Case Studies of Magnetizing Inrush Current Effect on Differential & REF Transformer Protection

Transformers is one of the most critical equipments for electrical energy transfers in a power system. In several cases, magnetizing inrush phenomenon can lead to mal-operation of differential & REF protection. The phenomenon of magnetizing inrush is a transient condition that occurs when any abrupt changes of the magnetizing voltage. It is not fault condition, and therefore transformer protection must remain stable during the transient condition.This paper describes and analyzes a case studies where protective relay with magnetizing inrush restraint still send tripping signal when the magnetizing inrush happen. This paper uses fault wave recording to discuss the operation behavior and reasons of corresponding protection relay. The results of the studies confirm that the magnetizing inrush current event affects not only differential protection but also REF protection.