Method of and apparatus for learning the phase error or timing delays within a current transducer and power measurement apparatus including current transducer error correction
Current transducers are widely used in current measuring systems. They provide good isolation between the supply voltage and the measurement equipment. However they can introduce small phase errors which can become significant sources of error if the current to a load is out of phase with the supply voltage for the load. This disclosure discusses a robust measurement apparatus and method that can be used in situ to monitor for and correct phase errors.
1. A method of improving power measurement accuracy based on an estimated phase measurement error resulting from a current transducer, the method comprising:
providing, to an input of a current transducer, an input signal having a first delay resulting from input signal generator circuitry;
receiving, from the current transducer, an output signal having a second delay resulting from the current transducer,
analyzing the output signal to determine a phase difference compared to the input signal based on the first and second delays resulting from the input signal generator circuitry and the current transducer, respectively; and
estimating a phase measurement error, resulting at least in part from the current transducer, from the phase difference determined based on the first and second delays.
2. A method as claimed in claim 1 , wherein the input signal generator circuitry is configured to generate a repeating signal as the input signal.
3. A method as claimed in claim 1 , wherein the signal generator circuitry is configured to generate a stepwise approximation to a continuous signal as the input signal.
4. A method as claimed in claim 1 , wherein the input signal generator circuitry is configured to generate at least one of a sinusoid, a triangle wave, a square wave with smoothed transitions, and a bandwidth limited noise source as the input signal.
5. A method as claimed in claim 1 , wherein the input signal generator circuitry is configured to generate a square wave like signal as the input signal, and wherein the first delay results from edges of the square wave like signal transitioning between high and low values.
6. A method as claimed in claim 2 , in which the current transducer comprises a current transformer, and the repeating signal approximates a slew rate limited square wave or a charge rate limited square wave.
7. A method as claimed in claim 6 , in which the slew rate or charge rate limited square wave transitions between first and second values, and where the first delay corresponds to a time to reach a midpoint between a transition from the first value to the second value.
8. A method as claimed in claim 7 , further comprising determining the midpoint by starting a counter or timer at the beginning of the transition from the first value to the second value and stopping the counter or timer at the end of the transition.
9. A method as claimed in claim 7 , further comprising adding a second phase correction to account for non-linearly of the rate of transition between the first and second values.
10. A method as claimed in claim 9 , in which the second correction is estimated or measured at manufacture and stored in memory.
11. A method as claimed in claim 1 , in which the transducer is a current transformer and the phase difference after applying a correction signal represents a phase shift resulting from the current transformer.
12. A method of estimating power consumption comprising measuring the potential at a first conductor, measuring the current flowing in the first conductor, applying a phase correction to the measurement of current using the method of claim 1 , and multiplying the potential and current measurements to estimate power.
13. A method as claimed in claim 12 , in which the correction to the measured current comprises estimating and applying a time shift to be applied to sampled values of the current compared to sampled values of voltage.
14. A method as claimed in claim 6 in which the slew rate or charge rate limited square wave has a substantially equal mark-space ratio.
15. A method as claimed in claim 1 in which the phase difference is estimated using a FFT or a Geortzel algorithm, or a phase detector circuit.
16. A method as claimed in claim 1 in which the input signal has a predetermined slew rate or transition time and is formed by a digital to analog converter and a correction required to account for the finite rate of change of the input signal is known because the slew rate or transition time of the input signal is predetermined.
17. The method of claim 1 further comprising computing power by applying a correction to the estimate of phase measurement error to compensate for a finite rate of change of the input signal based on the determined phase difference including compensation for the first and second delays.
18. The method of claim 1 further comprising storing or outputting the phase difference for monitoring performance of a power meter.
19. The method of claim 1 , wherein the phase difference is further determined based on a third delay resulting from at least one of a current sensor, a filter, or an analog to digital converter.
20. The method of claim 1 function comprising converting, using an analog to digital converter, the determined phase difference from analog to digital form for further processing.
21. An apparatus of estimating phase shifts in measurements of current, the apparatus comprising:
a current transducer for receiving, from input signal generator circuitry, an input signal having a first delay resulting from the input signal generator circuitry; and
a phase or time shift comparator for:
receiving, from the current transducer, an output signal having a second delay resulting from the current transducer; and
analyzing the output signal to deter mine a phase or time difference compared to the input signal based on the first and second delays resulting from the input signal generator circuitry and the current transducer, respectively,
wherein the phase or time shift comparator is arranged to estimate the phase shift based on the phase difference determined based on the first and second delays.
22. An apparatus as claimed in claim 21 , in which the input signal is an approximation to a square wave but having ramp like transitions, the input signal is formed by a digital to analog converter such that the duration of the ramp like transitions are known or predetermined, and a correction value to account for the duration of the ramp like transition is known or predetermined.
23. A power meter including an apparatus as claimed in claim 21 .
24. A power meter as claimed in claim 23 further including an interface for sending data back to a network operator, where the data includes estimates of performance of the power meter and/or information about load and voltage conditions at the power meter.
25. A power meter as claimed in claim 23 further arranged to account for harmonic signals in the current when calculating the power drawn.
26. A power meter as claimed in claim 23 in which the signal generator circuitry generates a repeating signal having the fundamental frequency and the frequency of the signal generator circuitry is adjustable.
27. An apparatus as claimed in claim 21 further comprising a circuit for measuring the transition time in the input signal corresponding to the first delay.
28. An apparatus as claimed in claim 21 , in which the input signal generator circuitry is a square wave generator.
29. An apparatus as claimed in claim 28 further comprising a measurement circuit configured to estimate the first delay based on a midpoint of a square wave transition and provide a timing signal to the phase or time shift comparator as the first delay.
30. The apparatus of claim 21 further comprising an analog to digital converter configured to convert the determined phase difference from analog to digital form for further processing.
31. The apparatus of claim 21 , wherein the phase difference is further determined based on a third delay resulting from at least one of a current sensor, a filter, or an analog to digital converter.
32. The apparatus of claim 21 further comprising circuitry for applying a phase correction to compensate for a finite rate of change of the input signal based on the determined phase or time difference including compensation for the first and second delays to enable power computation.
33. The apparatus of claim 21 further comprising circuitry for storing or outputting the phase difference for monitoring performance of a power meter.
34. The apparatus of claim 21 further comprising a counter or timer circuit, wherein the input signal comprises a periodic signal having a slew rate or charge rate that transitions between first and second values, and where the first delay is computed by the counter or timer circuit based on a time to reach a midpoint between a transition from the first value to the second value, wherein the midpoint is determined by starting the counter or timer circuit at the beginning of the transition from the first value to the second value and stopping the counter or timer circuit at the end of the transition.
35. A method of estimating a phase measurement error in measurement of current, the method comprising:
providing a repeating input signal to an input of a current transformer, where the repeating signal has nominally linear rising and falling edges, and wherein the input signal has a first delay resulting from input signal generator circuitry;
receiving, from the current transducer, an output signal having a second delay resulting from the current transducer;
analyzing the output signal to determine a phase difference compared to the input signal based on the first and second delays resulting from the input signal generator circuitry and the current transducer, respectively; and
estimating a phase measurement error from the phase difference determined based on the first and second delays.
36. The method of claim 35 further comprising computing power by applying a correction to the estimate of phase measurement error to compensate for a finite rate of change of the edges of the input signal, and the fundamental frequency of the input signal is adjustable to enable estimation of phase measurement error at one or more frequencies based on the determined phase difference including compensation for the first and second delays.