Novel techniques and procedures for the assessment of fault current withstand capability 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 find the time trace of the junction temperature using the curve of transient thermal impedance. The calculation procedure based on a superposition method has been in use for some 40 years. An improvement based on current state-of-the-art computer software is overdue. However, 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. First, it shows the application engineer how to use off-the-shelf computer software for more accurate and much easier prediction of junction temperature excursions. Second, it advises what to do with the results. Finally, it points to the pieces of information which are needed in the process and which should therefore be found in data sheets of all high-power thyristors. 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.