Isolated H-bridge DC–DC converter integrated transformerless DVR for power quality improvement

: This study presents a new H-bridge DC-DC converter-based transformerless dynamic voltage restorer topology (DVR). The proposed system can compensate balanced and unbalanced voltage sag/swell that are the most common electrical power quality problems and offers advantages over conventional DVR topologies by providing the isolation with high-frequency transformer (HFT) rather than bulky injection transformers and by employing shunt converter to eliminate the requirement of an energy storage unit. The system is composed of H-bridge DC–DC converter equipped with a HFT with one primary and three secondary windings and transformerless DVR. The single-phase shift modulation method is used for each series converter independently to provide the bidirectional power flow control of DC–DC converter, whereas in-phase compensation method with a hybrid detection algorithm is used to mitigate voltage sag/swell. An LC filter is employed to attenuate the switching ripple harmonics on the output of the DVR. The performance of the proposed system is verified experimentally on a three-phase, three-wire, 380 V, 10 kVA prototype.


Introduction
Widespread use of power electronics equipment causes various adverse effects in the grid and raises the phenomenon of electrical power quality. Since power quality is a consumer-driven issue, the reference is the end user. Variations in the amplitude of voltage and current, system frequency deviations and waveform distortions that cause damage to the power system equipment or end-user loads are defined as power quality problems [1]. Voltage sag and voltage swell are the most common electric power quality problems in electrical distribution systems. Sensitive loads such as communication devices, adjustable speed drives, manufacturing process, computers etc., which are connected to distribution systems at the end-user point, have a low tolerance to voltage deviations. To mitigate the effects of voltage sag and swell on the voltage sensitive users, one of the most well known solutions is the dynamic voltage restorer (DVR).
Several studies have been performed to improve the operating performance and to reduce the cost of DVR. Traditional DVR topologies and controllers are compared in [2]. The topologies with and without energy storage units are investigated in this paper. The traditional DVR topologies consist of voltage-source converter (VSC), output filter, injection transformers and DC-link capacitor that is connected to an energy storage unit or load/source side [3][4][5]. The voltage compensation is provided by VSC and DVR controllers. The compensating voltage is injected into the grid by injection transformers that achieve isolation between the series converter and grid. Since the injection transformers are operated at the line frequency, they are bulky and costly. Moreover, the impedance of the series injection transformer causes a voltage drop and load harmonics. Therefore, to eliminate the injection transformers from DVR topology, some studies are carried out. A transformerless single-phase DVR is suggested for low-voltage (LV) level in [6][7][8][9]. Owing to the single-phase operation, the isolation requirement between the series converters is eliminated and transformerless topology is performed in the proposed systems. In [10], a unidirectional power flow controlled LV DVR supported with the configuration of three-phase inverter and separated DC links is proposed. The use of separate DC links for each phase ensures isolation between phases and eliminates the need for injection transformers. The main aim of this paper is to restrict the DC-link voltage from exceeding its safe operational range in voltage swell events. Therefore, this paper focused on swell compensation and unidirectional power flow is performed. In [11], a transformerless LV DVR is realised by employing half-bridge VSC.
Recently, line transformers used for voltage matching and galvanic isolation in grid-connected topologies are eliminated by using DC-DC converter equipped with high-frequency transformer (HFT). Several studies have been performed on this issue in the literature. The related topology is used in the applications of smart transformers, grid-connected photovoltaic (PV) systems, PV/ battery hybrid systems etc. [12][13][14][15].
To eliminate the injection transformer and energy storage unit from the system, DVR topologies that are characterised by threephase rectifier, HFT and unidirectional isolated DC-DC converter have been developed in [16,17]. HFT provides the isolation of the system rather than a bulky line transformer. The usage of the threephase rectifier enables to eliminate the costly battery requirement. The DC-DC converters of the systems are formed with a topology that provides unidirectional power flow.
As it is understood from the literature review, it is aimed to both improve the performance and reduce the cost of the DVR. Table 1 shows the benchmarking of the DVR topologies available in the literature.
This paper proposes a new isolated H-bridge DC-DC converterbased DVR topology to compensate the voltage sag/swell. In the proposed topology, small-sized and low-cost HFT, that is designed to have one primary/three secondary windings, is used to provide galvanic isolation instead of bulky and costly line transformers.
Costly energy storage devices are eliminated from the DVR topology by a full-bridge diode rectifier connected in parallel to the load side. Single-phase shift (SPS) modulation method is used for bidirectional power transfer in the H-bridge DC-DC converter and is applied to each series converter to control them independently. The performance of the proposed system is verified with an experimental prototype. The remaining of this paper is organised in the following manner: the power circuit topology of the proposed system is described in Section 2. The control schemes of the DVR and DC-DC converter are presented in Section 3. The design details of the experimental prototype and the experimental results are presented in Section 4. Finally, Section 5 analyses the results and contributions of this paper.

Proposed H-bridge DC-DC converter-based transformerless DVR configuration
The power circuit configuration of the proposed system is illustrated in Fig. 1 consists of shunt converter, isolated H-bridge DC-DC converter, three single-phase H-bridge inverters and LC filters. The shunt converter, which is connected to the load side, composed of a three-phase converter and has a constant DC-link voltage, is used to eliminate the need for costly energy storage units. The H-bridge-based bidirectional DC-DC converter allows the DVR to compensate both voltage sag and voltage swell with the capability of bidirectional power flow. Use of isolated H-bridge DC-DC converter in DVR also makes it possible to eliminate the bulky line transformer in the point of common coupling (PCC). As shown in Fig. 1, the HFT is designed with one primary/three secondary windings. Thus, the isolation between the series converters in each phase is ensured by DC links. The usage of dual active bridge (DAB) topology makes it possible to use zero-voltage switching operation and phase shift operation to improve the performance of the system [26][27][28][29][30]. The output of the isolated Hbridge converter is composed of three DC-link capacitors that supply the required power for compensation of the sensitive load. LC filter is used at the output of the VSC to filter the oscillations.

Control scheme of the system
The controller of the proposed system consists of two parts: a DVR controller and DC-DC converter controller. The controller of DVR shown in Fig. 2 is designed to compensate the voltage sag/swell and to control each phase independently. The controller of the DVR monitors the voltages at the PCC for the detection of the missing/excessive voltage. The control process starts with the measurement of bus voltages. To detect the balanced and unbalanced voltage sag/swell, a hybrid method, which is developed by Koroglu et al. [31], consisting of improved Clarke transformation and enhanced phase-locked loop (EPLL) is  employed. The method of EPLL described in [32] is used to calculate the magnitude and the phase of the phase voltages for synchronisation. The improved Clarke transform is adopted to calculate the magnitudes of phase voltages. The two phases (α, β) of stationary reference frame are computed by using virtual phase voltages derived by using the reference phase of Vph,A (voltage of phase A) according to the equations below [33]

Fig. 1 Circuit configuration of the proposed system
The transformation of three-phase stationary frames (A, B, C) to two-phase stationary frame (α, β, 0) is performed by using (4). The magnitudes of phase voltages are calculated by using alpha and beta components of phases according to (5) [34]. The depth of the voltage disturbance is calculated according to (6) V αn where A Clarke,n are the magnitudes of the phase voltages that are calculated by the improved Clarke transform and A EPLL,n are the magnitudes of the phase voltages that are calculated by EPLL. The reference voltage of the distorted phase is derived from the voltage at PCC by using the in-phase compensation method [35]. The aforementioned method controls the phase voltages continuously and calculates the magnitude of injection voltage according to the pre-sag and sag magnitudes and phase angles.
The switching signals of the insulated-gate bipolar transistors (IGBTs) are generated by using the carrier-based pulse-width modulation technique. The reference voltage (V ref ) to be injected to the distribution system compared with a fixed frequency triangular carrier signal. The switching frequency of the series converters is selected as 10 kHz.
In the controller of the DC-DC converter, SPS modulation method is performed to provide the bidirectional power flow and voltage control at both sides. To be able to compensate for the balanced and unbalanced voltage sag/swell, each H-bridge converter on the secondary side is independently controlled. Therefore, independent phase shift angles are computed for each phase. The switching pairs of H bridges have a constant duty cycle (50%) and have 180° phase shift between two legs to provide the square wave AC voltage across transformer terminals. The bidirectional power flow between the DC links is provided by their own phase shift angles (ϕ 1,2,3 ), as follows [36]: where P D is the power transfer, V DC is the amplitude of the primary-side DC-link voltage, V DVR,DC−1,2,3 is the amplitude of secondary-side DC link, ω is the angular switching frequency and L is the sum of the transformer leakage inductance and auxiliary inductance (L p , L S−1,2,3 ). The phase shift angles are adjusted by proportional-integral (PI) controller. The desired value of the DVR-side DC link is subtracted from the measured DC-link voltage to calculate the error. The error signal is applied to the PI controller to generate the phase shift angle. The change of phase shift angle determines the current and voltage of equivalent inductors of the DC-DC converter. By adjusting the phase shift angle, power transfers between the DC links are achieved. The controller of the DC-DC converter is shown in Fig. 3. The overall control algorithm of the system is given in Fig. 4.

Experimental prototype and results
To evaluate the performance of the H-bridge DC-DC converterbased transformerless DVR topology and designed controllers, an experimental prototype is developed and realised for three-phase 380 V and 10 kVA ratings. The switching elements of power circuits are SEMiX-202GB12E4 s IGBT modules driven by SKYPER PRO 32 R. The control scheme of the system is implemented with TMS320F28335 digital signal processor-based (DSP) microcontroller unit. The reduced scale side view pictures of the experimental prototype of the proposed system are shown in Fig. 5.
To test and evaluate the performance of the developed compensating device, 20 kVA, three-phase, transformer-based DSP-controlled sag/swell generator developed by Savrun et al. is used [37].
A load group composed of heaters (resistive load) and inductors are used to investigate the performance of the system. The parameters of the load group are summarised in Table 2.
The objectives of the proposed system are (1) to compensate balanced three-phase 50% voltage sag for three periods and (2) to compensate 15%, three-phase voltage swell for three periods. The When the power demand of the load group is around 15 kVA, 4.5 kVA power transfer is needed to compensate 50% three-phase voltage sag for three cycles. This means that 1.5 kVA power transfer is needed for each phase. The DC-link capacitors are calculated according to the equations below: E dc is the energy transferred to the DC link during the voltage sag/ swell; 'S' is transferred power; 't' is the duration of the disturbance; V dci is the initial capacitor voltage; and V dcf is the final capacitor voltage. The DVR DC-link capacitors are determined as 20 mF for each series converter by theoretical calculations (V DVR,DC,1−2−3 ). In the proposed system, the LC-type inverter-side output filter equipped with passive damping method is used to attenuate the voltage harmonics and high-frequency fluctuations. Although the switching frequency harmonics can be eliminated more effectively in LCL and LLCL filters, additional voltage disturbances occur due to the load current passing through the inductance that is on the load side after the filter capacitor. So, the waveform of the load voltage is distorted. Therefore, in the proposed DVR topology, LCtype filter is preferred. The parameters of the filter are summarised in Table 3.
The isolated H-bridge DC-DC converter is used to regulate the DC-link voltages of the DVR during the voltage sag/swell and provide isolation of the system with HFT. The HFT takes the part of bulky injection transformers in the traditional DVR topology. Thus, the transformer with a volume of 1386 cm 3 (16.5×12×7) is employed rather than the transformer with a volume of 35,280 cm 3 (35×16×21×3 ph) as illustrated in Fig. 6. Thus, a significant reduction in DVR size is achieved with the proposed topology. Furthermore, this paper has the potential to reduce costs because of the high costs of energy storage units and bulky injection transformers as well as the downward trend in the costs of semiconductors.   The primary-side DC link of the DC-DC converter is fed by a full-bridge diode rectifier connected in parallel to the load side. The primary-side DC-link voltage is kept constant at its steadystate value (∼530 V) under normal condition by the three-phase converter (active rectifier). The secondary-side DC-link voltages remain constant at 160 V to perform successful compensation performance. The parameters of the developed system are shown in Table 4.
The presented system is tested under two case studies summarised in Table 5. Case 1 has been formed by considering the worst operating condition for the proposed system. Case 2 consists of both voltage sag and voltage swell.
During cases, three-phase source and three-phase load voltage measurements have been taken with Fluke 1760 PQ recorder. The three-phase source currents and single-phase (A phase) measurements have been captured with Hioki 1396 power quality analyser. The DC-link measurements have been taken with Tektronix MSO3034 oscilloscope. All measurements have been captured simultaneously.
Case 1: In this case, the effect of three-phase balanced 50% voltage sag for five periods is examined under a load combination of 13.5 kW resistive and 7 kVAR inductive. The voltage magnitude of phase A of the supply sags from 217 to 113 V RMS . The missing voltage should be detected and injected to the system to protect the load from disturbance as soon as possible. Fig. 7 shows the compensation performance of the proposed system.
As can be seen from this figure, the proposed system achieves the voltage sag compensation with a fast initial response for the specified case study. The compensation of the sagged phase to nominal value lasts less than a half cycle.
As illustrated in Fig. 7, when phase voltages sag, DC-link voltages of the DVR and shunt converter are kept under the desired voltage limits via bidirectional H-bridge DC-DC converter controller and shunt converter. The PI controllers of the DC-DC converter increase the phase shift angles of the related bridges according to the error value of the DC link. Thus, the DC-link voltages are held at a certain level.
The root-mean-square (RMS) trends of the supply, load and injected voltages are shown in Fig. 7. The RMS of supply voltage can vary due to other loads operating simultaneously such as air conditioners, lamps etc. The sag detection and reference extraction times cause instantaneous voltage fluctuations at point of voltage sag start and end. Total harmonic distortion (THD) of load voltage in phase A is about 2.8% and is always kept below voltage harmonic limits of the IEEE 519-1992 standard [38].
Case 2: In this case, the effect of unbalanced voltage disturbance for five periods is investigated under the same load combination. The voltage magnitudes of phase A and C sag from 217 V RMS to 133 V, while the voltage magnitude of phase B reached 120% of its nominal value (263 V RMS ). As illustrated in   Fig. 8, the missing and excessive voltages are detected in less than a half cycle and compensated by the proposed system. As illustrated in Fig. 8, the DC-link voltages of the DVR and shunt converter maintain within specified voltage limits during disturbance, thanks to bidirectional power flow capability of the system. The compensation of the excessive voltage is carried out by power transfer in the direction from DVR to shunt converterside DC link, while the compensation of the sagged voltages is carried out by power transfer in the opposite direction.
The RMS voltages of the loads are kept within the limits of the IEEE 1159-1995 standard despite the fact that the RMS voltages of the source change during the disturbance condition. The RMS trend of the source, load and injected voltages and THD of the load voltages are presented in Fig. 8. THD of load voltages in phases is about 2.1% and is always kept below voltage harmonic limits of the IEEE 519-1992 standard [34].
The sag/swell compensation performance of the developed prototype is summarised in Table 6.

Conclusion
In this paper, an isolated H-bridge DC-DC converter-based novel transformerless DVR topology and control method have been proposed. The main advantages of the proposed topology are as follows: • The usage of the isolated H-bridge DC-DC converter allows providing isolation in the system rather than bulky line transformers. • Transformerless topology allows for a significant reduction in the size and cost of the overall system. • Separate DC-link capacitors also provide isolation between the phases. • The shunt converter, which is connected in parallel to the load side, reduces the system cost by eliminating the need for energy storage units.
The performance results of the 380 V, three-phase/three-wire, 10 kVA system are examined. Experimental results show that the proposed topology and controller have compensated the voltage sag/swell under a half cycle of the AC waveform and the voltage THD can be kept within the limits of the IEEE standards. The reliability and realisation of the system have been verified with case studies.

Acknowledgments
The authors acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK) for their full financial support (Project no. 112R028).