Post on 05-Sep-2018
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Wisdom is not Virtue but Necessityhcmehta@powerlinker.org www.powerlinker.org
GROUP
POWER-LINKER
MR. H. C. MEHTABY
AT
1ST INDIA DOBLE PROTECTION AND AUTOMATION CONFERENCE, NOV 2008
EARTH FAULT PROTECTION VIS-A-VIS GENERATOR GROUNDING SYSTEM
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EARTH FAULT PROTECTION
VIS-A-VIS
GENERATOR GROUNDING
SYSTEM
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• IN AN ELECTRICAL SYSTEM,CLOSE TO 70% TO 80% OF FAULTS ARE EARTH
FAULTS.
• THE MAGNITUDE OF EARTH FAULT CURRENT IS
DEPENDENT UPON:
• SYSTEM (SOURCE) GROUNDING
• VECTOR GROUP OF TRANSFORMER AND
CORRESPONDING STAR NEUTRAL
GROUNDING METHOD.
• ARC RESISTANCE VALUE
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• GROUNDING METHODS
o FAULT CURRENT LIMITED TO 1000 AMPS
TO 2000 AMPS
• LOW (MEDIUM) RESISTANCE GROUNDING
• HIGH RESISTANCE GROUNDING
o FAULT CURRENT LIMITED TO 100 AMPS
• VERY HIGH RESISTANCE GROUNDING
o CURRENT LIMITED TO LESS THAN 15 AMPS
• SOLID GROUNDING
o FAULT CURRENT IN K AMPS
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• GROUNDING OF GENERATING SOURCE IS
• HIGH RESISTANCE GROUNDING, OR
• VERY HIGH RESISTANCE GROUNDING
• GENERALLY GROUNDING METHOD OF TRANSFORMER RECEIVING POWER FROM GRID IS
• SOLID GROUNDING OR
• LOW RESISTANCE GROUNDING
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• CASE STUDY :
o CEMENT PLANT
• RUNNING SYSTEM OPERATION IS WITH SOURCE
GROUNDED THROUGH LOW RESISTANCE
GROUNDING AT RECEIVING END.
• FAULT CURRENT MAGNITUDE IS LIMITED TO
1313 AMPS
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UTILITY
TR-215 MVA
132/6.6 kV10.59 % Z
5000 MVA
CB6
TR-115 MVA
132/6.6 kV10.59 % Z 2.9
OHMS1313 A
2.9 OHMS1313 A
CB7 CB8
CB10CB9
BUS4 132 kV
BUS7 6.6 kVBUS5 6.6 kV
CB11
NO
CB12 CB13
FIG 1: EXISTING SYSTEM SINGLE LINE DIAGRAM
RECEIVING GRID POWER
GROUNDED THROUGH LOW RESISTANCE
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• OUTGOING FEEDER C.T. RATIO IS 2000/1 ACCORDINGLY, EARTH FAULT PROTECTION RELAY SENSITIVITY IS GOOD.
SENSITIVITY = MIN.CURRENT FOR SENSING MAX. FAULT CURRENT
= 0.15 = 15 %
RELAYS OPERATE EFFICIENTLY FOR ALL EARTH FAULTS.
• CASE STUDY :
PU = 10% = 0.1
= 2001313
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• CAPTIVE POWER PLANT GENERATORS ARE
ADDED TO EXISTING ELECTRICAL
DISTRIBUTION.
• MODIFICATION IN SYSTEM :
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FIG. SLD AFTER ADDITION OF 2X15MW CAPTIVE POWER PLANT
TR-215 MVA
132/6.6 kV10.59 % Z
G ∼G ∼
CB6
TR-115 MVA
132/6.6 kV10.59 % Z 2.9
OHMS1313 A
2.9 OHMS1313 A
CB7 CB8
CB10
CB9
BUS4 132 kV
BUS7 6.6 kVBUS5 6.6 kV
CB11CB12 CB13CB4 CB5CB3
CB1
BUS1 6.6 kV
TG-115 MW6.6 kV1640 A
13.07 % Xd”
TG-215 MW6.6 kV1640 A
13.07 % Xd”
NGR100 A
NGR100 A
CB2
NOBUS3 6.6 kVNO
UTILITY 5000 MVA
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• GENERATORS ARE GROUNDED THROUGH HIGH RESISTANCE.
• CURRENT LIMITED TO 100 AMPS
• EARTH FAULT CURRENT CONTRIBUTION BY GENERATOR IS
SMALL
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FIG. SLD AFTER ADDITION OF 2X15MW CAPTIVE POWER PLANT
G∼
CB4 CB5CB3
CB1
BUS1 6.6 kV
TG-115 MW6.6 kV1640 A
13.07 % Xd”
TG-215 MW6.6 kV1640 A
13.07 % Xd”
NGR100 A
NGR100 A
CB2
BUS3 6.6 kVNO
G∼
HIGH RESISTANCE GROUNDING
MAX100A
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• PROBLEMS :
o GROUNDING METHODS ARE MIXED UP, HENCE,
o GRID CONTRIBUTES HIGH CURRENT (1313 AMPS)
TO GENERATOR FAULTS, AGAINST 100 AMPS
LIMITATIONS ENVISAGED BY GENERATOR
MANUFACTURER.
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FIG. SLD AFTER ADDITION OF 2X15MW CAPTIVE POWER PLANT
TR-215 MVA
132/6.6 kV10.59 % Z
G ∼G ∼
CB6
TR-115 MVA
132/6.6 kV10.59 % Z 2.9
OHMS1313 A
2.9 OHMS1313 A
CB7 CB8
CB10
CB9
BUS4 132 kV
BUS7 6.6 kVBUS5 6.6 kV
CB11CB12 CB13CB4 CB5CB3
CB1
BUS1 6.6 kV
TG-115 MW6.6 kV1640 A
13.07 % Xd”
TG-215 MW6.6 kV1640 A
13.07 % Xd”
NGR100 A
NGR100 A
CB2
NOBUS3 6.6 kVNO
UTILITY 5000 MVA
100 A
1313A1413 A
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• PROBLEMS :
o THIS CAN DAMAGE GENERATOR WINDING & CORE
GENERATOR CORE CAN WITHSTAND 100 A
FOR APP = 0.7 SEC.
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FIG: TYPICAL CORE DAMAGE CURVE
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o VARIATION IN EARTH FAULT CURRENT
FOR OUTGOING FEEDERS
• WITH GRID (1313 A + 100 A)
• WITHOUT GRID TRANFOFMER
• WITH ONE GENERATOR-100A
• WITH TWO GENERATORS (100 A +100 A).
• A PROBLEM FOR PROTECTION RELAY SETTING
• CHOOSE DMT CHARACTERISTICS
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o FEEDER WITH HIGHER C.T RATIO AND EXISTING
RELAYS MAY NOT PICK UP.
o FOR EARTH FAULT CURRENT MAGNITUDE OF 200 A
WITH TWO GENERATORS IN OPERATION.
o RELAY SETTING MIN 10%, CT RATIO = 2000/1
= 1 ≅ 100%200200
SENSITIVITY =
o IF FAULT HAS AN ARC RESISTANCE THEN RELAY MAY
NOT PICK UP.
EQUIPMENTS NOT PROTECTED
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• EARTH FAULT CURRENT ISOLATION SENSITIVITY
REDUCED DRASTICALLY.
• WITH ONE GENERATOR, MAX. EARTH FAULT
CURRENT = 100 A.
• RELAY DO NOT PICK UP. AS SENSITIVITY = = 200%200100
• DUE TO• MIX UP OF GROUNDING METHODS
• REDUCED EARTH FAULT CURRENT SENSITIVITY
• PROPOSED SYSTEM DISRIBUTION NOT ACCEPTABLE
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• POSSIBLE SOLUTION :
o RATIONALIZE GROUNDING SYSTEM
OR
o EXISTING EARTH FAULT PROTECTION
RELAYS TO BE REPLACED BY SENSITIVE
EARTH FAULT PROTECTION RELAY.
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• RATIONALIZATION OF GROUNDING SYSTEM
o SOLUTION : I
ADD GENERATOR TRANSFORMER (SOME TIMES UNIT
RATIO TRANSFORMER : URT) IN SERIES WITH
GENERATORS
VECTOR GROUP OF TRANSFORMER DELTA / STAR
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FIG. 4 RATIONALIZATION OF GROUNDING SYSTEM BY ADDITION OF UNIT RATIO GENERATOR TRANSFORMER
TR-215 MVA132/6.6
kV10.59 % Z
∇
G ∼∇
G ∼
CB6
TR-115 MVA
132/6.6 kV10.59 % Z
2.9 OHMS1313 A
2.9 OHMS1313 A
CB7 CB8
CB10CB9
BUS4 132 kV
BUS7 6.6 kVBUS5 6.6 kV
CB11CB12 CB13CB4 CB5CB3
CB1
BUS1 6.6 kV
2.9 OHMS1312.5 A
2.9 OHMS1312.5 A
GT-120 MVA
6.6/6.6 kV10 % Z
GT-220 MVA
6.6/6.6 kV10 % Z
TG-115 MW6.6 kV1640 A
13.07 % Xd”
TG-215 MW6.6 kV1640 A
13.07 % Xd”
NGR100 A
NGR100 A
CB2
NOBUS3 6.6 kVNO
UTILITY 5000 MVA
LOW RESISTANCE GROUNDING
GENERATOR TRANSFORMER OR
URT
EXISTING SYSTEM
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• LOAD SIDE STAR WINDING IS GROUNDED
THROUGH LOW RESISTANCE.
• NGR OF SAME RATING AS THAT OF GRID
TRANSFORMER TO BE INTRODUCED.
• FAULT CURRENT LIMITED TO 1313 AMPS.
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• ADVANTAGES WITH ADDITION OF GENERATOR TRANSFORMER (URT)
o EARTH FAULT CURRENT MAGNITUDE SUFFICIENT
TO MAINTAIN HIGH EARTH FAULT CURRENT
SENSITIVITY.
o GENERATOR CORE AND WINDING PROTECTED
AGAINST HIGH EARTH FAULT CURRENT FROM
GRID TRANSFORMER
o GENERATOR FAULT CURRENT LIMITED TO 100 A.
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• DISADVANTAGES :
o NO DISADVANTAGES.
o ONLY ADDITIONAL CAPITAL INVESTMENT ON
GENERATOR TRANSFORMER
• ADVANTAGES WITH ADDITION OF GENERATOR TRANSFORMER (URT)
o GENERATOR ISOLATED FROM DISTRIBUTION
SYSTEM’S PHASE AS WELL AS EARTH FAULT
CURRENT.
o DISTRIBUTION SYSTEM FAULTS ARE NOT SEEN AS
GENERATOR TERMINAL FAULTS.
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• SOLUTION : II
o RATIONALIZATION OF GROUNDING SYSTEM BY
MODIFYING EXISTING SOURCE GROUNDING.
• CONVERTING EXISTING LOW RESISTANCE
GROUNDING TO HIGH RESISTANCE GROUNDING.
• REPLACE EXISTING NGR.
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FIG. 5 RATIONALIZATION OF GROUNDING SYSTEM BY REPLACEMENT OF NGR
TR-215 MVA
132/6.6 kV10.59 % Z
G ∼G ∼
CB6
TR-115 MVA
132/6.6 kV10.59 % Z
100 A38.1
OHMS
CB7 CB8
CB10CB9
BUS4 132 kV
BUS7 6.6 kVBUS5 6.6 kV
CB11CB12 CB13CB4 CB5CB3
CB1
BUS1 6.6 kV
TG-115 MW6.6 kV1640 A
13.07 % Xd”
TG-215 MW6.6 kV1640 A
13.07 % Xd”
NGR100 A38.1
OHMS
NGR100 A
38.1 OHMS
CB2
NOBUS3 6.6 kVNO
UTILITY 5000 MVA
100 A38.1
OHMS
NEW HIGH RESISTANCE NGR
NGR VALUE SAME
EXISTINGNGR
REPLACED
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• NEUTRAL GROUNDING RESISTOR OF GRID
TRANSFORMER IS REPLACED.
• NEW NGR HAS THE SAME RATING AS THAT OF
GENERATOR NGR, CURRENT LIMITED TO 100 AMPS.
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• PRECAUTIONS.
o GRID TRANSFORMER GROUNDED THROUGH
LOW RESISTANCE.
NEUTRAL INSULATION CAN BE GRADED.
• TO CHANGE TO HIGH RESISTANCE GROUNDING
CHECK THAT TRANSFORMER NEUTRAL IS FULLY
INSULATED. (PRACTICALLY UNGROUNDED SYSTEM
INSULATION).
• TRANSFORMER NEUTRAL, WITH GRADED
INSULATION CANNOT BE HIGH RESISTANCE
GROUNDED.
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o THIS CEMENT PLANT CASE TRANSFORMER WAS 15
YEARS OLD, NOT FEASIBLE TO GET DESIGN DATA
VERIFIED.
o MANUFACTURER LATER CONFIRMED THAT
TRANSFORMER NEUTRAL WAS FULLY INSULATED .
o INSERTED HIGH RESISTANCE NGR, AS NEUTRAL WAS
FULLY INSULATED.
o RATIONALIZED THE GROUNDING SYSTEM BY LIMITING
THE EARTH FAULT CURRENT TO 100 AMPS FROM EACH
SOURCE.
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• ADVANTAGES :
o EARTH FAULT CURRENT MAGNITUDE LIMITED TO 100 AMPS,
o THUS IN CASE OF GENERATOR FAULT, DAMAGE TO GENERATOR WINDING & CORE IS LIMITED.
• DISADVANTAGE :o SENSITIVITY OF CLEARING EARTH FAULT CURRENT
REDUCED. o CONDITION IS CRITICAL WITH ONE GENERATOR IN
OPERATION. o MAXIMUM EARTH FAULT CURRENT IS 100 AMPS.o THIS REQUIRES REPLACEMENT OF EXISTING EARTH
FAULT PROTECTION RELAYS.
• SOLUTION TO PROBLEM :ADD SENSITIVE EARTH FAULT PROTECTION.
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FIG. 6 INTRODUCTION OF NDR WITH SENSITIVE EARTH FAULT RELAY
TR-215 MVA
132/6.6 kV10.59 % Z
G ∼G ∼
CB6
TR-115 MVA
132/6.6 kV10.59 % Z 38.1
OHMS100 A
CB7 CB8
CB10CB9
BUS4 132 kV
BUS5 6.6 kV
CB11CB12CB4 CB5CB3
CB1
BUS1 6.6 kV
TG-115 MW6.6 kV1640 A
13.07 % Xd”
TG-215 MW6.6 kV1640 A
13.07 % Xd”
NGR100 A
NGR100 A
CB2
NOBUS3 6.6 kV
38.1 OHMS100 A
50N/2R 50N/2R
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50N/25952/a
52 TC
3000/1 3000/1
PT
UTILITY 5000 MVA
NO
50N/2R
BUS7 6.6 kV
50N/2R 50N/2R
TYPICAL CONTROL CIRCUIT
50N/2R50N/2R
50N/2R
(-)
(+)
50N/2 : TIME DELAYED SENSITIVE EARTH FAULT RELAY
59 : NEUTRAL DISPLACEMENT RELAY
SENSITIVE EARTH FAULT PROTECTION
NDR
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o EARTH FAULT CURRENT MAGNITUDE LIMITED TO 100 AMPS. o THE HIGHEST RATING FEEDER IS 2000 AMPS.o MAXIMUM EARTH FAULT CURRENT WITH ONE
GENERATOR = 100A.o CT RATIO = 2000/1
CT SECONDARY CURRENT = 100/2000MINIMUM PU < 0.05 A
< 5%o NEW SENSITIVE EARTH FAULT RELAY RECOMMENDED WITHo PICK UP (SENSITIVITY) <5%o BUILT IN FUTURE TO FILTER 3RD HARMONIC CURRENTo HIGH RELAY RESET CURRENT
(GREATER THAN 95% OF OPERATING CURRENT)o SHORT TIME DELAY RANGE (0.1 – 9.9 SEC)
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• SUSPECTED MALOPERATION :
o SENSITIVE (VERY LOW) EARTH FAULT RELAY SETTING
(<5%).
o MAL-OPERATION OF THIS RELAY SUSPECTED DURING
TRANSIENT CONDITION TO ENSURE RELIABILITY AND
STABILITY.
o NEUTRAL DISPLACEMENT RELAY RECOMMENDED
o RELAY TO BE CONNECTED ON PT SECONDARY OPEN
DELTA WINDING.
o A CONTACT OF NDR WAS RECOMMENDED TO BE
CONNECTED IN SERIES WITH EARTH FAULT RELAY.
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• CONCLUSION :
o TWO POWER SOURCES WITH DIFFERENT
GROUNDING METHODS SHALL NOT BE OPERATED
IN PARALLEL.
o THIS CAN DAMAGE THE EQUIPMENT GROUNDED
THROUGH HIGH RESISTANCE GROUNDING
SYSTEM.
o GROUNDING METHODS OF TWO SOURCES SHALL
ALWAYS BE RATIONALIZED FOR CONTINUOUS
PARALLEL OPERATION.
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• RECOMMENDATION :
o RATIONALIZE GROUNDING SYSTEM OF SOURCES
AT POINT OF COMMON COUPLING (PCC) BY
• INTRODUCING GENERATOR TRANSFORMER, THUS
RETAINING,
SECONDARY GROUND FAULT CURRENT MAGNITUDE
SAME AS EXISTING &
SENSITIVITY OF DETECTING GROUND FAULT
CURRENT MAGNITUDE BY RETAINING EXISTING
EARTH FAULT PROTECTION SCHEME / RELAYS.
OR
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o REPLACE EXISTING SOURCE GROUNDING TO MATCH WITH THE EQUIPMENT GROUNDED THROUGH HIGH RESISTANCE.
o PROVIDE HIGH SENSITIVITY EARTH FAULT RELAY TO DETECT LOW MAGNITUDE EARTH FAULT CURRENT.
o TO PREVENT MAL-OPERATION,
RELAY SHALL HAVE PROTECTION AGAINST 3RD HARMONIC CURRENT.
SHALL HAVE HIGH RESET CURRENT.
PROVIDE CONTACT OF NEUTRAL DISPLACEMENT RELAY IN SERIES WITH SENSITIVE EARTH FAULT RELAY.
• RECOMMENDATION :
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Near Marol Metro Station,
Andheri (E), Mumbai-400 059, India.
E-Mail : hcmehta@powerlinker.org
Website : www.powerlinker.org