A directional protection strategy for multi-terminal VSC-HVDC grids

Protection issue is identified as the main drawback of the emerging multi-terminal HVDC grids. Multi-terminal HVDC grids demand fast short-circuit fault current interruption. The fast DC circuit breakers as a promising solution can be implemented as either bidirectional or unidirectional devices. In additional to less implementation cost, the unidirectional DC circuit breakers have less power losses as compared to the bidirectional ones. A protection strategy for multi-terminal HVDC grids based on unidirectional breaking devices is discussed and assessed in this paper. The performance of unidirectional protection strategy is examined under different fault scenarios in a four-terminal MMC-HVDC grid model. Furthermore, the impacts of unidirectional protection strategy on the power converters and also the current interruption and surge arrester ratings of the DC circuit breakers are discussed.