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INDIA
Journal
Vol. 10, No. 2 - July 2021 (Half Yearly Journal)
Join CIGRE
The World Forum for Electric Power System
India
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Restricted Earth Faults -A Complementary Protection to WURXEOHVKRRWLQJ RIDOOWKH+9D
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vital and
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3URWHFWLRQ PD\ to protect
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provides excellent protection against Ph to Ph faults and phase to earth faults for the faults within the WKH
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Differential zone protecting the winding covering 80% from the bushing, however it is not capable of
3UHYLRXVWRWKLVKHZRUNHGDVDQ$GYLVRU
providing protection against Single Phase to Earth faults near WURXEOHVKRRWLQJ
7HVWLQJ
the neutral in a starRIDOOWKH+9DQG(+9HTXLSPHQW
DW6287+&2D',6&20LQ
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connected VKRRWLQJ RI 3URWH
Transformer,
with solidlyWKH 6WDWH
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of 0
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protection is the only answer.
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Biased Differential protection as this protection too is based on Merz-price methodology. [1]
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1. REF WRZKROHZLQGLQJIURPEXVKLQJWXUUHWVWRQHXWUDORIWKH6WDUFRQQHFWHGWUDQVIRUPHUV
Protection Concept from bushing turrets to neutral of the Star connected
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1.1 REF ZLWKVROLGO\HDUWKHG transformers with solidly earthed Neutral.
is the SHUWDLQLQJWRWKH3RZHU7UDQVIRUPHUVDQGRWKHUHTLSPHQWVRISRZHUVHFWRU+HLV
acronym for Restricted
1HXWUDO Earth fault.
The area of protection is restricted to whole winding
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Fig.1 : Depicting unprotected15-20% of winding of Star connected transformers near the QHDU
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neutral against Phase to Earth Fault
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1.1.1 Type of REF Protections
(1) Low Impedance (2) High Impedance
In case of through earth fault the direction of the secondary of the winding only. High Impedance REF protection
loop current of ‘3Io’ from the affected phase line/turret CT takes care of the faults between phase winding and earth
would be from S2 to S1and the secondary of the Neutral against the unprotected 20% of the winding close to
current ‘In’ entering the Transformer through Neutral in neutral. In a star connected transformer the voltage also
the opposite direction shall have the direction from S1 reduces in proportion to the remaining turns (distance)
to S2.Since the loop currents are in opposition to each from the neutral.
other therefore shall cancel each other and there shall Thus whenever Phase to Earth fault occurs within 20%
not be any current though REF relay 64 R. distance from the neutral, the driving voltage of earth-
For clarity please refer to Fig. 4, the earth fault occurs fault current very low. The secondary replica of this low
in LV ‘B’ at the location ’F’. The secondary loop current earth fault current (3Io) is sufficient to activate the REF
(in pink colour) ‘Ib’ is balanced against ‘In’. relay 64 R.
Note : In this High impedance REF scheme it is essential to LV B Phase to earth fault occurs at a location F of the
have same ratio and characteristics of line/turret CTs and the winding in the vicinity of 15-20% of the winding from the
Neutral CT ie the knee-point voltage should be same and the neutral .The zero sequence
,QFDVHRIWKURXJKHDUWKIDXOWWKHGLUHFWLRQRIWKHVHFRQGDU\ORRSFXUUHQW earth
RIµ,R¶ fault current (3Io) shall
IURP
CTs should be of PS Class confirming to IS :2705(part-4) of
follow
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6 the pathWKH
6DQG shown in red RI
VHFRQGDU\ coloured
WKH dotted lines and
Indian standard or PX class confirming to IEC 61869( Earlier
1HXWUDOFXUUHQW µ,Q¶ HQWHULQJWKH7UDQVIRUPHU shall enter
WKURXJK1HXWUDOthe source through the
LQWKHRSSRVLWHGLUHFWLRQ Neutral N (please refer
IEC 600440)/IEEE Std. C57.13 class C of International
VKDOO KDYH WKH GLUHFWLRQ IURP 6 WR 66LQFH
to WKH ORRS FXUUHQWV
fig.5).The DUH LQ RSSRVLWLRQ
secondary replica of WR
zero seq.
Standards.[4,5]
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Primary Earth fault current IN (3Io) is (In) and is in the
2.1.2 REF protection-WKRXJK5()UHOD\5
In-zone fault (20% of unprotected
opposite direction of it (as per characteristic of a CT).
winding)
This’In’
)RU FODULW\ SOHDVH UHIHUWR ILJ WKH HDUWK IDXOW shall
RFFXUV circulate
LQ /9 µ%¶ DW WKHinORFDWLRQ
the loop
¶)¶ actuating the REF
The % Bias Differential relay with 20% bias setting and 64R relay and
7KHVHFRQGDU\ORRSFXUUHQW LQSLQNFRORXU µ,E¶LVEDODQFHGDJDLQVWµ,Q¶ in-turn give a trip command to the Circuit
with operating current setting of 0.2PU can cover 80-85% breaker.
Fig. 4 : Schematic
)LJ diagram of circuit for out of zone fault in REF Protection
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A question comes to mind that why there is no contribution 2.2.1 Protection of Delta winding in conjunction with
of the secondary currents from the turret CT? This is Zig-zag Transformer;
because the induced EMF from the HV Source winding The delta winding is protected in conjunction with the Zig-
B-Y , rises from LV Neutral to the LV B Phase bushing in zag Transformer as depicted in the schematic diagram
a healthy Transformer but in this instant case it is getting in Fig. 6.
earthed at point “F”, thus there is no contribution from 2.2.2 REF Protection in an Auto-transformer:
the Turret CT of LV -B phase.
The difference between an Auto transformer and 2
It is not out of place to mention here that the Star winding Star/Star transformeris an Auto transformer has
connected Transformer windings have graded insulation only one Neutral and 2 windingStar/star transformer has
ie insulation level decreases vis-à-vis decrease in 2 neutrals.ie one each in the HV side and LV side.
voltage of the winding as it approaches to the neutral In this protection secondary replica of neutral current
ie number of turns reduce. This concept is used in the ’3Io’ is balanced against the secondary of HV turret CTs
REF relay. paralleled together and also secondary IV Turret CTs
paralleled together against the High Z REF relay as
shown in the figure 7.
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Fig. 6FKHPDWLFGLDJUDPRISURWHFWLRQRI'HOWD
6 : Schematic ZLQGLQJ
diagram of protection of Delta winding as a whole DVDZKROHDQGDOVR
and also Zig-zag Transformer windings.
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Fig. 8 : Schematic diagram of High Z REF Protection for Out of ZoneIRU2XWRI=RQHVHYHUIDXOW
sever fault causing CT saturation.
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Restricted Earth Faults - A Complementary Protection to the Differential Protection 37
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Fig. 9 : Metrosils& Stabilizing registers mounted H 7KHUHIRUH6WDELOLVLQJ5HVLVWDQFH5VW
ୱ ଵǤ଼
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in a Relay panel = 178.6 Ωൌ Ǥଵ
6WDELOL]LQJUHJLVWHUVPRXQWHGLQD5HOD\SDQHO
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38 CIGRE India Journal
(f)Hence we have to set the Stabilising resistor at CTS It can work with CTs Requires all
178.6 Ω for HV REF Protection with 0.1 Amp of unequal Ratios. Identical CT ratios
Current pick up setting. Free of any need and turns ratio.
VII. Now Calculations for LV REF are as follows :- of matched CT knee point voltage
5FW 5Z PHDVXUHGGLUHFWO\DW&
(a) Adopting Current 5SDQHORQ7HUPLQDOZKHUH
Pick setting for REF relay ‘Is’ characteristics or and the winding
= 0.1 Amp. ratio or resistance. resistance of the
5()&DEOHLV&RQQHFWHG 7KLVLVWKHFRPELQDWLRQ+97XUUHW&7
EPHDVXUHGGLUHFWO\DW&
5FW 5Z PHDVXUHGGLUHFWO\DW& 5SDQHORQ7HUPLQDOZKHUH Turret CTs and the
5FW5FW
5Z5Z (b) Rct +Rw measured
PHDVXUHGGLUHFWO\DW& 5SDQHORQ7HUPLQDOZKHUH
directly at C&R panel on
5SDQHORQ7HUPLQDOZKHUH
QG+91HXWUDO7XUUHW&7 ZKLFKFRPHVDV neutral CT should
Terminal
5()&DEOHLV&RQQHFWHG
5()&DEOHLV&RQQHFWHG where REF Cable
5()&DEOHLV&RQQHFWHG is Connected(This
7KLVLVWKHFRPELQDWLRQ+97XUUHW&7
7KLVLVWKHFRPELQDWLRQ+97XUUHW&7
7KLVLVWKHFRPELQDWLRQ+97XUUHW&7 be same.
is the combination
୍୰ୟ୲ୢ HV Turret CT and HV
KURXJK)DXOWFXUUHQW
DQG+91HXWUDO7XUUHW&7 ൌZKLFKFRPHVDV
DQG+91HXWUDO7XUUHW&7
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Neutral Turret CT Low-impedance REF High-impedance
୮୳CT) which comes as 3.3 Ω
୍୰ୟ୲ୢ
୍୰ୟ୲ୢ ୍୰ୟ୲ୢ
Saturation protection has low REF protection
7KURXJK)DXOWFXUUHQW ൌൈ
ͷͲ ͳͲͲͲ
F (c)7KURXJK)DXOWFXUUHQW
7KURXJK)DXOWFXUUHQW
Through
ൌ
ൌ Fault
୮୳ current
୮୳ ൊ ͳͲǤͳͲΨ
If ൌ
= and inherent stability has high inherent
୮୳
ξ͵ ൈ ͵͵ Stability against CT saturation stability against
ൌͷͲͷͲൈൈͳͲͲͲ
ͳͲͲͲ
ൊ ͳͲǤͳͲΨ for external faults. CT saturation for
ൌ ͺͳǤ͵
ξ͵ ൈ ͵͵ ൊ ͳͲǤͳͲΨ
ͷͲ ൈ ͳͲͲͲ
ξ͵ ൈ ͵͵ ൌ ൊ ͳͲǤͳͲΨ external faults.
9ROWDJHDFURVV(OHPHQWൌ ͺͳǤ͵ ξ͵ ൈ ͵͵
=ൌ8661.37Amp
ͺͳǤ͵ Burden Imposes less burden I m p o s e s
9ROWDJHDFURVV(OHPHQW
ൌ(d)ͳʹͲͲ across
Voltage Elementൌ ͺͳǤ͵
ൌ ͳ on CTs. comparatively
9ROWDJHDFURVV(OHPHQW
high burden on
G ൌ9ROWDJHDFURVV(OHPHQW
ͳʹͲͲ
LV CT pry
= 1200
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ͳ
CT sec = 1 Amp
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ൈൌ ͳʹͲͲ
ሺ
ൌ ͳ References
ሻ
ൌ ൈ ሺ ଵ ሻ
ൈ ቀ
ൌ ͺͳǤ͵ ଵ ቁ ൈ
͵Ǥ͵ (1) Mohammad Gholami,Ehsan Hajipour & Mehdi
ൌ ͺͳǤ͵ ൈቀଵଶ
ൌ
ቁ ൈ
ൈ ͵Ǥ͵ ሺ ሻ Vakikian of Sharif University of technology, Tehran,
ൌ ͺͳǤ͵ ൈ ቀ
ଵ
ଵଶ
ቁ ൈ
͵Ǥ͵ Iran-High Impedance REF Protection: Fundamental
ൌ ʹ͵Ǥͺ ଵଶ
ൌ ʹ͵Ǥͺ ଵ concepts,Design and performance.International
ൌ9V,V
ൌ ʹ͵Ǥͺ Stabilising
KHUHIRUH6WDELOLVLQJ5HVLVWDQFH5VW
(e) Therefore ͺͳǤ͵ Rst
Resistance ൈ ቀ= Vs/Is
ቁ ൈ ͵Ǥ͵ Conference on Electrical Engineering inTehran Iran-
7KHUHIRUH6WDELOLVLQJ5HVLVWDQFH5VW 9V,V ଵଶ
2016.
ୱ ଶଷǤ଼
7KHUHIRUH6WDELOLVLQJ5HVLVWDQFH5VW 9V,V
ୱൌ ଶଷǤ଼ ൌ ʹ͵Ǥͺ (2) In consultation with Er. Sthitaprajnyan Sharma,
୍ୱ୍ୱ ൌଶଷǤ଼
ୱ Ǥଵ
Ǥଵ
ൌ
H 7KHUHIRUH6WDELOLVLQJ5HVLVWDQFH5VW
୍ୱ =Ǥଵ 238.18 Ω 9V,V
Application manager, Schweitzer Engineering
Laboratories Inc.
(f) Hence the ୱresistor
Stabilising ଶଷǤ଼works out to be (3) Meyer Kao and Garry Cobet(Tennessee Valley
+HQFHWKH6WDELOLVLQJUHVLVWRU
+HQFHWKH6WDELOLVLQJUHVLVWRU ൌ
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238.18 Ω for LVZRUNVRXWWREH
IRU
REF ProtectionIRU
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with 0.1 Amp
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Current pick up. implementing ground Differential protection for
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5. Comparisons between Low Z and Transformer protection at TVA.
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