Method and apparatus for measuring electrical parameters using a differentiating current sensor and a digital integrator

A power meter which outputs voltage and current samples at an output sample rate, where a voltage sample indicates a quantity of voltage across an electrical load at a sample time and a current sample indicates a quantity of current flowing through the electrical load at the sample time, the power meter comprising:

• a voltage sensor, electromagnetically coupled to the electrical load, which outputs a voltage sensor signal representing the voltage across the electrical load;
• voltage sampling means, coupled to receive said voltage sensor signal, for digitally sampling said voltage sensor signal and outputting a voltage waveform comprising digital samples at an input sample rate;
• a voltage filtering path, coupled to receive said voltage waveform from said voltage sampling means, which outputs a filtered voltage waveform;
• a current sensor electromagnetically coupled to the electrical load, which outputs a current sensor signal representing a derivative with respect to time of the current flowing through the electrical load;
• a differential adder coupled at a first adder input to receive said current sensor signal, said differential adder outputting a signal at an adder output which is a difference of said current sensor signal and a signal applied to a second adder input of said differential adder;
• current sampling means, coupled to said adder output, for digitally sampling said signal at said adder output at said input sample rate and outputting a time derivative current waveform comprising digital samples at said input sample rate;
• a current feedforward filtering path, coupled to receive said time derivative current waveform, which outputs a filtered time derivative current waveform, wherein said current feedforward filtering path comprises filters with characteristics equal to filters in said voltage filtering path;
• a current feedback signal path, coupled to receive said filtered time derivative current waveform from said current feedforward filtering path, which outputs a feedback signal to said second adder input of said differential adder, where said current feedback signal path comprises filters which cause the cancellation of direct-current (DC) components of said filtered time derivative current waveform;
• a digital integrator, coupled to receive said filtered time derivative current waveform, which integrates said filtered time derivative current waveform and outputs, at the output sample rate, samples indicating an instantaneous value of the current flowing through the electrical load;
• and
• a delay means, clocked by an intermediate clock having a rate of at least twice the output sample rate, for delaying current samples in said current feedforward filtering path by a time equal to one-half a period of the output sample rate, thereby providing a substantially equal delay for samples through said current feedforward filtering path combined with said digital integrator and through said voltage filtering path.