Inrush current restraint circuit for alternating power system switch

Abstract

An inrush current restraint circuit, for use with a typical overcurrent minimum pickup circuit of a power system switching device, such as a recloser or sectionalizer, for raising the initial overcurrent minimum pickup value upon closure of the switching device to a selected value, then linearly returning this pickup value to normal within a selected period of time. The minimum pickup circuit is actuated by the voltage drop across a current sensing resistor, through which a current signal proportional to current in the power system is passed. The initial pickup overcurrent value is increased by providing another current path in parallel with the current sensing resistor when the switching device is closed. A timing capacitor is discharged each time the switching device is open, then charged from a constant current source each time the switching device is closed. The voltage across this timing capacitor is applied to the input of an amplifier having a unity voltage gain and the output of this amplifier is connected in series with a second resistor across the current sensing resistor. When the timing capacitor is charged to a voltage higher than the voltage produced across the current sensing resistor, the amplifier is back biased to effectively open the additional current path through the second resistor, and return the minimum pickup current to its normal value.