Effect of Rooftop-PV on power transformer insulation and on-load tap changer operation

The uptake of Rooftop PV has been extremely rapid in Australia. An advantage is that it can reduce the load supplied by upstream power transformers. This in theory should make the transformer run cooler, and therefore extend the remaining life of its paper insulation. However, power transformers often fail on problems other than aged paper insulation, such as the on-load tap changer. This can be a problem when rooftop PV is installed because it changes the load profile throughout the day. In this article we present the findings of an analysis on the sub-transmission network of Townsville, a city of 200,000 people in northern Queensland. The load profile of eleven power transformers in six substations was investigated, with IEC thermal models applied to estimate hot-spot temperature. The relative effect of PV on the weighted hot-spot temperature was investigated, and then a determination was made on the likely loss of life. The effect of weather on the on-load tap changer was also investigated with the findings reported.