Watthour meter with isolation transformers having a feedback loop

Abstract

A voltage transducer in a watthour meter, for sensing AC line voltage across a load, includes a current transformer having its primary winding connected in series with a current-limiting resistor across the AC source. The transformer includes a secondary coil and a tertiary coil. The current through the secondary coil represents the sensed voltage and is signal conditioned through an integrator circuit for input to a solid-state watt transducer chip. An active circuit is connected across the tertiary coil, with an output feeding a compensating current into the input of the secondary coil. The active circuit includes an RC network at the output of the tertiary coil for providing a loop stability. A first non-inverting op amp amplifies the voltage output from the RC network to provide a phase correction, adjustable by a replaceable resistor connected between the inverting node of the first op amp and an electrical ground. An intergrating op amp is connected between the output of the first op amp and the input to the tertiary coil to form a feedback circuit to reduce DC offset signals from the output of the first op amp. The primary coil of the current transformer is composed of an electric conductor wire coated with an electrically insulating layer to withstand impulse voltages of at least 6 kV.