2011 2nd Power Electronics, Drive Systems and Technologies Conference
A Novel Variable-Speed Wind Energy System Using Permanent-Magnet Synchronous Generator and Nine-Switch AC/AC Converter M. Heydari1, Student Member, IEEE, A. Yazdian Varjani 1, Member, IEEE, M. Mohamadian1, Member, IEEE, and H. Zahedi2, Student Member, IEEE 1
Faculty of Electrical and Computer Engineering, Tarbiat Modares University, Tehran, Iran Department of Electrical Engineering, Islamic Azad University, Badrood Branch, Badrood, Iran Emails:
[email protected], yazdian@ modares.ac.ir,
[email protected],
[email protected] 2
[8] as multi output inverter for control of two motors. This converter is used as an AC/AC converter to deliver wind power
Abstract— Permanent-magnet synchronous generators (PMSGs) are attracting considerable attention in wind energy conversion systems (WECSs) due to their numerous advantages such as low weight and volume, high performance, and the elimination of the gearbox. In this paper, a new variable-speed WECS with a PMSG and a three-phase three-leg AC/AC converter as power electronic interface between PMSG and network is proposed. Characteristics of nine-switch AC/AC converter are used for maximum power tracking control under different wind speed and delivering power to the grid, simultaneously. This configuration has low manufacturing cost compare to conventional topologies. Two simulations are performed for proposed configurations and results are presented. The results verify effectiveness of the proposed interface configuration.
Figure 1. Connection of wind power generation system to grid through back to back inverters
Keywords; Adjustable speed generation system; Permanentmagnet synchronous generator (PMSG); Wind energy conversion system (WECS); Nine-Switch AC/AC Converter
I.
INTRODUCTION
Wind energy is becoming one of the most important renewable energy sources in the world. With growing application of wind energy conversion systems (WECSs), various technologies are developed for them. Permanentmagnet synchronous generators (PMSGs) are attracting considerable attention in WECSs due to their numerous advantages such as low weight and volume, high performance, and the elimination of the gearbox [1]–[6]. Extracting maximum power from wind and feeding the grid with highquality electricity are two main objectives for WECSs. To realize these objectives, the ac–dc–ac converter is one of the best topology for WECSs [2]–[7]. Fig. 1 shows a conventional configuration of ac–dc–ac topology for PMSG. This configuration includes a back-to-back PWM rectifier-inerter [6]. The PWM rectifier is controlled for maximum power point tracking (MPPT) and inverter is controlled to deliver highquality power to the grid [2]–[4]. This topology requires 12 active switches and 12 diodes.
Figure 2. Schematic diagram of the proposed topology
to grid. The new topology has cost advantages compare to back-to-back WECS. The proposed topology reduces number of switches from 12 to 9. With this topology, 3 active switches and 3 diodes is omitted without any change in the objectives of WECS. This configuration has low manufacturing cost compare to conventional topologies. Section II of this paper describes operation of nine-switch AC/AC converter. Then, power delivery and MPPT control of system are explained. Finally, simulation results are presented to verify the performance of the proposed system.
In this paper, a new PMSG-based WECS with three-phase three-leg AC/AC converter is proposed. The proposed topology is shown in Fig. 2. Nine-switch converter was first presented in
978-1-61284-421-3/11/$26.00 ©2011 IEEE
II.
NINE-SWITCH AC/AC CONVERTER
The nine-switch AC/AC converter is shown in Fig.2. The nine switch converter can be considered as two separate
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conventional inverters with three common switches, therefore a special PWM technique is needed as shown in Fig.3. This technique has two reference signals (Vref-in & Vref-out) for each phase legs which are related to input and output terminals respectively. These signals are expressed by: Vref −in = min sin(2π fin + ϕin ) + Offsetin
(1)
Vref − out = mout sin(2π f out + ϕout ) + Offsetout
(2)
voltage, vq will be equal to zero. Then, active and reactive power may be expressed as 3 (vd id ) 2 3 Q = − (vd iq ) 2 P=
B. Vector control for PMSG The torque equation of PMSG is given by [9]
The gate signal for upper switch of each leg is positive logic value generated by Vref-out and Carrier. The gate signal for lower switch of each leg is negative logic value generated by Vref-in and Carrier. The gate signal for mid switch is logic value generated by the logical NAND value of the gate signals for upper and lower switches.
T=
The structure of the control system is shown in Figs. 4 and 5. The control system is composed of three parts: 1) Control of power delivered to the grid, 2) Vector control for PMSG and 3) MPPT. These control parts generate Vref-in & Vref-out for PWM block of nine-switch AC/AC converter.
T=
(7)
3 P ( )(λds iqs ) 2 2
(8)
Fig. 5 shows the vector control block diagram for PMSG, where stator command current iqs* is derived from the speed control loop and (8).
A. Control of power delivered to the grid The relations for active and reactive power delivered to the grid are given by [7]: 3 (vd id + vq iq ) 2 3 Q = (vq id − vd iq ) 2
3 P ( )(λds iqs − λqs ids ) 2 2
where P is number of poles, λ and i are the air gap flux and the stator current, respectively. The subscripts ‘ds’ and ‘qs’ stand for direct and quadrature components in synchronously rotating reference frame, respectively. The magnetizing current ( ids* ) will be equal to zero because the rotor flux is supplied by the permanent magnet. The equation (7) indicates that if ids be equal to zero, the torque in PMSG is proportional to iqs .
CONTROL SYSTEM
P=
(6)
According to earlier equations, active and reactive power control can be achieved by controlling direct and quadrature current components, respectively. DC voltage is set by controlling active power (Fig. 4).
where min, mout are modulation indexes, fin, fout are frequencies and φin, φout are phases. Offsetin and Offsetout are related to implementation of switching method and usually are set to 0.5 and -0.5 respectively. In this case, min and mout are limited to 0.5.
III.
(5)
The gate signal for mid switch is logic value generated by the logical NAND value of the gate signals for upper and lower switches.
(3) (4)
where P and Q are active and reactive power respectively. v is grid voltage and i is the current to the grid. The subscripts ‘d’ and ‘q’ stand for direct and quadrature components, respectively. If the reference frame is oriented along the grid
Figure 4.
Figure 3. PWM method for nine-switch ac/ac converter
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Block diagram of control of power delivered to the grid
IV.
SIMULATION RESULTS
To verify the performance of the proposed WECS, several simulation tests are performed. The simulated system parameters are listed in Tables I and II. These simulations were performed using methods mentioned in Section III. TABLE I.
PARAMETERS OF PMSG
Parameter Rs Lq Ld P J
TABLE II.
SIMULATION PARAMETERS
Parameter Li Ri C Fs Vdc* Wind speed
Figure 5.
Value 0.05 0.795 mH 0.795 mH 4 0.011 kg.m^2
Value 5 mH 0.01 2000 uF 8kHz 1400 V 12 m/s
A. Operation of Constant Wind Speed In order to evaluate the performance of the proposed system, simulation is first performed using constant wind speed (12 m/s).
Block diagram of vector control for PMSG
C. Maximum power point tracking Due to incident wind speed, maximum output power of wind turbine is obtained in different speed of the turbine (Fig.6). The generator speed must be adjusted according to instantaneous wind speed to obtain incident maximum power. Optimum generator speed is determined by MPPT block of control system. Perturb and Observe (P & O) technique is used for MPPT. Optimum speed is used as reference speed for speed controller of PMSG.
Fig. 7 shows capacitor voltage, which is almost constant (1400V). Reactive power that is kept at zero (unity power factor) is also shown in Fig. 8.
Figure 7. Capacitor voltage
Figure 6. Mechanical power versus rotor speed with the wind speed as parameter
Figure 8. Reactive Power
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Fig. 9 shows active power delivered to the grid and the extracted mechanical power. The electrical power delivered to the grid is different from the extracted mechanical power due to electrical and mechanical losses. To obtain maximum power control, the rotor speed has changed using Perturb and Observe. PMSG rotor speed and the obtained rotor speed from MPPT are shown in Fig. 10. It is seen that PMSG rotor speed nearly equals with the rotor speed obtained from MPPT. Fig. 11 shows maximum mechanical power of turbine and the extracted mechanical power. It is seen that extracted mechanical power is tracking the maximum mechanical power after a short time. Figure 12. Injected three phase currents to the Grid
Figs. 12 and 13 show grid current and its spectra. THD of injected current is 4.8%.
Figure 9. power delivered to the grid and extracted mechanical power Figure 13. Spectra of grid current in proposed WECS
B. Operation of Variable Wind Speed To verify dynamic of the proposed system, the wind speed has been varied from 12 m/s to 10 m/s in t=0.7 sec. The previous simulation was repeated in operation of variable wind speed. Fig. 14 shows capacitor voltage, which is almost constant and reactive power that is kept at zero (unity power factor) is also shown in Fig. 15. Figs. 16–19 show active power delivered to the grid, rotor speed, maximum mechanical power of turbine and the extracted mechanical power and injected three phase currents to the Grid. These results verify effectiveness of proposed system under variable wind speed.
Figure 10. PMSG rotor speed and the obtained rotor speed from MPPT
Figure 14. Capacitor voltage
Figure 11. The maximum mechanical power of turbine and the extracted mechanical power from turbine
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Figure 15. Reactive Power
Figure 19. Injected three phase currents to the Grid
V.
CONCLUSION
In this paper, a PMSG-based WECS with nine-switch AC/AC converter is proposed. Nine-switch converter is used for maximum power tracking control and delivering power to the grid, simultaneously. The proposed system has cost advantages compare to conventional WECS with back-to-back converter, because the number of switching semiconductors is reduced from 12 to 9. With this topology, 3 active switches and 3 diodes is omitted without any change in the objectives of WECS. The control system is composed of three parts: 1) Control of power delivered to the grid, 2) Vector control for PMSG and 3) MPPT. PWM switching method for nine-switch AC/AC converter was presented. Simulation results confirmed validity of the proposed system.
Figure 16. Active power delivered to the grid and extracted mechanical power
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Figure 17. PMSG rotor speed and the obtained rotor speed from MPPT
Figure 18. The maximum mechanical power of turbine and the extracted mechanical power from turbine
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