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3
Operation
Theory
PCS-978 Transformer Relay
3-54
Date: 2013-01-16
3.4 Restricted Earth Fault Protection (64REF)
3.4.1 Application
The Restricted Earth Fault (REF) protection is meant to protect a single winding of a power
transformer. The winding which should be protected must be earthed. In the case of delta windings,
the winding must be earthed by an earthing transformer, which must be electrically placed
between the winding and the current transformers.
REF protection is a kind of differential protection, so it calculates differential current and bias
current. The differential current is a vectorial difference of the neutral current (i.e. current flowing in
the neutral conductor) and the residual current from the lines. For internal faults, this difference is
equal to the total earth fault current. REF protection operates on the fault current only, and is not
dependent on eventual load currents. This makes REF protection a very sensitive protection.
REF protection can be applied to protection of two-winding transformer, three-winding transformer
or auto-transformer.
3.4.2 Function
Maximum 4 group CT and 1 neutral CT inputs for REF protection, and the maximum current
inputs applied for an auto-transformer with two circuit breakers at HV and MV sides
respectively.
Maximum 3 REF protections for each side of a three-winding transformer
Inrush current has no effect on REF protection.
RFE protection has high sensitivity to internal earth faults because there is no load current in
the restraint current.
The tap of transformer has no effect on REF protection.
CT Transient detection function based on the ratio of residual current to positive current is
adopted to eliminate the influence of difference of transient characteristic to REF protection.
CT saturation detection function based on 2
nd
and 3
rd
harmonics is adopted to avoid
maloperation of REF protection during an external fault.
3.4.3 Protection Principle
3.4.3.1 Overview
The difference between current differential protection and restricted earth fault protection is that
the first one is based on adjusted phase current balance and the later is based on balance of
calculated residual current and residual current from neutral CT.
Three phase currents and neutral current are the inputs to the restricted earth fault protection of a
three-phase winding. Polarity of three-phase CT is at busbar side, and that of neutral CT is at
transformer side as shown in the following figure.
Содержание PCS-978
Страница 1: ...PCS 978 Transformer Relay Instruction Manual NR Electric Co Ltd ...
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Страница 6: ...Preface PCS 978 Transformer Relay IV Date 2013 01 16 ...
Страница 10: ...Preface PCS 978 Transformer Relay VIII Date 2013 01 16 ...
Страница 62: ...3 Operation Theory PCS 978 Transformer Relay 3 p Date 2013 01 16 ...
Страница 250: ...4 Supervision PCS 978 Transformer Relay 4 b Date 2013 01 16 ...
Страница 264: ...4 Supervision PCS 978 Transformer Relay 4 14 Date 2013 01 16 ...
Страница 266: ...5 Management PCS 978 Transformer Relay 5 b Date 2013 01 16 ...
Страница 272: ...5 Management PCS 978 Transformer Relay 5 6 Date 2013 01 16 ...
Страница 334: ...6 Hardware Description PCS 978 Transformer Relay 6 60 Date 2013 01 16 ...
Страница 370: ...8 Human Machine Interface PCS 978 Transformer Relay 8 d Date 2013 01 16 ...
Страница 404: ...8 Human Machine Interface PCS 978 Transformer Relay 8 34 Date 2013 01 16 ...
Страница 406: ...9 Configurable Function PCS 978 Transformer Relay 9 b Date 2013 01 16 ...
Страница 446: ...10 Communication PCS 978 Transformer Relay 10 26 Date 2013 01 16 ...
Страница 448: ...11 Installation PCS 978 Transformer Relay 11 b Date 2012 10 09 ...
Страница 456: ...11 Installation PCS 978 Transformer Relay 11 8 Date 2012 10 09 ...
Страница 458: ...12 Commissioning PCS 978 Transformer Relay 12 b Date 2012 12 07 ...
Страница 468: ...12 Commissioning PCS 978 Transformer Relay 12 10 Date 2012 12 07 ...
Страница 470: ...13 Maintenance PCS 978 Transformer Relay 13 b Date 2012 10 09 ...
Страница 474: ...13 Maintenance PCS 978 Transformer Relay 13 4 Date 2012 10 09 ...
Страница 476: ...14 Decommissioning and Disposal PCS 978 Transformer Relay 14 b Date 2012 10 09 ...
Страница 478: ...14 Decommissioning and Disposal PCS 978 Transformer Relay 14 2 Date 2012 10 09 ...
Страница 480: ...15 Manual Version History PCS 978 Transformer Relay 15 2 Date 2013 01 16 ...