Modeling transformer differential protection with harmonic restraint

In this paper, a differential relay model (87T) is developed in the Alternative Transient Program for power transformer protection. The relay model uses an anti-aliasing filter and the Discrete Fourier Transform to calculate the relay operation and restriction currents. The relay model includes second, fourth and fifth harmonic restriction for blocking the relay operation during transformer energization and overexcitation. The relay model was successfully tested for different operating and fault conditions in a power transformer connected to the system. The results obtained validate the relay model and confirm its accuracy.