Research On Grid-connected Control Of Direct-driven Wind Power System Based On An Improved Z-source Inverter

With the rapid development of photovoltaic, wind power and other new energy generation technologies, grid-connetced control of these new energy sources has become a hotspot of its applied research. In this field, the grid-connected inverter, which is regarded as the interface device between the generator and the power grid, can affect the performance of the whole system directly. As a new circuit topology structure in recent years, Z-source inverter is widely used due to its many advantages. For example, it has the ability of boost and buck, also allows the shoot-thrrough phenomenon of the same leg in the inverter. Although it can overcome the shortcomings of traditional voltage and current source inverters, it still remains many deficiencies. Inrush current which is easy to damage the devices of three-phase inverter bridge at startup. High capacitor voltage stresses on impedance network during steady state, therefore large volume of capacitors are required and then the cost increases for this inverter. In addition, due to the shoot-through zero vectors were replaced part of traditional zero vectors, and then the quality of AC output voltage is affected. So, a improved Z-source inverter whose performance is abviously superior to the traditional Z-source inverter is proposed in this dissertation,then used in the direct-driven wind power generation system and realized its grid connection control with suitable control strategy. The main research contents of this dissertation are shown as follows:(1) A wind turbine mathematical model and machine side rectifier mathematical model are established. What's more, in the machine side of the rectifier, double-closed loop strategy including outer DC voltage loop and inner current loop is introduced to achieve high quality electricity.(2) An improved Z-source inverter is introduced to overcome the disadvantages of the traditional Z-source inverter. In this topology, a controlled device IGBT is proposed to realize the separation between shoot-through zero vectors and traditional zero vectors, and thus a coupled inductor is used to replace one of inductor in tranditional Z-source network. Moreover, its position relative to the three-phase inverter bridge has changed. The simulation results show that in contrast with traditional Z-source inverter, it can obtain higher boost factor,achieve higher voltage gain as well as the quality of its output voltage waveform is much higher. Besides, no inrush current when it starts working and of capacitor voltage is lower during steady state.(3) As for modeling the DC-side of Z source network of this improved Z source inverter, a small signal model is established by using the state space averaging method, and a corresponding controller is designed on the premise of adding the capacitor voltage loop. With this control strategy, DC-link voltage peak can maintain a steady base level and be applied to the occasion of power fluctuation.(4) Under the basis of keeping the stability of the DC-link voltage, the mathematical model in two-phase rotating coordinate for AC side of this improved Z-source inverter is established. By analyzing of the model, it can be known that resonance oscillation may take place and the coupling of active current and reactive current can exist in the system.Therefore, aiming at the overall performance of the system, a double current closed-loop control strategy with the adoption of applied controllers whose parameters setting method is based on the logarithmic frequency characteristics of the systems is used to realize its grid connection. The simulation results show that under the effect of this control strategy, an improved Z-source system can obtain high quality power and achieve the decoupling control between active current and reactive current decoupled. Verifying that this grid connected system has good dynamic performance, steady state performance, stability and robustness.