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| State Feedback Control of Brushless Doubly-Fed Wind Power Generator under Symmetrical Grid Voltage Drop |
| Nie Pengcheng1, Wu Wenhui2 |
1. Department of Electrical Machinery Huazhong University of Science and Technology Wuhan 430074 China;
2. EAST Group Limited by Share Ltd Dongguan 523808 China |
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Abstract For the grid-connected wind power system with brushless doubly-fed generators (BDFG), the transient symmetrical drop of grid voltage is equivalent to a sudden impose of a reverse voltage source at the grid-connected point. As a matter of fact, there are some deficiencies in the current research on low voltage ride through of BDFG. To be specific, some literatures merely give theoretical research without optimization strategies, and in some other studies, the hardware equipment are installed to improve system performance, but it increased the system cost. What's more, there are no comparative experiments carried out in the other works. On this basis, the dynamic operation characteristics of the BDFG under transient symmetrical voltage drop are deeply studied in this paper. Moreover, an optimal control strategy, i.e., state feedback control, is proposed to improve the transient operation characteristics of the control system.
Firstly, the simplified dynamic mathematical model of the BDFG after converting the rotor circuit is listed. Then, the steady-state current before the voltage drop as well as the transient current after the transient voltage drop are obtained. Afterwards, the current of the power winding (PW) and control winding (CW) after the transient voltage drop is solved by applying the superposition principle. It is observed from the results that the overshoots and oscillation time of the transient current are related to the generator parameters, the degree and phase of the voltage drop, as well as the rotor speed.
However, the degree and the phase of grid voltage drop cannot be controlled for the operating wind power system with BDFG. Moreover, the generator speed could not change suddenly. In addition, the parameters of the generator is fixed after been manufactured. To improve the operation characteristics of the control system, the state model is further constructed regarding PW and CW current as state variables. Then, the state variables are added to the input through the feedback matrix. Therefore, the parameters of the generator system matrix, i.e., the equivalent parameters of the generator are changed. As a result, the pole distribution diagram shows that the poles under the state feedback strategy are significantly shifted to the left compared with the traditional control model. Finally, a simulation case is conducted with the grid voltage drops to 20% under the rated working condition. The experimental waveform shows that the overshoot of the generator physical quantity with the proposed state feedback control is significantly smaller than which with the traditional control strategy. What's more, the oscillation duration is shortened while the requirements of power devices are met. In summary, the anti-disturbance ability of the whole control system is enhanced with state feedback control method.
According to the experimental results, it is concluded that the parameters of the original system matrix of the BDFG is revised by introducing the state feedback control. Essentially, the negative real part of the eigenvalue and the transient coefficient of the output are changed. In the view of physical sense, it is equivalent to changing the equivalent resistance and inductance of the generator, which further changes the attenuation time constant and transient reactance of the PW and CW. Therefore, when the grid voltage drops suddenly, this strategy can effectively reduce the peak value of the oscillating current of the stator PW and CW. On this basis, the convergence time is shorten, and the oscillation as well as overshoot of the system are optimized. In conclusion, the effectiveness and practicability of the introduction of state feedback strategy have been verified.
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Received: 16 June 2022
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2023, Vol. 38 
