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  What if Transformer Fails

Action to be taken if a Transformer Fails

The action to be taken relies on the size of the transformer that protective relays have operated, whether tripping is accompanied through loud noise, smoke or expulsion of oil from the transformer, etc. Hence, the first thing to do is to make an extremely careful note of the full conditions of the failure. The following concise notes might be of help:

i) Go round the transformer and monitor the external condition of it, look for any break to the bushings, leads or cable box and any proof of squirting of oil. Note down the temperature of oil, at the initial, and test if the level of oil in the conservator is right. Take megger readings among the primary and secondary and as well of each to earth. If everything is right, carry on as noted below.

ii) The failure might possibly be because of sudden and heavy over-load or short-circuit. If a HRC fuse given for a small transformer has blown or a switch fuse has dropped out, test if its ampere rating is right. If not correct, replace through the right size and energise the transformer, after switching off the secondary circuit. If everything is okay, close the secondary circuit; if the fuse blows once again, the fault is noticeably in the outgoing lines that should be traced and rectified; if it does not blow the fault or overload has in fact cleared itself. If, alternatively, the primary circuit fuse blows out, even while the load is disconnected, an internal fault or a fault in the connecting cable is pointed out. The above remarks as well apply, if an over-current transmit alone has operated and tripped the breaker.

iii) If a differential relay runs while a transformer is first switched on, it may be because of a switching surge. Test harmonic-restraint circuit. If, alternatively, the relay operates while the transformer is in service, it is a certain indication of an internal fault.

iv) Any tripping of the Bucholz relay needs to be thoroughly looked into. If the lower assembly has tripped because of sudden evolution of large quantities of gas, a main internal fault is to be inferred particularly if either over-current or differential or earth fault relay has operated. If, alternatively, the upper assembly has tripped because of slow release of gas it is essential to its composition before any conclusions can be drawn. If it is just an air, there is no cause for worry, when air can enter into the transformer in several ways. While a transformer is first commissioned it occasionally occurs that the Bucholz relay upper assembly trips, after a few hours of run, because of the release of air bubbles entrapped in the windings, like when hand filling is employed for filling oil into the tank. To avoid this occurring, it is best to fill the oil under vacuum, use of a pump. If the gas that is accumulated is not air, an incipient fault is indicated. Analysis of the gas would assist in recognizing the nature of the fault, and this should be completed like a routine measure. If the Bucholz relay has tripped, with no any gas being provided out, it may be because of electrical fault in the wiring. To make sure that the relay does trip while there is release of gas, it is essential to conduct a test at the time of commissioning and as well periodically. This can be simply done through pumping in air by the pet cock by an ordinary cycle pump. It will close the electrical contacts, sound the alarm and trip the circuit breaker if the internal assembly is mechanically free.

v) Careful checking is needed if the Earth Fault relay has tripped or if there is any evolution of smoke / oil, and also if the explosion vent given in a large transformer blows out. In such types of cases re closure of supply should not be allowed as it might cause additional extensive damage. It will be essential to lift the core out and create a detailed inspection of the internal parts. Before doing so, though it is essential to take measurements of the resistance of the windings and as compare the figures with the test figures of Maker that incidentally should be carefully preserved through every office. A sample of oil should as well be drawn out and its breakdown voltage tested.

One method of finding an inner-layer or inter-coil short is to test the turns ratio through an accurate ratio meter.

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