Research on Control Strategy of Cascade STATCOM under Unbalanced System Voltage

Depending on faster response time, stronger Var support capacity under low system voltage, and less site area, STATCOM is applied widely in centralized Var compensation of new energy power station such as wind power plant. In the beginning of 2012, China's national standard GB/T 19963-2011 'Technical rule for connecting wind farm to power system' was published, which demand that the wind farm plant should have dynamic Var support capability when grid voltage drops caused by power system's short circuit fault. As we know, single-phase or phase-to-phase sc fault may cause severe three-phase unbalance while grid voltage drop happens. So it's required that STATCOM should be able to work well under grid voltage deep drop. At present, most of STATCOM with multi-level cascading topology adopts well-proved direct current decoupling control strategy, which is designed under balance system, so the following damage occurs under unbalanced condition: 1. The output AC current of STATCOM is unbalanced which results in STACOM's over-current fault; 2. DC capacitor's voltage of STATCOM diverges which results in DC capacitor's over-voltage fault. There for, based on direct current decoupling control, improving and optimizing control strategy against unbalanced system, to achieve sustained working ability under unbalanced system, will expand STATCOM's feasibility greatly, and spread its application area, to play more important role in new energy integration.