Design and implementation of numerical differential relay

Abstract

Differential protection is one of the principal power system protection elements. This widely used protection technique is concerned only with internal faults. It is a very sensitive technique that detects nonzero differential current and it consequently isolate the protected element. Moreover, differential relays are extremely fast. A failed element can be taken out of service in less than a cycle. However, some external abnormal conditions can be similar to internal faults and make the differential relay detects a nonzero differential current, resulting in an unnecessary tripping of the circuit breaker. In this project, an improved algorithm of a power transformer numerical differential relay has been proposed, implemented and simulated. The hybrid technique used to distinguish between the internal fault and the other unnecessary operating conditions is based on a combination of three performant techniques: Dual slope percentage differential characteristic method, even (second and fourth) harmonic restraint and odd (fifth) harmonic blocking. The behavior of the presented relay has been tested versus various situations such as transformer energization with inrush current, internal fault, external fault and transformer energization with an existing fault. The obtained results show that the implemented algorithm provides good result in term of security, dependability and speed.