High-Impedance Restricted Earth Fault Relay Settings: Impact of using Auxiliary Current Transformers on Phase Side for Ratio Matching

This work describes the impact on settings of high-impedance Restricted Earth Fault (REF) relays while using wound auxiliary CTs on the phase side (as different from neutral side) to compensate for the mismatch between phase and neutral CT ratios for the power transformers. The article covers a solid single line to ground fault without consideration of any high impedance in the neutral of power transformer. The idea of using auxiliary CTs is in itself not a new concept to compensate for the mismatch of current transformer ratios. However, the derivation of REF settings is not easy and straightforward while using auxiliary CTs using conventional procedures. This paper uses detailed worksheets including varied parameters for phase and neutral CTs and auxiliary CTs (on phase side) and derivation of REF settings therefrom. The necessity arises for use of auxiliary CTs while having to use an existing neutral CT of a different ratio from the phase CTs. In such a case, there are two procedures for derivation of high-impedance REF settings. One is to directly connect the REF relay across the phase and neutral CTs with a suitable REF setting. This procedure has already been described in an accompanying article (Practical Considerations For High Impedance Restricted Earth Fault Relay Settings. Journal of the Institution of Engineers (India), Series B, July 12, 2014). But the current paper describes a different procedure of using auxiliary CTs for compensating for the ratio mismatch between phase and neutral CTs and derive the REF settings suitably as described. In an article submitted recently for publication, the use of auxiliary CT with a bar primary in the neutral side has been described in detail for REF application. In this paper, the use of auxiliary CT on the phase side has been described as different from the neutral side to evaluate its impact on REF relay settings. It is claimed that this procedure with use of auxiliary CTs on phase side for REF applications with detailed worksheets and with detailed parameters for the CTs has so far not been used in REF applications. © 2021, The Institution of Engineers (India).