A thermal analogue of higher accuracy and factory test method for predicting and supporting thyristor fault suppression ratings

Because sophisticated computer modeling is not easily accessible, a one-dimensional thermal analogue is often used for routine calculations of virtual junction temperature occurring in thyristors when subjected to fault currents. The limitations of this procedure were found while establishing and improving fault suppression ratings of high-voltage (5 kV) thyristors. The thermal model was upgraded to include a number of heat sources within the silicon and some degree of two-dimensional heat spreading, while retaining the relative simplicity of a one-dimensional model. This upgraded model distinguishes the thermal impedance of the cathode junction from the thermal impedance of the anode junction. The supporting bench test was set up to use the calibrated off-state V/sub B0/ characteristic rather than the calibrated on-state drop of the signal current. It was found to be a more discriminating thermometer, responding to any existing hot spot temperature caused by surge current levels. Using certified thyristors, improved fault suppression ratings were established and subsequently proved under real power system conditions.<>