Inrush / Fault Current Detection for Accelerated Fault Clearance to Enhance Transformer Life

Energization of a transformer results in sudden flow of current which is an effect of core magnetization. This current will be dominated by the presence of 2nd harmonic, which in turn is used to segregate fault and inrush current, thus guaranteeing proper operation of the relay protecting it. This additional security in the relay sometimes obstructs or delays differential protection in a specific scenario, when the 2nd harmonic content was present during a genuine fault. Such a scenario can result into isolation of the transformer by Buchholz and pressure release valve (PRV) protection, which is acted when fault creates more damage in transformer. Such delays involve a huge impact on the insulation failure and chances of repairing or rectifying fault of problem at site becomes very dismal. Sometimes such delay can cause fire in the transformer and such situation becomes havoc for a sub-station. Such occurrences have been observed in field also when differential relay operation was delayed by 10-15 ms by second harmonic blocking in some specific conditions. These incidences have led to the need for an alternative solution to eradicate such unwarranted delay in operation in future. Modern numerical relay called as intelligent electronic device (IED), are embedded with advanced protection features which permits higher flexibility and better provisions for tuning of protection logic and settings. Such flexibility in transformer protection IEDs, enables incorporation of alternative methods such as dynamic switching of 2nd harmonic feature for blocking the differential protection with additional security. The analysis and precautionary measures carried out in this case, has been simulated and discussed in this paper to ensure that similar solutions can be adopted to inhibit analogous issues in future.