Zero crossing detection and electronic compensation of D. C. saturation magnetization in current transformers used in watthour meter installations

摘要

A watthour meter installation of the type including an input current transformer having an input winding for connection to an AC power supply line, an output winding for connection to a watthour meter for measuring AC power being consumed and a core on which the input and output windings are wound and which is susceptible to DC magnetization by a DC component superimposed on the AC being measured. In order to overcome the effect of the DC component on the magnetization characteristics of the input transformer, automatic compensation circuitry is provided which includes a sensor coupled to sense the current flowing through the input winding and for deriving a sensed output signal proportional to the sensed current which includes both AC and DC components of the input current. A sensed signal processing circuit is provided in the form of a zero crossing detection circuit together with an amplitude referencing circuit and averaging circuit for deriving from the sensed voltage signal a feedback signal which is proportional only to the superimposed DC component flowing in the input winding. This feedback signal is converted to a current feedback signal and supplied back through a compensating winding wound on the input transformer core in a direction to automatically cancel out the effects of the superimposed DC component on the saturation characteristics of the core of the input current transformer.