Multilevel optimal modulation and dynamic control strategies for STATCOMs using cascaded multilevel inverters

A multilevel optimal modulation strategy was developed for static synchronous compensators (STATCOMs) using cascaded multilevel inverters. In this strategy, besides the minimized harmonic distortion level, the inverter units' fundamental output voltages are also equalized. Therefore, all the inverter units in each phase leg can equally share the exchanged active and reactive power with the utility grid. This greatly helps the dc-link voltages balancing control. A special gating-pattern swapping scheme was developed to further maintain the dc capacitors' charge balance and equalize the current stress of the switching devices. The modulation strategy also includes a redundant operation scheme and a transformer dc magnetization prevention scheme. The performance of the proposed strategy was compared with the conventional fundamental frequency modulation strategy. To implement a fast and robust control of reactive power, a nonlinear feedback linearization controller with a direct voltage control capability was employed. The scheme of modulation index controlling and the operating modulation index range are presented. The effects of the system uncertainties on the closed loop dynamic performance are also discussed. A 30 kVAr 21-level STATCOM prototype was developed. All the modulation and dynamic control strategies were verified using the experimental results from the scaled prototype.