Research on fault protection of DC grid based on hybrid MMC

Abstract

The protection strategy of overhead direct current (DC) grid is studied based on the hybrid modular multilevel converter (MMC), which is composed of half bridge sub-modules and full bridge SMs (FBSMs). First, the topology, operating principle and DC voltage operating region of the hybrid MMC are analysed. Then, a control framework embedding three-channel control freedom and DC fault ride through control strategy are presented. Two protection schemes based on DC disconnectors and mechanical DC circuit breakers (DC CBs) are designed. In the first scheme, the hybrid MMC controls the DC fault to zero and a current injection method is adopted to restart the DC grid. In the second scheme, the mechanical DC CBs can block the DC fault current and has the ability to reclose for three attempts. At last, the feasibility of two schemes is validated in power systems computer-aided design/electro-magnetic transient design and control.