Page 24 - ECLECTIC MARCH-2022 INSIDE PAGES_p2.indd
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the three tripping operations for the said transformer 3. Main Tank PRV relay.
were analyzed critically and the details of the repairs and 4. Master trip relay (both HV & LV) and
analysis are furnished below: 5. NIDS (Nitrogen Injection Drain and Stir) system
TRIP – 1 operated.
Date & Time of Fault: 10:18 am on 18/05/2019 Details of Fault and its subsequent action for
restoration:
Relay Operation:
1. Differential Relay and ➤ During this tripping of the above transformer, oil
2. Master Trip Relay (both HV & LV) splashed out from the main tank PRV, thereby
operating the main tank PRV relay which indicates
Details of Fault and its subsequent actions for some severe fault inside the main tank.
restoration:
➤ Subsequently, dissolved gas analysis was carried out
➤ The 50 MVA T-3 at Majerhat S/S got tripped by on the oil sample of the above transformer. It was
the operation of differential relay at the above observed from the DGA results that concentration
mentioned date & time. of Hydrogen, Methane, Ethylene and Acetylene
gases had increased significantly as compared to
➤ Subsequently, the staff of Majerhat S/S informed the results obtained in February 2019 for the same
us that tap changing operation was carried out power transformer. (Refer Table- I)
about 30 seconds prior to the tripping of the power
transformer. ➤ The results indicate a possible damage at the
selector assembly of the OLTC.
➤ Inspection was carried out on the OLTC diverter
switch, after ensuring proper earthing for the ➤ It was decided to inspect the selector chamber of
windings of the transformer. the OLTC after draining out requisite amount of oil
from the main tank. Accordingly, about 60 barrels of
➤ Inspection of the OLTC diverter switch revealed no oil had been drained and inspection was carried out
abnormality. through the inspection covers.
➤ Subsequently, all the routine tests and DC HV test ➤ During this inspection, severe flash marks were
were carried out on the transformer by Testing observed at the collector rings and both the
Department and all the results obtained were fixed and moving selector contacts for the tap
satisfactory. changer. The cause for this type of severe damage
was investigated and after critical inspection for
➤ Based on the above inspection and test results, the selector chamber of the tap changer it was
clearance for charging the transformer was given concluded that it will not be possible to repair the
and the same was charged from high voltage side at fault at site and make the OLTC fully operational. The
6:03 am on 19/05/2019 at Tap no.6. transformer could be charged at fixed tap (tap no.-
5, as coveted by System Control Department), by
➤ After carrying out tap changing operation of the replacing the fixed selector contacts.
above transformer from tap no.6 to tap no.-5 and
finally to tap no.-4, as requested by System Control ➤ After replacement of damaged contacts, oil refilling
Department the transformer got tripped at 6:05 am. through vacuum filter and necessary air release,
all routine tests along with DC pressure test were
TRIP - 2 carried out and the results obtained were found
satisfactory.
Date & Time of Fault: 6:05 am on 19/05/2019
➤ Online oil filtration was carried out to remove the
Relay Operation: existing gases from the oil of the transformer and
1. Differential Relay
2. Main Tank Buchholz Relay
24
were analyzed critically and the details of the repairs and 4. Master trip relay (both HV & LV) and
analysis are furnished below: 5. NIDS (Nitrogen Injection Drain and Stir) system
TRIP – 1 operated.
Date & Time of Fault: 10:18 am on 18/05/2019 Details of Fault and its subsequent action for
restoration:
Relay Operation:
1. Differential Relay and ➤ During this tripping of the above transformer, oil
2. Master Trip Relay (both HV & LV) splashed out from the main tank PRV, thereby
operating the main tank PRV relay which indicates
Details of Fault and its subsequent actions for some severe fault inside the main tank.
restoration:
➤ Subsequently, dissolved gas analysis was carried out
➤ The 50 MVA T-3 at Majerhat S/S got tripped by on the oil sample of the above transformer. It was
the operation of differential relay at the above observed from the DGA results that concentration
mentioned date & time. of Hydrogen, Methane, Ethylene and Acetylene
gases had increased significantly as compared to
➤ Subsequently, the staff of Majerhat S/S informed the results obtained in February 2019 for the same
us that tap changing operation was carried out power transformer. (Refer Table- I)
about 30 seconds prior to the tripping of the power
transformer. ➤ The results indicate a possible damage at the
selector assembly of the OLTC.
➤ Inspection was carried out on the OLTC diverter
switch, after ensuring proper earthing for the ➤ It was decided to inspect the selector chamber of
windings of the transformer. the OLTC after draining out requisite amount of oil
from the main tank. Accordingly, about 60 barrels of
➤ Inspection of the OLTC diverter switch revealed no oil had been drained and inspection was carried out
abnormality. through the inspection covers.
➤ Subsequently, all the routine tests and DC HV test ➤ During this inspection, severe flash marks were
were carried out on the transformer by Testing observed at the collector rings and both the
Department and all the results obtained were fixed and moving selector contacts for the tap
satisfactory. changer. The cause for this type of severe damage
was investigated and after critical inspection for
➤ Based on the above inspection and test results, the selector chamber of the tap changer it was
clearance for charging the transformer was given concluded that it will not be possible to repair the
and the same was charged from high voltage side at fault at site and make the OLTC fully operational. The
6:03 am on 19/05/2019 at Tap no.6. transformer could be charged at fixed tap (tap no.-
5, as coveted by System Control Department), by
➤ After carrying out tap changing operation of the replacing the fixed selector contacts.
above transformer from tap no.6 to tap no.-5 and
finally to tap no.-4, as requested by System Control ➤ After replacement of damaged contacts, oil refilling
Department the transformer got tripped at 6:05 am. through vacuum filter and necessary air release,
all routine tests along with DC pressure test were
TRIP - 2 carried out and the results obtained were found
satisfactory.
Date & Time of Fault: 6:05 am on 19/05/2019
➤ Online oil filtration was carried out to remove the
Relay Operation: existing gases from the oil of the transformer and
1. Differential Relay
2. Main Tank Buchholz Relay
24
