Current transformer with impedance compensation and associated method

Abstract

An impedance-compensated current transformer comprising a primary winding, a power source electrically coupled through the primary winding; a secondary winding magnetically coupled to the first primary winding, the secondary winding having a secondary winding impedance and a secondary winding resistance; and an impedance-compensation circuit electrically coupled across the secondary winding; wherein the impedance-compensation circuit actively reflects to the secondary winding a virtual impedance which is equal in magnitude and opposite in polarity to the secondary winding impedance.

1 . An impedance-compensated circuit transformer comprising:
 a primary winding;   a power source electronically coupled through said primary winding;   a secondary winding magnetically coupled to said primary winding, said secondary winding having a secondary winding magnetizing inductance and a secondary winding resistance;   an impedance-compensation circuit electrically coupled across said secondary winding and having a single amplifier stage; and   wherein said impedance-compensation circuit actively reflects to said secondary winding a virtual impedance which is equal in magnitude and opposite in polarity to said secondary winding resistance such that the current-driving voltage across said secondary winding magnetizing inductance is driven to zero independently of the value of said secondary winding magnetizing inductance.   
   
   
       2 . The current transformer of  claim 1  wherein said impedance-compensation circuit comprises:
 an operational amplifier having a positive input, a negative input, an output, a positive supply and a negative supply;   a first resistor having a resistance approximately equal to said secondary winding resistance;   a second resistor having a resistance which is much greater than the resistance of said first resistor; and   a third resistor having a resistance approximately equal to the resistance of said second resistor;   wherein said first resistor is electrically coupled across said operational amplifier negative input and said operational amplifier output;   said second resistor is electrically coupled across said operational amplifier positive input and said operational amplifier output; and   said third resistor is electrically coupled across said operational amplifier positive input and ground.   
   
   
       3 . The current transformer of  claim 2  wherein said impedance-compensation circuit further comprises:
 a current boosting circuit electrically coupled in series between said operational amplifier output and said first and second resistors, said current boosting circuit comprising an NPN transistor and a PNP transistor each having a base, a collector and an emitter;   wherein said bases of said NPN and said PNP transistors are electrically coupled to said operational amplifier output;   said emitters of said NPN and said PNP transistors are electrically coupled to said first and second resistors;   said collector of said NPN transistor is electrically coupled to said operational amplifier positive supply; and   said collector of said PNP transistor is electrically coupled to said operational amplifier negative supply.   
   
   
       4 . The current transformer of  claim 3  wherein said power source is an alternating current power source. 
   
   
       5 . The current transformer of  claim 3  wherein said power source is a rectified alternating current power source such that only the positive half or the negative half of the alternating current is driven through said primary winding. 
   
   
       6 . The current transformer of  claim 3  wherein said power source is a combination of an alternating current power source and a rectified alternating current power source. 
   
   
       7 . The current transformer of  claim 1  wherein said impedance-compensation circuit includes means to vary said virtual impedance of said impedance-compensation circuit in direct proportion to variations in the temperature of said secondary winding. 
   
   
       8 . A method for compensating the impedance of a current transformer comprising:
 electrically coupling a power source through a primary winding;   magnetically coupling a secondary winding to said primary winding, said secondary winding having a secondary winding magnetizing inductance and a secondary winding resistance; and   electrically coupling an impedance-compensation circuit across said secondary winding and having a single amplifier stage; and   actively reflecting to said secondary winding by said impedance-compensation circuit a virtual impedance which is equal in magnitude and opposite in polarity to said secondary winding resistance such that the current-driving voltage across said secondary winding magnetizing inductance is driven to zero independently of the value of said secondary winding magnetizing inductance.   
   
   
       9 . The method of  claim 8  further comprising varying said virtual impedance in direct proportion to variations in the temperature of said primary winding and said secondary winding.