Grid-Connected Control Strategy Of Doubly Fed Induction Generator (DFIG) Based On Matrix-Converter-Excited

Nowadays, Back-to-back PWM converter is widely used as power supply of excitation system in DFIG. But this converter needs a big capacitor for its DC energy storage, so it is very big and heavy, and it is difficult to maintain. Meanwhile, the electrolyte is easy to volatilize, which shortens the service life. Matrix converter has no large capacitor or large inductor. And it is small and easy for maintaining. Therefore, it is very significant to research matrix-converter-excitation in order to increase power density and decrease maintenance costs. The DFIG based on matrix-converter-excitation is difficult to cut in and lack of performance operating, because matrix converter has complicated structure and is difficult to control. Thus, the grid-connected control strategy of DFIG based on matrix-converter-excitation is studied. The effectiveness has been proved by experiments and simulation. The major achievements include:(1) After investigating the DFIG model and matrix converter modulation strategy, the grid-connected control strategy of DFIG based on matrix-converter-excitation is proposed for no-load condition, normal operating condition and abnormal operating condition. Thus, the strategy satisfies the grid-connected requirements under different running conditions.(2) Considering the large impact of the stator current of the DFIG based on matrix-converter-excitation at the instantaneousness of cutting-in, a closed-loop control strategy for stator voltage is proposed. This control system regards the grid voltage amplitude and phase as given value, and the stator voltage as feedback to control the virtual inverter-side voltage amplitude and phase of the matrix converter, so that the DIFG stator voltage precisely follows the grid voltage and the solution of complex decoupling control is simplified. The experimental results show that the impact current of the cutting-in is little.(3) Considering the problem of long response time and large overshoot of traditional control strategies for matrix-converter-excitation, a direct power control method for matrix-converter-excitation is proposed. Firstly, rotor voltage signal is directly calculated using the stator active power error, the reactive power error, stator voltage, and the location of rotor in the strategy. Secondly, the rotor voltage is modulated using the matrix converter space vector modulation strategy. Then the active power error and the reactive power error are eliminated in a fixed period. Meanwhile, the rotor current loop is not needed. The strategy is able to improve the response speed and diminish the overshoot.(4) Regarding to the incapability of the traditional control when the grid voltage is disturbed or imbalanced, nonlinear optimal control and voltage unbalance control are proposed separately. The Simulation results show that the duration of time to modulate the stator voltage and active power under the case of the grid voltage disturbance is reduced after adopting the nonlinear optimal control strategy, and the pulses of the active power and reactive power are decreased or the stator negative sequence current is eliminated by applying the voltage unbalance control strategy when the grid voltage is imbalanced.(5) The experimental equipment of matrix-converter-excitation system is developed. The system consists of a DSP controller as master control system, a main circuit in which the IGBT module is the switching device, and an inverter-driven induction motor to simulate fans. It achieves that the DFIG transmits active power and reactive power to the grid under normal operating conditions.