cables. Copyright © 2013 John Wiley & Sons, Ltd.

Reactive power optimization plays an important role in safe and economic operation of power systems. However, with multiple mixed variables, this issue is well known as complex, nonlinear and multi-constrained. In this paper, combined particle swarm optimization (PSO) algorithm with chaos and multi-agent system, a novel algorithm (MACPSO) is developed and applied to reactive power optimization of power system. It synthesizes advantages of the swarm search of PSO and the intelligent search of agents. Seen as a particle of PSO during the search process, each agent competes and cooperates with the neighboring agents so as to obtain solutions of high quality swiftly. To strike a balance between intensification and diversification strategy and to reduce the probability of falling into local optimum, chaos optimization algorithm is adopted in the proposed algorithm. In order to verify the effectiveness of the proposed algorithm, the performance of MACPSO is evaluated on four classical testing functions and reactive power optimization. Experiment results show that MACPSO has excellent search ability and highly accurate convergence. Copyright © 2013 John Wiley & Sons, Ltd.

This research presents a method for convexifying the non-linear constraints and the objective function for the Transmission Network Expansion Planning Problem (TNEP). The TNEP seeks to identify the best set of transmission capacity additions to meet a future electric power demand. The TNEP is a non-convex Mixed Integer Non-linear Programming problem for which a variety of primal solution methods have been designed. In this paper, the TNEP is formulated as a bilinear programming problem subject to bilinear constraints. This allows resolving the non-linearities through convexification of the non-linear objective function and the constraints for a relaxation of the TNEP in which integrality requirements are removed. The final solution for the original TNEP problem is obtained by using a branch and bound strategy. It was found by using a two-node test case that the convexification method presented in this paper in conjunction with a brand and bound strategy identifies the optimal solution. Successfully solving a small-scale test problem through convexification presents a promising scenario to solve larger scale problems. This primal-dual method allows identifying the duality gap which is a limitation of primal methods. Further multidisciplinary research is needed to extend the method presented here to larger cases and real life networks. Copyright © 2013 John Wiley & Sons, Ltd.

A novel optimal reactive power flow (ORPF) model using augmented rectangular coordinates is established. The load tap changing (LTC) transformer is represented by an ideal transformer and its series impedance with a fictitious node located between them. The ideal transformer is replaced by the current injections at the two end nodes, and the constraint equations relating the current injections and voltages between two sides are formed. Meanwhile, the nodal voltages, injection currents, reactive power injections, and transformers turn ratio are considered as optimization variables. By the procedure, the ORPF model turns into quadratic and can be expressed in the blocked-matrix form.

The predictor-corrector primal-dual interior point method is used to implement the optimization. The quadratic model results in the constant Hessians that all have elements of 1 or −1,and mostly being zero, and multiplication is not required to form the full Hessian, thereby accelerating the entire optimal process significantly. Column Approximate Minimum Degree algorithm is used to order the coefficient matrix, resulting in great reduction of the number of the fill-in elements. The proposed and the conventional methods are programmed in MATLAB for comparison. Extensive numerical simulations on test systems ranging from IEEE 14 to 300 buses have shown the validity and higher efficiency of the proposed model than the conventional one. Copyright © 2013 John Wiley & Sons, Ltd.

The basic idea of the fractional frequency transmission system (FFTS) is to use lower frequency, e.g. 50/3 Hz, to reduce the electrical length of the AC transmission line, thus to increase its transmission capacity. FFTS is particularly suitable for transmitting electricity produced by the renewable energy, such as hydro power and wind power, because the rotation speeds of their generators are very low. The paper first introduces the characteristics of integrating large wind farm via FFTS. Then, a case study for feasibility of integrating wind farm via FFTS is carried out in considerable details. The results show that the proposed FFTS-based approach for wind power integration is effective with high potential for practical implementation. Copyright © 2013 John Wiley & Sons, Ltd.

Designing an efficient and effective maximum power point tracking (MPPT) method is a vital yet challenging task for modern large-scale variable speed wind power systems. In this paper, an improved control and extremum seeking strategy is proposed to construct a novel MPPT scheme, and the merit of this scheme is twofold. On the one hand, a nonlinear speed tracking controller is developed on the basis of the dynamic model of wind turbines, which cannot only track a desired rotor speed trajectory but also provide the extremum seeking module with estimation of the varying rotor torque. On the other hand, an extremum seeking module blended with lookup-table module is designed on the basis of a novel hill climbing searching method, which is more efficient because of its online updating structure and independence of the measurement of wind speed and output power. For simulation validation, a 10-MW direct driven wind turbine is used as a benchmark to test the proposed strategy. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents a new solution approach for optimal substation expansion planning (SEP) within electric power distribution networks. A modified fuzzy membership matrix as well as a memorable cost index vector is introduced to find the optimal substation service areas. Besides these, a Learning Automata-based algorithm is introduced for simultaneous determination of optimal service areas and capacities of the distribution substations. Electrical constraints such as voltage drops, power flow, radial flow constraints, as well as all prevalent cost indices are taken into consideration. The developed method is conducted to solve the distribution substation allocation problem for an actual distribution network with about 200 000 customers, and obtained results are compared to those of other methods. Detailed numerical results and comparisons presented in the paper show that the proposed solution approach could noticeably improve the quality of problem solutions with low computational burden and can be used as an effective tool for SEP in large distribution networks. Copyright © 2012 John Wiley & Sons, Ltd.

Plug-in electric vehicles (PEVs) will spread as environmental awareness continues to grow. As a result, many PEV batteries are likely to be charged in residential areas overnight, but this will cause a significant voltage decrease in the distribution network. As a countermeasure that does not require capital investment to strengthen the network, this paper proposes an autonomous decentralized voltage control scheme using a PEV battery charger. The scheme can continuously estimate household loads consumption and calculate the reactive power required for compensating the voltage decreases caused by household load consumption and PEV charging. Because only the measured voltage and current at the connecting point of the PEV battery and the impedance between this point and the nearest distributing substation are required to calculate the necessary amount of reactive power compensation, the control scheme can be implemented merely by modifying the control logic circuit built into the PEV battery charger at almost no extra cost. A realistic model of the distribution network and the standard daily demand curve in residential areas in Japan are used to validate the performance of the new scheme. Copyright © 2012 John Wiley & Sons, Ltd.

The work carried out in this paper is to deal with a proposed conductor temperature rise computation algorithm. This simple algorithm gives more accurate calculation, making use of the load current and the weather data parameters, particularly the amount of solar incidental radiation upon the conductor and the conductor material properties. The calculation is based on the heat transfer mechanisms occurring in the stranded exposed overhead transmission lines (OHTL) conductors in a natural environment. In this paper, the mathematical models for the use of the natural and forced convections are presented by using Numbers of Grashof and Reynolds, respectively, in the determination of the temperature rise of the OHTLs. Skin effect and iron losses are also taken into account in order to determine the alternating current resistance of an Aluminium Conductor Steel Reinforced in the process of iteration's heat balance equation. The surface area factor of the stranded conductors that increases the loss by radiation from the conductor surface is also used to calculate the temperature rise of the stranded OHTL. Copyright © 2012 John Wiley & Sons, Ltd.

Distributed generation (DG) systems are small-scale power generation technologies that offer better reliability of supply, reduced transmission, and distribution losses and cost. The benefits of DG greatly depend on their placement and size. Most of the existing analytical methods developed so far are applicable for single DG installation only. Further, in meta-heuristic-based methods, location and sizing problem of DG were dealt separately in which optimal locations are identified with sensitivity-based methods, and optimal sizing is obtained using meta-heuristic algorithms which may lead to incorrect location and suboptimal sizing. In this paper, a novel method that employs harmony search algorithm with differential operator is proposed to install multiple DG units optimally in distribution system with an objective of minimizing active power loss and improving voltage profile. The proposed method is tested on 33-bus and 69-bus radial distribution systems to demonstrate its performance and efficacy. Results show that the proposed method is superior to existing methods in quality of solution. Copyright © 2012 John Wiley & Sons, Ltd.

Placement of phasor measurement units (PMUs) in power systems has often been formulated for achieving total network observability. In practice, however, the installation process is not implemented at once, but at several stages, because the number of available PMUs at each time period is restricted due to financial problems. This paper presents a novel integer linear programming framework for optimal multi-stage PMU placement (OMPP) over a preset schedule, in order to improve the network observability during intermediate stages, in addition to its complete observability by the end of the PMU placement process. To precisely evaluate the network observability and fully exploit the potential of circuit rules, the observability function is formulated by Boolean algebra. Next, it will be utilized to formulate the optimal PMU placement problem and then construct a unified optimization model for OMPP. As such, among various strategies for installing PMUs over the scheduled horizon, the one achieving the maximum total observability of the system during intermediate stages is found. Moreover, the unified optimization model allows for considering the network expansion scenarios as well as the diverse importance of critical buses during the installation process so that these buses may be equipped with PMUs at primary stages. Finally, to illustrate the applicability and efficiency of the proposed method, it is applied to several IEEE test systems and two practical large-scale power systems, and the results are studied. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, the principles of the three-phase saturated transformer coupling new-type solid state fault current limiter (SSFCL) are studied using the equivalent circuit method; in addition, considering the nonlinearity of the iron core material in the saturated transformer, the circuit field-coupled 3-D finite-element analysis is employed to investigate the transient performance of the proposed SSFCL. The analysis and simulation results show that the novel SSFCL has negligible impact on the power system during the normal operating stage, automatic response to the short-circuit fault, and especially low secondary voltage level during the current-limiting stage, which are of great significance to the reduction of the thyristor voltage level, the DC reactance, as well as the volume and cost of the whole SSFCL. The current-limiting effect is also tested in the simulation of a multi-machine power system with the protection of SSFCL. Last, the comparison of the simulation and experimental results proves the feasibility of the proposed SSFCL. Copyright © 2012 John Wiley & Sons, Ltd.

Distribution Companies (DisCos) play a pivotal role in developing new planning strategies for maintaining the proper operation and efficiency of distribution networks. Moreover, power system deregulation, demand increasing, shortage of transmission capacities and interests in renewable sources have led to the increased studies on integration of distributed generation (DG) into the distribution systems. Hence, proper planning of DG units is important for obtaining their maximum potential benefits from the perspective of DisCos. This paper aims to present a new DG planning model. This new model would help planners to find optimal planning of different types of DG units, dispatchable or non-dispatchable, in distribution systems for minimizing system's annual energy loss. The planning problem is formulated to determine the optimal hourly production profile, power factor and penetration of DG units. A particle swarm optimization algorithm, implemented in MATLAB software, is used for finding the optimal solution of planning problem. The proposed approach has been evaluated on 69-Bus Test System in matpower 4.0, with different scenarios in view of DG units' combination. The results show that a significant reduction in energy loss is achieved and the constraints including voltage limits and the feeders' capacity are satisfied. Copyright © 2012 John Wiley & Sons, Ltd.

The main contribution of this paper is the study of a new feeding system for direct current (DC) electric arc furnaces. This system includes pulse-width modulation-alternating current (PWM-AC) choppers along with six-pulse diode rectifier. In contrast with traditional power supplies such as 12-pulse thyristor-based rectifier and 12-pulse diode rectifier–DC chopper-based power supplies, the proposed configuration stands out because of the reduced number of rectifier units and output reactors. Furthermore, the proposed power supply system is free of any current/power-sharing problems, and only one proportional-integral controller is sufficient to control the DC arc current or its power. It is shown through analysis and simulation that in comparison with traditional feeding systems, the proposed topology can lead to smaller input filter size, total unbalance and harmonic distortion reduction, and power factor improvement. Copyright © 2012 John Wiley & Sons, Ltd.

To increase reliability of distribution networks, it is necessary to consider reserve capacity in substations and lines. The reserve capacity is used to supply interrupted loads through sectionalizing switches. This paper presents a method to determine optimal reserve capacity margin of ST substations through modeling the interruptions on medium voltage lines. In this method, it is assumed that each substation supplies a certain number of load points and there are sectionalizing switches on lines to restore interrupted load points. The objective function consists of investment cost and energy loss cost of substation transformers and also customer interruption cost. Genetic algorithm is used to find optimal solutions. The effectiveness of the proposed method is evaluated on a real 95 bus distribution network. Copyright © 2012 John Wiley & Sons, Ltd.

With increasing levels of distributed generation (DG) penetration, especially variable output ones, new challenges have appeared in electric power systems. The outputs of these generators are variable and determined largely by nature of the power sources such as wind, wave, and sunlight. One solution to decrease problems caused by the variable output of these resources is to aggregate them into a virtual power plant (VPP). The VPP is an energy management system tasked to aggregate the capacity of some DGs, storage facilities, and dispatchable loads (DL) for the purpose of energy trading and/or providing system support services. The economically optimal control of distributed energy resources (DERs) included in the VPP is complicated by uncertainties, nonlinearities, and inter-temporal constraints. Therefore, a novel two-stage dispatch framework is presented here for the integration of the stochastic and the controllable DERs in the distribution network through the VPP concept. The first stage deals with the VPP optimal bidding model (VOBM). A probabilistic price-based unit commitment using point estimate method (PEM) is used in the VOBM to allow the VPP to decide on the unit commitment of its DERs and the optimal bids to the day-ahead market. The second stage deals with active distribution network management. The distribution system operator uses input from the VOBM to manage any local network constraints and to determine the characteristics of the entire local network at the grid supply points, while seeking to minimize the hourly operation cost. The proposed optimization algorithm is applied to an 18-bus system. Copyright © 2012 John Wiley & Sons, Ltd.

The wind turbine is subjected to lightning due to its geographical location and size. The struck wind turbine suffers from backflow current-overvoltages affected by the grounding system. This issue is highlighted in this paper, and, therefore, the grounding system of the wind turbine is modified. In order to accurately examine the proposed grounding system, we present a model that takes into account both soil ionization and frequency-dependence effect. Also, wind farm components, such as wind turbine down conductor, transformers, surge arresters, overhead, and distributed line, are modeled using high-frequency models. More factors affecting backflow current-overvoltages are analysed such as the lightning inception angle, in-service and out-of-service wind farm, and the number of wind turbines. The simulation is carried out using ATP/EMTP software. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, an improved micro-particle swarm optimization (μPSO) algorithm is proposed and applied to the development of a computational method for solving Transient Stability Constrained Optimal Power Flow (TSCOPF) problem. The improvement includes two novel intelligent strategies: the dynamic space adjustment and the selective re-initialization. The first strategy is designed to accelerate the convergence speed by confining the search in a reduced space, while the second strategy is to reduce the possible overabundance by re-initializing the particles only in a non-overlapping space. The simulation results demonstrate the superiority of the improved μPSO algorithm over both the original μPSO algorithm and the standard PSO algorithm.

The proposed method is validated through case studies for solving TSCOPF problems on IEEE 39-bus system and IEEE 162-bus system. The results show that the algorithm can utilize a small population to obtain an equally good or better optimization outcomes compared to those of the original μPSO algorithm and the standard PSO algorithm. Copyright © 2012 John Wiley & Sons, Ltd.

This paper introduces an energy storage capacity optimization method and a new software tool for energy storage system designing (DESS). First, this paper proposes a new method of energy storage capacity optimization, which is the basis of a main function of DESS. The third-order Butterworth low-pass filter model can effectively smooth power fluctuations. Compared with traditional methods, the proposed method considers the external characteristics of energy storage and system losses, which produces more accurate capacity. Second, DESS is developed under the background of China Southern Power Grid MW-level storage station demonstration project. The functions of DESS include data analysis, capacity optimization, structural design and scheme evaluation. DESS can assist engineers to obtain the optimal scheme by comparing different schemes. Third, the characteristics of DESS are summarized through comparing with HOMER and Hybrid2. Finally, DESS has shown its effectiveness in the case of designing the first MW-level battery storage station in China. Copyright © 2012 John Wiley & Sons, Ltd.

This research article presents a novel approach based on an instantaneous active and reactive current component (i_d-i_q) theory for generating reference currents for shunt active filter (SHAF). Three-phase reference current waveforms generated by proposed scheme are tracked by the three-phase voltage source converter in a hysteresis band control scheme. The performance of the proposed control strategy has been evaluated in terms of harmonic mitigation and DC link voltage regulation under various source conditions.

In order to maintain DC link voltage constant and to generate the compensating reference currents, we have developed type-1 fuzzy logic controller (T1FLC) and type-2 FLC (T2FLC) triangular M.F. The SHAF with proposed type-1 and type-2 FLCs using triangular M.F is able to eliminate the uncertainty in the system. By using T2FLC triangular M.F, SHAF gains outstanding compensation abilities. The detailed simulation results using MATLAB/SIMULINK software are presented to support the feasibility of proposed control strategy. To validate the proposed approach, the system is also implemented on OPAL-RT (real-time) simulator, and adequate results are reported for its verifications. Copyright © 2012 John Wiley & Sons, Ltd.

Power Distribution System (PDS) is spread on different places. Therefore, PDS has many laterals and load taps. Accurate fault locating in PDS causes to improve reliability indices and its efficiency. In this paper, an improved method is suggested for fault location in PDS, which has a high accuracy. In the proposed algorithm, by using phase domain of distributed-parameter line model, a fifth-order algebraic equation of fault distance is obtained, which can improve the accuracy of determined fault distance for all types of faults. The proposed method is tested under different fault resistances in which the results show low sensitivity to this parameter. To evaluate the accuracy of the proposed method, the modified IEEE 34 Node Test Feeder is used, and its efficiency and accuracy is proved. Copyright © 2012 John Wiley & Sons, Ltd.

The task of modelling and locating steady-state unified power-flow controllers (UPFCs) for static voltage stability enhancements in power system planning is addressed. A hybrid UPFC model for power flow studied is proposed. The series side of the UPFC is represented by an ideal voltage source, while the shunt side is represented by an idea current source model. Equivalent relationships among our new models and existing models will also be investigated. Integrations of this hybrid UPFC model into existing Newton-type power-flow solvers are presented. Extensions of continuation methods for locating the voltage collapse point of power systems embedded with UPFCs are studied. Strategies of determining the UPFC rating and locations for voltage stability enhancements will also be discussed. Numerical demonstrations of IEEE 300-bus test system and Taiwan power system are performed to verify the correctness of the proposed UPFC model and the computational tool. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a new concept for the grid integration of large-scale offshore wind farms. In the offshore wind farm, the wind turbines are integrated into the offshore DC collector grid by a cascaded converter. The high-voltage level for the high-voltage direct current (HVDC) transmission purpose is built by series connections of low voltage modules, without the use of an offshore step-up substation. The output power of each module is controlled by the switching of the related half-bridge, and a modified straightforward voltage balancing strategy is proposed to balance the storage capacitor voltages in the turbines. Supplementary balancing strategies are also presented to assist in balancing the capacitor voltages, when the differences in the wind velocities flowing to the turbines become larger. Because of the stable capacitor voltages, self-commutated voltage source converter (VSC) is adopted as the onshore inverter station connecting the HVDC link and the onshore AC grid. The VSC station not only regulates the DC-bus voltage of the HVDC link, but also provides reactive power compensation to the onshore grid. The time-domain simulation results show that the proposed offshore wind-power collection and transmission system provides a new option for integrating large-scale offshore wind farms. Copyright © 2012 John Wiley & Sons, Ltd.

Adverse weather has considerable effects on reliability of electric power systems. On the other hand, using the Bayesian networks (BN) in reliability studies provides the possibility of analyzing different factors, especially the importance evaluation of system components. In this paper, a novel approach is presented to consider the weather conditions and load level variation in construction and implementation of the BN associated with the composite power systems. In this approach, the geographical division model is used to model weather conditions in various regions of the given power system and also different sections of overhead transmission lines. The obtained BN provides an effective framework for comprehensive reliability study of composite power systems. Copyright © 2012 John Wiley & Sons, Ltd.

This paper considers the incorporation of battery energy storage systems (BESS) into wind farms, for the purpose of achieving economic dispatch for the wind power generators in a manner similar to conventional power plants. A BESS operation optimization model (BOOM) is introduced, and through utilizing unit commitment results, BOOM maximizes a predetermined objective that measures economic benefits obtained from the wind power dispatchability against the BESS cost. Due to the requirements of the formulated problem, a revised genetic algorithm (GA) is developed to solve the combinational-optimization problem, in which the 0–1 variables represent BESS charge–discharge states and continuous variables pertain to BESS power. With the incorporation of the BESS, the resulting wind power stations attain bidirectional regulation capability as the stations can not only export but also absorb energy. Simulation study demonstrates the performance of the proposed design and the efficacy of the developed algorithm. Copyright © 2012 John Wiley & Sons, Ltd.

A feature extraction method for gray intensity image of partial discharge (PD) is applied to recognize the insulating defects in high-voltage cross-linked polyethylene power cable joint. The method is based on a two-directional and two-dimensional (2-D) maximum margin criterion (MMC). A 2-D orthogonal projection of gray intensity image of PD was performed in horizontal and vertical directions. Projected image data were taken as discriminant vector of different gray intensity images to solve the high dimensional and small sample size of PD gray intensity image. The nearest neighbor classifier was used to classify the PD gray intensity image to recognize different insulating defects in the joint. The recongnition results of four typical insulating defects in the laboratory indicated that the extraction speed of the gray intensity image feature and recognition rate of insulating defects are superior compared with the common principal component analysis and Fisher discriminant analysis with MMC. Copyright © 2012 John Wiley & Sons, Ltd.

With the advent of large-scale wind farms and their integration into electrical grids, more uncertainties, constraints and objectives must be considered in power system development. It is therefore necessary to introduce risk-control strategies into the planning of transmission systems connected with wind power generators. This paper presents a probability-based multi-objective model equipped with three risk-control strategies. The model is developed to evaluate and enhance the ability of the transmission system to protect against overload risks when wind power is integrated into the power system. The model involves: (i) defining the uncertainties associated with wind power generators with probability measures and calculating the probabilistic power flow with the combined use of cumulants and Gram-Charlier series; (ii) developing three risk-control strategies by specifying the smallest acceptable non-overload probability for each branch and the whole system, and specifying the non-overload margin for all branches in the whole system; (iii) formulating an overload risk index based on the non-overload probability and the non-overload margin defined; and (iv) developing a multi-objective transmission system expansion planning (TSEP) model with the objective functions composed of transmission investment and the overload risk index. The presented work represents a superior risk-control model for TSEP in terms of security, reliability and economy. The transmission expansion planning model with the three risk-control strategies demonstrates its feasibility in the case study using two typical power systems. Copyright © 2012 John Wiley & Sons, Ltd.

The objective of this paper is to investigate the torque–speed characteristics of single-phase axial flux squirrel cage induction motors using a two-dimensional (2-D) electromagnetic field analysis. In this study, the effect of saturation and anisotropy of electromagnetic cores is taken into account. Furthermore, the effect of harmonics is considered in the analysis. Then, 2-D and three-dimensional time-stepping finite-element (FE) analysis is employed to validate theoretical results. Finally, the motor prototype is constructed and tested. Good agreement between analytic, FE and experimental results shows the accuracy and success of proposed model. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents an N-1 restoration model and optimization algorithm for the solution of maximizing load transfer capability with the interconnection of DG and DESS in distribution networks. Through a deep analysis of the time-varying characteristics of DG and DESS, the load restoration model is established and LTC indicator is proposed. With the application of searching area expansion and network topology simplification in consideration of topology connectivity constraint, LTC indicator can be conveniently obtained. Case study results show that the proposed LTC indicator effectively assesses the power supply capability in distribution networks under different circumstances of DG and DESS interconnection. Copyright © 2012 John Wiley & Sons, Ltd.

Distortion levels on power systems have been continuously growing because of the increasing presence of loads, producing interharmonics as well as harmonics. Because in some loads such as arc furnaces, the interharmonic pollution is remarkable, the traditional power system unbalance indices and different apparent power quantities, in which only harmonic components were considered, create extraordinary errors. This paper proposes a beneficial method for extracting the actual amount of unbalance and the accurate apparent power quantities under nonsinusoidal, unbalanced conditions and in the presence of interharmonic components. In this regard, appropriate unbalance and harmonic distortion factors are defined. First, five case studies are presented to show the validity of the proposed indices. Then, in order to show the practicality of the introduced method, the proposed quality indices are calculated for data acquired through simulation of two typical steel industrial plants with a DC electric arc furnace. The results are compared with cases in which the interharmonic components have been neglected. On the basis of simulation studies, precision of the proposed method is represented, and it is shown that the obtained results are more accurate than those obtained by eliminating the interharmonic components. Copyright © 2012 John Wiley & Sons, Ltd.

Sensitivity analysis oriented by optimal power flow problem is applied in this work to estimate nodal technical losses in power transmission networks in a smart grid environment. This approach aims to efficiently provide active power loss indices as a result of load variations at each single load bus connected to the system. Unlike the power flow algorithm, this technique does not require an iterative process to find the solution, which results in a faster methodology with high accuracy and reduced computational cost, even when considering all the characteristics of the system. The methodology starts from a known optimal solution obtained by optimal power flow, considered as the base case. From the base case solution, new solutions can be evaluated by sensitivity analysis when system load bus perturbations occur. Tests were carried out on the IEEE 14 and 118 bus systems in order to evaluate the proposed technique performance. Copyright © 2012 John Wiley & Sons, Ltd.

The paper deals with thermal modelling of transformers with automatic control of the cooling system. The case study was done on 725 MVA transformer with OFAF cooling, having eight compact coolers, each of them with their own pump and hysteresis control: four coolers, when top oil temperature drops below 30 °C, and six coolers, when top oil temperature rises above 45 °C. The consequence of operation with different number of coolers is change of thermal characteristics of the transformer. Additional phenomenon causing change of thermal characteristics is fouling of the coolers. These variable thermal characteristics are included in mathematical models used for the calculation of top oil and hot-spot temperatures in all applications of such calculation algorithms: protection, monitoring, estimation of overload capability, calculation of thermal ageing, etc. Copyright © 2012 John Wiley & Sons, Ltd.

In this research, a hybrid particle swarm optimization with time-varying acceleration coefficients (HPSOTVAC) and bacteria foraging algorithm (BFA) are presented for solving a complex economic load dispatch problem. Basically, there are many realistic constraints that affect feasible operation such as generation limitation, ramp rate limits, prohibited operating zone, nonlinear cost functions, and transmission loss that are considered in this research. The effectiveness of the proposed HPSOTVAC/BFA is tested in 6-, 15-, and 40-unit generating systems. Also, for 6-unit case study, the valve-point effect is considered too. The numerical results demonstrate that the proposed hybrid technique gives less total generation costs than genetic algorithm (GA), particle swarm optimization (PSO), hybrid GA/PSO, active power optimization, iteration PSO, and self-organizing hierarchical PSO methods. Copyright © 2012 John Wiley & Sons, Ltd.

Current and force calculations in different short-circuit conditions are required for short-circuit performance analysis of a multi-winding traction transformer which is one of the most important requirements in its design process. This paper extends the available low-frequency three-winding star equivalent circuits to develop a novel equivalent circuit for the four-winding traction transformers. The leakage inductances of the traction transformer are determined and employed to calculate the parameters of this developed star model. It is shown that the star equivalent circuit is a valid and appropriate model to simulate the steady-state and dynamic performance of the traction transformer under different short-circuit conditions. To analyze the short-circuit performance of the transformer, the short-circuit currents (simulated by the introduced model) are employed to compute the electromagnetic forces for a typical dry-type traction transformer. The analytical and numerical methods are used for short-circuit force calculations, and those results are compared against each other. Copyright © 2012 John Wiley & Sons, Ltd.

Transmission network expansion planning (TNEP) is one of the most important parts of power system studies. An optimal expansion of a transmission system should also capable of facilitating forecasted load patterns as well as future generation. Mathematically, TNEP is a mixed integer nonlinear programming as a non convex problem that can be solved by either conventional or heuristic optimization algorithms. This paper proposes a methodology for choosing the optimal transmission expansion plan considering less computational effort. The proposed methodology is composed from combination of Modified Scatter Search Algorithm and Constructive Heuristic Algorithm (CHA) considering New Load Shedding Index (NLSI). The proposed NLSI is implemented without generation rescheduling in order to decrease the number of linear programming obtaining an optimal solution. The algorithm is applied to different cases: Garver, 24-Bus IEEE systems and Southern Brazilian System of 46 Buses system. The simulation result shows a significant performance of the proposed method in comparison with some studies addressed in literature. Copyright © 2012 John Wiley & Sons, Ltd.

This paper proposes a model for the computation of core losses and excitation current, in a lamination by lamination method, for wound core distribution transformers. The model was developed based on the finite-element method (FEM). The results obtained by applying the proposed model were compared with the FEM results and with the measurements of the no-load test. The no-load losses obtained by the proposed model present a difference of 4% with respect to measured values, while they are almost the same with respect to FEM. The proposed model contributes in the research of new techniques that improve transformer design. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents an efficient and fast method, using topological and numerical procedures, for observability analysis in state estimation with phasor measurement units as well as conventional (SCADA) measurements. Power and current flow measurements are used to build topologically flow islands that, in turn, are used to construct a reduced network and an associated gain matrix. To build this reduced order gain matrix, boundary power injections, and voltage phasors at flow islands are considered. Observability checking is carried out analyzing the pivots encountered during the triangular factorization of the gain matrix, and the observable islands are identified in a noniterative manner, by performing back substitutions with its triangular factors. Additional measurements that will render the system barely observable are directly provided, using the triangular factors of a reduced Gram matrix associated with existing and candidate power injections and candidate phasor measurements at boundary buses of observable islands. The dimension of the gain and Gram matrix is equal to the number of flow islands and boundary measurements, respectively, which is relatively low even for very large systems, and that makes the proposed algorithms fast and computationally efficient. The IEEE 14 and 30-bus systems are used to illustrate the steps of the proposed algorithms. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, an adaptive nonlinear controller is designed for regulating the output voltage of DC-DC buck converters, which is stable and robust with subject to widely changes of load demands and input voltage variations. The proposed controller is developed based on adaptive backstepping approach and can be applied for both Continuous and Discontinuous Conduction Modes (CCM and DCM). Using a stand-alone TMS320F2810 digital signal processor from Texas instruments, some simulations and experimental results have been obtained to verify the proposed control approach. These results are in good agreement and prove the effectiveness and capability of the controller for both CCM and DCM operations. Copyright © 2012 John Wiley & Sons, Ltd.

Time domain reflectometry (TDR) has been used for localization of discontinuities in transmission lines in diverse applications. In this paper, its potential as an alternative to frequency response analysis for the detection and localization of mechanical changes (damages) in transformer windings is investigated. This is done by computer simulations of a lumped-element circuit model of a disc winding which has been verified with measurements on an experimental coil. We also suggest that the idea of TDR can be used to visualize the results of frequency response measurements in time domain, where they are easier to interpret. When applying TDR to transformer windings instead of transmission lines, two main new problems have to be addressed, namely (i) the basic inhomogeneity of most winding designs, and (ii) the strong effect of dispersion (distortion of signals as they travel along the winding). We argue that TDR is still useful under these circumstances if it is applied as a differential method and in the right frequency window, i.e. using signals of suitable shapes. Under these conditions, it allows the detection and, to some extent, localization of certain mechanical changes in a transformer winding. Copyright © 2012 John Wiley & Sons, Ltd.

To block auto-reclosing operation for permanent earth faults on extra-high-voltage transmission lines, an accurate numerical algorithm is developed in this paper. The earth fault location and its nature (arcing or arc-less) are estimated, only using the sequence quantities of the local terminal and their harmonics. The proposed algorithm calculates the amplitude of arc voltage and compares it with a given threshold value; then, decision is made if it is transient or not. A square wave arc model is considered to represent the long electric arc in the free air. One main contribution of the algorithm is its independence from the remote terminal information such as phase difference and infeed current. The successful simulation results using the square wave and dynamic arc models show that this simple and powerful algorithm can be effectively utilized for implementing a single-pole auto-reclosing scheme. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a new algorithm for harmonics estimation in power systems. Because of the nonlinearity of phases of sinusoids, the estimation of harmonic parameters is a nonlinear problem. However, nonlinear solving for amplitude estimation decreases speed of convergence. Thereby, hybrid methods decompose the harmonics estimation problem into two problems, linear for amplitude and nonlinear for phase. The objective of this paper is to introduce an accurate approach for harmonic parameters estimation. This approach is based on a stochastic search method, that is, clonal selection, to estimate the phases and a linear estimator, that is, least squares (LS), to estimate the amplitudes. This paper also indicates high accuracy of proposed algorithm in comparison with discrete Fourier transform and LS–Adaline (hybrid of LS and Adaline neural network) methods, especially in multiple frequency and highly noisy situations. Performance of the algorithm in noise rejecting even in interharmonics presence is shown by extracted simulation results of MATLAB (MathWorks Inc., Natick, MA, USA). Copyright © 2012 John Wiley & Sons, Ltd.

Power transformers are key elements in a power system. Hence, their maintenance is essential for uninterrupted power supply. In line with this demand, a portable, online diagnostic analyzer is designed which is able to collect temperature data and quantify the concentration of some of the dissolved gases in transformer oil with the help of a non-invasive sensor fabricated by nanotechnology methodology. After conditioning the signals appropriately, the data are transmitted to the substation for storage. The computer also has the ability to remotely access the data for analysis and assess the health of the transformers. From the data of three gases, different faults, if they are occurring inside the transformer, can be predicted. The fault diagnosis is done by dissolved gas analysis, which is one of the proven methods widely used during the last two decades. Copyright © 2012 John Wiley & Sons, Ltd.

Higher penetration of distributed generation in distribution systems makes the islanding operation more realistic. In this paper, an adequacy assessment is presented in the presence of wind energy sources in distribution system for an islanded area. Wind speed simulation is performed using auto-regressive moving average time series. For adequacy assessment, well-being indices are introduced and calculated for an islanded area through an islanding algorithm. Then, sensitivity analysis is conducted to investigate the dependency of well-being indices on key parameters of analysis. Finally, the results are presented and discussed. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, a new framework is presented for multi-objective transmission expansion planning which expansion options are combination of transmission lines and phase shifter transformers. This framework is based on a multiple criteria decision making whose fundamental elements are reliability and market. Investment cost, congestion cost, users' benefit and expected customer interruption cost are considered in the optimization as four objectives. The proposed model is a non-convex optimization problem having a non-linear mixed-integer nature. Hence, a hybrid genetic algorithm and quadratic programming is used followed by a fuzzy satisfying method to obtain the final optimal solution. The planning methodology has been demonstrated on the IEEE 30-bus test system to show the feasibility and capabilities of the proposed model. Also, to compare the practical expansion plan and the expansion plan developed by the proposed method, it was implemented to the part of a real life 400-kV transmission grid. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a new approach to coordinate the decisions of transmission and generation capacity expansion planning for a competitive electricity market in which only the generation sector is deregulated. The independent system operator (ISO) as transmission planner has a regulatory role in the strategic behavior of generation companies. To reach coordinated decisions, the model relies on an interactive and iterative process between generation expansion planning and transmission expansion planning. This repetitive process continues until reaching a converging point or fulfilling the stopping criterion assigned by the ISO. In order to consider random outages of network components as well as uncertainty of load and bid prices of generating units, the Monte Carlo simulation method is applied. The simulation results confirm the efficacy of the proposed model in the coordinated generation and transmission planning problem considering the uncertainties. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a method for treating transmission network bottlenecks in a stochastic market model, through a flow-based market clearing process. The market model is designed for long-term and medium-term scheduling of hydrothermal power system operation, where generators and loads are allocated into regional subsystems or price areas. In addition to detailed hydropower, the model allows the use of detailed wind power data and start-up costs on thermal generators.

A description of the model's overall simulation logic is presented, emphasizing on the introduction of network constraints in the system simulation part. Simulation results at price-area level are disaggregated to a transmission network model. Potential overloads are identified through linearized (DC) power flow analyses. In case of overloads, network constraints are generated by use of aggregate sensitivity factors and added to the market model.

A four-area test case is presented to illustrate the impact of including detailed network analyses on the simulation results. Copyright © 2012 John Wiley & Sons, Ltd.

In this contribution, a control strategy in phase coordinates abc is proposed as the reference current generator to be used by the shunt active power filter (APF) compensation device. The development of the proposed reference current generator is described in detail. Its control design is kept simple, and transformation between different frameworks is not needed, thus allowing the control strategy to be applicable for both three-phase and single-phase electric systems maintaining theoretical and practical advantages. The proposed control strategy for the reference current generator is tested using Matlab/Simulink® with the incorporation of the Real-Time Workshop® for the simulation of the shunt APF compensation in real time. Based on the reported real-time application, it is possible to obtain simulation conditions that are close to the true real-time environment under study using only a single computer, giving accurate results with relatively low-cost hardware and providing a more complete evaluation of the applied control strategy. The dynamic behavior and performance of the shunt APF, using the proposed reference current calculator, have shown remarkable results. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a finite-element frequency domain model for numerical solution of the coupled nonuniform transmission line problem. Based on the finite-element method, a novel numerical procedure for solution of a system of the nonuniform multi-conductor transmission line equations in the frequency domain is presented. The results obtained by the proposed method have been compared to the analytical solution. Copyright © 2012 John Wiley & Sons, Ltd.

Useful lifetime of power cables and related aging failure rates mostly depends on thermal stresses that they experience because of the loss-resulted heat inside the cable in normal or abnormal currents of the network. Knowing the real values of the cable aging failure rate is a key point, and the network operators should enable to estimate the related aging failure rates of their network cables. The already provided methods for cable transient temperature estimation require several different inputs as well as cable installation configuration data that are difficult to collect for operators. Hence, in this article, an artificial neural network–based approach is applied for estimation a cable maximum temperature, which serves a certain daily load curve. The artificial neural network only requires four inputs that are easy to provide. An experimental equation is then used for estimating the cable minimum temperature, and finally, a three-level temperature curve is formed for cable aging failure rate estimation. The life fractions lost during each level of the predicted temperature curve are evaluated by resorting to an already existing combined electrothermal life model held for cable insulation. This method uses the life model and the probabilistic failure model to predict the failure rate of power cables for a future period. The results show that failure rate estimations are in good accordance with exact results, remarking that the estimated three-level stepwise curve of cable temperature is a good approximation of cable thermal transients during cyclic loads. Copyright © 2012 John Wiley & Sons, Ltd.

In recent years, nanometaloxide/epoxy resin composites have attracted interest because of their wide application in the electrical industry. Research on certain nanocomposites indicates that they may be advantageously used in the electrical power industry and as novel dielectric materials. Earlier work in the author's group showed improvement in dielectric properties of nanocomposites prepared with nanosized alumina fillers, e.g. resistance to surface degradation, modification of complex permittivity at low frequencies. In this work, we study the dielectric permittivity of epoxy-based nanocomposites with various metal oxide fillers including spherical alumina (Al₂O₃), titania (TiO₂) and zinc oxide (ZnO) particles. Dielectric spectroscopy measurements and analysis are carried out in the frequency range of 10⁻⁴ to 10³ Hz and, in the temperature range of 25°C to 90°C. To understand the effect of water, measurements are carried out in humid environments of 25% and 75% RH. It is shown that the dielectric behaviour changes significantly at high temperatures and humidities, especially for the nanocomposites. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, the application of the hyperbolic S-transform is proposed for detecting internal incipient faults of transformers during the impulse test. This test is performed on the under consideration transformer and during this test, the input current and applied voltage are recorded. The S-transform is applied to the recorded input current. An index is proposed and computed considering absolute deviations of the S-matrix. Faulty and non-faulty windings are discriminated by using the suggested index. The data obtained from field tests of a 66 kV/25MVA interleaved transformer winding and computer simulations, are recorded. Then, these data are fed to the proposed method, and faulty and normal conditions are discriminated, accordingly. Also, a wavelet-based method is implemented in MATLAB environment and the proposed method is compared with the wavelet-based method. According to simulations and experimental results, the proposed method is superior to wavelet transform-based method. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a novel real-time load modeling method to minimize the impact of load variation in large-disturbance and short-term voltage stability analysis. The proposed approach utilizes the strong tracking filter and the dominant parameter techniques to achieve the goal of real-time load modeling. A historic database of load parameters is built for the load modeling. Two example systems are illustrated to validate the proposed method. The results show that the new approach is able to track the variation of the load online and provide more accurate load model to predict the system voltage stability. Copyright © 2012 John Wiley & Sons, Ltd.

Transmission line is the primary facility for power delivery and is mainly composed of twisted aluminum wires. During the long-term outer door field service, overhead transmission lines will encounter strikes of lightning, corrosion of chemical contaminants, ice shedding, wind vibration of conductors, lines galloping, damage due to external forces and so on which will generally bring a series of latent faults such as aluminum strands fracture. It will induce some fatal accidents such as broken transmission lines which have a very strong impact on the safe operation of power grids if the latent faults can not be recognized and restored as early as possible. The method of detecting broken strands in transmission line by inspection robot with detectors is a method with good prospect. In this paper, a kind of eddy current transducer (ECT) for detecting broken strands in transmission line is developed. The developed ECT can be integrated to the transmission line inspection robot. Simulation and field experiment results show that the magnitude field induced by transmission current has no influence on the detection of broken strands in transmission line. Hence, the developed ECT carried by robot can identify broken strands with high reliability and sensitivity. At the same time, the proposed structural style and design method of the ECT can be applied to different types of transmission line. Copyright © 2012 John Wiley & Sons, Ltd.

Wind farms with fixed speed asynchronous generators generate periodic voltage fluctuations and flicker therefore mainly due to the so-called tower shadow effect. An analytical expression of the current supplied by the wind farm (and the voltage at the connection point) can be derived from a simplified model of the farm with a term that quantifies the relative fluctuation of the electrical variable considered. By its very nature, this term is a random variable since it is the result of the fluctuations introduced by each of the generators, and these depend on the angular positions of the individual turbines. In this paper, the authors, based on the aforementioned expression of the total current of the farm, have analyzed the resulting fluctuation phenomenon as a statistical problem by a new theoretical approach. This has allowed the calculation of the distribution and probability density functions as well as the mean values, the variance and the standard deviation, as expressions depending on the number of generators. Copyright © 2012 John Wiley & Sons, Ltd.

Strikes of lightning, corrosion of chemical contaminants, ice shedding, wind vibration of conductors, lines galloping and damage due to external forces may induce some fatal accidents such as broken transmission lines. The common means to be used for latent faults inspection in transmission lines is manually and periodically examined by workers from commercial electrical power company. With the development of both the artificial intelligence technologies and the smart grid, the method of detecting broken strands in transmission line by inspection robot with detectors is a method with good prospect. In this paper, a method with eddy current transducer carried by robot for detecting broken strands in transmission line is developed and the broken strands in transmission line identification approach based on S-transform is proposed. The proposed approach utilizes S-transform to extract the module and phase information at each frequency point from detection signals. Through module phase and comparison, the characteristic frequency points are ascertained, and the fault information of the detection signal is constructed. The broken strands identification confidence degree is defined with Shannon fuzzy entropy (SFE-BSICD). The proposed approach combines module information while utilizing phase information, SFE-BSICD and the energy, so the reliability is greatly improved. These characteristic qualities of broken strands in transmission line are the input of support vector machine with multi-classification, and then the number of broken strands can be determined. Through field experimental verification, it can be concluded that the proposed approach shows high accuracy and the SFE-BSICD is defined reasonably. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents an optimal planning of passive harmonic filter using the hybrid differential evolution method considering variation of system impedance, harmonic current sources and filter tuning points. The proposed method minimizes an objective composed of the total harmonic distortion of current and voltage, selects different filters topologies, and determines their optimal parameters and tuned points under a constant cost. The harmonic currents and voltages in a power system are expressed as probabilistic models which are obtained using the statistical simulation method. Monte-Carlo simulation is employed to verify the accuracy of the obtained probabilistic models. The method proposed is applied to solve the harmonic problems in a practical chemical plant, and the results show that the proposed method is effective for passive filter planning in industrial distribution systems. Copyright © 2012 John Wiley & Sons, Ltd.

Phase-locked loop schemes are widely used in induction heating applications for controlling the converters. Most authors use total or partial analog solutions to implement the control schemes, which are less sturdy and flexible to changes than digital schemes. In this paper, an all-digital DSP-based PLL is presented. It has fewer variations due to the tolerances of the analog components and provides great flexibility in a wide range of conditions just by reprogramming. Moreover, it allows changing dynamically some of the parameters during the process and opens up the possibility of more complex control algorithms. The all-digital PLL has been simulated and experimentally tested, demonstrating the feasibility and reliability of this type of control in a real industrial equipment. Copyright © 2012 John Wiley & Sons, Ltd.

This paper develops a new approach to fairly clear the market and assess the amount of each participant's revenue in a deregulated power system, through modifying the system nodal prices. This approach equalizes the economic profit of each participant group to its fair and rational value. Transco behavior is modeled as an independent entity without direct supervision of an independent system operator, causing the possibility of either making profit or incurring a loss for it. With this method applied, three main results for better system design and operation are produced:

1. The rational and acceptable profit/loss values for all participant groups (producers, customers, and Transco) are determined through fair apportionment of total system profit among them based on their optimality and contribution of profitability to the whole system.
2. The profit/loss value for each of producers and consumers in the system is allocated according to their proportional efficiency and extent of transmission use.
3. Transmission revenue assessment is performed through defining a criterion for evaluating the transmission network optimality.

So this approach can correct wrong and unfair economic signals in the previous electricity pricing and participants' revenue assessment methods, presenting such a fairer mechanism for it that each participant's action will be oriented in maximizing the system's total economic efficiency. Copyright © 2012 John Wiley & Sons, Ltd.

Holidays load forecasting is one of the most challenging topics in the short-term load forecasting area. This is mainly due to different load behaviors and insufficient samples of holidays compared with the normal weekdays. This paper proposes an effective method for holidays short-term load forecasting that is robust to face with holidays of lunar year and simple for practical applications. The first step is to identify the essential characteristics of the holiday's profiles. For each characteristic, one similar day is selected. The essential load characteristics of the intended holiday are derived from these similar days. The holiday's load is forecasted by combining these characteristics and then will be modified using a fuzzy method. The proposed method has been implemented on Isfahan grid. Results show the proper performance of the proposed method especially for the holidays from the lunar year, which are varying compared with the solar year. Copyright © 2012 John Wiley & Sons, Ltd.

As the main protection of HVDC line, the protection zone of conventional travelling wave protection depends on the predetermined setting value. It also can't detect the high-impedance grounding fault due to its using of data from only one end. To improve the protection performance, a travelling wave-based pilot direction comparison protection is proposed based on the fact that the amplitude of the first backward travelling wave is greater than that of forward travelling wave for a forward fault. The magnitudes of forward and backward travelling wave are integrated, respectively, within a short period following the relay starter has picked up. The ratio of the integral values is used as the fault direction discriminator. If both relays at the two ends of the HVDC line detect it a forward fault, it will be judged as an internal fault, and the faulty pole can be determined by the polarity comparison of aerial and earth mode backward traveling waves. The RTDS-based simulation tests are carried out with consideration of dc filter effect, and the results demonstrate that the proposed scheme can discriminate the internal and external faults rapidly and reliably. It is tolerant of a high resistance coverage and has a specific protection zone. Copyright © 2012 John Wiley & Sons, Ltd.

The EMTDC/PSCAD simulation can be used to study the closed-loop transfer characteristics of HVDC system, but it is very difficult to understand analytically the transfer characteristics of each system element, i.e. transfer characteristics of valves of HVDC system. Thus, the model based on the modulation theory has been widely used to describe the open-loop valves transfer characteristics of thyristor-based convertors. However, it is found that there is discrepancy between the EMDC/PSCAD simulations and the open-loop valves transfer model based on the modulation theory in subsynchronous oscillation analysis. It is further revealed that when employing the model based on the modulation theory, the assumption that the valves conduct at equal time intervals is no longer true in low and subsynchronous frequency band. Hence, in this paper, a new model is proposed to describe the valves transfer characteristics of the thyristor-based HVDC convertor, which helps understand the behavior of valves conducting at unequal time intervals. An EMTDC/PSCAD-based simulation system is employed to verify the new model and compare the results of the new model with that of the model based on the modulation theory in the low and subsynchronous frequency band. The new model is useful to understand how the additional spectral components are generated by the HVDC system considering the transfer characteristics of the converter in low and subsynchronous frequency band. Copyright © 2012 John Wiley & Sons, Ltd.

Because of the variable nature of loads and the presence of harmonics in a network, the variation of capacitor bank steps results in the reduction of capacitor life. Previous studies calculated the optimum capacitor in networks with variable loads to maximize the power factor. This paper calculates the optimum capacitor in harmonic networks, taking into consideration the effect on capacitor lifetime of load variation and harmonics. For this purpose, genetic algorithm was employed to determine the optimum capacitor. The proposed method was applied to an actual network in the Province of Markazi in Iran. The results showed this method to be flexible and reliable. Copyright © 2012 John Wiley & Sons, Ltd.

Fault diagnosis following a disturbance in a power system is of great importance for the operators in the control center as a prerequisite for system restoration. In this article, for the first time, an analytic method for fault diagnosis, using the wide area measurement system (WAMS) and employing circuit rules, is developed. Instead of conventional information about status of protective relays and circuit breakers, voltage and current phasors at different points of the power network after fault occurrence are utilized. Because most of the power systems will be equipped with WAMS consisting of high-speed sampling features, the proposed method introduces a new application of WAMS for fault diagnosis without the need for additional hardware. In this method, with the aid of network impedance matrix, analytic voltage and current phasors resulted after a fault in each of the fault candidate points are compared with those monitored by WAMS. The faulted point is where the best fitting coincides with the monitored voltage and current phasors. The proposed method is applied on IEEE 118-bus test system where the results confirm its ability in wide-area fault diagnosis of large power networks. Copyright © 2012 John Wiley & Sons, Ltd.

The maintenance strategy of power transformer, which is determined by the operational condition, is very important to ensure safety for power system. But there exists prevalently the problem of lacking theoretical basis in preventive maintenance strategy to power transformer at present. Failure mode and effects analysis (FMEA) is a methodology for the analysis of potential failure modes within a system and has been extensively utilized for examining potential failures of power transformer. The critical issue of FMEA is the determination of risk priorities of potential failure modes. In order to provide the basis for risk assessment and preventive maintenance decision making, this work proposes the FMEA based on cloud model for prioritizing failure modes, which can combine randomicity with ambiguity and realize the transform between qualitative evaluation and quantitative numerical value, intending to overcome limitations of traditional FMEA. Two numerical examples are utilized to illustrate the potential applications of the proposed FMEA and the detailed computational process of the risk priority number based on cloud model. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents details on the implementation and application of power hardware in-the-loop simulation for several physical high voltage direct current (HVDC) simulators using transmission line modeling (TLM) method. The implementation of the TLM method to the decoupled simulation of HVDC links is presented. A small test system is then used to demonstrate the feasibility of the TLM decoupling method. A real-time interface method relying on the propagation delay of transmission lines is developed and verified. Case studies presented in this paper indicate that it is possible to have interactions between the HVDC terminals in an AC system. A PHIL simulation similar to what is presented in this paper will provide an alternative to study the muti-infeed power grid. Copyright © 2012 John Wiley & Sons, Ltd.

With the continued development of renewable energy generation technologies and increasing pressure to combat the global effects of greenhouse warming, plug-in hybrid electric vehicles (PHEVs) have received worldwide attention, finding applications in North America and Europe. When a large number of PHEVs are introduced into a power system, there will be extensive impacts on power system planning and operation, as well as on electricity market development. It is therefore necessary to properly control PHEV charging and discharging behaviors. Given this background, a new unit commitment model and its solution method that takes into account the optimal PHEV charging and discharging controls is presented in this paper. A 10-unit and 24-hour unit commitment (UC) problem is employed to demonstrate the feasibility and efficiency of the developed method, and the impacts of the wide applications of PHEVs on the operating costs and the emission of the power system are studied. Case studies are also carried out to investigate the impacts of different PHEV penetration levels and different PHEV charging modes on the results of the UC problem. A 100-unit system is employed for further analysis on the impacts of PHEVs on the UC problem in a larger system application. Simulation results demonstrate that the employment of optimized PHEV charging and discharging modes is very helpful for smoothing the load curve profile and enhancing the ability of the power system to accommodate more PHEVs. Furthermore, an optimal Vehicle to Grid (V2G) discharging control provides economic and efficient backups and spinning reserves for the secure and economic operation of the power system. Copyright © 2012 John Wiley & Sons, Ltd.

Active power filters have demonstrated effective harmonic compensation of dynamic nonlinear loads in DC traction systems. In this paper, it is proposed a practical methodology for allocation of active power filters in DC traction systems using the least number of filters and minimizing the size of them. Active compensation is placed at the most sensitive locations in the power system with the aim to meet the harmonic standard levels presented in the IEEE 519 standard. The proposed methodology has the advantage that the decision of allocation is based on the dynamic behavior of traction load. The usefulness of the methodology is illustrated in a model of DC traction system. Copyright © 2012 John Wiley & Sons, Ltd.

Conventional feeding systems for DC electric arc furnace steel making plants usually include a 12-pulse rectifier that is fed by Δ/Δ and Δ/Y three-phase transformers. In this paper, with the use of the PSCAD/EMTDC software (Developed by Manitoba HVDC Research Center Inc., Canada), the advantages of applying multi-phase transformers for supplying such loads are evaluated and compared with the traditional three-phase power supply systems. A well-designed secondary winding for multi-phase transformer is proposed, and then the modeling of such transformers is discussed. With some proper power quality indices applied, it is shown that utilizing multi-phase transformers can lead to power quality improvement such as current/voltage harmonic reduction and unbalance mitigation. Copyright © 2012 John Wiley & Sons, Ltd.

The work reported in this paper adds damping to the inter-area mode of oscillation in a power system with relatively high penetration of wind power, by modulating active power output of the permanent magnet synchronous generator wind turbines (PMSG-WTs) using local information. The dc-link voltage of the voltage source converter is used as a local signal for controller input. This input signal is processed through phase compensator blocks, and the output of the controller is used to vary the active power reference signal of the machine side converter controller. The results are compared to the cases of the same supplementary controller loop with a global signal as its input, added to the both PMSG-WTs and doubly fed induction generator wind turbines. An optimization-based approach is proposed to adjust and coordinate various control loops of the grid-connected PMSG-WTs. The performance of the supplementary damping control loop is evaluated, employing modal analysis and time domain simulations of a power system test case. Copyright © 2012 John Wiley & Sons, Ltd.

The dual-channel switched reluctance machine (DCSRM) is a kind of special machine, which is developed to enhance reliability in some safety critical applications. When the DCSRM works under dual-channel operation (DCO) mode, the two channels of DCSRM would have strong interactions between each other due to the magnetic mutual coupling for each channel. This paper proposes a fast analytical model considering the mutual coupling for the DCSRM based on the magnetic charactersitics of self- and mutual inductances. The rapid analytical phase current models are deduced in several different inductance regions, and the different analytical phase current and torque models for the DCSRM under single-channel operation (SCO) and DCO modes are also compared. Using this proposed model, the simulation results of steady-state phase current, average torque-speed and average torque-copper loss characteristics are compared with the experimental results at the same conditions in a 12/8 DCSRM systems under the SCO and DCO modes, which verify this proposed modeling and analysis. Copyright © 2012 John Wiley & Sons, Ltd.

Partial discharge (PD) activity of the inter-turn insulation of electric motor winding is investigated under different rise time of the square wave voltage, and the superposition effect of the polarized charges on PD activity is also analyzed for the specimen. Under different rise time of the square wave, tests of the PD activity (e.g., PD inception voltage (PDIV) and phase resolved PD (PRPD) pattern) and the thermal stimulated depolarization current are both performed. Comparing the results of the PRPD patterns at different rise time of the square wave, most of PD moves to the zero point of square wave voltage with the reduction in rise time. Also, PDIV decreases with the reduction in rise time. Through the analysis of a complete polarization process at the square wave voltage, the superposition effect of the polarized charges leads to the enhancement of electric field in the specimen when the polarity of square wave voltage rapidly reverses. With the reduction in rise time at square wave, more polarized charges can not follow changes of the applied voltage since the relaxation time of these dipoles is longer than the rise time of square wave, which leads to the enhancement of the electric field across a cavity. For this reason, they result in the reduction in PDIV and the movement of PRPD patterns to zero point of square wave voltage. Copyright © 2012 John Wiley & Sons, Ltd.

This paper investigates the characteristic performance of broken bar faults in induction motors at standstill by 2D time-stepping finite-element and experiential approaches. Three identical rotors with the same stator are employed to generate one healthy-rotor, one-broken-bar and two-broken-bar fault cases in a three-phase 1.1-kW squirrel-cage induction motor. The focus is placed on how the broken bar affects air-gap flux density, stator and rotor currents, torque and magnetic force, as well as iron and rotor bar losses. Numerical results are validated by some experimental measurements obtained from a dedicated test rig. Copyright © 2012 John Wiley & Sons, Ltd.

This article presents a comparative study among four control strategies for the energy management of a fuel cell (FC) battery–powered hybrid system applied to a surface tramway of 400 kW, which, at present, operates as catenary-powered tramway in the center of Seville, Spain. The proposed hybrid system is composed of a polymer electrolyte membrane FC with a unidirectional dc/dc boost converter and a rechargeable nickel–metal hydride battery with a bidirectional dc/dc converter, both of which are coupled to a traction dc bus. This configuration allows a suitable control of the energy of the hybrid tramway and the traction dc bus voltage.

The polymer electrolyte membrane FC and nickel–metal hydride battery models are designed from commercially available components. The control strategies presented in this work are as follows: (i) control strategy based on operating states of the tramway (state machine control), (ii) control strategy based on cascade control loops (cascade control), (iii) control strategy based on fuzzy logic (fuzzy logic control), and (iv) control strategy based on the fuel consumption minimization (equivalent consumption minimization strategy. The control strategies are tested and compared for the real driving cycle of the tramway. The simulation results reflect the optimal performance of the presented control systems. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a methodology for fault analysis of multiphase systems with the following innovations: (i) a breaker-oriented modeling approach that enables the computation of fault currents through any circuit breaker in a system and (ii) the use of a three-phase model with explicit representation of neutrals, ground wires, and grounding systems. Explicit grounding models allow the simulation of faults to neutrals and to ground wires and help evaluate ground potential rise. The proposed breaker-oriented fault analysis is utilized in a Monte Carlo simulation to provide a statistical profile or distribution of fault currents through any circuit breaker. Such fault current profiles permit an evaluation of the risk of breaker failures caused by increased fault current levels that result from the expansion of power systems. The proposed fault analysis methodology is illustrated using a three-phase, breaker-oriented 24-substation test system. Copyright © 2012 John Wiley & Sons, Ltd.

A multi-string grid-connected two-stage photovoltaic system acting as an active power filter with anti-islanding and fault protection is presented. The inverter is current controlled for harmonic compensation. Harmonics in the load current are detected by a time-domain technique. A time-domain technique based anti-islanding method is proposed. Moreover, our time-domain technique is also utilized to detect faults in the dc bus of the inverter. The simulations were performed in PSCAD/EMTDC, and experimental results are provided to validate the concept. Copyright © 2012 John Wiley & Sons, Ltd.

The new tendencies in the power system organization and the fast-changing technologies in the power industry dictate the need to keep track of the international experience and activities in the field of the modern economic dispatch problem. The goal of this paper was to provide a detailed account for papers published after 1990, the year that saw the beginning of major transformations in the power system organization. A comprehensive survey on mathematical formulations and a general background of methods, analyses, and developments in the field of economic dispatch is presented for the past 20 years based on more than 150 publications. The research literature in the field is classified into sections covering economic dispatch in both regulated and deregulated (reregulated) energy markets and where variable, partial predictable electricity generation is part of the generation portfolio. A database of the most common test systems used in the literature to test different economic dispatch methodologies is also provided. Copyright © 2012 John Wiley & Sons, Ltd.

In recent years, because of structure renewal, development, and progressive distribution, many of the economical and technical characteristics of power systems in the various parts of production, transmission, distribution, and consumption have been changing. Especially, the transmission systems have been affected to a great degree because of the increase in demand and limitation in the new lines formation. One of the shortcomings of this affair is the risk of losing stability after turbulence occurrence. The controllability and stability of power systems can be increased by FACTS devices. One of the FACTS devices is Interline Power-Flow Controller (IPFC) by which the voltage stability, dynamic stability, and transient stability of power systems can be improved. In the present paper, the convenient operation and control of IPFC for transient stability improvement are considered. Considering that, the system's Lyapunov energy function is a relevant tool to study the stability affair. IPFC energy function optimization has been used in order to access the maximum of transient stability margin. In order to control IPFC, ANFIS which is a neuro-fuzzy controller has been used. The designing results have been studied by the simulation of a single-machine system with infinite bus and another standard 9-buses system (Anderson and Fouad, 1977). Moreover, with simulating of other FACTS devices, such as unified power flow controller (UPFC), static synchronous series compensator (SSSC), and static synchronous compensator (STATCOM) and comparing the results, the superiority of IPFC is proved. Copyright © 2012 John Wiley & Sons, Ltd.

There are various types of fault current limiters (FCLs), which are made of different superconducting materials and have different designs. The bridge-type FCL is mainly composed of a diode bridge rectifier and a DC reactor. This FCL can control the fault current by controlling the DC reactor current. To enhance this capability, a resistor parallel with a semiconductor switch has been used in series with DC reactor. In this article, hysteresis technique has been proposed and integrated in this structure, which not only limit the fault current but also reduce total harmonic distortion (THD) at the point of common coupling (PCC). Therefore, the performance of all loads connected to PCC will not be affected during fault. The theory and properties of the proposed bridge-type FCL for limiting the fault current and producing sinusoidal voltage at PCC during fault has been presented, and its model has been simulated by power systems CAD/electromagnetic transients including DC. It is shown that the proposed FCL can effectively reduce the THD at PCC and limit the fault current. Copyright © 2012 John Wiley & Sons, Ltd.

The paper presents an unbalanced three-phase power-flow model for wind turbine generating systems (WTGSs). The model takes into account voltage unbalance factor which exists at the point of common coupling. The developed model is integrated with the unbalanced forward/backward sweep power-flow method. The model comprises of three main components: they are the wind turbine, induction generator, and interface transformer to the grid. Due to their design symmetry, the generator and the transformer are modeled using symmetrical sequence networks. The results show that the developed model has robust convergence characteristics. The solution of the IEEE unbalanced radial feeders shows that the injected powers per phase due to the WTGS are not equal and strongly dependent on the voltage unbalance factor. The results also show that the simplified models based on positive sequence network lead to inaccurate overall power-flow solution. Copyright © 2012 John Wiley & Sons, Ltd.

This paper proposes a hybrid particle swarm optimization and simulated annealing (PSO-SA) method to solve the dynamic optimal power flow (DOPF) problem while the prohibited zones, valve-point effects, and ramp rate constraints are taken into account. In the conventional optimal power flow, the total load is constant and the problem is solved for just one period, but in the proposed approach, the multiperiod OPF is considered. Also, the operating region of the units having prohibited zones is broken into isolated feasible sub-regions, which results in multiple decision spaces for the discontinuous OPF problem. Also, nonlinear characteristics of the alternative current (AC) power flow as well as technical constraints, for example, valve-point effect and transmission constraints, are all considered for the realistic operation, and they further complicate the proposed problem. These features make the DOPF as a complicated nonlinear and non-convex optimization problem. This paper proposes a hybrid PSO-SA algorithm to solve the DOPF problem. The hybrid PSO-SA algorithm can efficiently search and explore solution space while it profits from privileges of both PSO and SA algorithms. The IEEE 30-bus test system is implemented to illustrate the application of the proposed modeling framework. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a polynomial approximation method to give an explicit expression for the boundaries of small-signal stability region (SSSR) based on the implicit function approach. Different from most of the current methods, the proposed method solves the problem of SSSR boundary approximation directly in the high-dimensional parameter spaces. To settle the problem of local validity due to the polynomial approximation, we further put forward an optimization formula to estimate the credible region of the approximation with a given accuracy. The correctness and effectiveness of the proposed methods are verified through the case studies on a rudimentary power system and the real Hainan power grid of China. Copyright © 2012 John Wiley & Sons, Ltd.

The main goal of Generation Expansion Planning (GEP) is to minimize total costs associated with new power generating units’ installation subject to technical and economical constraints. This paper addresses the GEP with probabilistic reliability criteria. The Loss-of-Load Probability reliability index is explicitly augmented as a new constraint which takes into account the reserve requirements. The outage cost is represented by the Expected Energy Not Served index. Due to nonlinear nature of these reliability indices, the GEP optimization problem with reliability criteria is very complicated to solve. Accordingly, the focus of this work is to deal with the reliability constrained multi-period GEP problem as a Mixed Integer Linear Programming (MILP). The results in the case study indicate the effect of reliability considerations on decision-making process. The simulation results also show the superiority of the proposed MILP-based method in comparison with the well-known metaheuristic algorithms and Dynamic Programming approach in the viewpoints of the accuracy and computational speed. Copyright © 2013 John Wiley & Sons, Ltd.

This paper analyzes two control strategies of doubly fed induction generator (DFIG)-based wind turbine under unbalanced voltage which were introduced in the previous works and then proposes a new control strategy to achieve more improved performance of DFIG. Under unbalanced conditions, the rotor side converter (RSC) and grid side converter (GSC) are controlled in two positive and negative ((dq)⁺ and (dq)^–) reference frames. The control of these converters in the (dq)⁺ frame is the same as the conventional control under balanced grid conditions. In the proposed control strategy, the control of the RSC in the (dq)^– frame is done to balance the stator currents, and also the total active power oscillations are removed by the control of the GSC in this frame. In this strategy, the oscillations of both the torque and stator active power are properly reduced even when the voltage unbalance is somewhat severe. Consequently, the elimination of the stator active power oscillation by the active power output from the GSC is accomplished with fewer constraints. To provide the accurate coordinated control of the RSC and GSC, this proposed control strategy is implemented in the stator-voltage oriented frame. The simulation results obtained for a 7.5-kW DFIG-based wind turbine validate the enhancement of system operation using the proposed control strategy during unbalanced voltage network. Copyright © 2012 John Wiley & Sons, Ltd.

High penetration of distributed energy resources (DERs) has led to considerable evolution in the operational aspects of distribution systems. As a result, distribution companies (DISCOs) tend to utilize optimization in order to schedule their DERs to meet their demand as well as participation in the electricity markets. This study presents a comprehensive operation model for a distribution system which involves DISCO participation in energy production and reserve providing activities. The uncertainties are modeled by means of a chance constraint representing the confidence level of serving load by DISCO. The presented model can incorporate DERs (both dispatchable and non-dispatchable units) along with network constraints and load and wind uncertainties in order to achieve optimal decisions in both day-ahead energy and reserve markets. A modified 32-bus distribution network including dispatchable generators, electric energy storage, wind turbine units, interruptible loads, and interties is employed to illustrate the effectiveness and feasibility of the proposed method. Copyright © 2012 John Wiley & Sons, Ltd.

This contribution deals with the analysis of synchronous generator loss of excitation (LOE) protection. This paper presents a new method for detecting LOE in synchronous generator based on the variation of the magnetic flux linkage in the air gap. To show the effectiveness of the proposed protection, comparisons are made with the conventional LOE protection methods by considering different generator sizes. The simulation results show that the proposed method is useful and effective to detect LOE conditions. The main advantage of this method is its speed and sensitivity. Copyright © 2012 John Wiley & Sons, Ltd.

In this article, the self-recurrent wavelet neural network (SRWNN) is used as a controller in both direct and indirect adaptive control structures to damp the low-frequency power system oscillations when only the inputs and outputs of synchronous generator are accessible for measurement. The gradient descent method using adaptive learning rates (ALRs) is applied to train all weights of SRWNN. The ALRs are derived from the discrete Lyapunov stability theorem, which was applied to guarantee the convergence of the proposed control schemes. Finally, the proposed control schemes are evaluated on a single machine infinite bus power system under different operating conditions and disturbances to demonstrate their effectiveness and robustness. Copyright © 2012 John Wiley & Sons, Ltd.

In order to extend the range of a constant-power operation regulation of the AC drive system, the electronic pole changing (EPC) technique for multiphase induction motor (IM) is proposed based on multiphase system transformation theory and the vector control (VC). This proposed technique can extremely expand a constant-power operation range without increasing the volume or current of an IM. The harmonic currents are generated in different orthogonal planes by VC to realize EPC without the poweroff. The EPC process is continuous and smooth, and the torque impulse is not obvious. An EPC experiment of a nine-phase IM is demonstrated in this paper. The experimental results verify that the proposed EPC technique has the ability to extend the operation range of multiphase IM. Copyright © 2012 John Wiley & Sons, Ltd.

When fiber optic probes were taken into use to record local hot-spots in windings and oil ducts, it was noticed that the hot-spot temperature rise over top-oil temperature due to load changes is a function depending on time as well as the transformer loading (overshoot time dependent function). This results in winding hottest spot temperatures higher than those predicted by the old loading guide during transient states after the load current increases, before the corresponding steady states have been reached.

The new loading guide, (IEC 60076–7), takes into account these findings by introducing two thermal models. The first one, which is given as an exponential equation solution, is suitable for a load variation according to a step function. This method is particularly suited to determination of the heat transfer parameters by test, especially by manufacturers. The second one, which is given as a differential equations solution, is suitable for arbitrarily time-varying load factor K and time-varying ambient temperature θa. This method is particularly applicable for on-line monitoring. This method is a mathematical variation of the exponential model. A significant advantage of the suggested thermal models is that they are tied to measured parameters that are readily available (i.e., data obtained from a non-truncated heat run test performed by the transformer manufacturer).

However, it has been reported recently by many users that this new approach causes a lot of technical hitches. It is unclear when and why the exponential method should be used instead of or the differential method and vice versa. Also, is there any difference between the recommended models, especially in their output? How the new thermal constants, (k₁₁, k₂₁, and k₂₂)_, should be applied and how they could be derived? Answers to these questions and more are given in the text below. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents a new synthetic methodology for detection and classification of different Power Quality Disturbances (PQDs). Gabor Transform (GT) integrated by a Probabilistic Neural Network (PNN) model is designed for implementation of a pattern recognition system. The approach uses features extracted from the output Matrix of GT as an input vector of a PNN classifier. The key attribute of GT is that it yields good time-frequency resolution with low computation burden. The PNN learner without any iteration for tuning weights classifies nine types of the most common PQDs including simultaneous events. The performance of the algorithm based on the combination of the GT and PNN (GT-PNN) is evaluated by generated data using parametric equations and a simulated network in the PSCAD/EMTDC software environment. The obtained numerical results confirm the effectiveness of GT-PNN approach for recognition of the different PQDs. Moreover, the classification accuracy is evaluated in the noisy conditions. Copyright © 2012 John Wiley & Sons, Ltd.

Most of the modern home appliances use power electronic circuits as a power supply to interface with the distribution grid. They have become a major source of harmonic distortions in power distribution systems. This article presents a study of the harmonic current characteristics of such appliance through a measurement-based comparative approach. Magnitude-related indices are introduced to compare the harmonic effects of the various appliances and to quantify their relative severity. Phase angle–related indices are introduced to study the diversity of the current harmonics phase angle among the devices and to understand its effect on harmonic cancellation. The proposed approach can be used to establish a database of harmonic characteristics for home appliances and other small, distributed harmonic-producing loads. Copyright © 2012 John Wiley & Sons, Ltd.

This paper describes a hitherto unstudied malfunction that can occur in the protection system of distribution networks (DNs) in the presence of large photovoltaic plants (LPVPs). The focus here is on the phase-balance current (PBC) protection of overhead or blended feeders back-fed by LPVPs. This study used the DNs belonging to ENDESA Distribución Eléctrica S.L.U. In the ENDESA utility company, PBC protection (ANSI 46) is an obligatory DN protection practice for overhead or blended feeder types. Low power quality or two ferroresonant states were found to be the causes of the malfunction of PBC protection, which led to its untimely tripping.

The results of this study highlight the need to modify current DN protection practices and to elaborate new power quality requirements for PV-distributed generation (PV-DG). Since PV-DG penetration in DNs is steadily increasing, regulators and DN operators should implement more effective practices and guidelines to offer better power quality to customers. Copyright © 2012 John Wiley & Sons, Ltd.

Power systems as one of the key infrastructures play a crucial role in any country's economy and social life. A large-scale blackout can affect all sectors in a society such as industrial, commercial, residential, and essential public services. However, the frequency of large-scale blackouts across the world is not being reduced, although advanced technology and huge investment have been applied into power systems. Given a single blackout, it is possible to analyze the causes with the traditional engineering methods. What we want to do is not to explain the causes of blackouts but to find what are the most critical elements of the power system to improve the resilience of the system itself. As blackout can happen in different load conditions, we do not want a method that depends on the load/generation level. We want a method independent from these factors: This is the structural perspective.

When the interconnection between European and Russian power grids will create the largest interconnected power grid throughout the world in terms of the scale, transmission distance, and involved countries, analyzing the vulnerability of a large-scale power grid will be useful to maintain its reliable and secure operation. To analyze the vulnerability of the interconnected power grid, in this article, we first created the interconnected transmission network between continental Europe and the Commonwealth of Independent States (CIS) and Baltic countries; then, the structural vulnerability of the interconnected power grid was analyzed from a topological point of view using our proposed extended topological method, which incorporates some electrical engineering characteristics into complex network methodology. We found that these power grids of continental Europe, the Baltic states, and the CIS countries can benefit from the interconnection because the interconnected power grid can not only improve the overall network performance of these power grids in the Baltic states and the CIS countries but also increase their structural robustness. Copyright © 2012 John Wiley & Sons, Ltd.

Sub-synchronous resonance (SSR) is a frequent adverse phenomenon in series-compensated lines, threatening either or both mechanical facilities and power system stability. It is widely accepted that flexible AC transmission system (FACTS) can provide an effective solution to relieve the SSR. On the other hand, the advent of distributed FACTS (D-FACTS) technology has led to more economical deployment of the flexible devices in the power system control. This article focuses on verifying the capability of the distributed power flow controller (DPFC) to alleviate the SSR. As a member of the D-FACTS family, the DPFC operates in the same way as the unified power flow controller (UPFC). Compared with UPFC, the DPFC has some advantages like higher control capability, lower price, and more reliability. The DPFC impact on SSR mitigation is interrogated in various case studies, namely, with the SSR damping (SSRD) controller on the series converter, with SSRD controller on the shunt converter and with both simultaneously operating. The best case for SSR mitigation is achieved when the DPFC is supplied with two SSRD controllers concurrently. Simulations are carried out with Power Systems Computer Aided Design/ Electromagnetic Transients including DC (PSCAD/EMTDC) on the Institute of Electrical and Electronics Engineers (IEEE) first SSR benchmark model aggregated with DPFC on the transmission line. Furthermore, a comparison between the SSRD controller on DPFC converters and the supplementary excitation damping controller or the conventional power system stabilizer is also carried out. Copyright © 2012 John Wiley & Sons, Ltd.

Considering the economical and environmental significance of the integration of wind units into power systems, this article proposes a stochastic market clearing model for joint energy and reserve auctions wherein wind power producers can participate in the electricity market along with other participants. The proposed model takes into account the uncertainty of wind power and forced outages of generating units and branches. Uncertain resources in the power system are modeled using a new scenario generation technique. After scenario reduction, the selected scenarios are included in the stochastic joint market clearing model with AC network constraints, leading to a mixed integer nonlinear programming formulation. The proposed stochastic programming model is tested on the IEEE Reliability Test System. Obtained results confirm the validity of the developed approach. Copyright © 2012 John Wiley & Sons, Ltd.

This article presents a new approach for power transformer protection based on a hybrid pattern recognition scheme. The hyperbolic S-transform (HST) is a very powerful tool for analysis of the nonstationary waveforms because it is able to extract the information from transient signals simultaneously in both time and frequency domains. Magnitude, frequency, and HST contours are the main attributes obtained from the output matrix of HST. At first, differential current waveforms of different conditions such as normal, internal and external faults, inrush, and over-excitation conditions are analyzed by HST and some potential useful features are extracted from the abovementioned contours. To decrease the dimension of feature vector and to increase the classification accuracy of the proposed algorithm, the most effective features are selected by using well-known feature selection methods namely sequential forward selection, sequential backward selection, and genetic algorithm. Selected features are trained by a probabilistic neural network as an effective classifier core, which has advantages regarding learning speed and generalization capability compared with feed-forward neural network. The classification accuracy of the proposed algorithm has been used as a criterion function for the selection of the best subset features. The proposed protection scheme is evaluated for various operating conditions of three different transformers using the PSCAD/EMTDC package. Extensive simulation results show that the proposed algorithm relies only on the waveshape properties, and it is independent of the value of transformer parameters and consumed power. Copyright © 2012 John Wiley & Sons, Ltd.

Measurement algorithm is the important element of Intelligent Electronic Devices (IEDs) used in protection and monitoring of transmission and distribution networks. The function of the measurement algorithm is to estimate fundamental frequency component (phasor) of current and voltage signals recorded during network faults. The estimated fundamental component is used by variety of protection algorithms for the successful operation of the IEDs. The most popular measurement algorithm is the Discrete Fourier Transform (DFT). However, the estimation of the fundamental frequency component by the DFT algorithm will produce error if input signal contains nuisance components such as a decaying DC offset. This paper evaluates the performance of DFT measurement algorithm when its input signal is influenced by variety of the nuisance components. Various random factors will impact the amount of nuisance components present in a measured signal. Therefore, we propose a novel global uncertainty and sensitivity analysis technique to determine in a systematic way the impact of nuisance components on the performance of measurement algorithms. In addition to the full-cycle DFT, we analyze performance of half-cycle DFT and full-cycle cosine filter algorithms. Impact of nuisance components due to current transformer (CT) saturation and without saturation was studied. Steady-state and transient performance indices are defined to evaluate the performance of the algorithms. Copyright © 2012 John Wiley & Sons, Ltd.

This paper aims to determine unique features in voltage and current waveforms of a given disturbance event so as to automatically identify its root cause. In particular, the paper focuses on short-circuit faults caused by external factors such as animal and tree contacts, lightning-induced events, and cable failures. The proposed methodology consists of analyzing sets of known events caused by similar external agents to identify unique features characterizing the set and at the same time discriminate the remaining event subset. The proposed methodology has been implemented and tested using real-world fault events with a classification rate of 93.4%. This result demonstrates a good performance in identifying the cause of the events. In addition, the methodology rejects those events that do not follow any cause. Copyright © 2012 John Wiley & Sons, Ltd.

This article proposes a novel approach for price-based market partitioning to mitigate the congestion problem. The conventional fuzzy C-means clustering algorithm is reported in previous works for market partitioning, but it may be inefficient or unstable especially in large-scale power systems. Intrazonal congestion and fragmented zones are two important reported deficiencies of market partitioning algorithms. To enhance the congestion management, the proposed algorithm takes directly the effect of congested lines using a suitable index. In this approach, the fuzzy C-means is used in a new manner to stabilize and improve the effectiveness of the algorithm and to mitigate the reported deficiencies of previously market partitioning methods. Numerical studies on IEEE 118-bus test system show the algorithm efficacy. The results do not include any intrazonal congestion or fragmented zone. Copyright © 2012 John Wiley & Sons, Ltd.

In this paper, a modified simulated annealing technique is proposed for the simultaneous improvement of power quality and optimal placement and the sizing of fixed capacitor banks in a modern distribution network. The latter is supplying a mix of linear and nonlinear loads imposing voltage and current harmonics. Besides, the network includes integrated variable-speed wind turbines as distributed generation. The stochastic power output of the wind generation is considered by performing the Monte Carlo simulation of the distribution power flow. Operational and power quality constraints include the bounds of node voltage magnitude, the node voltage total harmonic distortion, and the number/size of installed capacitors. The net annual economic saving is defined as the associated objective function that accounts for the cost of the released power capacity because of the power losses reduction, the cost change of energy losses, and the extra cost of the searched capacitor banks. Solution results are discussed for a model-distorted distribution system under different conditions. Copyright © 2012 John Wiley & Sons, Ltd.

Because of the oligopoly structure of the electricity markets, the Generation Companies (GenCos) may exert market power by using bidding strategies such as capacity withholding. This brings the market clearing price higher and the quantity generated of the market lower. In this article, capacity withholding in an oligopolistic electricity market in that all GenCos bid in the supply function equilibrium model is analyzed and the capacity withheld index, the capacity distortion index and the price distortion index are formulated. Demand elasticity and forward contracts can significantly affect the GenCos' capacity withholding in the market. The potential ability of market for capacity withholding is measured by the distortion–withheld index proposed in this article. The finding in this article is helpful for market power monitoring and mitigating. A two-settlement market including a forward market and a spot market is used to describe GenCos' strategic forward contracting and spot market competition. Copyright © 2012 John Wiley & Sons, Ltd.

Different radial eccentricities in salient pole machine are well documented in previous studies. In reality, the most probable case is the rotor eccentricity, which is not uniform down the axial length. This article determines a precise model of salient pole synchronous machine for general axial eccentricity condition. Proposed model allows calculating salient pole machine inductances with different static, dynamic, and mixed axial eccentricities in a unified technique. By taking into account machine geometry, slots effects, and type of winding connection, this model is able to consider most of the important features of a salient pole machine. A variant of modified winding unction approach has been used to calculate the inductances under axial eccentricity conditions. The effects of several rotor asymmetries on the inductances are shown, and the inductances are evaluated. The evaluated inductances are compared with those obtained from experiments. There is a good agreement between the simulated and the experimental inductances profiles. Copyright © 2012 John Wiley & Sons, Ltd.

As a high proportion of the total generated electricity energy and the power system costs is due to the power losses in lines, allocating the loss and associated cost to the system loads and generating units is a substantial issue in deregulated power systems. The procedure to accomplish this cost allocation is designated as ‘loss cost allocation’ (LCA). So far, a number of LCA methods have been proposed for transmission networks. A few papers have compared different loss allocation methods, which are the methods which calculate the loss (and not necessarily the loss cost) allocated to the loads and generating units. However, this paper presents a comparative study of LCA methods. In this paper, the existing LCA methods are compared from different aspects. The comparison is made by implementing these methods on the IEEE-RTS, and the results will be discussed. Copyright © 2012 John Wiley & Sons, Ltd.