A Fast and Safe Discharging Method for MMC Submodule Capacitors

Abstract:
The Δ connected full-bridge Modular Multilevel Converter (SDBC-MMC) has a wide use particularly for medium voltage STATCOM applications. One of the major challenges of an MMC is to safely discharge the Submodule (SM) capacitors during shutdown. Several discharging methods, especially based on the use of bulky resistors, have already been proposed in the literature. This paper proposes using circulating current to discharge the SDBC-MMC through equivalent resistor of each phase until a minimum SM voltage is reached, while simultaneously limiting the discharging current. The bleed resistors are used later for the remaining voltage. The advantages of this method are that it provides safe operation, low cost, and fast discharging time. The proposed method was simulated for the discharge of a 22 KV Δ connected MMC based STATCOM with 22 cells in each phase in three different configurations: I. phase-by-phase discharge, II. cell-by-cell discharge, and III. overlapped cell-by-cell discharge.