CASE STUDIES PROTECTION OF
POWER DISTRIBUTION
SYSTEM - I
by R. N. Kumar
GM , NDPL
CASE STUDY 1
Tripping Analysis for fault on
11 KV Swatantra Nagar
Feeder at Ghogha Mod in
Zone 514 (NRL)
Ghoga KIosk S/Stn
Swatantra
Nagar
Feeder
Bawana
CWW Grid
S/Stn
X
/
/
X
Holambi Nagar
Feeder
Schneider
make RN2C
RMU
A/R
Sectionalizer
Sectionalizer
Sectionalizer
3
Occurrences
o
o
o
o
o
Auto Recloser Locked out
Sectionalizer did not trip
Fuses of the DT’s were getting blown off
The fault was of transient nature. It happened three to four times
Load on the feeder was found more than 250 Amps
Observations
o
o
o
o
o
Relay settings of RMU at kiosk S/Stn are found as per approved relay settings.
Relay setting of AR are also found as per the approved relay settings..
Auto recloser relay settings were 180Amps for O/C and 100Amps for E/F elements.
Sectionalizers current settings were 500Amps.
Fault current was found between 350 Amps to 450 Amps as per relay data.
4
Investigations
o
o
o
o
Auto recloser was working properly
Load on the feeder was detected more than the O/C set value of Auto Recloser relay
Sectionalizers current setting was more than that of the fault current , hence never isolated
the faulty feeder
Fuses were under rated
CAPA
o
o
o
o
The Auto Recloser O/C current setting was enhanced from 180 Amps to 200 Amps
The Auto Recloser E/F current setting was enhanced from 100 Amps to 150 Amps
The Sectionalizers current setting was reduced to 275Amps from 500Amps.
Zonal staff was asked to reduce the load on the feeder and install proper/standard rating of
the fuses on the DT’s.
4
CASE STUDY 2
Cascade Trippings in Zone
1303 (Motinagar)
Single Line Diagram
Sudarshan
Park grid
Trip
Gadaria S/Stn X
Achpal S/Stn
Trip X
Trip
Basai Darapur
Ramgarh S/Stn
X
Rattan Park S/Stn
F
7
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
8
Occurence
For any fault beyond Ramgarh S/Stn, O/G CB of all the S/Stns in this circuit were
getting tripped in cascading.
INVESTIGATIONS
o
o
o
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
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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CASE STUDY 3
Transformer Tripping at AIR
KHAMPUR
Occurrence
Power Transformer tripping on Differential Protection
Observations
The Numerical relay indicated that Transformer has tripped on Saturation Discrimination
element in Differential Relay.
11
Investigations
A new feature of Saturation Discrimination has been added in relay to pinpoint tripping of the
transformer due to saturation of CT Cores. Saturation of CT Cores causes flow of differential
current through operating coil of the relay.
In CT Saturated condition, tripping generally occurs when transformer is loaded up 50 to 75%
loading capacity. At this load current discrimination is sufficient enough to cause tripping.
Findings
o
o
o
Knee point voltage of PS Class CT Core has got reduced on account of Saturation.
Difference of Saturation curve is indicated in Diagram.
Tripping current waveforms also shows fault details.
CAPA
o
o
Saturated CT’s were replaced
All three CT’s were replaced to keep Identical CT Magnetizing curve
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CT SATURATION AT 33 KV AIR KHAMPUR
GRID
SATURATED CORES
13
FAULT DATA EVENTS for AIR Khampur
Created by
AREVA / S&R-103
Output date
02.02.2007
Fault records file (SFD format)
Fault date
29.01.07
Fault time
13:38:30.114
synchronized
Yes
Reference time for tRel(s)
tAbs
Recording start time
| tRel(s) | xxx.yyy Description
| Value
-----------------------------------------------------------------------13:38:30.114 |
0.000 | 041.210 DIFF Enabled
| Present
13:38:30.114 |
0.000 | 036.090 PSS PS 1 active
| Present
13:38:30.114 |
0.000 | 034.046 LOGIC Enabled
13:38:30.114 |
0.000 | 034.075 LOGIC 1 set externally
| Present
13:38:30.114 |
0.000 | 034.076 LOGIC 2 set externally
| Present
13:38:30.114 |
0.000 | 034.077 LOGIC 3 set externally
| Present
13:38:30.114 |
0.000 | 034.078 LOGIC 4 set externally
| Present
13:38:30.114 |
0.000 | 034.079 LOGIC 5 set externally
| Present
13:38:30.114 |
0.000 | 034.080 LOGIC 6 set externally
| Present
13:38:30.114 |
0.000 | 034.081 LOGIC 7 set externally
| Present
13:38:30.114 |
0.000 | 034.082 LOGIC 8 set externally
| Present
13:38:30.171 |
0.057 | 041.116 DIFF Sat.discr. 2 trigg. | Start
13:38:30.171 |
0.057 | 041.117 DIFF Sat.discr. 3 trigg. | Start
13:38:30.171 |
0.057 | 041.125 DIFF Meas.system 2 trigg. | Start
13:38:30.171 |
0.057 | 041.126 DIFF Meas.system 3 trigg. | Start
13:38:30.178 |
0.064 | 041.115 DIFF Sat.discr. 1 trigg. | Start
| Present
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CASE STUDY 4
Tripping of 11 KV Incomer-2
Tripping without indication
on Outgoing fault in 33 KV
Observations
1. It was found that at high currents with unbalance, the 11 kV Incomer-2 was tripping without
indication.
2. It was found that the tripping was occurring through series trip coil mounted on the 11 kV
Incomer breaker.
3. It was found that the series trip relay (CDG), though not operated gave a trip signal to the
breaker when secondary injection was carried out on Phase to neutral CT.
4. On checking the wiring of the series trip relay and the tripping circuit, one wire was found
broken as shown in diagram. Breaker was tripping through series trip coil which was operating
on spill current due to unbalance in system.
Analysis For Wazirabad Inc. 2 tripping without indication during outgoing fault.
C11
C31
C51
C71
=> CDG relay Coil
=> Series Trip Coil
=> CDG Relay NC
contact.
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Conclusion & Corrective Action
1. As the interconnection between NC Contact and Neutral terminal was disconnected due
to defective lug crimping, the unbalance current, whenever present, was not being
bypassed by the NC contact of the relay. Instead, the current was going through the series
trip coil, thereby making relay settings redundant. This was leading to the tripping of 11 kV
Inc.2 breaker tripping on any unbalance or fault.
2. Necessary correction wiring of contacts was done and Unbalance was simulated to check
the correct operation of the relay .Now Control circuit and relay functionality found OK.
17
CASE STUDY 5
Tripping Analysis report for
11 KV Mithila Vihar feeder
along with 11 KV Incomer
Critical Analysis
11 KV Incomer tripping alongwith 11 KV Outgoing
Due to fault summation at RG-22 Grid.
11 KV Incomer-1
11 KV Incomer-2
Incomer sees vector
summation of both the
fault currents and cause
tripping
Nithari feeder
Mithila Vihar
Both feeders running on same pole generally
face external faults simultaneously
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Case Study
Tripping Analysis report for 11 KV Mithila Vihar feeder alongwith 11 KV Incomer
Details of Event
11 KV Mithila Vihar feeder tripped alongwith 11 KV Incomer
Observations
o
o
o
o
o
o
Total load of the station is around 800 Amps and System is keeping total load on any of
the transformer keeping the other transformer on no load.
Mithila Vihar feeder tripped on over current High set.
11 KV Incomer-2 tripped on over current High set.
Fault data record of the Incomer show’s that Fault seen by 11 KV
Incomer is 9.1 KA while fault seen by Mithila Vihar feeder is 6.2 KA.
It is observed that Nithari feeder was also seen the fault but not tripped due to isolation of
the fault. Protection settings were Coordinated and CT Ratio adoption was found
correct.
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Conclusions
After going through the tripping events it was observed that both feeders were getting
tripped simultaneously.
After going through the fault data record, it is very clear that fault seen by Incomer is always
on higher side as compared to the outgoing feeder.
The tripping in the incomer occurs as the fault current “seen” by the incomer breaker is a
summation of the outgoing faults.
It has been learnt from Concerned Zonal staff, that both the outgoing feeders in question, viz.
Mithila Vihar feeder and Nithari feeder, are routed on the same poles and on same cross
arm. Thereby, the occurrences of birdages in both the feeders occur simultaneously.
21