Research On The Key Technologies Of Variable-Speed Constant-Frequency Wind Power Generation

Nowadays the problems of energy lack and environment pollution are more and more serious, wind power generation becomes an important approach as green regenerative energy. Variable speed constant frequency (VSCF) wind power generation of doubly fed induction generator (DFIG) is realized by controlling the rotor circuits. Power flows through rotor circuits is decided by slip, so synchronous speed can be set in the middle of running range of speed. If slip range is±30% , the maximum slip power is 30% of generator rated power. Thus, the capability of AC excited converters is reduced greatly, cost is decreased considerably.If good control strategy is added to the converter, superior steady-state and transient-state operation characteristics will be obtained. So it is especially suitable for the renewable energy generation such as wind power generation. In this paper some key technologies,such as flexible cutting in and off, maximum power point tracking(MPPT) control strategy for variable-speed constant-frequency wind power generation and PWM rectifier operating in unbalanced voltage condition, fault ride-through are studied deeply. The contributions of this dissertation are as follows:According to characteristic of AC-excited variable speed constant frequency wind power generation, vector control technique is applied in doubly fed induction generator. Flexible cutting-in control strategy is studied based on grid voltage orientation without overshoot current, decoupling control of active power and reactive power is obtained. The modes of flexible cutting in and running in ready state are established. Flexible cutting in and its inverse process cutting off are studied by simulations and experiments.A novel maximum power point tracking control strategy is presented in this paper to deliver the most power to grid. The maximum power point can be tracked automatically not measuring wind speed in this control strategy and the control is independent of optimal turbine power curve, which has excellent dynamic, static performance. Simulation and experimental results confirm the accuracy and validity of this control strategy.Direct current control and phase-amplitude control strategies are investigated on grid-connected power converter. The algorithm of phase-amplitude control strategy is simple and practical, by which sinusoidal current can be get, harmonic components are small, unity power factor operation is achieved. But speed of response is relatively slow. Direct current control with current closed-loop control makes the dynamic and static performances of grid side current improved, is not sensitive to system parameters, and the robustness of control system is enhanced. But the algorithm is relatively complex.Under the unbalanced voltage condition, PWM rectifier designed according to balanced condition can not operate in normal state. To enhance the operating performance of the three-phase PWM rectifier, the control strategies in unbalanced input case are improved in this paper, which aims to suppress negative-sequence current and the secondary harmonic of input instantaneous power, to assure that line current is sinusoidal, component of negative sequence is zero, total reactive power is minimum.In order to improve the running performance of VSCF wind power generation, low voltage ride-through is investigated in this paper. A excitation control strategy is proposed for the rotor side converter in a doubly fed induction generator(DFIG) to allow the system to ride through a grid fault without changing system hardware. The machine and converter remain in the safe operating state, the stability and the security of system is thus enhanced.