EARTH FAULT PROTECTION VIS-A-VIS GENERATOR GROUNDING SYSTEM

Transcription

1 EARTH FAULT PROTECTION VIS-A-VIS GENERATOR GROUNDING SYSTEM BY MR. H. C. MEHTA AT 1 ST INDIA DOBLE PROTECTION AND AUTOMATION CONFERENCE, NOV 2008 POWER-LINKER Wisdom is not Virtue but Necessity hcmehta@powerlinker.org 1

2 2 EARTH FAULT PROTECTION VIS-A-VIS GENERATOR GROUNDING SYSTEM

3 3 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 OF TRANSFORMER AND CORRESPONDING STAR NEUTRAL GROUNDING METHOD. ARC RESISTANCE VALUE

4 4 GROUNDING METHODS SOLID GROUNDING o FAULT CURRENT IN K AMPS LOW (MEDIUM) RESISTANCE GROUNDING o FAULT CURRENT LIMITED TO 1000 AMPS TO 2000 AMPS HIGH RESISTANCE GROUNDING o FAULT CURRENT LIMITED TO 100 AMPS VERY HIGH RESISTANCE GROUNDING o CURRENT LIMITED TO LESS THAN 15 AMPS

5 5 GENERALLY GROUNDING METHOD OF TRANSFORMER RECEIVING POWER FROM GRID IS SOLID GROUNDING OR LOW RESISTANCE GROUNDING GROUNDING OF GENERATING SOURCE IS HIGH RESISTANCE GROUNDING, OR VERY HIGH RESISTANCE GROUNDING

6 6 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

7 UTILITY 5000 MVA CB6 RECEIVING GRID POWER BUS4 132 kv CB7 CB8 GROUNDED THROUGH LOW RESISTANCE TR-1 15 MVA 132/ % Z TR-2 15 MVA 132/ % Z 2.9 CB9 2.9 OHMS 1313 A BUS5 NO CB10 BUS7 OHMS 1313 A CB12 CB11 CB13 FIG 1: EXISTING SYSTEM SINGLE LINE DIAGRAM 7

8 8 CASE STUDY : OUTGOING FEEDER C.T. RATIO IS 2000/1 ACCORDINGLY, EARTH FAULT PROTECTION RELAY SENSITIVITY IS GOOD. PU = 10% = 0.1 SENSITIVITY = MIN.CURRENT FOR SENSING MAX. FAULT CURRENT = 0.15 = 15 % = RELAYS OPERATE EFFICIENTLY FOR ALL EARTH FAULTS.

9 9 MODIFICATION IN SYSTEM : CAPTIVE POWER PLANT GENERATORS ARE ADDED TO EXISTING ELECTRICAL DISTRIBUTION.

10 UTILITY 5000 MVA CB6 BUS4 132 kv NGR 100 A CB1 BUS1 TG-1 G 15 MW 1640 A % Xd NO NGR 100 A CB2 G TG-2 15 MW 1640 A % Xd BUS3 CB7 CB8 TR-2 15 MVA 132/ % Z 2.9 OHMS 1313 A TR-1 15 MVA 132/ % Z 2.9 CB9 BUS5 NO CB1 0 BUS7 OHMS 1313 A CB4 CB3 CB5 CB12 CB11 CB13 FIG. SLD AFTER ADDITION OF 2X15MW CAPTIVE POWER PLANT 10

11 11 GENERATORS ARE GROUNDED THROUGH HIGH RESISTANCE. CURRENT LIMITED TO 100 AMPS EARTH FAULT CURRENT CONTRIBUTION BY GENERATOR IS SMALL

12 HIGH RESISTANCE GROUNDING NGR 100 A NGR 100 A G TG-1 15 MW 1640 A % Xd G TG-2 15 MW 1640 A % Xd MAX 100A CB1 CB2 BUS1 NO BUS3 CB4 CB3 CB5 FIG. SLD AFTER ADDITION OF 2X15MW CAPTIVE POWER PLANT 12

13 13 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.

14 UTILITY 5000 MVA CB6 BUS4 132 kv NGR 100 A CB1 BUS1 TG-1 G 15 MW 1640 A % Xd NGR 100 A 100 A NO 1413 A CB2 G TG-2 15 MW 1640 A % Xd BUS3 CB7 CB8 TR-2 15 MVA 132/ % Z 2.9 OHMS 1313 A TR-1 15 MVA 132/ % Z A CB9 BUS5 NO CB1 0 BUS7 OHMS 1313 A CB4 CB3 CB5 CB12 CB11 CB13 FIG. SLD AFTER ADDITION OF 2X15MW CAPTIVE POWER PLANT 14

15 15 PROBLEMS : o THIS CAN DAMAGE GENERATOR WINDING & CORE GENERATOR CORE CAN WITHSTAND 100 A FOR APP = 0.7 SEC.

16 FIG: TYPICAL CORE DAMAGE CURVE 16

17 17 o VARIATION IN EARTH FAULT CURRENT FOR OUTGOING FEEDERS WITH GRID (1313 A A) WITHOUT GRID TRANFOFMER WITH ONE GENERATOR-100A WITH TWO GENERATORS (100 A +100 A). A PROBLEM FOR PROTECTION RELAY SETTING CHOOSE DMT CHARACTERISTICS

18 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 SENSITIVITY = = 1 100% EQUIPMENTS NOT PROTECTED o IF FAULT HAS AN ARC RESISTANCE THEN RELAY MAY NOT PICK UP. 18

19 EARTH FAULT CURRENT ISOLATION SENSITIVITY REDUCED DRASTICALLY. WITH ONE GENERATOR, MAX. EARTH FAULT CURRENT = 100 A. 200 RELAY DO NOT PICK UP. AS SENSITIVITY = = 200% 100 DUE TO MIX UP OF GROUNDING METHODS REDUCED EARTH FAULT CURRENT SENSITIVITY PROPOSED SYSTEM DISRIBUTION NOT ACCEPTABLE 19

20 20 POSSIBLE SOLUTION : o RATIONALIZE GROUNDING SYSTEM OR o EXISTING EARTH FAULT PROTECTION RELAYS TO BE REPLACED BY SENSITIVE EARTH FAULT PROTECTION RELAY.

21 21 RATIONALIZATION OF GROUNDING SYSTEM o SOLUTION : I ADD GENERATOR TRANSFORMER (SOME TIMES UNIT RATIO TRANSFORMER : URT) IN SERIES WITH GENERATORS VECTOR OF TRANSFORMER DELTA / STAR

22 FIG. 4 RATIONALIZATION OF GROUNDING SYSTEM BY ADDITION OF UNIT RATIO GENERATOR TRANSFORMER 22 LOW RESISTANCE GROUNDING GENERATOR TRANSFORMER OR URT UTILITY 5000 MVA NGR 100 A NGR 100 A BUS4 CB6 132 kv EXISTING SYSTEM GT-1 20 MVA 6.6/ 10 % Z TG-1 15 MW G 1640 A % Xd GT-2 20 MVA 6.6/ 10 % Z 2.9 OHMS A G TG-2 15 MW 1640 A % Xd 2.9 OHMS A CB7 TR-1 15 MVA 132/ % Z 2.9 OHMS 1313 A CB8 TR-2 15 MVA 132/6.6 kv % Z 2.9 OHMS 1313 A CB1 CB2 CB9 CB10 BUS1 NO BUS3 BUS5 NO BUS7 CB4 CB3 CB5 CB12 CB11 CB13

23 23 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.

24 24 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.

25 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. DISADVANTAGES : o NO DISADVANTAGES. o ONLY ADDITIONAL CAPITAL INVESTMENT ON GENERATOR TRANSFORMER 25

26 26 SOLUTION : II o RATIONALIZATION OF GROUNDING SYSTEM BY MODIFYING EXISTING SOURCE GROUNDING. CONVERTING EXISTING LOW RESISTANCE GROUNDING TO HIGH RESISTANCE GROUNDING. REPLACE EXISTING NGR.

27 NGR VALUE SAME UTILITY 5000 MVA CB6 NEW HIGH RESISTANCE NGR NGR 100 A 38.1 OHMS TG-1 G 15 MW 1640 A % Xd NGR 100 A 38.1 OHMS G TG-2 15 MW 1640 A % Xd CB7 TR-1 15 MVA 132/ % Z BUS4 132 kv 100 A 38.1 OHMS CB8 TR-2 15 MVA 132/ % Z 100 A 38.1 OHMS EXISTING NGR REPLACED CB1 CB2 CB9 CB10 BUS1 NO BUS3 BUS5 NO BUS7 CB4 CB3 CB5 CB12 CB11 CB13 FIG. 5 RATIONALIZATION OF GROUNDING SYSTEM BY REPLACEMENT OF NGR 27

28 28 NEUTRAL GROUNDING RESISTOR OF GRID TRANSFORMER IS REPLACED. NEW NGR HAS THE SAME RATING AS THAT OF GENERATOR NGR, CURRENT LIMITED TO 100 AMPS.

29 29 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.

30 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. 30

31 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. 31

32 50N/2 : TIME DELAYED SENSITIVE EARTH FAULT RELAY 59 : NEUTRAL DISPLACEMENT RELAY BUS4 UTILITY 5000 MVA CB6 132 kv (+) (-) 50N/ /a 52 TC TYPICAL CONTROL CIRCUIT NGR 100 A NGR 100 A CB7 CB8 CB1 BUS1 TG-1 G 15 MW 1640 A % Xd R 50N/2 NO CB2 TG-2 G 15 MW 1640 A % Xd R 50N/2 BUS3 TR-1 15 MVA 132/ % Z 38.1 OHMS 100 A TR-2 15 MVA 132/ % Z 38.1 OHMS 100 A 3000/1 R 50N/2 R 50N/2 3000/1 SENSITIVE CB9 CB10 EARTH FAULT PROTECTION BUS5 NO BUS7 CB4 CB5 CB12 59 CB3 PT CB11 R 50N/2 R R 50N/2 R 50N/2 NDR FIG. 6 INTRODUCTION OF NDR WITH SENSITIVE EARTH FAULT RELAY 50N/2 32

33 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 = 100/2000 MINIMUM PU < 0.05 A < 5% o NEW SENSITIVE EARTH FAULT RELAY RECOMMENDED WITH o PICK UP (SENSITIVITY) <5% o BUILT IN FUTURE TO FILTER 3RD HARMONIC CURRENT o HIGH RELAY RESET CURRENT (GREATER THAN 95% OF OPERATING CURRENT) o SHORT TIME DELAY RANGE ( SEC) 33

34 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. 34

35 35 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.

36 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 36

37 RECOMMENDATION : 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. 37

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