Restricted Earth Fault Protection

A Comparison Between High-Impedance and Low-Impedance Restricted Earth-Fault Transformer Protection
Casper Labuschagne, Schweitzer Engineering Laboratories, Inc. Izak van der Merwe, Eskom Enterprises
Abstract—Restricted earth-fault (REF) protection on a transformer is a subject for which there has been little attention and, compared to other types of protection, very little literature exists. Depending on the method of transformer earthing and fault location, some transformer earth faults result in only a small increase in phase current, which transformer differential protection may not detect. Conversely, the amount of current in the neutral may be sufficient to detect most or all earth faults, again depending on the earthing method. By connecting an REF relay to CTs installed in correct locations on the transformer, one can use REF protection to complement differential protection in detecting transformer earth faults. Obtaining maximum benefit from REF protection requires that one consider many factors, including whether to select high-impedance REF or lowimpedance REF relays. In making this selection, one should understand the theory behind each option. Historically, only high-impedance REF protection was available, because of equipment and technology limitations. Today, numerical protection relays include low-impedance REF elements for transformer protection. Both types of protection have advantages and disadvantages; the relays do not perform equally well in all applications. One key advantage of low-impedance REF protection included in a numerical relay is the ability to use CTs with different ratios and specifications without the need for interposing CTs. One key advantage of high-impedance REF is proven immunity (relay security) to CT saturation for external faults. Key to either type of protection is the ability to provide maximum winding coverage against earth faults. There is also speculation, as yet unsubstantiated, that a high-impedance REF element

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