Novel techniques and procedures for the assessment of fault current withstand of power thyristors

In applications using high power thyristors, the designer has to make sure that the selected thyristor will withstand stresses caused by overloads and fault currents. If the surge current characteristics found in the thyristor data sheet do not provide sufficient information, he has to find the transient excursions in junction temperature that will be caused by the worst expected fault current and then make a judgment on whether or not they can be tolerated. The standard way of predicting changes in junction temperature due to a known current waveshape is to determine the corresponding power loss using the on-state conduction characteristic and then to find the time trace of the junction temperature using the curve of transient thermal impedance. The main problem facing the designer is that the information found in contemporary data sheets is often neither sufficient for a meaningful calculation, nor for deciding whether or not the calculated temperature excursions can be tolerated. This paper deals with three subjects. It shows the application engineer how to use off-the-shelf computer software for more accurate and much easier prediction of junction temperature excursions. It advises what to do with the results. And finally, it points to the missing pieces of information which should be made available in all future data sheets. The proposed method for calculation of temperature excursions in high power thyristors is also applicable to other electrical apparatus such as ZnO arresters, transformers, electric machines etc