Ratios-Based Universal Differential Protection Scheme for Power Transformers

Abstract

Power transformers are one of the most important and costly components in power systems. It is subject to short circuit faults similar to any other component of the power system. Failure to detect these faults may result in transformer blaze and may subsequently lead to property damage. Therefore, transformer protection is of vital significance to provide reliable operation of power systems. To detect fault conditions and isolate the transformer in the shortest time possible is the main challenge in power transformer protection. Percentage differential relay is reportedly the most commonly used approach to protect transformers rated from 10 MVA and above. However, it is known that the conventional differential protection could mal-operate owing to the characteristics associated with the nonlinearities in the transformer core. The current research enhances the conventional differential protection algorithm applied to the transformer to overcome the shortcomings of the existing algorithm. The algorithm with the proposed supplements presents an improved differential protection scheme based on current and voltage ratios. The current and voltage ratios are the ratios of the absolute difference and absolute sum of the primary and secondary per unit currents and voltages of each phase, respectively. Avoiding mal-operation of differential algorithm during various transient phenomena, such as transformer energization, simultaneous energizing under internal fault, current transformer saturation during external fault, is the major challenge that faces power transformer protection. The proposed algorithm is evaluated by simulation with Y-Y and Y-Δ configurations and experiments on a physical transformer with studies such as inrush conditions, internal fault, external fault combined with CT saturation and simultaneous inrush with internal fault. Moreover, the performance of the proposed approach is evaluated for different fault cases in the presence of superconductor fault current limiter. The results show that the proposed algorithm can provide fast, accurate, secure and dependable protection for power transformers.