Abstract
In this paper, a wind energy conversion system (WECS) is presented for the electrification of rural areas with wind energy availability. A three-phase AC-DC converter based on a bridgeless Cuk converter is used for power extraction from the permanent magnet synchronous generator (PMSG). The bridgeless topology enables the elimination of the front-end diode bridge rectifier (DBR). Moreover, the converter has fewer components, simple control, and high efficiency, making it suitable for a small-scale WECS. A squirrel cage induction motor (SCIM) is used to emulate a MOD-2 wind turbine to implement the PMSG-based WECS. A direct-drive eight-pole PMSG is used in this study; thus, a low-input-voltage system is designed. The converter is designed to operate in the discontinuous inductor current mode (DICM) for inherent power factor correction (PFC) and the maximum power point tracking (MPPT) is achieved through the tip-speed ratio (TSR) following. The performance of the developed system is analyzed through simulation, and a 500 W hardware prototype is developed and tested in different wind speed conditions.
EXTENSIVE studies have been conducted on the adverse effects of renewable energy integration into the power grid [
The grid-connected wind energy conversion system (WECS) is based on partial-scale power converters in the case of a doubly fed induction generator (DFIG) and full-scale power converters in the cases of PMSG and SEIG [
Power electronic converters are the most vulnerable part of the WECS; thus, a robust, efficient, and inexpensive AC-DC converter is required [
Converter | No. of switches | No. of diodes | No. of inductors | No. of capacitors | Complexity | Power flow |
---|---|---|---|---|---|---|
SVPWM controlled TPTPR [ | 12 | 0 | 3 | 2 | Complex | Bi-directional |
Bi-directional three-phase AC-DC converter [ | 10 | 0 | 7 | 2 | Complex | Bi-directional |
Three-phase unfolded circuit based converter [ | 12 | 6 | 3 | 1 | Very complex | Bi-directional |
Matrix based AC-DC converter [ | 8 | 2 | 6 | 4 | Complex | Unidirectional |
Multiport converter with independent control [ | 12 | 0 | 9 | 2 | Complex | Bi-directional |
Proposed converter | 3 | 3 | 3 | 2 | Simple | Unidirectional |
Note: SVPWM stands for space vector pulse width modulation; and TPTPR stands for three-port three-phase rectifier.
This study aims to implement this converter in the WECS for rural and isolated areas with a reduced component count and high efficiency. The topology of the proposed three-phase AC-DC bridgeless Cuk converter is illustrated in

Fig. 1 Topology of proposed three-phase AC-DC bridgeless Cuk converter.
The main contributions of this study are summarized as follows.
1) The developed converter has less switches and a simple control, and can operate in a broader voltage range.
2) A methodology for developing a wind turbine emulator using a squirrel-cage induction motor (SCIM) with field-oriented control (FOC) is presented.
3) The tip-speed ratio (TSR) following the MPPT technique is used.
The remainder of this paper is organized as follows. The design and operation of converter are briefly discussed in Section II. Section III describes the WECS setup used in this study. Simulation results with brief discussions are presented in Section IV. Section V presents the hardware results. The conclusions are presented in Section VI.
As shown in
The proposed converter has the following advantages over the conventional bridgeless Cuk converter.
1) There are no diodes connected directly to the source; thus, generator leakage inductance can be used effectively as the input inductance.
2) The switching control is simple and thus needs less components, as a single gate driver can be used to switch all three switches.
3) The topology of three-phase bridgeless Cuk converter ensures only 50% working cycle for capacitors, output inductors, and diodes.
4) The back diodes of the switches are used in the negative half-cycle of the source voltage in the respective phase, which leads to further component reduction.
The converter is designed to operate in the discontinuous inductor current mode (DICM) of the input inductances. This ensures an inherent PFC and allows the converter to operate more efficiently by drawing only active power from the generator. The converter operation is defined in the following three modes.
In Mode-1 of the converter operation depicted in

Fig. 2 Mode-1 of converter operation.

Fig. 3 Mode-2 of converter operation.
The mode-3 of converter operation is depicted in

Fig. 4 Mode-3 of converter operation.
The converter is designed to keep the input inductor in the deep DICM. Thus, a converter analysis is also needed with the same consideration. To find the converter voltage gain , it is necessary to find the voltages across each input inductor.
(1) |
(2) |
(3) |
where , , and are the voltages of phases R, Y, and B, respectively; is the amplitude of the three phases; and is the speed angle frequency.
By adding the squares of , , and , we can obtain:
(4) |
Now, as the energy absorption is performed in Mode-1 of the converter operation, the energy absorbed in Mode-1 by input inductors in “dt”, i.e., , can be written as:
(5) |
where is the three-phase average root mean square (RMS) voltage; and is the current at time t.
Thus, the total energy absorbed in each phase in time DT can be expressed as:
(6) |
(7) |
where D is the duty cycle; and T is the total time cycle.
The total energy absorbed can be calculated using (4) and (7). The energy of inductors with negative currents are also discharged to capacitors. This energy is then transferred to the output capacitor. Thus, the total energy can be written as:
(8) |
(9) |
(10) |
(11) |
(12) |
(13) |
where is the coefficient of power. Thus, the output voltage can be expressed as:
= | (14) |
(15) |
The converter is analyzed using the current-injected equivalent circuit method [
(16) |

Fig. 5 Equivalent circuit for small-signal analysis.
where , and D1 is the duty cycle of Q1; and is the input inductor current.
The derivative of the input inductor current is given by:
(17) |
Let the effective load on the capacitor C be . Then, we can obtain:
(18) |
(19) |
where is the output current.
From the output part of the circuit, the derivative of the inductor current is given by:
(20) |
Perturbing (16)-(19) around the steady state and taking the Laplace transform, the small-signal linear equations are determined as:
(21) |
(22) |
(23) |
where ; and is the perturbed value of .
Solving (5)-(7) yields:
(24) |
In the output part of the converter, the state of the output capacitor is not perturbed, as the converter is employed in a DC microgrid of 230 V. The stiff DC voltage at the output is constant. Thus, (20) is perturbed to obtain:
(25) |
Thus, solving (24) and (25) yields:
(26) |
where .
As the output power is proportional to the output current , the control-to-output current transfer function is presented here by substituting in (26).
(27) |
The converter is designed to maintain the input inductor in the DICM and keep the input inductance of the three-phase circuit maximum at the power line frequency. The design parameters assumed in this study are listed in
Parameter | Value |
---|---|
100 V | |
500 W | |
350 V | |
f | 10 kHz |
It is convenient to perform the calculations in per-phase quantities. The design methodology is as follows: ① the maximum input voltage per phase is ; ② the maximum input current =; ③ the saturation current of the input inductors is taken as 8 A; ④ the maximum operating duty cycle for the DICM operation is taken as 70%; and ⑤ the load RL for the converter is taken as 250 .
The input inductance is calculated as:
(28) |
The input inductance is taken to be 560 (commercially available value) to maintain the inductor in the deep DICM.
The critical value of is calculated as:
(29) |
The chosen values of the output and input voltages are 350 V and 100 V, respectively. From (2), is calculated to be 0.0412; thus, for the DICM mode, a lower value of is taken as 0.03. The value of the equivalent inductance is calculated as:
(30) |
(31) |
Taking the value of from (28) and using (30) and (31), the output inductance can be calculated as:
(32) |
The capacitance is taken as 1 , where the resonant frequency of the LC oscillation with the input inductor, output inductor, and combined is not near the switching frequency or the input power frequency, which ranges from 25 Hz to 100 Hz.
The DC link capacitance is selected to obtain the minimum voltage ripple at the rated output power, and the output capacitance is chosen as a reasonable value of 1000 . At this value, the voltage ripple is found to be less than 2%.
Converter parameter | Value |
---|---|
560 | |
1120 | |
1 | |
1000 |
To test the proposed converter, a WECS is realized, which consists of a wind turbine emulator, the proposed converter, and a DC load. The emulator is developed using an SCIM driven by decoupled FOC. In this study, a MOD-2 turbine type is emulated, as it has a simpler mathematical model and a higher value of TSR, denoted as . A higher TSR is more suitable for the direct-drive PMSG-based WECS because a higher rotor speed is achieved at all wind speeds.
The power coefficient , which depends on the TSR and blade pitch , determines the amount of energy extracted from the wind by the turbine, denoted as . For a blade of radius R rotating with an angular velocity of at wind speed , the value of TSR is given as:
(33) |
(34) |
Based on the calculations performed by Betz, the maximum amount of wind power that can be extracted using a turbine is expressed as:
(35) |
By gathering the field data and plotting - curves, according to [
(36) |
where for MOD-2 wind turbine, the values of - are selected as , , , , and, and .
A 5 kW, two-pole SCIM is selected to emulate the wind turbine. The control actions are implemented using CP1104 dSPACE. Using the turbine model given in Section III-A, a MATLAB model is developed to generate the torque reference for the SCIM. Using (36), the torque reference can be calculated at rotor speed as:
(37) |
The stator and rotor leakage inductances and , mutual inductance , steady-state rotor flux , and stator and rotor resistances and are calculated by performing no-load and block-rotor tests on the SCIM. The FOC is implemented on the SCIM as follows.
1) The stator currents and are measured and converted to the d-q reference frame as and using the Park transformation.
2) is used to calculate the slip speed in the SCIM using (38) and (39).
(38) |
(39) |
where is the motor time constant; and is the rotor resistance.
This calculated slip speed is added to the measured rotor speed of the machine to obtain the electrical angle of the stator currents :
(40) |
By using the measured value of , the value of rotor flux can be estimated by:
(41) |
For the implementation using a digital system, the Laplace transform is changed into the z-domain using the values given in
(42) |
Motor parameter | Values |
---|---|
0.0138 H | |
0.0138 H | |
1.09 Ω | |
3.917 Ω | |
1.182 H | |
0.305285 s |
The torque of the machine needs to be estimated accurately to implement the torque control for turbine emulation.
(43) |
where P is the number of pole pairs in the SCIM and .
The values of different motor parameters used in (38), (39), and (43) are listed in
The maximum power extracted from an ideal wind turbine is given by (35). Ideally, a maximum of 59.26% of the input wind power can be extracted. This makes the use of an MPPT system pivotal in a WECS. For fixed-blade-pitch turbines used in small-scale wind generation, the TSR value determines the value of ; thus, the TSR-MPPT technique is used in this study.
The schematic of the wind energy emulation and coversion system is shown in

Fig. 6 Schematic of wind energy emulation and conversion system.

Fig. 7 System description for motor control.
The system represented in
The simulated results of the three-phase input voltages and currents of the steady-state converter are shown in

Fig. 8 Three-phase input voltages and currents of steady-state converter.
The starting characteristics and response to a step change in wind speed are analyzed below. A simulation for a step change in wind speed from 10 m/s to 15 m/s at is performed, and the results of TSR following and controller output (duty cycle) for MPPT are presented in

Fig. 9 TSR following and controller output for MPPT.
The rotor speed and electromagnetic torque of the SCIM are shown in

Fig. 10 Rotor speed and electromagnetic torque of SCIM.
The DC output voltage of the converter and the voltage generation of PMSG at a step change in wind speed from 10 m/s to 15 m/s at is shown in

Fig. 11 DC output voltage of converter and voltage generation of PMSG.
The efficiency of the three-phase AC-DC conversion system used in [

Fig. 12 Comparative efficiency of different converters.
The developed converter is tested using a PMSG (BLS-143) driven by a wind turbine emulator, which is developed using a three-phase 5.5 kW induction motor (1722 J). The components used to develop the converter are presented in
Component | Part number | Quantity |
---|---|---|
Inductors | PCV-2-564-08L | 9 |
Capacitor (1 ) | MKP C.4C | 3 |
Capacitor (1 mF) | ALC70C 102 FP | 1 |
Insulated gate bipolar transistors (IGBTs) | K40H1203 | 3 |
Diodes | DSEI60-10A | 3 |
The steady-state operation of the converter at the wind speed of 12 m/s is shown in

Fig. 13 Steady-state operation of converter at wind speed of 12 m/s.
During the wind speed variation, the load resistance is maintained constant with the controller tracking the optimum TSR value. This working condition helps maintain the system operation at the maximum power point (MPP) even after the induced step change in the wind speed.

Fig. 14 System performance at a sudden drop in wind speed from 12 m/s to 10 m/s.
A step change in wind speed from 10 m/s to 12 m/s is simulated, and the result is presented in

Fig. 15 System performance at a step change in wind speed from 10 m/s to 12 m/s.
To test the system in the wind shear condition, a step change in wind speed from 10 m/s to 15 m/s is introduced in the system. The results are shown in

Fig. 16 System performance in wind shear condition at a change in wind speed from 10 m/s to 15 m/s.
The hardware results for the DICM operation of the converter with a duty cycle lower than 70%, at a high wind speed of 15 m/s, are presented in

Fig. 17 DICM operation of converter with duty cycle lower than 70%.
The hardware results for CICM operation of the converter with a duty cycle higher than 70% are shown in

Fig. 18 CICM operation of converter with duty cycle higher than 70%.
Figure A1 in Appendix A shows the laboratory setup for the wind turbine emulation and the PMSG-based WECS. The turbine is emulated using a three-phase SCIM that is mechanically coupled with the BLS-143 PMSG. The developed converter is fed with a variable-frequency three-phase supply from the PMSG, which is converted to a DC supply feeding the resistive load.
A PMSG-based WECS using a new three-phase AC-DC converter is designed and tested in this study. The developed WECS is designed and tested as a standalone system. It is envisaged that the system can be easily integrated with a photovoltaic system or battery bank. The functioning of the emulator is tested by employing the TSR-MPPT technique, and the system is experimentally validated at different wind speeds. The elimination of the DBR and the compatibility of the designed converter with the PMSG make the system more efficient with fewer components. The simulation and laboratory prototype confirm the effectiveness of using this system in WECSs.
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