Thermal model and temperature control of a 30 MVA distribution transformer applied in low frequency drying process

In drying process of distribution transformers with low frequency heating (LFH) method, the heating currents are injected in windings while active parts are kept in vacuum. At this environment, the heat flow between winding layers is carried out by radiation, which is a poorer mean of heat transfer, compared with conduction and convection. Therefore there is a risk of non-uniform temperature distribution between the winding layers. In this paper a thermal model is developed for a 30 MVA distribution transformer and the effect of inject current levels in temperature distribution is investigated by this model. Because of non-linearity and high order of the plant, it is too complicate to design the proper controller. The model is simplified by RLS method, and a control system is proposed for closed loop control of the drying process. The effectiveness of the proposed method is investigated by simulation.