Design and development of a new three-phase AC-DC single-stage wind energy conversion system

The prevalent use of the diode bridge rectiﬁer in three phase wind energy conversion systems decreases the efﬁciency of the power generation. In standalone wind energy generation, bridgeless converters can address this problem by eliminating the front-end diode bridge rectiﬁer. This work presents a novel single-stage three-phase bridgeless AC to DC c`uk converter for wind energy conversion systems. The proposed converter is designed to work in discontinuous inductor current mode because of its inherent power factor correction and less harmonic distortion. Using body-diodes of the MOSFETs per phase component reduction is also done in this converter. A unique maximum power point tracking technique is developed for this converter using three-phase voltages and currents, instead of the DC link states. A wind energy emulation system using a DC motor and self excited induction generator is used to test the laboratory prototype of the proposed converter.


INTRODUCTION
Non-conventional energy sources are the only viable option to alleviate the increasing energy demand with sustainable development [1]. Compared to other renewable energy sources, wind energy is economical and more suitable for rural standalone application [2]. In standalone WECS, a DBR is convertionally used for AC to DC power conversion, followed by a DC-DC converter for MPPT. The front-end DBR decreases the conversion efficiency due to conduction losses; it is reported in [3] that the elimination of input diode bridge can increase the efficiency by around 2% . Thus, to increase efficiency of the WECS bridgeless AC-DC converters are optimal [4][5][6][7][8][9][10][11][12]. The active power quality improvement by eliminating the DBR has already been successfully achieved using a single-phase bridgeless buck [12], boost [9,13], cùk [5,6,8,11], singleended primary-inductor converter (SEPIC) [4,14] converters. A single-phase bridgeless AC-DC converter with three-port output has been reported in [15]. The currently available bridgeless AC-DC converters are mostly designed for single-phase conversion systems. These converters increase the efficiency of the system by eliminating the front-end DBR [4,5,10,12]

but are lacking in
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. IET Power Electronics published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology system performance, which can be achieved using three-phase systems. Three-phase power generation using permanent magnet synchronous generator or self-excited induction generator (SEIG) in a standalone system has a lesser cost and higher efficiency than a single-phase power generator [16] maximum power point tracking (MPPT) techniques for WECS are widely used by sensing torque and turbine speed or by sensing the DC current and voltages obtained after rectification [2,[17][18][19]. In the proposed system there is no intermediate DC link after the three-phase generator; thus, a new MPPT technique is implemented.
Bridgeless boost converters exhibit excellent PFC, efficiency, ease of control and low cost. However, the lack of a wide range of voltage control makes them unsuitable for MPPT across the available range of wind power. To alleviate this problem, a buck stage is used in [20,21], which leads to decrease in system efficiency.
In this paper, a novel single-stage three-phase bridgeless cùk converter is proposed, the converter is designed with the input inductors working in the DICM, while in the output inductors in continuous inductor current mode (CICM). This improves power quality in the source side and has a low DC voltage ripple  in the output. A DC motor is used for emulating wind turbines, which is coupled with a SEIG. This paper is organized as follows: The circuit description, operation, design, and analysis are collated in Section 2. The implementation technique of the wind turbine emulation is presented in Section 3. The discussion on simulated results is done in Section 4. Hardware results are given with a discussion in Section 5. The conclusion regarding this work is drawn in Section 6.

Circuit description
The proposed three-phase bridgeless cùk converter is designed to operate in a wide range of input voltage and frequency fluctuations and has the capability to maintain the desired output voltage. The wind energy is highly stochastic; thus, the widerange voltage tracking capability of this converter makes it ideal for MPPT in WECS. The Bridgeless cùk and SEPIC converters are generally used with two diodes per phase in typical configurations. In the proposed topology the number of diodes per phase is reduced to one by using the body diodes of the MOSFETs as shown in Figure 1   This converter is designed to operate at 100 kHz and discontinuous input inductor current; thus has small inductance requirement, which leads to lower weight and size. Conventional bridgeless converters require DBR to be connected before the input inductors [7,8,11]. The low inductance requirement and

FIGURE 5
Key waveforms of the proposed converter absence of the input side DBR makes it possible to use the leakage inductance of the generator to be used as the input inductors in the proposed converter.

Circuit operation
It is shown in Figure 1 that the input to the converter is a threephase voltage source. The input inductors are L 1 , L 2 and L 3 . These input inductors are being switched by three MOSFETs namely S 1 , S 2 and S 3 .
As the MOSFETs are switched to "ON" state, the input inductors start to get energized, and capacitors C 1 , C 2 and C 3 begin discharging into the output capacitor C o and load R L through the output inductors L 11 ,L 22 and L 33 .The operation of the proposed converter can be broadly divided into three parts as mode-1, mode-2 and mode-3, discussed as follows. The operation of the converter is represented in the Figure 5.

2.2.1
Mode-1 In this mode, all the switches are in "ON" state, as shown in Figure 2. The input inductors start to charge according to the applied phase voltage across them. The inductors with positive voltage start to build up a positive current, while the inductor with a negative potential difference starts to build negative current across them. The capacitors having a higher voltage than the output capacitor starts to discharge through the switches and the output inductors. Figure 2 shows the modes of operation for a particular condition in which voltages at terminal R and Y are positive with terminal R at a higher voltage. While the terminal B is at a negative voltage.

Mode-2
In mode-2, switches S 1 , S 2 and S 3 are simultaneously switched to "OFF" state; this marks the end of Mode-1. Figure 3 shows an instance of Mode-2, where phase R and Y have positive, while phase B has negative voltage. The inductors with positive current(L 1 and L 2 ) and negative current-carrying inductor (L 3 ) discharges in capacitors (C 1 and C 2 ) through diodes (D 1 and D 2 ) and body-diode of S 3 . The voltage of capacitors(C 1 and C 2 ) becomes higher than the output capacitor (C o ). The output inductors (L 11 and L 22 ) discharges in the output capacitor(C o ) through diodes (D 1 and D 2 ). The capacitor (C 3 ), diode (D 3 ) and output inductor (L 33 ) carry no current as long as the polarity of phase B is negative. The active components change with polarity of source voltage.

Mode-3
The input inductors are fully discharged in this mode. The increased voltage across the capacitors is sustained as long as the switches are in "OFF" state. Only the output inductors and the diodes are conducting in this mode. The stored energy in the output inductor is being transferred to the output capacitor. The switching of the MOSFETs to "ON" state marks the end of this mode. The sinusoidally varying voltages across three-phase supply ensure equal current sharing by the switches and diodes and thus increases the reliability of this converter.

Converter design
For designing the circuit parameters, it is mathematically convenient to change the system in per phase quantity for doing the calculations as we do with three-phase electrical machines. The design parameters are given in Table 1. The average voltage can be calculated using these parameters: For efficient DICM operation, the duty cycle is taken as 0.6. For three phase system this duty becomes 0.6∕3 = 0.2.
For DICM in L 1 , Current Ripple (ΔI ) = 2 × average current.   therefore, input inductor For Deep DICM operation of converter the value of input inductor is taken as 47 H.
As, Ka < (Ka) critical Hence, Ka is taken as 0.25. After determining Ka, the equivalent Inductance of the circuit can be calculated by using: = 156.25 H Hence, the value of output inductor can be calculated as:  The capacitor value for Fs = 5000, which is also a good selection for filter comes around 2.2 F. Thus taking the amount of leakage current into account, the value of capacitors is chosen at the commercially available 1 F. The designed parameters of the circuit are summarized in Table 2.
The Value of Output capacitor is taken as 1000 F as it is high enough to bring down the voltage ripples to less than 2% of the output voltage at rated load.

Converter analysis
The designing is done in order to keep the input inductor in deep DICM. Thus converter analysis is also done with the same consideration. For finding the converter voltage gain Vo∕V in , its necessary to find the voltages across each input inductor.
On adding the squares of the above voltages we get; Thus, total energy absorbed per phase in DT time is: Thus, total energy absorbed per phase in DT time is: The total energy absorbed can be calculated by using Equations (12) and (16). The inductors with negative currents also discharge their energy in the capacitors. This energy is then transferred to the output capacitor. Thus the total energy can be written as: Which can be written as: The Equation (18) represents total energy input to the converter in DT time [4,10]. It can be noted here that, the energy absorption is only happening for the DT time. As the Total time cycle is of T seconds. The input power can be written as: ∴P out = P in (21)

WIND TURBINE EMULATION
A separately excited DC motor is used for wind turbine emulation in this work. The armature current control method is used for generating the torque-speed characteristic of a wind turbine. In the developed system, a two-blade wind turbine (MOD2) is emulated, as these turbines have the maximum power coefficient (CP) at around tip speed ratio ( ) of 7. This value of lambda ensures voltage generation from the mechanically coupled SEIG. Figure 6 shows the schematic diagram of the developed system. Mechanical power output of the turbine is: Where, Cp is a well established function of and pitch angle ( ). An approximate model of performance coefficient for MOD 2 wind turbine is given in [22] as a non-linear function given by: where,

MPPT and control
The availability of wind energy is stochastic in nature. This necessitates implementation of an MPPT technique to decrease overall cost by maximizing utilization. Generally, MPPT tech-niques in wind systems are based on the DC link voltage and current, or by measuring the blade speed and torque of the generator [18]. With miniaturization and mass production of voltage and current sensors, they are extensively being used for condition monitoring and system control.
In this paper, a unique MPPT technique is implemented by measuring the three-phase voltages and currents of the generator. Figure 7 shows a flowchart showing the MPPT technique used. As shown in Equation (12), a DC value proportional to three-phase current can be calculated. Using these proportional DC values of voltage and current of the generator, the duty cycle of the AC-DC converter can be adjusted accordingly to get maximum power. The Control of the emulator can be easily implemented by measuring the speed of the DC motor to generate a reference torque using Equation (25). The torque applied by the DC motor is calculated by multiplying the measured armature current by the torque constant. The DC motor torque is then controlled by using a PI controller. The assumed parameters for the turbine, and values of K p and K i , are given in Table 3.

SIMULATION AND DISCUSSION
The simulation of the proposed converter is done in Matlab Simulink 9.0 as per the parameters given in Tables 2 and 3. The system performance on the basis of the input voltage, MPPT technique, output voltage, inductor current, and switching stress is done. It is evident from the Figures 8 and 9 that the proposed system is working as per the design considerations. Figure 9 shows the voltage and current at the input side of the filter. A low pass LC filter is used in this circuit, with inductors of 560 H and AC capacitors of 10 F. As the circuit is connected with the generator at 0.7 s, current waveforms exhibits the transients as the LC filter is energized. The MPPT technique  is dependent on the calculated values of the voltage and current maxima. These values are shows in Figure 10.
The input inductor is working in deep DICM, and the calculated voltage and current maxima values are DC in nature. The output inductor is intended to work in the CICM for output DC voltage to have low ripples. Figure 11 shows that the output inductor current is in CICM around the maxima and in DICM around zero crossing of the input voltage. This drawback is compensated by the 120 deg phase-shifted output currents in the three output inductors, which results in lower output voltage ripple.

FIGURE 15
Current waveforms at 10% duty. Current in R phase (channel-3 at 2 A/div), Y phase (channel-1 at 2 A/div), B phase (channel-2 at 2 A/div) and output voltage on channel-4 at 20 V/div Figure 12a shows the variation of the duty as per the methodology shown in Figure 7; the generator is connected with the converter at 0.7 s. In the simulations, a system consisting of DC motor based wind emulator and SEIG is considered, which increases the overall settling time. The MPPT technique is found to be robust enough to respond satisfactorily. The variation of the output voltage with duty variation is shown in Figure 12b. Figure 13 shows the tracking of the reference torque by the DC motor. The higher value of Ki ensures stable operation of the controller with high ripple current drawn by the DC motor.

FIGURE 16
Voltage of R phase (channel-3 at 50 V/div) and input current (channel-2 at 10 A/div) and output voltage of the converter (channel-1 at 20 V/div)

FIGURE 17
Generator voltage (channel-1 at 50 V/div) and current (channel-2 at 2 A/div) with output voltage of converter (channel-4 at 20 V/div) In Figure 12b the simulated current and speed waveform off the DC motor is shown. The sudden decrease in the speed is due to loading of the SEIG at 0.7 s. After loading the generator, MPPT technique controls the converter duty and the emulation logic controls the current from the buck converter in order to follow the intended torque-speed characteristic of the wind turbine.

HARDWARE RESULTS AND DISCUSSION
DSpace-CP1104 is used to develop the hardware prototype for this work. NTHL040N65S3F MOSFET transistor is used for switching with FOD3184 optocoupler. The details of the components used are given in Table 4. Figure 14 shows the input current waveforms of the converter under loaded condition. The input inductor currents in  Figure 15, where phase A has a positive voltage and thus has positive current (channel-3), while the other two phases carry negative currents (channel-1 and 2), and output voltage on channel-4 is constant.
The PFC in the proposed converter is shown in the Figures 8  and 16, the per phase voltages and currents are found to be in phase. The high frequency switching of the input current can be easily filtered by a small LC lowpass filter. Figure 17 shows variation of the voltage and current drawn from the SEIG (channels-1 and 2), and the output voltage on channel-4. The transients in voltage and current on connection of the converter to the generator is shown in Figure 18. It can be observed that the transient phase passes in under 15 ms.
In Figure 19, the voltage build-up of the SEIG is shown. A change in generated voltage (channel-1) is observed on connecting the converter to the generator, which is marked by an increase in the output voltage (channel-4) and generator current (channel-2). The speed is measured by an optical encoder and then converted to analog signal using a DAC (1 V/400 RPM), which is shown on channel-3.

CONCLUSION
A new three-phase bridgeless cùk converter is proposed, analyzed, and validated in this work. The proposed single-stage converter is found suitable for three-phase generators as it eliminates front-end DBR. The DICM operation of input inductor ensures lesser switching stress and inherent PFC. The wide range of output voltage control of the cùk converter leads to better MPPT technique implementation. An MPPT control based on three-phase voltages and currents is also reported, which eliminates the need for an intermediate DC stage. The satisfactory results obtained from the SEIG based wind generation is indicative of the suitability of the converter's application in standalone WECS.