Action to be taken if a transformer fails
The action to be taken depends upon the size of the transformer, which protective relays have operated, whether tripping is accompanied by loud noise, smoke or expulsion of oil from the transformer, etc. Therefore, the first thing to do is to make a very careful note of the full circumstances of the failure. The following brief notes may be of help:
- Go round the transformer and observe its external condition, look for any damage to the bushings, leads or cable box and any evidence of squirting of oil. Note the temperature of oil, at the earliest, and check if the level of oil in the conservator is right. Take megger readings between primary and secondary and also of each to earth. If everything is right, proceed as noted below.
- The failure may possibly be due to sudden and heavy over-load or short-circuit. If a HRC fuse provided for a small transformer has blown or a switch fuse has dropped out, check if its ampere rating is right. If incorrect, replace by the correct size and energise the transformer, after switching off the secondary circuit. If everything is all right, close the secondary circuit ; if the fuse blows again, the fault is obviously in the 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.
- 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.
- 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 Buchholz 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.
- 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.
- One way of finding an inner-layer or inter-coil short is to check the turns ratio by an accurate ratiometer.
