Determination of Hot-Spot Temperature for ONAN Distribution Transformers with Dynamic Thermal Modelling

High temperature rise problem, one of the most serious problems for transformers. Loading cycles and ambient conditions of transformers can be different than design conditions. Transformer top-oil temperature and winding hot spot temperatures are most critical parameters for transformer because it causes aging and therefore affects life time of transformer. It is very important to determine hot-spot temperature and oil temperature accurately which depends on ambient conditions and loading conditions to avoid loss of life on transformer. IEEE C57.91-2011 and IEC 60076-7 standards present thermal models for defining critical temperatures in transformers. These models are widely used in industry. On the other hand these methods are solved with exponential equations and they do not take into account the effects of change on oil temperature. Dynamic thermal model which is proposed by D. Susa, requires similar input parameters with IEEE C57.91-2011 model and IEC 60076-7 model, in addition to this, this thermal model takes into account the effects of change on oil temperature. Scope of this study will be to verify three different temperature rise models for distribution transformers to allow optimum loading of natural oil cooled transformers by using the data which could be obtained from manufacturer. In this study, on a 1000 kVA 33/0.4 kV ONAN cooled distribution transformer, two temperature rise tests will be realized. Significant advantage of this study will be, necessary parameters for all thermal models will be determined with proper measurements of tested unit, therefore all three models will be compared under same conditions.