Power transformer protection improvement

Abstract

A power transformer is mostly protected against internal faults using a differential protection which is sensitive and a fast clearing technique. This technique of protection detects nonzero differential current, and then activates a circuit breaker that disconnects the power transformer. However, this nonzero differential current may be produced by transformer magnetization, due to so called inrush current or over-excitation, and may cause the relay to operate unnecessarily. This magnetization current is a transient current that appears only when a transformer is first energized or after clearing external fault. Even though, it can be as great as 8 times the full load current and it contains harmonic components, however, it is harmless due to rapidity. During periodic magnetization condition due to over-excitation the third and fifth harmonic components are largely noticed; however, during the normal apperiodic inrush conditions, the second harmonic is relatively high The transformer differential protection scheme has to be improved so that it can distinguish between nonzero differential current produced by magnetization current and that produced by internal fault. Several methods have been proposed to blind the differential protection system during magnetization current where the harmonic components have been used as means of detection. However, the digital computer based protection offers a number of advantages over the conventional ones. So, the security and reliability have been improved; it remains only to develop an efficient algorithm requiring less time consuming calculations. In this research project, a new approach applied to digital differential protection relay for a large power transformer is proposed