Control Strategy Improving Fault Ride-through Capability of Cascade Multilevel Inverter Based STATCOM

A 10 MVA Synchronous Static Compensation (STATCOM) is being designed for the wind farm owned by the Seawest and connected to the Bonneville Power Administration (BPA) network in U. S. The five-level cascade multilevel inverter based STATCOM will be used to mitigate the voltage fluctuation. The fault ride through is regarded as the main challenge not only to the wind-turbine, but also to the STATCOM. The common control strategy for the fault ride through is to limit the currents within the rating values. With this strategy, however, we find out big over currents that will trig the over current protections during transient processes. We also find out the over voltages on DC capacitors that will trip the system. In the paper, we propose the improved control strategy for boosting the fault ride through capability. By degrading the power rating during faults, the big over current is avoided. Moreover, by lowering the reference of DC capacitor voltages, the DC capacitors are safe during faults. Simulation results are given to verify the proposed control strategy.