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 provides superior sensitivity and coverage against earth faults. This paper summarizes the theory of classical high-impedance REF protection and new low-impedance REF protection. It also discusses issues such as relay sensitivity requirements, transformer fault current distribution, impact of fault location on relay performance (winding coverage), CT requirements, the impact of CT saturation response on REF protection elements, and application considerations for the two protection methods.
current changes very little, but large current flows in the neutral conductor  . REF takes advantage of the large current in the neutral conductor to provide sensitive and fast protection for transformer faults close to the earth point. REF protection applied to transformers may be referred to as “unit earth-fault protection,” and the “restricted” part of the earth-fault protection refers to an area defined between two CTs. Generally, REF protection can be applied in one form or another to all transformer windings, even delta-connected windings (see Delta Winding—NEC/R Earthed). On solidly earthed star windings, we will show that fault coverage is possible from the first turn above the star point, provided the REF element connects to a CT in the transformer neutral. This high winding coverage is possible because the relay operates on the high fault current in the neutral conductor instead of on the small fault current in the phase. On an unearthed star winding or a delta-connected winding without a neutral earthing compensator (NEC), winding coverage is reduced because of the lack of a neutral CT. Unearthed star windings or delta-connected winding installations provide phase CTs only (see Delta Winding—NEC/R Earthed), and the REF element operates on the change in phase current only. II. EARTH-FAULT CURRENT AND IMPACT ON SENSITIVITY A. Earth-Fault Currents in a Transformer for Different Connections When operating from the neutral CT, REF protection provides more sensitive...
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