Simplified Estimation of Critical Quantities for Short-Line Fault Interruption

From analyses of equations derived in a previous paper for critical interrupting conditions for an arc shunted by both resistance and capacitance in parallel, or for short-line fault interruption with capacitance shunting, further analytical expressions useful for prediction of limiting conditions are derived. Examples, based on observed relations of Cassie-Mayr arc model parameters to current rate of change for a particular SF6 blast circuit breaker, show realistic plots of limiting short-line fault currents versus line length with capacitance shunting. Also shown are required capacitance shunt values as functions of busfault current, fault fraction, line length, and number of series breaks. Included in the equations are circuit voltage and frequency and line surge impedance. Finally, it is shown that similar relations can be derived by using an approach suggested by A. M. Cassie in 1939.