VSC-HVDC can improve the stability of AC grid, but it also changes its fault characteristics. Thus, it is necessary to re-verify the AC line protection connected with HVDC. Based on the different operation modes of VSC-HVDC, the fault characteristics on DC near side are analyzed: the magnitude of positive and negative sequence fault current is greatly influenced by DC control strategy and the negative sequence fault current is completely suppressed. However, the zero sequence fault current characteristic is not affected. Based on the derived fault characteristics, an optimization strategy of starting elements is proposed, which is based on AC strong fault side commanding VSC-HVDC weak fault side and voltage reduction criterion. Meanwhile, the influence range of VSC-HVDC interconnection on fault characteristics of AC lines is given. Finally, a simulation model is built in PASCAD/EMTDC software and the effectiveness of the proposed optimization strategy is validated.

VSC-HVDC; fault ride-through; differential protection; starting element