A New Method of Improving Transformer Restricted Earth Fault Protection

Abstract

A new method of avoiding malfunctioning of the transformer restricted earth fault (REF) protection is presented. Application of the proposed method would eliminate unnecessary operation of REF protection in the cases of faults outside protected zone of a transformer or a magnetizing inrush accompanied by current transformer (CT) saturation. On the basis of laboratory measurements and simulations the paper presents a detailed performance assessment of the proposed method which is based on digital phase comparator. The obtained results show that the new method was stable and precise for all tested faults and that its application would allow making a clear and precise difference between an internal fault and: (i) external fault or (ii) magnetizing inrush. The proposed method would improve performance of REF protection and reduce probability of maloperation due to CT saturation. The new method is robust and characterized by high speed of operation and high reliability and security.

Keywords

• power transformer
• power system protection
• fault discrimination
• digital phase comparator
• current transformer saturation