Analysis of Transformer Inrush Current and Comparison of Harmonic Restraint Methods in Transformer Protection

It is recommended that differential protection be used for the protection of transformers of 10 MVA (self-cooled) and higher. Differential protection generally is considered the best protection for transformers. However, inrush current due to transformer energization exists mostly only in one winding of the transformer; therefore, the relay sees the energization condition as a fault. To improve security while maintaining the required levels of dependability, many restraint methods have been proposed to block the operation of the differential element due to the inrush current. Methods using harmonic restraint, wave-shape recognition, and artificial neural networks (ANN) have been proposed to discriminate magnetizing inrush and internal faults. The second harmonic restraint method is the most common one used by various relay manufacturers and application engineers. There are a few variations of harmonic restrained differential protection. This paper will analyze factors affecting the second harmonic ratio in inrush current, and describe various harmonic restraint methods and compare their performance.