Extra‐sensitive impedance differential protection immune to CT saturation

Abstract

For extra‐high voltage transmission lines configured with a communication channel, the current differential protection is facing a dilemma between sensitivity and security. On the one hand, the resistive coverage will not be sufficiently high if security is guaranteed. Although the sequence‐current‐based differential elements are more sensitive, it may cause a longer time delay and deteriorate sensitivity under the open‐phase operating condition. On the other hand, if high sensitivity is guaranteed at the cost of security, the main protection element will be blocked in the current transformer (CT) saturation scenario. To solve this problem, a novel type of impedance differential protection (IDP) is proposed. Under the open‐phase condition awaiting an automatic reclosing, the proposed IDP can assist in accelerating the speed of fault‐clearance for healthy phases, which is faster and more sensitive than using the zero‐sequence current differential protection. For an external fault with heavy CT saturation, the proposed IDP can take over the fault identification capability before the recovery of CT, in which duration the current differential protection is blocked. Simulation results indicate that the proposed IDP coordinate perfectly with the current differential protection in the above two scenarios. The performance of the proposed IDP is also assessed using field data.