POWER-LINKER POWER-LINKER GROUP CO’s GROUNDING SYSTEM … 1.pdf · 3 power-linker power- linker...

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POWER-LINKERPOWER- LINKER

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POWER-LINKER GROUP CO’sPOWER- LINKER

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Wisdom is not Virtue but [email protected] www.powerlinker.org

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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


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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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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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



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



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



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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POWER-LINKER POWER - LINKERS

Times Square, 602B, “B” Wing,

Near Marol Metro Station,

Andheri (E), Mumbai-400 059, India.

E-Mail : [email protected]

Website : www.powerlinker.org

POWER-LINKER POWER-LINKER GROUP CO’s GROUNDING SYSTEM … 1.pdf · 3 power-linker power- linker...

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