A New Technique for Improvement Differential Relay Performance in Power Transformers

Transformer protection devices are often used to identify internal or external transformer problems and act to either prevent damage or unnecessarily disconnect power transformers. This study proposes a new differential element that combines harmonic restraint, security, and reliability with harmonic blocking speed to improve the relay performance in a power transformer. Under high load, a negative-sequence differential element adds more sensitivity for internal turn-to-turn failures. External fault detection monitoring enhances security in an external problem involving current transformer (CT) saturation. Furthermore, overcurrent elements may be configured to vary dynamically in operation is provided. This element enhances protection coordination for various operating conditions without requiring modifications to the transformer group settings. The balance of the paper discusses the use of an under-load tap changer using a time-synchronized phasor monitoring system to reduce loop current and losses in parallel transformer applications.

I. Introduction

An important consideration is whether or not a relay can detect internal faults during times of high inrush currents when used to protect a transformer[1]. A traditional differential protection system with common harmonic blocking can detect these problems when the inrush current harmonic content is less than the relay harmonic blocking threshold. This may be achieved in a few cycles[2]. However, due to the extended time duration of inrush currents, more damage to the transformer occurs, increasing the repair costs and posing a severe risk to personnel and equipment. Due to the decreased tripping time, differential elements with separate harmonic constraints may identify faults under inrush conditions in merely two cycles.