Variable DC voltage based reactive power enhancement scheme for MMC-STATCOM

Constrained by the AC voltage amplitude modulated by a modular multilevel converter-based static synchronous compensator (MMC-STATCOM), its reactive power output may be subject to oscillations under grid contingencies, posing a threat to the grid stable operation. To solve this problem, this paper proposes a variable DC voltage (VDCV)-based reactive power enhancement scheme for MMC-STATCOM. In this scheme, a novel variable DC voltage control is designed, which can increase the DC voltage in a transient state for relaxing the constraint of the AC voltage amplitude modulated by MMC-STATCOM and improving its reactive power output capability (RPC). At the same time, to make full use of the improved RPC of MMC-STATCOM, a VDCV scheme also proposes an optimisation algorithm of its reactive current-AC voltage droop coefficient using the established reactive power model of the MMC-STATCOM. Based on small signal modelling and analysis, the key parameters of the proposed VDCV scheme are optimised. The performance and reactive power enhancement of the VDCV scheme is evaluated through the hardware-in-the-loop experiment under grid disturbances.