Load Capacity Evaluation of Power Transformer via Temperature Rise Characteristics

Power transformers are key components in the power grid, so it is of great importance to protect them from an unplanned outage. With the increase of social electricity consumption, the overload accidents of transformer have become increasingly frequent. In order to avoid the overload accidents, we investigate the loading capacity of transformer based on temperature rise characteristics. Firstly, considering the influence of sunshine radiation, we establish the improved thermal model. Because the DC resistance of winding and oil viscosity change with oil temperature, the algorithms of load loss and thermal resistance are modified to improve the calculation accuracy of hot-spot temperature (HST). Compared with conventional IEC standard method, the relative error of the improved model of hotspot temperature is reduced by about 2.5%. Further, the constraints of loading capacity for transformer are studied. Taken temperature rise characteristics with load current and the loading constraint factors of transformer into account, the evaluation model for loading capacity of transformer is proposed. The results show that the main constraint factor is the relative life loss of transformer under the normal periodic load; the main constraint factor is the capacity of auxiliary equipment of transformer under the long-term emergency load. The capacity of short-term emergency load is not only limited by the top-oil temperature and hot-spot temperature, but also affected by the initial load rate of transformer and the ambient temperature.