Solid state circuit protection system and method

Claim 1

A control for monitoring current flow in a circuit and selectively actuating protective apparatus therefor, comprising: sensing means for sensing current flow in the circuit and outputting sample signals representative thereof at periodic intervals; first comparison means for comparing the value of successive sample signals to a value representative of previously-sensed current level; first and second registers; means for incrementing said first register in response to a sample signal whose value differs from said previously-sensed current level by less than a predetermined proportion thereof; means for incrementing said second register in response to a sample signal whose value differs from said previously-sensed current level by more than said predetermined proportion thereof; second comparison means for comparing the increments accumulated by said registers and outputting a first current threshold signal I t whose value is at least in part a function of the ratio of the increments accumulated by said first and second registers; arithmetic means for algebraically adding said current threshold signals to individual sample signals to provide net current signals; accumulator means for algebraically accumulating said net current signals; and trip signal means for producing a trip signal when the value of the accumulated signals attains a TRIP threshold value.

Claim 7

The method of detecting an overcurrent condition in a polyphase AC system comprising the steps of deriving an indication of composite current flow in all phases; producing a digital signal representative of composite current flow at regular intervals at a frequency f s which is a non-harmonic frequency of the current being monitored; comparing the value of said digital signals to the value of previous ones of said digital signals; incrementing a first register if the value of said signals depart from the value of said previous signals by a predetermined proportion; generating a current threshold component signal when the contents of the first register represents a predetermined proportion of said digital signals; subtracting a current threshold value including said current threshold signal from succeeding ones of said digital signals and incrementing an accumulator register in accordance with the difference; and outputting a trip signal when the contents of said accumulator register attains a TRIP threshold value.

Claim 8

The method of detecting an overcurrent condition in a polyphase AC system comprising the steps of deriving an indication of composite current flow in all phases; producing a digital signal representative of composite current flow at regular intervals at a frequency f s which is a non-harmonic frequency of the current being monitored; comparing the value of said digital signals to the value of previous ones of said digital signals; incrementing a first register if the value of said signals depart from the value of said previous signals by a predetermined proportion; generating a current threshold component signal when the contents of the first register represents a predetermined proportion of said digital signals; subtracting a current threshold value including said current threshold signal from succeeding ones of said digital signals and incrementing an accumulator register in accordance with the difference; and outputting a trip signal when the contents of said accumulator register attains a TRIP threshold value.