A cross connected cell based modular multilevel converter for HVDC system with DC short‐circuit fault ride‐through capability

Abstract

SummaryThe non‐permanent dc short‐circuit fault is a crucial issue in overhead high voltage dc transmission (HVDC) lines. In this paper, a cross connected (CC) cell also known as hexagonal switch cell (HSC) based modular multilevel converter (MMC) with dc short‐circuit fault ride through feature is addressed for an HVDC system. During normal operation, the CC cell‐MMC is capable of producing an identical number of levels while using fewer power devices than full‐bridge (FB) and clamp double sub‐module (CDSM)‐based MMCs. Similarly to the FB‐MMC and CDSM‐MMC, the provided CC cell‐MMC not only blocks the dc fault current but can also function as a static synchronous compensator (STATCOM) during a DC‐Bus fault. The Proposed control technique uses MMC as a STATCOM to inject reactive power during a pole‐to‐pole DC‐Bus fault to keep the grid stable. The control approach comprises injecting zero‐sequence voltage into each phase to balance the mean voltage of capacitors and arm currents. The fundamental zero‐sequence voltage is obtained by a simple stationary reference frame‐based controller. The control strategy's efficacy is evaluated in MATLAB and confirmed in the Opal‐RT environment.