Influence of Large-scale Synchronous Condenser on Relay Protection

In order to solve the problems of insufficient dynamic reactive power compensation and small short-circuit capacity of ultra-high voltage direct current (UHVDC) transmission system, it is a practical and effective scheme to install a certain number of synchronous condensers (SCs) at the receiving end of UHVDC. However, the connected SCs will provide a lot of sub-transient and transient currents in the case of AC system fault, which may affect the operation of AC protection. This paper takes distance and current differential protections of high-voltage transmission network as examples to analyze the influence of the large-scale SC on relay protection. Firstly, the system modeling is carried out. Secondly, the influences of SC's output current on the measuring resistance and differential current are analyzed. Finally, in PSCAD, the electromagnetic transient simulation model of the AC power grid including the SC and its excitation control system is established, and the theory analysis results is verified by simulation. The research shows that the branch coefficient of the adjacent lines will be increased with the connection of the SC, which will increase the measurement impedance, reduce the sensitivity of the distance protection and even cause the protection's refusing action. The connection of the SC will increase the differential current slightly, and the differential protection can still operate reliably.