79. Transformer Protection against Inrush Currents

Inrush currents are frequently encountered when a transformer is switched ON. Though an inrush current is a transient phenomenon it may trip a differential relay as the inrush current flows only in the primary circuit of a transformer. Inrush is not a fault in a transformer & hence the differential relay shouldn't operate. As tripping of the differential relay can create havoc in an industry leading to higher downtime & production loss, therefore, its necessary that transformer protection remains stable during this transient. Transformers are key elements in electrical distribution systems. They are usually protected by relays and surge suppressors to guard against various abnormal conditions such as internal and external short circuits, over-loading & overvoltages. Reliability and stability in protection of a transformer is an important task in distribution systems. Large size transformers (above 100 MVA) apart from other protection schemes, are usually protected by means of differential protection. The differential protection protects the transformer against internal faults. Inrush currents are frequently encountered during the switching process of transformers. Due to the magnetizing current the differential relay may trip as the magnetizing currents flows only in the primary side of the transformer. This article deals in details with the understanding of various parameters that govern the amplitude & nature of inrush currents. It also talks about some of the techniques to minimize the amplitude of the inrush current. As tripping of the differential relay can create havoc in an industry leading to higher downtime & production loss, therefore, its necessary that transformer protection remains stable during this transient.