Control and performance of a cascade low-frequency high-power multilevel converter for SSR mitigation

Large turbine generator units connected to series-compensated system may cause the sub-synchronous resonance (SSR) problem. It would result in the damages to the turbine-generator shaft. To solve the problem, the controllable sub- and super-synchronous current is injected into generator stators. So the 10 kV, 10MVA cascade low-frequency high-power multilevel converter is designed to output the sub- and super-synchronous current. Control algorithm and circuit structure design method are presented in this paper. To validate the effectiveness of proposed control method as well as the control logic, large capacity sub-synchronous tracing field tests are conducted. The result demonstrated that the proposed method can completely realize referenced low-frequency current tracing, the DC voltage capacitor stability and control with excellent steady and dynamic characteristic. It will play important role in practical engineering of the mitigation of SSR.