Improvements in transformer protection and control

Abstract

This paper describes protection elements that detect transformer faults quickly and avoid unnecessary transformer disconnections. The paper introduces a differential element that combines the security and dependability of harmonic restraint with the speed of harmonic blocking to optimize relay performance. An additional negative-sequence differential element improves sensitivity for internal turn-to-turn faults under heavy load conditions. External fault detection supervision adds security to this negative-sequence differential element during external faults with CT saturation. The paper also describes a dynamically configurable overcurrent element that improves protection coordination for different operating conditions but does not require settings group changes. In addition, the paper discusses an underload tap changer control system that uses time-synchronized phasor measurements to minimize loop currents and losses in parallel transformer applications.