Application of interactive multiple models in thermal modeling of power transformers
SUMMARY Power transformers are one of the most expensive equipments in power systems. The efficiency and optimal utilization of these equipments are very important issues. Some factors, like temperature, moisture and oxidation can affect the solid and liquid insulations and reduce the useful life of power transformer. Temperature of a power transformer depends on the ambient temperature, present and past loading and cooling classes. Proper thermal model is essential to predict temperature in different conditions of cooling system. Such model leads to analyze useful life and improve loading and utilization of a power transformer. In this contribution a thermal model for power transformers based on the new technique of Interactive Multiple Modeling (IMM) is suggested to predict the top oil temperature. Practical validation is carried out with measured load and temperature data of a power transformer. Consequently the prediction error for the conventional thermal model and the new suggested model based on IMM technique are compared. Copyright 2011 John Wiley Sons, Ltd. Temperature plays a key role in insulation deterioration of power transformers. Thermal control during service life is very important for optimum loading of these equipments. Critical points for thermal stress in transformers are hottest spots which can predict loss of life of insulation systems. Precise temperature estimation of the hottest spot is significant for evaluating and monitoring thermal stresses of insulation system. Aging wave of the electrical infrastructures and increasing failure rate of the electrical systems have caused an increasing tendency for utilities to redesign their maintenance approaches. They are changing maintenance strategy from fault and time based maintenances toward the condition based maintenance applying on line monitoring and diagnostic systems. Temperature of the hot spot and top oil in transformer have direct relation with the ambient temperature as well as the transformer loading condition. Nowadays direct measurement of the hot spot temperature is possible using the new optical fiber sensors. But usage of these sensors has not still become popular in manufacturing process and is not applicable for old transformers. Therefore, measurement of winding temperature for thermal stress control and optimum loading of transformer are performed indirectly using top oil temperature sensor[1] This sensor is installed adjacent to a thermal element (heater) and its temperature is proportional to the transformer current as depicted in Figure 1. Described method for temperature measurement is not precise enough in the new generation of monitoring and diagnostic systems and novel methods based on precise thermal modeling are necessary. Thermal aging is dependent on the operating temperature and a precise thermal model is