Study Of Three-Phase Four-Leg Inverter In A Small Scale Wind Power Generation System

In small scale wind power generation systems, the loads are stochastic and unbalanced sometimes, which put forward special requirements to the inverter. Three-phase four-leg inverter brings forward to power unbalanced three-phase loads. The topology is known to produce balanced voltages even under unbalanced load conditions. And compared to other three-phase four-wire inverters, its size and weight is small.In this dissertation, the mathematic model of three-phase four-leg inverter is established in both the a-b-c and bisynchronous coordinates. Because in the a-b-c coordinate, PI adjustor can't achieve isochronous control for AC components, this dissertation introduces three control strategies for the three-phase four-leg inverter system under synchronous frames. For the first strategy the voltage control loop and the current control loop are both designed in the traditional single synchronous d-q-0 coordinate. This control strategy is most simple, but it is difficult to achieve isochronous control for negative component, which is second harmonious in the traditional single synchronous d-q-0 coordinate. In order to control the positive sequence, negative sequence and zero sequence components independently, the last two control strategies based on bisynchronous coordinates are introduced. Because the second control strategy needn't separate the positive sequence and the negative sequence, this control strategy is simpler than the last. But the zero sequence component is AC component, which is difficult to achieve isochronous control. The last control strategy can control the three components independently, and all of them are DC contents, so this control strategy is most accurate.Using MATLAB software, the simulation of system control strategies validate consistency with theory analysis under open loop and closed loop. The three control strategies are all feasible. Finally, the experiments are completed using the TMS320LF2406 digital controller. Experimental results demonstrated the control strategies can accomplish the output voltage balance in unbalanced loads. These control strategies is verified.