Loading Capability of Transformer in Over-excitation Condition

Hot spot temperature is the main limiting factor of transformer loading. On the other hand, supposing the frequency being constant, voltage increase will result in transformer over-excitation. When the transformer is over-excited, the no load loss and subsequently the hot spot temperature will increase. Thus, the transformer loading will be restricted. In this paper, the problem of loading capability determination considering over-excited operation of the transformer is addressed. Necessary improvements are proposed in order to adapt the IEEE thermal model to this condition. Then, over-excitation effect on transformer loadability is illustrated using improved model. Simulation results show that ignoring the over-excitation effects may lead to transformer insulation lifetime reduction up to 20%.