Analysis of the Effects of Grid-Connected Charging/Discharging Stations on Relay Protection

The grid-connected operation of charging/discharging stations changes the original load, power supply, and network structures of the distribution network. It also affects the power flow level and direction and leads to a reduction in the sensitivity and reliability of the relay protection system. The grid connections may even lead to improper operation or failure of the relay protection protocols. In order to solve the above problems, this paper proposes a method that constructs the protection criteria by using the characteristics of the positive sequence comprehensive impedance after internal and external faults. Specifically, based on the difference in amplitude of the differential positive sequence impedance in the feeder after the charging/discharging station is connected to the grid, positive sequence comprehensive impedance is proposed for the purpose of establishing longitudinal differential protection. The results show that the vertical protection principle based on the positive sequence comprehensive impedance has a certain versatility for the protection of distribution networks with charging/discharging stations. Additionally, the protection principle has high sensitivity and is generally unaffected by transition resistance. The applicability of the proposed positive sequence comprehensive impedance vertical protection principle is verified by simulation