Dynamic Thermal Model for Oil Directed Air Forced Power Transformers With Cooling Stage Representation

Power transformers are critical assets for generation and grid. Controlling their aging is an essential task to ensure the availability of installations, and to optimize maintenance and operational costs over their lifetime. Large power transformers do not always operate at their full cooling capacity, and the cooling efficiency is often adjusted by steps (each different step being also called a “cooling stage”) based on the top-oil, winding temperature indicator or hot-spot temperature. Various dynamic thermal models exist in the literature, but none of them provide easy-to-use correction factors to consider the effect of the cooling stage on the coolers’ thermal resistance as well as on top-oil and hot-spot gradients. The purpose of this article is to propose a simple yet accurate model for the thermal monitoring of Oil Directed Air Forced power transformers. Several applications of the model are given such as cooling efficiency monitoring, evaluation of the load capability (or loadability), and coolers temperature control improvement.