Theoretical Study on Doubly-Fed Induction Motor Wind Power Generation System

1 Introduction

As energy consumption grows and environmental pollution becomes increasingly serious, wind energy, as a renewable green energy, has become an energy source that is widely valued by countries around the world, and wind power generation technology has also become a hot topic for scholars from all over the world to study. The variable speed constant frequency generation method in wind power generation technology is the current development direction of wind power generation technology. The wind power generation system with variable speed constant frequency control technology is mainly composed of wind turbines, double-fed generators, inverter excitation systems, and control and detection systems, as shown in Figure 1. The wind power generation system using double-fed generators allows the prime mover to operate at variable speeds within a certain range, simplifies the adjustment device, and reduces the mechanical stress during speed regulation; at the same time, it makes the unit control more flexible and convenient, improves the unit operation efficiency, and the application of vector control can achieve independent regulation of active and reactive power [1][2].

This paper attempts to analyze some basic theories of doubly-fed wind power generation systems to provide a theoretical basis for further simulation, experimental and other research work.

Figure 1 Basic structure of variable speed constant frequency wind power generation system

2 Basic structure and working principle of double-fed asynchronous generator

The structure of a doubly-fed asynchronous generator is similar to that of a wound-rotor asynchronous motor, with two sets of windings, the stator and the rotor. In normal operation, the stator winding of the doubly-fed generator is connected to the industrial frequency grid, and the rotor winding is powered by a three-phase variable frequency power supply with adjustable frequency, amplitude, and phase. The output of the inverter is connected to the grid directly or through an isolation transformer. Since both the stator and rotor of the generator are involved in the excitation, the meaning of "doubly-fed" is derived from this [3].

From the above formula, it can be seen that when the generator speed changes, the stator output power frequency f1 can be kept constant by adjusting the rotor excitation current frequency f2. This is the principle of variable speed constant frequency operation. When the generator is running subsynchronously (nrn1), the phase sequence of the rotor winding is changed, and the direction of the rotating magnetic field speed n2 generated by it is opposite to the direction of the rotor, so n1=nr-n2; when the generator is running at synchronous speed (nr=n1), f2=0, and the rotor is DC excited [4].

3 Control strategy of doubly-fed machine[5][6][7]

Based on the stator winding being connected to an infinite power grid, the angular velocity of the stator rotating magnetic field is the synchronous angular velocity. Therefore, the variables in the d-q-0 coordinate system rotating at a constant synchronous speed in space are selected to replace the real variables in the three-phase stationary coordinate system to analyze the motor. In steady state, the space vectors of various electromagnetic quantities are stationary relative to the coordinate axis, and these electromagnetic quantities are no longer sinusoidal AC quantities in the d-q-0 coordinate system, but DC quantities. The nonlinear and strongly coupled mathematical model of the doubly fed motor becomes a constant coefficient differential equation in the d-q-0 synchronous coordinate system, and variables such as current and flux linkage also appear in the form of DC quantities, as shown in Figure 2.

Figure 2 Physical model of the doubly-fed machine under the d-q axis

It can be seen from formula (6) that under the stator flux orientation, the active power P1 and reactive power Q1 output by the doubly fed machine stator are proportional to the components ids and iqs of the stator current on the d and q axes respectively. By adjusting ids and iqs, P1 and Q1 can be adjusted independently respectively, and the two can achieve decoupling control.

Substituting into the stator flux equation, we have:

This rotor three-phase voltage component value can be used as the command signal required to generate the PWM wave rotor excitation power control, which is used to control the on and off of the inverter main circuit switch tube to generate the required frequency, magnitude, and phase of the three-phase AC excitation power supply. The above relationship constitutes the vector control equation of the doubly fed machine under stator flux orientation.

4 Stator flux directional control

According to the basic equation of vector control obtained in the previous section, the system block diagram of the vector control side of the doubly-fed generator wind power generation system under stator flux orientation can be designed, as shown in Figure 4.

Figure 4 Block diagram of doubly-fed machine stator flux directional vector control

5 Conclusion

In short, the doubly-fed generator is applied to the wind power generation system. The rotor winding of the doubly-fed generator is also connected to the power grid through a bidirectional inverter. In addition to the adjustable current frequency, the current amplitude and phase of the rotor winding can also be adjusted. Preliminary research shows that by adjusting the excitation through vector control, the active power and reactive power output of the generator can be independently adjusted. While achieving maximum wind energy capture, the power factor of the power grid can also be adjusted, improving the dynamic and static performance and stability of the power system.

References

[1] Sha Fei, Ma Chenglian. Research on variable speed constant frequency wind power generation system and its control technology [J]. Power Grid and Clean Energy, 2009(1): 44-47

[2] Liu Wankun. Wind energy and wind power generation technology[M]. Beijing: Chemical Industry Press, 2007.

[3] Lin Bo, Song Pinggang, Zhao Fang. Theoretical analysis of doubly-fed generator in variable speed constant frequency wind power generation system [J]. Electrical Engineering, 2008(5): 71-73

[4] Sun Dongsheng. Modeling and simulation of AC excitation variable speed constant frequency wind power generation system [J]. Computer Simulation, 2007(11): 235-239.

[5] Chen Boshi, Chen Minxun. AC Speed ​​Control System[M]. Beijing: Machinery Industry Press, 2005.

[6] Bian Songjiang. Research on key technologies of variable speed constant frequency wind power generation [D]: [PhD dissertation]. Zhejiang: Zhejiang University, 2003

[7] Chen Jian. Mathematical model and speed control system of AC motor [M]. Beijing: National Defense Industry Press, 1989.

[8] R. Pena. J. C. Clare. and G M. Asher. Doubly fed induction generator using back-to-

back PWM converters and its application to variable speed wind-energy generation

[J]. IEEE Proc.B, vol 143. no. 3, May 2002: 231-241.

[9] Yu Shitao, Yan Xiangwu, Liu Shuyong, Li Heming. Stator field oriented vector control of AC excitation generator [J]. Journal of North China Electric Power University, 2005, 32 (2): 10-14.

[10]Wang Q,chang L.An Intelligent Maximum Power Extraction Algorithm for Inverter-Based Variable Speed ​​Wind Turbine System.IEEE Transaction on Power Electronics, 2004,19(5):1242～1249

Notes

School-level project: Research on frequency conversion system and control strategy of small wind turbine generator (Project No.: K200912)

About the Author

Fang Ning (1972-), male, associate professor, research direction is automation and new energy