Transient Characteristics of VSC-HVDC System and its Effects on Distance Protection under AC Grid Fault

As the voltage source converter (VSC) based high voltage direct current (HVDC) system has advantages of high stability and controllability, it becomes the main developing direction of the HVDC system. Dynamic control characteristic of VSC-HVDC system differ a lot from those of conventional AC and DC system. The VSC-HVDC exports various short-circuit current to AC grid during grid fault, which may lead to inaccurate computation of the apparent impedance and malfunction of the distance relay. Few research efforts have been made to investigate the impacts of VSC-HVDC system on the distance protection of AC transmission lines. This paper analyzes the detailed impact mechanism of the VSC-HVDC system on the distance protection operation characteristics of the interconnected AC system and provides a reference for the relay protection configuration of the AC/DC interconnected power system. Based on the control strategy of VSC-HVDC system, the magnitude and phase features of the short-circuit current contributed from VSC-HVDC are analyzed. The variation of measured impedance in the presence of VSC-HVDC system is investigated. Then, the impact factors of VSCHVDC on distance protection are proposed. The validity of the study are verified by PSCAD/EMTDC simulations.