CASE STUDIES - PROTECTION OF POWER DISTRIBUTION SYSTEM - II

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CASE STUDIES - PROTECTION OF POWER DISTRIBUTION
SYSTEM - II by R. N. KumarGM , NDPL
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CASE STUDY 1 Tripping of 33/66 kV Transformers
on OLTC OSR Relay Indication
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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 DATE 14/09/07 2. Rewari
Line 03/11/07 3. Rohtak Rd
04/12/07 4. RG4 05/12/07
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Analysis Observation

• Following are observations for all tripping
  events mentioned above
• All transformers tripped in early hours of the
  day
• All transformers tripped due to low oil level in
  OLTC conservator tank.
• 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

• 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.
• Oil level of 70 of OLTC conservator will
  sustain any sudden change in temperature due to
  change in weather condition.
• Check up of OLTC conservator oil level should be
  monitored regularly and especially during change
  in weather condition.

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CASE STUDY 2 Tripping of 11kV O/G Feeder at
Azadpur Grid Station
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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 P/F IDMT 300A/0.1
HS 3.0KA/200ms SLD
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Event List
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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.
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CASE STUDY 3 Tripping from Two Different
Sources at Ashok Vihar
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Occurrence

• GRID Ashok Vihar 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
• 33 KV O/G Tri Nagar circuit from AZP Grid Grid
• 33 KV WP2-Ashok Vihar circuit from both ends.
• 33 KV SMB220kV-WP2 circuit-2 from both ends.

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General Observations

• Flash Marks at the Upper Limb Of CB
• Flash had occurred in R Y Phase CTs of AZP-Tri
  Nagar feeder at Ashok Vihar Grid station.
• All the isolators i.e. line as well as front bus
  isolator were in closed condition due to
  unmanning of Grid.
• Flash marks observed at R,Y B phases of the
  upper limb of AZP-Tri Nagar Breaker.
• 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.

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Events, FDR Investigations
GRID Ashok Vihar Feeder AZP-Tri NgrFAULT
11/09/08 050237.900 G1, PHASE A LS IDMTL HS1
IA 5.56xIn, PHASE B LS IDMTL ltHS1gt IB 5.35xIn,
PHASE C IC
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Events, FDR Investigations
GRID Ashok Vihar Feeder I/C WP2FAULT 1
11/09/08 050238.300 G1, PHASE A LS IDMTL IA
9.81xIn, PHASE B LS IDMTL ltHS1gt IB 10.69xIn,
PHASE C LS IDMTLIC9.76xIn, EARTH FAULT IDMTL
IE 1.86xIn, CB ON
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Events, FDR Investigations
GRID WP2 Feeder O/G Asok Vihar FDR FAULT 1
11/09/08 050240.5300 G1 PHASE A LS IDMTL HS1
IA10.63xIn PHASE B LS ltIDMTLgt HS1 IB11.53xIn
PHASE C LS IDMTL HS1 IC11.45xIn EARTH FAULT IE
0.01xIn
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Events, FDR Investigations
GRID WP2 Feeder I/C SMB ckt2 FAULT 1
11/09/08 050240.5850 G1 PHASE A REV LS
IDMTL IA10.97xIn PHASE B REV LS ltIDMTLgt
IB11.45xIn PHASE C REV LS IDMTL IC10.93xIn
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
• 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
• 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.
• 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.
• 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
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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

• 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).
• 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)
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Single Line Diagram
Trip
X
X
Trip
Trip
Basai Darapur
X
F
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Single Line Diagram
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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
• As per protection philosophy circuit breaker is
  required to be installed only at first switching
  S/station for O/G feeder.
• 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
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FDR
33KV Tx-2 tripped on circuit Breaker failure
protection
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FDR
Date 24/04/07 Time 00.2911KV I/C-2 tripped
on O/C Hi-Set element. Fault currents are as
follows Ia 4.55 KAIb 4.34 KAIc 4.24
KA33KV Tx-2 tripped on circuit Breaker failure
protection
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FDR
33KV Tx-2 tripped on circuit Breaker failure
protection
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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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