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High Impedance Restricted Earth Fault Relay Settings: Impact of using Auxiliary Current Transformers at Neutral End for Ratio Matching

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Abstract

This paper describes the impact on settings for high impedance Restricted Earth Fault Relays (REF) while using auxiliary CTs at neutral end to compensate for the mismatch between phase and neutral ratios for the power transformers. 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 work sheets including varied parameters for phase and neutral CTs 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 (ref 10). But the current paper describes a different procedure of using auxiliary CTs at neutral end for compensating for the ratio mismatch between phase and neutral CTs and derive the REF settings suitably as described. It is claimed that this procedure with use of auxiliary CTs with detailed work sheets with detailed parameters for the CTs has not so far been used (Ref 1–23) in REF applications.

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Abbreviations

CTR1:

CT Turns ratio on the phase side

CTR3:

CT Turns ratio of the neutral CT

IG:

Ground Current (considered as reference)

Isec1:

Secondary current phase side = IG/CTR1

Isec2:

Secondary current through auxiliary CT 

= Say (Isec21 + j Isec22)—Derived

Isec3:

Secondary current on neutral side = IG/CTR3

Xm3:

Excitation impedance, neutral CTs

Ie3:

Excitation amps, neutral CT secondary

Ie1:

Excitation amps (total) of phase side CTs

Xm1:

Excitation impedance of all phase CTs in parallel

RCT1:

Secondary resistance of CTs at phase end

Isec1:

Secondary current through phase CTs in parallel

RL1:

Total lead resistance from CTs to relay at phase end

RL:

Relay location

Rstab:

Stabilising resistor value (indicated as R in Fig. 1)

RCT2, CTR2 RL2, Ie2, Xm2:

similar parameters of the aux CT side

I1′, I2′:

Circulating currents described w.r.t Fig. 1

(I1′–I2′):

Relay current

References

  1. Alsthom Network Protection and Automation Guide (Protective Relays Measurement and Control), May 2011 edn. Micom 30 series, REF Protection application guide, Issue B1, Alsthom

  2. A comparison between high impedance and low impedance REF Transformer protection by Casper Labuschagne and Izak Vander Merwe, pp. 1–9. Eskom Enterprises and Schweitzer Engg (2005–2007)

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  5. Application Notes for KCGG High Impedance Protection by Alsthom T&D Protection and Control Ltd., Publication R6142B

  6. Application Guide for High Stability Circulating Current Relay, Alsthom Publication Ref R6136D

  7. Application Notes for High Impedance REF Protection, Section 2.3, Micom application guide NO. P64X/EN-AP/A32

  8. High impedance differential relaying. GE Power Management Document No: GE 3184 9. SIPROTEC Differential Protection Manual, 7UT612, V4.0, C53000, G1176-C148–1, Siemens

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Acknowledgements

The authors are thankful to the Ministry of Electricity and Water Authority (KINGDOM OF BAHRAIN) for their encouragement and support during the preparation of this manuscript for publication.

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  1. Independent Print Ltd, Pune, Maharashtra, India

    Subramanian Vanchier Payyalore & Sridhar Palanisamy

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  1. Subramanian Vanchier Payyalore
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Correspondence to Subramanian Vanchier Payyalore.

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Payyalore, S.V., Palanisamy, S. High Impedance Restricted Earth Fault Relay Settings: Impact of using Auxiliary Current Transformers at Neutral End for Ratio Matching. J. Inst. Eng. India Ser. B 103, 469–475 (2022). https://doi.org/10.1007/s40031-021-00638-x

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