Mitigation of Communication Failures on Line Current Differential Relays by adding Automatic Function Switching Logic to Improve Protection System Reliability, Study Case in PT PLN (Persero)

Line current differential (LCD) relay is a kind of transmission line protection system which uses Kirchhoff Current Law as its fundamental principle. It compares measured current from two relays that installed on each substation. The two relays "talk" to each other using certain communication media, e.g. optical fiber, pilot wire, etc. Communication is one of the important components that greatly determines the reliability of the LCD relay. Without proper communication media, this relay cannot work as a protection system. No matter how sophisticated the media is, sometimes experiences communication failure. And this condition is very risky because it can lead to widespread power outage if a system failure occurs on the related transmission line. Recently, many of our transmission lines are provided LCD relay as main protection, OCR/GFR as local backup protection, and Z2/Z3 Distance relay as remote backup protection. If there is system failure on the transmission line, LCD would trip instantaneously to shutting down the line. However, if a system failure occurs when LCD communication failure, LCD would be blocked and the line would be shutting down by remote backup which has delayed for fault clearing time. It can cause more impact such as widespread power outage or equipment damage. This paper deals how to provide instantaneous protection function while LCD blocked by communication failure. This paper shows the implementation of automatic function switching to activate Z1 distance function while communication failure detected. This Z1 would operate as main protection while LCD blocked and whenever occurs a system failure on the transmission line, this protection would detect and clear fault instantaneously. Thus, widespread power outage and equipment damaged can be avoided.