Highly Efficient Fault-Tolerant Modular Embedded Thyristor Directed Converter for HVDC Applications

Modular multilevel converter containing half-bridge submodules (SMs) is the most preferred topology for high voltage dc (HVDC) applications. However, the inability to block/limit dc fault current and large number of capacitors requirement are some of its limitations. Recently, hybrid topologies utilizing thyristors for such applications have gained significant interest as they offer lower on-state voltage drop, higher reliability, and power handling capability. In this article, the modular embedded thyristor directed converter (METDC) is proposed, which uses series-connected thyristors as the high voltage switches and chain-links comprising of IGBT based half and full-bridge SMs. A control technique is proposed for force-commutation of thyristors by using full-bridge SMs for lagging loads, while they are naturally commutated for leading loads. The METDC offers wide operating range besides being fault tolerant. The results obtained from the METDC-HVDC model developed in PSCAD substantiate its ability to provide independent active and reactive power control required for HVDC application. The fault studies carried out validate the dc fault tolerant capability of METDC. The results obtained from the various ac fault/unbalanced case studies substantiate the efficacy of proposed controller to force-commutate the thyristors even during contingencies. Loss computation validates the ability of converter to provide higher efficiency.