AN ALGORITHM FOR PROTECTING POWER TRANSFORMERS

Abstract

Electric power utilities have traditionally used electromechanical and solid-state relays for protecting power transformers. Differential protection is the conventionally used scheme for the purpose.' With the advent of digital technology, researchers and designers have made remarkable progress in the development of microprocessor-based relays. These relays use algorithms similar in principle to their electromechanical counterparts. Several algorithms, based on differential principle and suitable for implementation on microprocessors, have been proposed in the past. However, these algorithms are adversely affected by current transformer saturation and ratio-mismatch conditions . This thesis describes a technique that provides a new approach for protecting power transformers. The technique uses positive- and negative-sequence models of the power system in a fault-detection algorithm. While phase voltages and currents at the transformer terminals are used' to detect fault, no information concerning parameters of the transformer and power system is required. The performance of the proposed technique was studied for a variety of operating conditions using data generated by simulations on the electromagnetic transient program, EMTDC. Performance of the developed technique was also evaluated for saturation and ratio-mismatch conditions of the current transformers. A theoretical basis explaining the stable performance during such conditions is also established. The proposed algorithm can be adapted to protect the transformer which is connected to a source but is not supplying any load. Occasionally, switch-on faults may occur when a transformer is energized. A criterion to detect such faults is proposed and test results verifying the same are also included in the thesis.