Specifications

186
outgoing lines, which should be traced and rectified; if it does not blow the fault or
overload has apparently cleared itself. If,on the other hand, the primary circuit fuse
blows out, even when the load is disconnected, an internal fault or a fault in the connecting
cable is indicated. The above remarks also apply,if an over-current relay alone has
operated and tripped the breaker.
iii) If a differential relay operates when a transformer is first switched on, it may be due to
a switching surge. Check harmonic-restraint circuit. If, on the other hand, the relay
operates when the transformer is in service, it is a sure indication of an internal fault.
iv) Any tripping of the Bucholz relay requires to be carefully looked into. If the lower assembly
has tripped due to sudden evolution of large quantities of gas, a major internal fault is to
be inferred especially if either over-current or differential or earth fault relay has operated.
If, on the other hand, the upper assembly has tripped due to slow release of gas it is
necessary to find out its composition before any conclusions can be drawn. If it is air
only, there is no cause for worry, as air can enter into the transformer in many ways.
When a transformer is first commissioned it sometimes happens that the Bucholz
relay upper assembly trips, after a few hours of run, due to the release of air bubbles
entrapped within the windings, such as when hand filling is employed for filling oil into
the tank. To avoid this happening, it is best to fill the oil under vacuum, making use of a
pump.
If the a ccumulated gas is not air, an incipient fault is indicated. Analysis of the gas
would help in identifying the nature of the fault, and this should be done as a routine
measure. If the Bucholz relay has tripped, without any gas being given out, it may be
due to electrical fault in the wiring. To ensure that the relay does trip when there is
release of gas, it is necessary to conduct a test at the time of commissioning and also
periodically. This can be easily done by pumping in air through the pet cock by an
ordinary cycle pump. If the internal assembly is mechanically free, it will close the
electrical contacts, sound the alarm and trip the circuit breaker.
v) Thorough checking is required if the Earth Fault relay has tripped or if there is any
evolution of smoke or oil, and also if the explosion vent provided in a large transformer
blows out. In such cases reclosure of supply should not be permitted as it may cause
further extensive damage. It will be necessary to lift the core out and make a detailed
inspection of the internal parts. Before doing so, however, it is necessary to take
measurements of the resistance of the windings and compare the figures with the
Maker
’
s test figures, which incidentally should be carefully preserved by every office. A
sample of oil should also be drawn out and its breakdown voltage tested.
vi) One way of finding an inner-layer or inter-coil short is to check the turns ratio by an
accurate ratiometer.
16.11 Points to be attended during periodical overhaul :
After lifting out the core and placing it on a trestle, the interior of the tank should be thoroughly