An Investigation of Thermal Performance of Transformers Using Optical Fibre Technology

The hot-spot temperature of the winding is assumed to represent the thermal limitation of the transformer, which often governs the transformer loading. The IEC standard stipulates limiting value of winding hot-spot rise. In the past, it was not feasible to directly measure the hot-spot in the windings due to non-availability of reliable devices. The standards defined a hot-spot factor ‘H’ to be applied as a multiplication factor on the measured winding gradient (difference in average winding temperature and average oil temperature) for deriving the value of winding hot-spot rise to demonstrate the compliance. It is known that towards the upper end of the winding there is usually concentration of eddy current losses, which lead to higher temperature in local area. As a result, the actual local temperature difference between conductor and oil is higher by a factor called hot-spot factor (H). IEC 60076-2 and IEC 60354 stipulate H factors of 1.1 for distribution transformers and 1.3 for medium and large power transformers.

Recognizing that depending on design, there could be considerable variation in H factor in case of large transformers, experimental estimation of the H factor was considered important for realistic determination of the thermal parameters of a transformer.

This paper presents findings of exploratory investigations conducted on a 100 MVA, 220 kV Class auto transformer that involved measurement of hot-spot in the windings under rated load conditions. Besides the use of conventional OTI, WTI and pocket thermometer for temperature measurements, 8 fibre-optic temperature sensors (Lumasense make) were used additionally inside the transformer tank. This included 6 sensors embedded at discreet locations inside the different windings in an effort to record winding hot-spot temperature. Further, in order to study the heating behaviour of a particular winding in the 3 different phases under same loading conditions, one sensor each in all 3 phases of the series winding was used at the pre-estimated hottest spots locations facilitated by a software program.