Transformer field drying improvement by applying low-frequency-heating

Water is damaging for transformer insulation, and its presence increases the probability of unexpected failures. Field drying is currently being employed by utilities to reduce the transformer water content when it reaches an excessive value. One of the key aspects to achieve an adequate drying level, is the temperature attained by the solid insulation during the process. Low-frequency heating (LFH) consists of heating the transformer active part using the heat generated in the windings when they are fed with current at low frequency. This method is becoming popular and some companies are using trying to improve the efficiency of the drying process. In this paper, the improvement in drying obtained when LFH is applied is analyzed. Simulations have been conducted using theoretical models to predict the real response of the transformer when LFH is applied. Experimental tests have been also performed in the laboratory to quantify the improvement achieved when LFH is applied. Finally an experimental study was carried out on a real transformer dried under controlled conditions with and without LFH to compare the effectiveness of both processes.