Abnormality and analysis report of 25MW STG SCCT Diode failure and replacement

1. ABNORMALITY REPORT OF TG-2
Page 1 of 4
CPSU/Elect./M-103/2022
Date: 25/05/2022
Sub: Abnormality report of TG-2 for SCCT over current due to Diode failure.
Date& Time of Event: At 13:33Hrs. on 19/05/2022
Effect on Plant: Nil
Description of events:
 Before event TG-1 was in off condition. TG-2 was running at @ 25.0 MW, GT was tripped
on Rev Power Protection (13:09.40) & power import was @ 27.25 MW. IMP-EXP
controller was in “OFF” condition.
 Common annunciation operated at 13:33Hrs. on 19/05/2022 on TG-2 GCP panel i.e.
SCCT O/C / REG DC FAIL / FB SUPPLY FAIL & also Common LED no: 12 “S/C O/C /
REG. DC FAIL / FB SUPPLY FAIL” operated on BCU relay.
 In the view of above, DAVR LED checked by shift technician & found that DAVR LED No:
09 on TIER 65 - “Series compounding O/C” operated. Also checked that SCCT DC output
voltage zero volt (0.0V) even at 25.0MW on GCP panel. Also at SCCT cubicle, analog
voltmeter was showing zero volt.
 There is no any abnormality observed & TG-2 was running normal, however in the view
of event, we have checked actual current of SCCT which was 744A secondary side
against rated 665A (checked on BMR i/p i.e. 6.2A, CTR 600/5), relay setting (5A), CT
secondary current which was 6.2A & SCCT DC output voltages which was 10V.
 At 16.33Hrs after GT start-up it’s load increased to 20.07 MW & subsequently TG-2 load
decreased up to 14.0MW in order to avoid thermal heating & damage of SCCT
 At 14 MW load of TG-2, SCCT secondary current measured and found 588A against
rated 665A (checked on BMR i/p i.e. 4.9A, CTR 600/5)
 On Next day i.e. 20/05/2022, fault finding job carried out. After necessary arrangements
of spare material (Diode stack), TG-2 de-synchronized to replace faulty diodes @
09:02Hrs.
 After completion of job, TG-2 re-synchronized at @10:03Hrs.
 After GCB ON, it was observed that TG-2 has taken base load of 1.45 MW but eventually
it’s load reduced to 0.45 MW. Load was also not increasing from DCS panel. After that,
tried to increase load from “Freq Raise” PB on GCP but load increased after load
controller active.
 Problem was referred to instrument & process for its analysis in accordance with valve
response. Refer Annex-A, for details.

2. ABNORMALITY REPORT OF TG-2
Page 2 of 4
Action Taken:
 In the view of SCCT over current alarm, measured secondary current of SCCT on BMR
in SCCT cubical at 25.0 MW load. BMR setting : 5A; CTR 600/5
Sr. No. Phase
Current at CT Sec.
(Amp.)
Current at CT Pri.
(Amp.)
1 R 6.2A 744A
2 Y 5.8A 696A
3 B 6.0A 720A
(SCCT Details: Rating:150 KVA, Voltage Ratio: 108V / 130V & Current :1804A / 665A)
 As SCCT secondary current rating is 665A only & it was running at 744A in R-Phase, so
reduced load of TG-2 gradually after GT start-up and kept TG-2 load at 14MW in
consultation with process.
 TG-2 SCCT CT secondary & primary current readings are as below at 14.6MW load:
Sr. No. Phase Current at CT Sec.
(Amp.)
Current at CT Pri.
(Amp.)
1 R 4.9A 588A
2 Y 4.8A 576A
3 B 4.9A 588A
 Above activities completed @16:40Hrs. & kept TG-2 load below 15.0 MW throughout the
night & GT load increased up to @ 21 MW & Import kept @ 16MW to avoid thermal
heating and any damage of SCCT & job planning done for next day.
 On 20/05/2022, TG-2 de-synchronize at 09:02Hrs. After grounding all live bus, SCCT
primary side was clamped to earthing rod for safety. SCCT all 6 nos. diode (D1, D2, D3,
D4, D5 & D6) were checked with the help of ERS, found diode D2, D4 & D6 short circuited
and Diode D1, D3 & D5 found healthy. Reading from multi-meter on Diode mode was as
follows :
D1: 0.247V; D2: 0V; D3: 0.247V; D4: 0V; D5: 0.248V & D6: 0V;
 D2, D4 & D6 diodes were replaced from spare healthy stack with heat-shrink unit.
Installed diodes were of “Ruttonsha” make flat pack type Diode, Cat- R1600B22C. SCCT
all 6 nos. diode (D1, D2, D3, D4, D5 & D6) were checked with the help of ERS, found o.k.
Reading from multi-meter on Diode mode of attended diode stack was as follows :
D1: 0.292V; D2: 0.263V; D3: 0.292V; D4: 0.263V; D5: 0.293V & D6: 0.263V;
 TG-2 synchronize at 10.03hrs.

3. ABNORMALITY REPORT OF TG-2
Page 3 of 4
 After synchronizing SCCT DC voltage contribution checked at 14MW load and its found
o.k. i.e. 63V. Also, SCCT CT secondary & primary current readings were taken and are
as below at 14.6MW load.
Sr. No. Phase Current at CT Sec.
(Amp.)
Current at CT Pri.
(Amp.)
1 R 2.53A 303A
2 Y 2.54A 304.8A
3 B 2.53A 303A
 Also TG-2 SCCT diode D1, D2, D3, D4, D5 & D6 temperature checking done with
temperature gun found 75, 81, 71 , 78 , 76 & 83 ºC respectively at @ 15.9MW load.
 Also TG-2 SCCT transformer core, diode heat sink & O/G cable temperature measured
for future record point of view.
Relay events:
Date : 19/05/2022 (GPR-M Relay)
Event Time Event Description Remarks
13:33:00.974 S/C,O/C OR REG. DC FAIL/F.B SUPPLY FAIL_ON
16:01:09.421 S/C,O/C OR REG. DC FAIL/F.B SUPPLY FAIL_OFF
16:01:10.232 S/C,O/C OR REG. DC FAIL/F.B SUPPLY FAIL_ON
16:01:12.061 S/C,O/C OR REG. DC FAIL/F.B SUPPLY FAIL_OFF
16:01:13.148 S/C,O/C OR REG. DC FAIL/F.B SUPPLY FAIL_ON
16:52:06.252 S/C,O/C OR REG. DC FAIL/F.B SUPPLY FAIL_OFF
16:52:21.866 RESET LED_ON
Date : 20/05/2022 (GPR-M Relay)
Event Time Duration Event Description Remarks
09:02:40.520 00.00.00.000 52G1 Close_OFF TG-2 de
synchronize
09:03:16.710 00:00:36.190 FCB + EXCI ON_OFF
10:02:42.404 01:00:01.884 FCB + EXCI ON_ON
10:02:43.401
01:00:02.881
32F P< stage picked up_ON
Set : 1%;
450ms
10:02:43.849 01:00:03.329 32F P< stage TRIP_ON
10:04:30.056 01:01:49.536 32F P< stage picked up_OFF
10:04:30.056 01:01:49.536 32F P< stage TRIP_OFF
10:04:30.084
01:01:49.564
52G1 CLOSE
TG-2
synchronize

4. ABNORMALITY REPORT OF TG-2
Page 4 of 4
Action taken & Analysis:
 Following alarm /annunciations operated:
1.) GCP-2 Annunciation: SCCT O/C / REG DC FAIL FB SUPPLY FAIL
2.) DAVR LED 09 on TIER 65 : “Series compounding O/C”.
3.) BCU relay LED no: 12 “S/C O/C / REG. DC FAIL / FB SUPPLY FAIL”
Remedial action:
 NA
QEHSEn (ISO 14001, 45001, 9001 & 50001) related impact:
 Nil
SEE(AAS)
Thro: M(PLP)-------------SM (BGS) ------------------------- CM(KKB)
To: AGM(RPT)

5. ABNORMALITY REPORT OF TG-2
Page 5 of 4
0
1
2
3
4
5
6
7
0
10
20
30
40
50
60 10:04:22
AM
10:04:32
AM
10:04:42
AM
10:04:52
AM
10:05:02
AM
10:05:12
AM
10:05:22
AM
10:05:32
AM
10:05:42
AM
10:05:52
AM
10:06:02
AM
10:06:12
AM
10:06:22
AM
10:06:32
AM
10:06:42
AM
10:06:52
AM
10:07:02
AM
10:07:12
AM
10:07:22
AM
10:07:32
AM
10:07:42
AM
10:07:52
AM
10:08:02
AM
10:08:12
AM
10:08:22
AM
10:08:32
AM
10:08:42
AM
10:08:52
AM
10:09:02
AM
10:09:12
AM
10:09:22
AM
10:09:32
AM
10:09:42
AM
10:09:52
AM
10:10:02
AM
10:10:12
AM
10:10:22
AM
TG-2 MW v/s EHTC Valve
EHT valve % TG-2 MW
Syncronised, GCB ON
Annexure-A
TG-2
MW