Method and apparatus for digitally measuring alternating current

An apparatus for measuring and monitoring an alternating current including:

• means for developing a voltage waveform directly related to the alternating current to be measured and monitored;
• first sensing means for sensing when the voltage waveform crosses a predetermined negative reference voltage;
• memory means for remembering the point in time t.sub.1 when this occurs;
• second sensing means for sensing when the voltage waveform crosses a predetermined positive reference voltage;
• said memory means being operable to remember the point in time t.sub.2 when this occurs;
• comparing means associated with said memory means and operable therewith for comparing t.sub.2 -t.sub.1 with a predetermined .DELTA.T and, when t.sub.2 -t.sub.1 is less than .DELTA.T, said comparing means causes a control signal to be generated for actuating a circuit interrupter in the line carrying the alternating current.

An apparatus for measuring a sinusoidal alternating current of known frequency including:

• means for developing a voltage waveform directly related to the alternating current to be measured;
• first sensing means for sensing when the voltage waveform crosses a predetermined negative reference voltage -R;
• memory means for remenbering the point in time t.sub.1 when this occurs;
• second sensing means for sensing when the voltage waveform crosses a predetermined positive reference voltage +R;
• said memory means being operable to remember the point in time t.sub.2 when this occurs;
• and data processing means associated with said memory means and operable therewith for computing the RMS value of the current by solving the equation:
• ##EQU14##

An apparatus for measuring an alternating waveform comprising first sensing means for sensing when the waveform exceeds a predetermined negative reference value;

• memory means for remembering the point in time t.sub.1 when this occurs;
• second sensing means for sensing when the waveform exceeds a predetermined positive reference value;
• said memory means being operable to remember the point in time t.sub.2 when this occurs;
• said second sensing means being operable to sense when the waveform falls below the predetermined positive reference value and said memory means being operable to remember the point in time t.sub.3 when this occurs;
• said first sensing means being operable to sense when the waveform falls below a predetermined negative reference value and said memory means being operable to remember the point in time t.sub.4 when this occurs;
• said first sensing means then being operable to sense when the waveform again exceeds the negative reference value and said memory means being operable to remember the point in time t.sub.5 when this occurs;
• and data processing means for determining from t.sub.5 -t.sub.1 the frequency of the waveform, and for determining from the negative and positive reference values and the time increments t.sub.2 -t.sub.1, t.sub.3 -t.sub.1, and t.sub.5 -t.sub.1 the size of any DC offset present in the waveform and the amplitude of the fundamental of the waveform.

An apparatus for digitally measuring an alternating current comprising:

• means for transofrming the alternating current waveform to a voltage waveform;
• a first sensing means for sensing when the instantaneous value of the voltage waveform crosses a first reference voltages;
• a second sensing means for sensing when the instantaneous value of the voltage waveform crosses a second reference voltage;
• latch means coupled to the output of said sensing means;
• a counter coupled to the input to said latch means for supplying a count to said latch;
• said latch being operable to latch simultaneously a voltage reference crossing sensed by said sensing means and the count in the counter at the time of the crossing;
• data processing means coupled to the output of said latch means;
• logic circuitry coupled to and between said sensing means, said latch means and said data processing means first for causing, after being actuated by a reference voltage crossing, said latch means to latch a reference voltage crossing and the count at that point in time in said latch means and second for causing said data processing means first to read the values latched in said latch means and second to send a reset signal to said logic circuitry for resetting same for the next reference voltage crossing.

A method for measuring the fundamental and second harmonic of an alternating waveform which has no other harmonics of any sighificance and no DC offset comprising the steps of:

• sensing when the waveform exceeds a predetermined negative reference value and remembering the point in time t.sub.0 when this occurs;
• sensing when the waveform exceeds a predetermined positive reference value and remembering the point in time t.sub.1 when this occurs;
• sensing when the waveform falls below the predetermined positive reference value and remembering the point in time t.sub.2 when this occurs;
• sensing when the waveform falls below a predetermined negative reference value and remembering the point in time t.sub.3 when this occurs;
• determining, such as by means of Cramer's method, from t.sub.0, t.sub.1, t.sub.2 and t.sub.3 the fundamental and second harmonic of the waveform.

An apparatus for measuring the fundamental and second harmonic of an alternating waveform which has no other harmonics of any significance and no DC offset comprising first sensing means for sensing when the waveform exceeds a predetermined negative reference value;

• memory means for remembering the point in time t.sub.0 when this occurs;
• second sensing means for sensing when the waveform exceeds a predetermined positive reference value;
• said memory means being operable to remember the point in time t.sub.1 when this occurs;
• said second sensing means being operable to sense when the waveform falls below the predetermined positive reference value and said memory means being operable to remember the point in time t.sub.2 when this occurs;
• said first sensing means being operable to sense when the waveform falls below a predetermined negative reference value and said memory means being operable to remember the point in time t.sub.3 when this occurs;
• and data processing means for determining, such as by applying Cramer's method, from t.sub.0, t.sub.1, t.sub.2, and t.sub.3, the fundamental and second harmonic of the waveform.