CASE STUDIES PROTECTION OF
POWER DISTRIBUTION
SYSTEM - II
by R. N. Kumar
GM , NDPL
CASE STUDY 1
Tripping of 33/66 kV
Transformers on OLTC OSR
Relay Indication
Occurrence
HV breaker of Power Transformers of following mentioned NDPL Grid stations
were tripped on OLTC OSR & 11KV breaker tripped on inter-trip:
GRIDS
1. Jahangirpuri
2. Rewari Line
3. Rohtak Rd
4. RG4
DATE: 14/09/07
03/11/07
04/12/07
05/12/07
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Analysis & Observation
Following are observations for all tripping events mentioned above:
1. All transformers tripped in early hours of the day
2. All transformers tripped due to low oil level in OLTC conservator tank.
3. It has been observed in all above cases that during early hours of the day, oil in the
tank/conservator contracts and hence oil level reduces
4. It is also observed that this phenomenon is more predominant in OLTC conservator as the
capacity of the same is very low and is of the tune of 20 liters.
5. All above cases clearly indicate that these tripping's are due to:
a. Low oil level in OLTC conservator
b. Change in atmospheric temperature due to change in weather conditions.
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Recommendations
1. Tripping of power transformer on OLTC OSR due to weather change can be prevented
by keeping oil level of OLTC conservator up to 70% of its capacity.
2. Oil level of 70% of OLTC conservator will sustain any sudden change in temperature
due to change in weather condition.
3. Check up of OLTC conservator oil level should be monitored regularly and especially
during change in weather condition.
5
CASE STUDY 2
Tripping of 11kV O/G
Feeder at Azadpur Grid
Station
Occurrence
Tripping of 11KV GTK G feeder on IDMT & HS indication at 33kV Azadpur Grid station.
SLD WITH CTR, FAULT CURRENT RECORDED & RELAY SETTINGS:
11KV O/G GTK G feeder:
CTR 400/5
Relay Settings
SLD
P/F IDMT 300A/0.1
HS 3.0KA/200ms
Azadpur
25 MVA
TX#2
CTR
1200
/5
GTK
G
CTR 400/5
33KV Bus
Tripped
11KV
Bus
7
Event List
8
Analysis & Observation
On investigation it was observed that S-20 relay of O/G feeder has issued tripping command
on its IDMT P/F & HS elements. The date and timing of the relay is also differing from the
actual time.
EVENTS, INVESTIGATIONS
As it is clear from event list that feeder is tripping at the current which is less than its setting.
Hence relay is tested by injecting current upto 300A and found ok.
On further investigation it was found that CT secondary metering and protection control
wiring was got interchanged due to wrong ferruling. ( metering core ferule i.e D and
protection core ferule i.e C were got interchanged. This led to the wrong connection at the
relay input resulting the tripping of the feeder
CAPA
CT secondary control wiring was rectified as per standard drawing.
9
CASE STUDY 3
Tripping from Two Different
Sources at Ashok Vihar
Occurrence
GRID: Ashok Vihar
I/C WP-2
DATE: 11/09/08
TRIPPING / OCCURANCE : R &Y Phase CT of 33
KV Azad Pur – Tri Nagar Ckt T-Off, Flashed at
ASHOK VIHAR Grid, caused following trippings:
CTR-800/1
o/c HS-3.5In/0.02
o/c Idmt-0.75In/0.1
CLOSE
CTR-750/1
o/c HS-10In/0.02
o/c Idmt-1.5In/0.05
CLOSE
CLOSE
•33 KV O/G Tri Nagar circuit from AZP Grid
Grid
•33 KV WP2-Ashok Vihar circuit from both
ends.
I/C AZP-TRINAGAR
CLOSE
OPEN
CLOSE
FRONT BUS
OPEN
OPEN
REAR BUS
•33 KV SMB220kV-WP2 circuit-2 from both
ends.
33 KV ASHOK VIHAR GRID
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General Observations
Flash Marks at the Upper Limb Of CB
1.Flash had occurred in R & Y Phase CTs of AZP-Tri Nagar feeder at Ashok Vihar Grid station.
2. All the isolators i.e. line as well as front bus isolator were in closed condition due to
unmanning of Grid.
3. Flash marks observed at R,Y & B phases of the upper limb of AZP-Tri Nagar Breaker.
4. This shows that the flash of CT fault had reached up to breaker also and caused the
tripping of I/C WP 2 Ckt at Ashok Vihar.
12
Events, FDR & Investigations
GRID: Ashok Vihar
Feeder: AZP-Tri Ngr
FAULT 11/09/08 05:02:37.900 G1, PHASE A LS IDMTL HS1 IA= 5.56xIn, PHASE B LS IDMTL <HS1>
IB= 5.35xIn, PHASE C IC=
13
Events, FDR & Investigations
GRID: Ashok Vihar
Feeder: I/C WP2
FAULT 1 11/09/08 05:02:38.300 G1, PHASE A LS IDMTL IA= 9.81xIn, PHASE B LS IDMTL <HS1> IB=
10.69xIn, PHASE C LS IDMTL
IC=9.76xIn, EARTH FAULT IDMTL IE= 1.86xIn, CB ON
14
Events, FDR & Investigations
GRID: WP2
Feeder: O/G Asok Vihar FDR
FAULT 1 11/09/08 05:02:40.5300 G1 PHASE A LS IDMTL HS1 IA=10.63xIn PHASE B LS <IDMTL> HS1
IB=11.53xIn PHASE C LS IDMTL HS1 IC=11.45xIn EARTH FAULT IE= 0.01xIn
15
Events, FDR & Investigations
GRID: WP2
Feeder: I/C SMB ckt2
FAULT 1 11/09/08 05:02:40.5850 G1 PHASE A [REV] LS IDMTL
IA=10.97xIn PHASE B [REV] LS <IDMTL> IB=11.45xIn PHASE C [REV]
LS IDMTL IC=10.93xIn EARTH FAULT IDMTL IE= 0.26xIn
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Conclusion
Following is the conclusion of all the trippings due to R & Y Phase CT Flash at
Ashok Vihar Grid:
o
33 KV O/G Tri Nagar Ckt tripped at Azad Pur Grid because it was the first upstream
breaker to trip for isolating the CT fault at Ashok Vihar Grid
o
33 KV I/C WP 2 CKT at Ashok Vihar end tripped as it has come into the flash range of
faulty CT. All the isolators were in closed condition, only the breaker was off. The flash
Spark had engulfed the both sides of the Azp-Tri Nagar breaker. This has been confirmed
as breaker is having flash marks on upper limbs of all phases.
o
33 kV O/G Ashok Vihar ckt and I/C SMB ckt2 at WP2 tripped due to very high intensity of
the fault . The fault current was of the order of 9.5 kA.
o
33 KV O/G WP2 Ckt2 at 220 KV SMB end had tripped under racing condition.
CAPA
This is a rarest kind of fault which was fed from two different sources (AZP-Tringr CKT and WP2).
Leading to the cascade tripping. The only way to avoid these types of tripping is by not
keeping a CB to be charged from two different sources.
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CASE STUDY 4
Transformer Tripping at
BADLI
Occurrence
Reporting: Power Transformer tripping on Differential.
Observations
Basic testing of the transformer was carried out and following are the observations
1. No fault was observed on HV Side of the transformer
2. CT Circuit was found healthy
3. Stability of the transformer was OK
4. Load at the time of tripping was 510 Amps on LV Side i.e 40% of rated capacity of
Tansformer.
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Investigations
o
It was observed from the waveforms of the tripping event that stray DC Current
component was present in R Phase current at the time of event. (Please refer to snapshot
of disturbance in badli-1doc attached along with).
o
It was also confirmed from last tripping event dated 8.3.2006 that DC Ingress has caused
the tripping earlier also.
Complete circuit was checked to find out the source of DC Ingress in the Relay, It was
ensured that no dc ingress was there through external cabling. Even after isolating
external circuit, Relay was generating trip command. Waveform of this particular event
indicated that tripping was purely due to DC Component of the current in CT Circuit. It
also shows that after decay of the DC Current Output relay got reset.
On further investigations, 4 ma DC leakage current was detected on 220 V Battery
charger, rendering Positive to earth voltage to be 12 volts and negative to earth voltage
to be 220 V DC.
This leakage was found in adjacent Line feeder panel.
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Conclusions
1. It is confirmed that Tripping was due to intermittent DC Ingress in R Phase
secondary of HV CT Circuit .
2. In modular type Duo-bias relay, two separate modules are there for power supply as well
as CT Circuit. It seems DC was getting mixed up in CT circuit within the relay.
CAPA
Corrective Action:
As It could not be concluded that where from DC is getting ingresses in CT Circuit, It was
strongly suspected that it was an internal phenomenon within the relay.
Based on this relay was replaced with new one.
Future course of action:
Expert comments from Relay manufacturer (ER) were solicited.
Enclosed: Snapshots of disturbance records
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Transformer Tripping at Badli
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CASE STUDY 5
Cascade Tripping in Zone
1303 (Motinagar)
Single Line Diagram
Sudarshan
Park grid
Gadaria S/Stn
Trip
X
Achpal S/Stn
Trip X
Trip
X
Basai Darapur
Ramgarh S/Stn
Rattan Park S/Stn
F
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Single Line Diagram
S/Stn Gadaria
/
X ABB
X
/
I/C Sudarshan
Park Grid
S/Stn Achpal
I/C Bali
Nagar
/
,WIC1
O/G
Achpal
630
KVA
X
I/C
Gadaria
/
X
/
O/G
Rattan
Park
1000
KVA
/
Schneider
RN2C
VIP300
2X250
KVA DT
S/Stn Rattan Park
X
I/C Achpal
X
/
X
/
Tie
Spare
630
KVA
X
/
O/G
Ram
Garh
630
KVA
X
/
630
KVA
/
O/G Old
Bsai Dara
Pur
/
I/C
Bali
Nagar
CG Lucy
WIP1
S/Stn Ram Garh
/
Schneider
RM6
VIP300
X
/
I/C Rattan
Park
O/G
Achpal
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Occurrence
For any fault beyond Ramgarh S/Stn, O/G CB of all the S/Stns in this circuit were getting
tripped in cascading.
Investigations
1.
As per protection philosophy circuit breaker is required to be installed only at first
switching S/station for O/G feeder.
2.
Purpose of RMU is defeated.
3. Cascade tripping of circuit breakers installed in series can not be avoided as there is a
little time margin to discriminate between these breakers.
CAPA
Zonal staff has been advised to connect the O/G feeder of the S/Station other than that of
First Switching S/Station (Rattan Park S/Station) on isolator to avoid cascade tripping of
breakers.
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Tripping Analysis, SLDs, FDRs & CAPA
Tripping of 33/11 kV Transformer on CBFP protection at DIFR
GRID: DIFR
DATE: 23/04/07 & 24/04/07
OCCURRENCE : Tripping of 11KV I/C-2 on O/C Hi set indication along with Tx-2 on CBFP on
both the occurrences.
SLD showing exact location of the fault:
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EVENTS , FDR & INVESTIGATIONS
Date 23/04/07
On investigation, it was observed that 11KV I/C tripped due rat electrocution on the circuit
breaker limb on cable side resulting in the flashover at the poles of the CB and its chamber.
33KC Tx-2 was tripped on CBFP due to continuous fault feeding from source end. Fault was
cleared when 33KV CB was tripped on CBFP.
Date 24/04/07
On 24/04/07 also exactly same occurrence was repeated. Again 11KV I/C tripped due rat
electrocution inside the circuit breaker resulting in the flashover in the CB and CB chamber.
33KC Tx-2 was tripped on CBFP due to continuous fault feeding from source end. Fault was
cleared when 33KV CB was tripped on CBFP.
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FDR
Date :23/04/07 Time: 2.54
11KV I/C-2 tripped on O/C Hi-Set element. Fault currents are as follows:
Ia= 4.36 KA
Ib= 4.36 KA
Ic= 4.34 KA
6
FDR
33KV Tx-2 tripped on circuit Breaker failure protection
6
FDR
Date: 24/04/07 Time: 00.29
11KV I/C-2 tripped on O/C Hi-Set element. Fault currents are as follows:
Ia= 4.55 KA
Ib= 4.34 KA
Ic= 4.24 KA
33KV Tx-2 tripped on circuit Breaker failure protection
6
FDR
33KV Tx-2 tripped on circuit Breaker failure protection
6
CONCLUSIONS
On both the occurrences 11KV I/C-2 was tripped on O/C Hi-Set element within 100ms which is
meant for bus fault. 33KV CB of Transformer tripped on circuit breaker failure protection
because even after the tripping of11KV CB fault was not cleared as fault occurred on the
cable side limb of 11KV CB. Hence to clear the fault 33KV CB tripped on CBFP. Hence this is well
coordinated tripping.
CAPA
The 11KV I/C-2 CB was heavily carbonized due to fault on 23/04/07. M/S Schneider have
replaced the CB by new one. Same has been tested and taken into service.
The necessary vermin proofing /sealing of panels was undertaken by Schneider engineer in the
presence of project engineer.
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