General:-
Transformer is a static device and does not consist of any moving parts except OLTC.
Therefore, the possibility of any sort of damage by way of wear does not arise. Copper and Iron Core can last indefinitely. It is, however, subjected to many strains and stresses under operation such as occurrences like lightning surges, short circuits and over voltages; each of which leaves its effect on the winding structure. Yet being a sturdy equipment, a transformer does survive all these and gives a long life, if it is installed, operated and maintained intelligently.
The normal life of a transformer manufactured by a concern of repute can be taken to be
about 30 years, provided it is well looked after. It can last much longer, say for about 50
years, if operated with care. Gross negligence in its proper operation and maintenance may
cause damage prematurely and its life span reduced to even 10 years. There should be a
vigilant check on temperature rise and humidity as the excessiveness of these is quite harmful and should never be allowed beyond permissible limits. At 75ºC, the insulation lasts indefinitely. As per past experience, it has been estimated that every 8ºC rise in temperature above the permissible limit halves the life of the insulation. Abnormal over loading can certainly char the insulation and damage the transformer badly in an hour or so. The overloading beyond the capacity can be a very costly affair and it should not be allowed in any case.
The life expectancy of transformers depends upon the rate of deterioration of the insulation.
Generally the rate of deterioration of insulation at various operating temperatures doubles
with each 8ºC temperature rise above 100ºC. The deterioration is also proportional to the
time for any given temperature above 100ºC.
Since the actual temperature is the sum of the ambient temperature and temperature rise, it is apparent that the ambient temperature very largely determines the load at which the
transformer can be operated in service.
Therefore,
i. The Shift-in-charge / engineer should ensure that the transformer shall not run at a
load more than the rated load, except for an unavoidable shorter period.
ii. The Shift-in-charge shall observe/notice the sound of the transformer. Any
abnormality observed is to be reported to the Incharge (GSS)
iii. The Shift-in-charge shall check that the fans are running, if the winding temperature
of transformer is 60º C or above. In case the fans are not running then they shall be
switched on manually and non-operation of auto system shall be reported to the
Incharge (GSS) If the winding temperature is below 60º C , he shall check the
operation of fans by switching them on manually and after checking, the Auto /
Manual switch be returned to Auto position. Intimation of defective fans, if any, be
reported to Incharge (GSS) for rectification.
iv. The winding / oil temperature of the transformer shall not be allowed to be abnormal looking to the load and ambient temperature. Difference in the oil and winding
temperature meters of the transformer shall not be more than 15º C. If it is more, it
shall be reported to the Incharge (GSS).This may be either due to the defective
temperature meters or due to some abnormality in the transformer.
v. The oil level of the transformer, as shown by the magnetic oil gauge, should be
corresponding to the oil temperature of the transformer. This can be checked from the
temperature mark given on the Magnetic Oil Gauge. If this is not so, the matter be
reported to Incharge (GSS) for immediate action.
vi. The oil level in sight glass should be full in air cell type conservator. If not full, the
matter should be reported to Incharge (GSS)
vii. The Shift-in-charge, on assuming the charge of shift, shall check the breather for the
following:-
a. The colour of silica gel should be blue. If not, it should be reported to the Incharge
(GSS).
b. The oil level in the breather oil cup (at bottom) shall be up to required level. If not, he
should arrange to get transformer oil filled up to the marked level.
viii. The Shift-in-charge, on assuming charge of shift, shall check the relief vent pipe for
the following:
a. No oil is visible in the sight glass, where provided on the relief vent pipe. If visible,
the matter be reported to the Incharge (GSS) for immediate action.
b. The diaphragm provided on the top of relief vent pipe is intact. If not, matter be
reported to the Incharge (GSS) for immediate action.
ix. In case the high oil temperature or high winding temperature alarm is received, it
indicates that the oil temperature or winding temperature is approaching the set limits.
Normally a difference of 5ºC/ 10ºC is kept between the setting for alarm and the
setting for tripping. If either or both of these alarms are received, then the following
shall be checked:-
a. That all the fans and oil pumps are running. Any fan and pumps which is not running
should be switched on. If any fan & pump is defective, the load should be restricted
to avoid unnecessary tripping of the transformer. Information of defective fan be
intimated to the Incharge (GSS)
b. If the load on the transformer is more than the rated load, the load should immediately
be restricted to rated load.
c. That the temperature should not be allowed to be high corresponding to the load on
the transformer and the ambient temperature.
If it is high, then the load is to be restricted and intimation given to the Incharge
(GSS) to analyze the cause of abnormal temperature rise. If the temperature rise is
corresponding to the load and the ambient temperature, then the trend of the rise is to
be observed. If the rate of rise is abnormal, then the load should be reduced to
prevent tripping of transformer and Incharge (GSS) be informed.
If the rate of rise is not abnormal, the transformer can be run upto rated load and
vigilant watch be kept on the temperature of the transformer.
x. In case the over-flux alarm is received, the tap position of the transformer should
immediately be lowered to a tap corresponding to the voltage of High Voltage side. If the voltage is above the rated voltage of tap No.2, then message should be given to the feeding 400 kV/220 kV/132 kV GSS and Load Dispatch Center to reduce the system /
supply voltage. If the voltage is more than 6% higher then the rated voltage at tap No.
2, the transformer should be switched off and Load Dispatch Center intimated.
xi. Transformer tripping on operation of buchholz relay or OLTC Surge Relay (OSR).
a. In the event of the operation of buchholz alarm, buchholz trip, OLTC buchholz surge
trip etc., the transformer is to be isolated from the system immediately. Where the
buchholz alarm is not connected to trip the HV/LV breaker, both the HV/LV breakers
shall be tripped manually. Transformer shall be isolated from system by opening the
required HV/LV isolators and intimation be given to the Incharge (GSS)
b. After isolating the transformer, the Shift-in-charge shall carry out the physical
inspection of main buchholz relay and OLTC surge relay to observe the collection of
gas in the relays or for indication of their operation and shall record his findings. Incase of NGEF make transformer, the operation of micro switch as provided on the
OLTC top cover is to be confirmed as in such transformers, instead of gas operated
relay, micro switch is provided for OLTC protection. Gas collected in relay is not to
be released till the same is tested.
c. The transformer is not to be energized till it is thoroughly tested and the possibility of
internal damage to the transformer is ruled out.
xii. In the event of operation of differential protection of the transformer, the transformer
is to be isolated from the system immediately. The Shift-in-charge shall carry out the
physical inspection of all the equipments (such as LAs of 400 kV/ 220 kV/ 132 kV/
33 kV/ 11 kV, Cable Boxes and transformer) installed between 400 kV, 220 kV, 132
kV and 33 kV or 11 kV transformer CTs and also 400kV,220kV,132 kV, 33 kV or 11
kV CTs themselves. The buchholz relay of the transformer shall be checked for any
collection of gas. The relief vent diaphragm is to be checked. The Incharge (GSS) is
to be informed immediately. The transformer is not to be energized till reason of
operation is diagnosed.
xiii. In the event of the transformer tripping on HV side O/C and E/F only, the following is
to be checked:-
a. If tripping is due to over current as a result of over loading, the transformer can be
charged after shedding the excess load.
b. If it is due to non-tripping of the outgoing / incoming breaker due to mechanical
defect, the transformer can be taken back into service either after taking the defective
breaker out of service or rectifying the defective breaker. The report of the defective
breaker is to be made to the Incharge (GSS)
c. It may also be due to the improper coordination of the protection. The matter, in this
case, is to be reported to the Incharge (GSS) and by him to the Executive Engineer
(Prot.) but the transformer can be charged if no abnormality is observed.
d. Before energization of the transformer, all the feeders / incoming breakers are to be
tripped manually. In the event of the transformer tripping again on charging, it is to
be isolated from the system and intimation given to the Incharge (GSS) The
transformer is not to be re-energized till the same is thoroughly tested and the
possibility of internal damage to transformer is ruled out.
xiv. In the event of the transformer tripping on the oil / winding temperature trip, the
following is to be investigated:-
a. If it is due to the over loading during the high ambient temperature, the transformer
can be taken back into service after load shedding and cooling.
b. If it is due to defective fans, the transformer can be charged after cooling, and
arrangements are made for repair of defective fans. Till fans are repaired, load is to
be restricted so as not to cause the tripping of transformer on high winding
temperature. Details of defective fans are to be reported to the Incharge (GSS) for
immediate repairs / rectification.
c. If it is due to defective winding temperature indicator, replacement should be
arranged immediately. Till the replacement is received, the transformer is to be run
on reduced load. The defective winding temperature meter is to be immediately
reported to the Incharge (GSS) for replacement / rectification.
d. If it is due to the closing of the radiator valves, then the transformer can be charged
after opening the valves and cooling the transformer.
e. However, if all other conditions are normal, then the temperature rise may be due to
some abnormality in the transformer. In that case, the matter is to be reported to the
Incharge (GSS) and the transformer is not to be energized till the same is thoroughly
tested and the possibility of internal damage to the transformer is ruled out.
xv. In the event of transformer tripping on over-flux protection, the system voltage should
be observed. The tap position of the transformer should be brought down to tap No.2.
The transformer is to be charged only if the voltage is not more than 6% above the
rated voltage of tap No.2.
OVER LOADING OF TRANSFROMERS:
Transformers may be overloaded when any other unit fails, when a spare unit is not available and when load shading is not possible. The cooling system in such cases should be made effective by:
i. Increasing the circulating oil supply, if the transformer is of the oil cooled type or,
ii. Increasing the cooling air supply by blowers.
It is, however, important that overloads should not be allowed on transformers
without an investigation of the various limitations involved. Among these limitations,
which should be checked in the field, are
(i) Oil expansion,
(ii) Heating of bushing
lead,
(iii) Soldered connections,
(iv)Tap changers,
(v) Heating of the associated
equipments such as cables, reactors, circuit breakers, disconnecting switches, current
transformers, etc. Any one of these may constitute the practical limit in load carrying
ability. Rapid and wide changes in operating temperature should be avoided, as much
as possible since, these aggravate the changing effects carried by temperature
expansion and contraction of the copper on the winding insulation.
iii. When system voltage is low taps should be increased considering both HV and LV
side currents of the transformer. In such conditions the HV side of the transformer
may become overloaded even if the LV side load is within limits.
Parallel operation of transformers in sub station:
If transformers are to operate in parallel in a sub station, they must have:
a. The same voltage ratio,
b. The same polarity,
c. The same percentage impedance (or impedance voltage) to its own base MVA rating,
d. The same phase rotation i.e. vector diagram,
e. The difference in KVA capacity should not exceed the ratio of 3:1.