STATCOM OPERATION OF PARALLEL-HYBRID MODULAR MULTILEVEL CONVERTER

The Parallel Hybrid MMC (PH-MMC) is characterized by a fixed modulation index (MI). The MI can be varied by injecting a third harmonic voltage component to the chain-links (CLs), which increases the voltage and current ratings of the converter. Moreover, the range of MI obtained with third harmonic injection method (THIM) is limited. This paper analyses the reactive power injection capability of the PH-MMC during balanced and unbalanced loading conditions. The PH-MMC as static compensator (STATCOM) provides wide MI range without any requirement of THIM. During unbalanced loading conditions, the sum of the capacitor voltages of the CLs tends to diverge from its reference value owing to unequal exchange of power by each CL with the grid. An expression for negative sequence current is derived, which regulates the capacitor voltages by redistributing cluster powers within CLs. The proposed analyses are substantiated using the developed models in PSCAD. Results obtained prove the excellent performance of PH-MMC-STATCOM.