Motor drive ground fault detection

Abstract

A motor drive ground fault detection device operates by computing the total RMS voltage of all the phases, and comparing the total RMS voltage to a threshold to determine if a ground fault has occurred.

1 . A method for detecting a ground fault in an AC electrical system comprising:
 computing a combined Root Mean Square (RMS) voltage of all phases of an electrical system;   comparing the combined RMS voltage to a threshold; and   performing a specified action when the combined RMS voltage exceeds the threshold.   
   
   
       2 . The method of  claim 1  wherein said computing a combined RMS voltage additionally comprises:
 computing a phase RMS voltage for each of the at least one phases; and   summing each of the phase voltages, thereby computing a combined RMS voltage.   
   
   
       3 . The method of  claim 2  comprising the additional step of processing a raw AC voltage input prior to the step of computing a combined RMS voltage. 
   
   
       4 . The method of  claim 2  wherein the step of computing a combined RMS voltage further comprises filtering the RMS voltage of each of the at least one phases. 
   
   
       5 . The method of  claim 4  wherein the step of computing a combined RMS voltage further comprises filtering a combined RMS voltage value prior to the step of comparing the combined RMS voltage value to a threshold. 
   
   
       6 . The method of  claim 2  comprising the additional step of processing a raw AC voltage input prior to the step of computing a combined RMS voltage, and
 wherein the step of computing a combined RMS voltage further comprises filtering the RMS voltage of each of the at least one phases.   
   
   
       7 . The method of  claim 6  wherein the step of computing a combined RMS voltage further comprises filtering a combined RMS voltage value prior to the step of comparing the combined RMS voltage value to a threshold. 
   
   
       8 . The method of  claim 1  wherein said performing a specified action when the combined RMS voltage exceeds the threshold comprises transmitting a ground fault detected signal to a controller. 
   
   
       9 . The method of  claim 8  further comprising said controller disconnecting a generator from the electrical system when a ground fault detected signal is received. 
   
   
       10 . A computer readable medium containing instructions for performing the steps of
 computing a combined RMS voltage of all phases of an electrical system;   comparing the combined RMS voltage to a threshold; and   performing a specified action when the combined RMS voltage exceeds the threshold.   
   
   
       11 . An electrical system comprising;
 a ground fault detection device for detecting phase to ground fault conditions, said ground fault detection device comprising a voltage sensor connected to each phase of an electrical system;   a voltage summer capable of summing a Root Mean Square (RMS) voltage of all phases of the electrical system;   a voltage comparator for comparing a summed RMS voltage against a threshold; and   a ground fault detected signal transmitter for transmitting a ground fault detected signal when the summed RMS voltage exceeds the threshold.   
   
   
       12 . The device of  claim 11  wherein each of said voltage sensors are capable of computing a phase RMS voltage based at least in part on the sensed voltage. 
   
   
       13 . The device of  claim 11  wherein said voltage summer additionally comprises at least a first filter for filtering sensed voltage values. 
   
   
       14 . The device of  claim 13  wherein said voltage summer additionally comprises a second filter for filtering at least partially processed voltage values. 
   
   
       15 . The device of  claim 11 , additionally comprising a switch operable to disconnect a generator from an electrical system in response to said ground fault detected signal.