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

Claim 1

In a watthour meter installation of the type including an input transformer having an input winding for connection to an alternating current power supply line, an output winding connected to the watthour meter for measuring alternating current power being consumed and a core on which said input and output windings are wound and which is susceptible to magnetization saturation by a DC component superimposed on the alternating current power being measured; the improvement comprising compensation circuit means for compensating for the effect of the DC component on the power measurement achieved with the watthour meter, said compensation circuit means comprising: (a) sensing means coupled to sense the current flowing through said input winding and for deriving a sensed output signal proportional to the sensed current and which includes the AC and DC components of the current flowing through said input winding; (b) sensed signal processing circuit means coupled to said sensing means for deriving from said sensed output signal a feedback signal which is proportional to the DC component of the current flowing through said input winding, said sensed signal processing circuit means comprising; (i) current to voltage signal conversion circuit means supplied with said sensed signal for converting the same to a voltage signal, (ii) zero crossing sensing circuit means supplied with the voltage signal for sensing the zero crossings of the voltage signal in either a positive going or negative going direction relative to a reference voltage level and for deriving aperiodic zero crossing signals indicative of the occurrence of the zero crossings; (iii) amplitude referencing circuit means responsive to said aperiodic zero crossing output signals for deriving a series of essentially constant amplitude varying duration positive and negative going square waveshape signals representative of the distortion of the waveshape of the sensed signal due to the superimposed DC component, (iv) averaging circuit means responsive to said amplitude referenced varying period positive and negative going square wave signals for deriving an average output voltage signal representative of the superimposed DC component, and (v) voltage to current signal conversion circuit means responsive to the average output voltage signal for converting the same to a current feedback signal; and (c) compensating winding means wound on said transformer core and having said current feedback signal supplied thereto in a direction to automatically cancel out the effects of the DC component on the magnetization characteristics of said input transformer.

Claim 8

The method of automatically compensating for the effect of a DC component superimposed on a supply alternating current being measured by a watthour meter installation comprising; (a) sensing the supply alternating current having the DC component superimposed thereon and deriving a sensed signal output representative of both the supply AC and superimposed DC component current values, (b) converting the sensed signal to a sensed voltage signal, (c) sensing the zero crossings of the sensed voltage signal in either a positive going or negative going direction relative to a reference voltage level and deriving aperiodic zero crossing signals indicative of the zero crossings, (d) amplitude referencing the zero crossing signals to derive a series of substantially constant amplitude varying duration positive and negative going square waveshape signals representative of distortion in the waveshape of the sensed signal due to the superimposed DC component, (e) averaging the positive and negative square waveshape signals of varying duration to derive an average output voltage signal representative of the superimposed DC component, (f) converting the average output voltage signal to a current feedback signal, and (g) feeding back the current feedback signal ahead of the watthour meter installation in a direction to null out the effect of the DC component superimposed on the supply alternating current being measured.

Claim 9

The method of automatically compensating for the effect of a DC component superimposed on a supply alternating current being measured by a watthour meter installation comprising; (a) sensing the supply alternating current having the DC component superimposed thereon and deriving a sensed signal output representative of both the supply AC and superimposed DC component current values, (b) converting the sensed signal to a sensed voltage signal, (c) sensing the zero crossings of the sensed voltage signal in either a positive going or negative going direction relative to a reference voltage level and deriving aperiodic zero crossing signals indicative of the zero crossings, (d) amplitude referencing the zero crossing signals to derive a series of substantially constant amplitude varying duration positive and negative going square waveshape signals representative of distortion in the waveshape of the sensed signal due to the superimposed DC component, (e) averaging the positive and negative square waveshape signals of varying duration to derive an average output voltage signal representative of the superimposed DC component, (f) converting the average output voltage signal to a current feedback signal, and (g) feeding back the current feedback signal ahead of the watthour meter installation in a direction to null out the effect of the DC component superimposed on the supply alternating current being measured.