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
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
References: GEC Alsthom Measurements Limited, Protective Relays Application Guide, 3rd edition, 1990. [2] D. Robertson, ed. Power System Protection Reference Manual, Reyrolle Protection, Chapter 6, Stockfield: Oriel Press. [3] P. Bertrand, B. Gotzig, and C. Vollet, “Low Impedance Restricted Earth Fault Protection,” in Developments in Power System Protection, Conference Publication No. 479, IEE, 2001. [4] SEL-387 Relay Training, Restricted Earth Fault Protection, Schweitzer Engineering Laboratories Inc., Rev 0.0, July 2001. [5] P. E. Sutherland, PE (SM), “Application of Transformer Ground Differential Protection Relays,” presented at the Industrial and Commercial Power Systems Technical Conference, Sparks, NV, 1999. [6] A. Guzmán and L. S. Anderson, “Restricted Earth Fault Protection for Auto-Transformers Using a Directional Element.” Available at www.selinc.com [7] SEL-387-0, -5, -6 Instruction Manual, Current Differential Relay, Overcurrent Relay, Data Recorder, Schweitzer Engineering Laboratories, Date Code 20040628. [8] SIPROTEC, Differential Protection Manual, 7UT612, V4.0, C53000– G1176–C148–1, Siemens. [9] Application Manual, ProtectIT Transformer Protection Terminal, RET521*2.5, 1MRK 504 037-UEN, ABB. [10] T60 Transformer Management Relay, UR Series Instruction Manual, T60 Revision 4.0x, Manual P/N: 1601-0090-G1 (GEK-106490), GE Multilin, 2004 [11] P. A. Gerber, SCSAGAAG0 Rev 3, Transformer Protection Philosophy, Eskom Distribution Division, 2001. [12] Mini APPS Course (Analysis & Protection of Power Systems), vol 1, Section 9, Transformer Protection, 25 February to 1 March 1996. [1] Protection Field Engineer. He is currently a Chief Engineer: Protection Specialist in Resources and Strategy, a Corporate Division of Eskom Holdings Limited. He is responsible for Distribution Division National Contracts for protection schemes and equipment, general protection technology direction setting and technology management, and the implementation of Distribution Automation and Substation Automation in Eskom’s Distribution Division. He has authored a number of protection and substation automation related papers. He is a Registered Professional Engineer in South Africa. Casper Labuschagne earned his Diploma (1981) and Masters Diploma (1991) in Electrical Engineering from Vaal Triangle Technicon, South Africa. After gaining 20 years of experience with the South African utility Eskom, where he served as Senior Advisor in the protection design department, he began work at SEL in 1999 as a Product Engineer in the Substation Equipment Engineering group. Presently, he is Lead Engineer in the Research and Development group. He is registered as a Professional Technologist with ECSA, the Engineering Counsel of South Africa, and has authored and coauthored several technical papers. IX. BIOGRAPHIES Izak van der Merwe obtained his B. Eng (Electrical) degree from the University of Stellenbosch in 1991. He started to work for Eskom in 1993 as a © 2005, 2007 by Eskom Enterprises and Schweitzer Engineering Laboratories, Inc. All rights reserved. 20070711 • TP6207-01