Winding Temperature Protection

Transformer Protection Part 10 
Winding Temperature Protection
       Large Transformer with forced cooling are usually fitted with winding temperature devices to detect overloading of the transformer or failure of the cooling equipment. 
       The winding temperature device is of the type described in (3 phase transformer). The bulb is situated in a special pocket located in the flow of the hot oil and is, in addition, heated by a small heater energised from a current transformer connected to measure the transformer winding current. The device thus indicates the top oil temperature of the transformer plus an increment proportional to the load on the transformer, this increment being arranged to match the difference between top oil and winding hot spot temperatures. Full use is made of the transformer overload capability by arranging the thermal time constant of the equipment to be similar to that of the transformer.
       Two winding temperature instruments are generally fitted to each transformer ; each instrument is fitted with two mercury switch contacts. Operation of one instrument is arranged to start cooling fans and pumps, and to give an alarm. The other instrument is arranged to give the same alarm and to trip the l.v. circuit breaker.
       Typical setting employed are :

                                 Instrument 1                                                           Instrument 2 
Coolers                   In :    90oC                                    Alarm                       110o C
                               Out :  65oC
Alarm                          110oC                                        Trip                         125oC

       At one time, transformers were fitted with separate oil and winding temperature instruments. The former measured the oil temperature temperature only which did not give a reliable indication of hot spot temperature because of the long thermal time constant of the oil (approximately 10 times that of the winding). In the case of transformers without forced cooling, an oil temperature alarm only is sometimes fitted.

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