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On load tap changer (OLTC) explode?
Transformer
Vector group: Ynd11
Rated Power: 353MVA
Rated Current: 693A
Voltage:294/20KV
OLTC
Type: T III Y1000 110/c 12193w
Max Operation Current: 295A
Step Voltage: 550V
Arrangement in Tx windings: Y Connection
Problem: Damage of Unit 1 Generator Transformer On Load Tap
Changer (OLTC)
Description of Incident:
9.00 a.m. :- Generator No Load test on excitation system
started
10.30 a.m :- Carried out AVR change over
test
11.30 a.m :- Preparation for Generator Manual Synchronizing
test
12.00 a.m :- Manual Synshronizing test started.
Observed incoming generator voltage higher than running voltage.
CECR operator operated Generator Transformer OLTC to bring down
the incoming generator voltage
12.03 a.m :-Generator Transformer abnormal alarm operated at
Gen.Control Desk (CECR)
CECR operater at Generator transformer bay informed that the
transformer
oil splashed out from OLTC.
CECR operater informed that he heard explosion inside OLTC.
Observed Oil level & Oil Flow alaram operated at Tranformer
local annunciator.
12.05 am :- CECR operator 'Open Gen Field Breaker”
12.10 a.m. :- Elec1 team inspected the OLTC and found the
OLTC rupture disk was broken
and oil spilled at OLTC area.
- Elec1 found the OLTC top cover bulging.
Do anyone know why and possibility cause happen in this OLTC.
- 9 Answers
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Answers were Sorted based on User's Feedback
Answer / mushtaq ahmed
Dear Sir, The OLTC of Generator Transformer should not be operated when Generator is in parallel with Grid. OLTC step change introduces a step voltage difference, which results in very high circulating currents. Generator AVR takes some time to adjust to this new voltage. But by that time, a very high current has flown thru the circuit. OLTCs are not rated for this, especially during changeover.
| Is This Answer Correct ? | 6 Yes | 1 No |
Generator voltage is decided by excitation and not by OLTC which is provided on HV side . If the voltage on Generator side ( LV ) is reduced with OLTC ,then the voltage difference will be more resulting in more current due to poor lagging pf . AVR maintains the set voltage if apfc mode is not selected . This happened due to wrong operation of OLTC and loose contacts in OLTC . You could have equalized the voltage operating AVR voltage control .
| Is This Answer Correct ? | 1 Yes | 1 No |
Answer / jayesh n patel
failure is happend after tap operation , chances of contact stucking in between positions and full load current passes more time through transition resistance( as per design current passes for 60 to 90 mili seconds only, while snape action)and it got burnt ,and voltage has cut with big spark, sudden pressure has develop in side the tank and oil has got expand which result in bulging of top cover and oil spread,(mechanical failure result in electrical failure).
| Is This Answer Correct ? | 0 Yes | 0 No |
Answer / sakono
May be explode from
1. During moving the OLTC contact, some part was lossen and
OPEN circuit occure.
2. Some material of OLTC was fail and loosen. then the OPEN
circuit was occure in OLTC.
3. Contaminate inside OLTC chamber
4. Oil was contaminated
5. TAP Position of OLTC was not in step.
| Is This Answer Correct ? | 1 Yes | 2 No |
Answer / jayesh n
while contact changing if it stuck in between position and if full load current is pass more time from transition resistance than resistance got break and heavy spark has occure, due to which sudden pressure develope in side the oltc tank and top cover got bulged and oil spread out side the tank (transition resistance is carrying current for short time only that is 60 to 90 miliseconds only, while contact snapping action).
| Is This Answer Correct ? | 0 Yes | 1 No |
Answer / j.kesavan
oltc always problem,why its opration is motorised in auto
cotrol ,thats why disc breake every time
| Is This Answer Correct ? | 3 Yes | 5 No |
Answer / j.ikesavan
my friend please check your inter lock system how to
working motor on load tap changer your winding failure its
going up and down voltage thats why this disc is breake.
| Is This Answer Correct ? | 1 Yes | 4 No |
Answer / brad stroh
where did this happen at? what is your location? we
specialize in failure analysis.
Brad Stroh
alineeds.com
| Is This Answer Correct ? | 1 Yes | 5 No |
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