Negative sequence current feed-forward control for three-phase four-leg STATCOM under unbalanced loads

The three-phase four-leg STATCOM is based on four-leg voltage source converter. It can compensate reactive currents, negative sequence currents and neutral currents of the power grid. Compensating the negative sequence currents will results in the second harmonic wave voltage on the DC-link voltage of the STATCOM. The second harmonic wave voltage finally results in negative sequence currents and the third harmonic currents in output currents. In this paper, the mathematical model of the three-phase four-leg STATCOM is derived. The negative sequence currents feed-forward control strategy for three-phase four-leg STATCOM is presented. The simulation analysis is made to discuss the performance of the strategy and a laboratory prototype has been developed to illustrate the performance of the feed-forward control strategy. The model shows that when the voltage is balanced, only the active current has an influence on DC-link voltage. When the STATCOM compensates the negative sequence currents accurately, the active current will contain second harmonic sequence. In this case, the DC-link voltage of STATCOM will contains second harmonic wave. The second harmonic wave have an influence on the voltage control loop of the STATCOM and finally results in the third harmonic currents and negative sequence currents in output currents. The negative sequence currents feed-forward control strategy presented in this paper calculates the second harmonic wave on the DC-link voltage of STATCOM with the negative sequence currents of the net. The second harmonic wave that has been calculated feed back to the voltage control loop of the STATCOM. The feedback cancels the second harmonic wave on the output of the STATCOM voltage control loop. Without the second harmonic wave on the output signal of the STATCOM voltage control loop, the negative sequence currents and third harmonic currents in output currents are suppressed effectively. Results of Simulation analysis and prototype experiment shows that the feed-forward control strategy cancels the negative sequence currents and reduces the third harmonic currents from 8.2% to 1.4% of the foundational component. The Simulation and experimental results demonstrate that the proposed control strategy is effective.