The temperature profile of a transformer is an important characteristic in establishing its satisfactory performance. The information it provides can be used in a variety of different ways. These include:- ·Condition based maintenance decisions. · Making decision on overload operation. · Insulation ageing and residual life assessment. · In conjunction with oil quality and gas in oil analysis. In order for temperature profiles to be meaningful, the thermal model from which they are derived needs to accurately reflect operating conditions. The basic formulas for calculating temperature rise contained in the various international transformer loading guides are adequate for this purpose provided operation is at or near full load and that steady-state conditions have been reached. In many practical situations, however, loading patterns do not stay constant long enough for steady-state analysis to apply. A more sophisticated mathematical model is therefore needed in order to cope with the dynamics of transient load conditions. This has particular implications for a generating company who is providing say frequency keeping or who operates predominantly in a peaking type role where frequent load changes can occur. The thermal model developed in this paper, models the heat transfer processes in a transformer as a simple electrical RC circuit. In this form it can be more readily modified to accommodate changes in temperature dependant parameters. The electrical analogy also helps to provide a physical meaning to the various elements of the model.
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