Monitoring the health of an equipment is invariably practised by all users and the utilities. The more is the value of the asset, the more involved is the monitoring process. Composite monitoring of a transformer installation includes the monitoring of the various components such as the transformer, the On Load Tap Changer [OLTC] and the bushings. More often, the interpretation of the results obtained in a composite monitoring technique needs greater caution and a detailed understanding of the component behaviour. This paper discusses the pitfalls in the measurement of transformer winding resistance combined with the OLTC and suggests remedial measures to account for and neutralise the possible error. The film formation on the OLTC Contact surface due to the presence of hot transformer oil, can often cause temporary change in the winding resistance recorded. Adopting a relatively higher dc current for the winding resistance measurement & a few dummy mechanical operations of the OLTC might eliminate the aberration of inaccurate resistance caused by the film layer on the OLTC contact surface. A case study is discussed to bring-in the relevance and importance of the issue. It is hoped that the observations recorded in the paper provide required guidelines to the practising engineers, the asset management team & the diagnostic engineers.
