A reliable differential protection algorithm for delta hexagonal phase-shifting transformers

Abstract

Phase-shifting transformers (PST) are exhibited to continuous variations in phase angle. This continuously-varied phase shift can cause miss operation for the current-based differential protection relay by generating false trip signal. This paper discusses the experimental validation of a proposed current differential protection technique. The currents of the exciting winding are calculated based on the relationship between the magnetic and electric characteristics of the transformer. Then, these currents are provided for calculating the restraint and operating currents of the protective relay. The first stage is to design and fabricate the delta hexagonal (Delta/Hex) phase-shifting transformer and, then, constructing a complete laboratory system. PST terminal currents and tapping value are sensed and, then, they are provided to the relay for calculating the excitation current. To check the relay security, different types of external faults are considered. Similarly, extra internal faults, both with and without impedance, are represented to test the relay dependability and sensitivity. Also, the protective relay is checked to discriminate between fault currents that occur during the transformer energization and healthy inrush currents.