US 20050288884 A1 Abstract A method compensates for phase differences between sampled values of first and second AC waveforms. The method employs a phase angle compensation factor and sequentially samples a plurality of values of each of the waveforms. For a positive compensation factor, second sampled values are adjusted to correspond with first sampled values by employing, for a corresponding second sampled value, a preceding second sampled value plus the product of: (i) the compensation factor and (ii) the difference between the corresponding second sampled value and the preceding second sampled value. Alternatively, for a negative compensation factor, the second sampled values are adjusted by employing, for the corresponding second sampled value, the preceding second sampled value minus the product of: (i) the sum of one plus the compensation factor and (ii) the difference between the preceding second sampled value and the second sampled value preceding the preceding second sampled value.
Claims(22) 1. A method of compensating for phase differences between sampled values of first and second alternating current waveforms, said method comprising:
employing a phase angle compensation factor; sequentially sampling a plurality of values of each of said first and second alternating current waveforms; and adjusting the sampled values of said second alternating current waveform to correspond with the sampled values of said first alternating current waveform by employing, for a corresponding one of said sampled values of said second alternating current waveform, a preceding sampled value of said second alternating current waveform plus the product of: (i) said phase angle compensation factor and (ii) the difference between said corresponding one of said sampled values and said preceding sampled value, when said phase angle compensation factor is positive, or alternatively adjusting the sampled values of said second alternating current waveform to correspond with the sampled values of said first alternating current waveform by employing, for said corresponding one of said sampled values, said preceding sampled value minus the product of: (i) the sum of one plus said phase angle compensation factor and (ii) the difference between said preceding sampled value and the sampled value of said second alternating current waveform preceding said preceding sampled value, when said phase angle compensation factor is negative. 2. The method of employing a meter; receiving said first and second alternating current waveforms at said meter; applying said phase angle compensation factor at said meter; and employing a circuit internal to said meter to calibrate said phase angle compensation factor. 3. The method of employing a meter; receiving said first and second alternating current waveforms at said meter; applying said phase angle compensation factor at said meter; and employing a circuit external to said meter to calibrate said phase angle compensation factor. 4. The method of employing as said first alternating current waveform a voltage alternating current waveform; and employing as said second alternating current waveform a current alternating current waveform. 5. The method of sequentially sampling the values of each of said first and second alternating current waveforms at a rate of about 512 samples per alternating current cycle; and employing said phase angle compensation factor, which has an absolute value that is smaller than one. 6. The method of acquiring a plurality of samples of said first and second alternating current waveforms before applying compensation to the sampled values of said second alternating current waveform. 7. The method of applying a direct current offset to the samples of said first and second alternating current waveforms before applying compensation to the sampled values of said second alternating current waveform. 8. The method of acquiring a plurality of sets of voltage samples and current samples as sampled values of each of said first and second alternating current waveforms; determining a plurality of zero crossings in said voltage samples; calculating a plurality of zero crossing sample times for said voltage samples; determining a plurality of zero crossings in said current samples; calculating a plurality of zero crossing sample times for said current samples; calculating a plurality of differences between the zero crossing sample times for said voltage samples and the zero crossing sample times for said current samples; and averaging said differences to provide said phase angle compensation factor. 9. The method of applying a direct current offset to said voltage samples and said current samples before determining the zero crossings in said voltage samples and before determining the zero crossings in said current samples. 10. The method of incrementing and storing a count for each of said sets of voltage samples and current samples; calculating the zero crossing sample times for said voltage samples by employing, for a corresponding one of said zero crossing sample times and a corresponding one of said voltage samples, the stored count of said corresponding one of said voltage samples immediately before a corresponding one of said zero crossings plus the voltage of the voltage sample immediately before said corresponding one of said zero crossings divided by the difference between: (i) said voltage of the voltage sample immediately before said corresponding one of said zero crossings and (ii) the voltage of the voltage sample immediately after said corresponding one of said zero crossings; and calculating the zero crossing sample times for said current samples by employing, for a corresponding one of said zero crossing sample times and a corresponding one of said current samples, the stored count of said corresponding one of said current samples immediately before a corresponding one of said zero crossings plus the current of the current sample immediately before said corresponding one of said zero crossings divided by the difference between: (i) said current of the current sample immediately before said corresponding one of said zero crossings and (ii) the current of the current sample immediately after said corresponding one of said zero crossings. 11. The method of determining the count of one of said voltage zero crossings and said current zero crossings; determining a plurality of differences between each of said zero crossing sample times for said voltage samples and corresponding ones of said zero crossing sample times for said current samples; summing said differences between each of said zero crossing sample times for said voltage samples and corresponding ones of said zero crossing sample times for said current samples; and dividing the sum of said differences by said count of one of said voltage zero crossings and said current zero crossings to determine said phase angle compensation factor. 12. A meter apparatus comprising:
a first input adapted to receive at least one first alternating current waveform; a second input adapted to receive at least one second alternating current waveform; an analog to digital converter circuit adapted to sequentially sample and convert said received at least one first alternating current waveform to a plurality of first digital values and adapted to sequentially sample and convert said received at least one second alternating current waveform to a plurality of second digital values; a processor adapted to receive and process the first and second digital values from said analog to digital converter circuit, said processor including a compensation routine having a phase angle compensation factor, said compensation routine being adapted to adjust said second digital values to correspond with said first digital values by employing, for a corresponding one of said second digital values, a preceding one of said second digital values plus the product of: (i) said phase angle compensation factor and (ii) the difference between said corresponding one of said second digital values and said preceding one of said second digital values, when said phase angle compensation factor is positive, or said routine being adapted to alternatively adjust said second digital values to correspond with said first digital values by employing, for said corresponding one of said second digital values, said preceding one of said second digital values minus the product of: (i) the sum of one plus said phase angle compensation factor and (ii) the difference between said preceding one of said second digital values and the second digital value preceding said preceding one of said second digital values, when said phase angle compensation factor is negative, in order to compensate for phase differences between said first and second digital values. 13. The meter apparatus of 14. The meter apparatus of 15. The meter apparatus of 16. The meter apparatus of 17. The meter apparatus of 18. The meter apparatus of 19. The meter apparatus of 20. The meter apparatus of 21. A method of compensating for phase differences between sampled values of first and second alternating current waveforms, said method comprising:
employing a phase angle compensation factor; sequentially sampling a plurality of values of each of said first and second alternating current waveforms; and adjusting the sampled values of said second alternating current waveform to correspond with the sampled values of said first alternating current waveform by interpolating between a corresponding one of said sampled values of said second alternating current waveform and a preceding sampled value of said second alternating current waveform, when said phase angle compensation factor is positive, or by interpolating between the preceding sampled value and a sampled value of said second alternating current waveform preceding said preceding sampled value, when said phase angle compensation factor is negative. 22. The method of interpolating between said corresponding one of said sampled values of said second alternating current waveform and said preceding sampled value of said second alternating current waveform by a percentage defined by said positive phase angle compensation factor; and alternatively interpolating between the preceding sampled value and said sampled value of said second alternating current waveform preceding said preceding sampled value by a percentage defined by said negative phase angle compensation factor. Description 1. Field of the Invention This invention pertains generally to meter apparatus and, more particularly, to such apparatus receiving one or more first alternating current waveforms and one or more second alternating current waveforms. The invention also pertains to a method for compensating for phase differences between first and second alternating current waveforms. 2. Background Information In power measurement systems employing, for example, current transformers, it is very important to correct the phase angle of related signals (e.g., current and voltage signals for one or more power line phases), in order to achieve relatively high levels of accuracy. Previous known methods of phase angle correction involve analog calibration, relatively difficult digital-signal processing, and/or relatively high-speed sampling. While various analog adjustments are possible, it is believed that this analog proposal lacks the precision and consistency of digital approaches. It is also believed that known digital-signal processing proposals are not ideal. While a phase-shifting digital filter is possible, it is believed that the computation of coefficients is relatively complicated for calibration and the real-time requirements are relatively excessive. Another known digital-signal processing or “digital shift” approach requires a re-sampling process in which a number of zeros are inserted into the digital data stream and the high-frequency content is digitally removed with a low-pass digital filter. It is believed that this proposal is relatively computationally intense and could interfere with real-time performance. In a relatively high-speed digital sampling approach, in order for the sampling rate to be high enough for a suitable resolution (e.g., about 0.05 degree resolution), at least 7200 samples/cycle are required. However, such an approach increases cost and complexity. Accordingly, there is room for improvement in meter apparatus and methods for compensating for phase differences between alternating current waveforms. These needs and others are met by the present invention, which employs a phase angle compensation factor and adjusts sampled values of one alternating current waveform to correspond with sampled values of another alternating current waveform by interpolating between pairs of sampled values of such one alternating current waveform based upon the phase angle compensation factor. In accordance with one aspect of the invention, a method of compensating for phase differences between sampled values of first and second alternating current waveforms comprises: employing a phase angle compensation factor; sequentially sampling a plurality of values of each of the first and second alternating current waveforms; and adjusting the sampled values of the second alternating current waveform to correspond with the sampled values of the first alternating current waveform by employing, for a corresponding one of the sampled values of the second alternating current waveform, a preceding sampled value of the second alternating current waveform plus the product of: (i) the phase angle compensation factor and (ii) the difference between the corresponding one of the sampled values and the preceding sampled value, when the phase angle compensation factor is positive, or alternatively adjusting the sampled values of the second alternating current waveform to correspond with the sampled values of the first alternating current waveform by employing, for the corresponding one of the sampled values, the preceding sampled value minus the product of: (i) the sum of one plus the phase angle compensation factor and (ii) the difference between the preceding sampled value and the sampled value of the second alternating current waveform preceding the preceding sampled value, when the phase angle compensation factor is negative. The method may sequentially sample the values of each of the first and second alternating current waveforms at a rate of about 512 samples per alternating current cycle; and employ the phase angle compensation factor, which has an absolute value that is smaller than one. The method may acquire a plurality of sets of voltage samples and current samples as sampled values of each of the first and second alternating current waveforms; determine a plurality of zero crossings in the voltage samples; calculate a plurality of zero crossing sample times for the voltage samples; determine a plurality of zero crossings in the current samples; calculate a plurality of zero crossing sample times for the current samples; calculate a plurality of differences between the zero crossing sample times for the voltage samples and the zero crossing sample times for the current samples; and average the differences to provide the phase angle compensation factor. The method may increment and store a count for each of the sets of voltage samples and current samples; calculate the zero crossing sample times for the voltage samples by employing, for a corresponding one of the zero crossing sample times and a corresponding one of the voltage samples, the stored count of the corresponding one of the voltage samples immediately before a corresponding one of the zero crossings plus the voltage of the voltage sample immediately before the corresponding one of the zero crossings divided by the difference between: (i) the voltage of the voltage sample immediately before the corresponding one of the zero crossings and (ii) the voltage of the voltage sample immediately after the corresponding one of the zero crossings; and calculate the zero crossing sample times for the current samples by employing, for a corresponding one of the zero crossing sample times and a corresponding one of the current samples, the stored count of the corresponding one of the current samples immediately before a corresponding one of the zero crossings plus the current of the current sample immediately before the corresponding one of the zero crossings divided by the difference between: (i) the current of the current sample immediately before the corresponding one of the zero crossings and (ii) the current of the current sample immediately after the corresponding one of the zero crossings. The method may determine the count of one of the voltage zero crossings and the current zero crossings; determine a plurality of differences between each of the zero crossing sample times for the voltage samples and corresponding ones of the zero crossing sample times for the current samples; sum the differences between each of the zero crossing sample times for the voltage samples and corresponding ones of the zero crossing sample times for the current samples; and divide the sum of the differences by the count of one of the voltage zero crossings and the current zero crossings to determine the phase angle compensation factor. As another aspect of the invention, a meter apparatus comprises: a plurality of first inputs adapted to receive at least one first alternating current waveform; a plurality of second inputs adapted to receive at least one second alternating current waveform; an analog to digital converter circuit adapted to sequentially sample and convert the received at least one first alternating current waveform to a plurality of first digital values and adapted to sequentially sample and convert the received at least one second alternating current waveform to a plurality of second digital values; a processor adapted to receive and process the first and second digital values from the analog to digital converter circuit, the processor including a compensation routine having a phase angle compensation factor, the compensation routine being adapted to adjust the second digital values to correspond with the first digital values by employing, for a corresponding one of the second digital values, a preceding one of the second digital values plus the product of: (i) the phase angle compensation factor and (ii) the difference between the corresponding one of the second digital values and the preceding one of the second digital values, when the phase angle compensation factor is positive, or the routine being adapted to alternatively adjust the second digital values to correspond with the first digital values by employing, for the corresponding one of the second digital values, the preceding one of the second digital values minus the product of: (i) the sum of one plus the phase angle compensation factor and (ii) the difference between the preceding one of the second digital values and the second digital value preceding the preceding one of the second digital values, when the phase angle compensation factor is negative, in order to compensate for phase differences between the first and second digital values. The processor may further include a calibration routine adapted to receive and save a plurality of first and second digital calibration values from the analog to digital converter circuit, to communicate the saved first and second digital calibration values to an external calibration circuit, and to receive from the external calibration circuit the phase angle compensation factor. The processor may further include a calibration routine adapted to calibrate the phase angle compensation factor. The compensation routine of the processor may be a first compensation routine when the phase angle compensation factor is positive and a second different compensation routine when the phase angle compensation factor is negative. As another aspect of the invention, a method of compensating for phase differences between sampled values of first and second alternating current waveforms comprises: employing a phase angle compensation factor; sequentially sampling a plurality of values of each of the first and second alternating current waveforms; and adjusting the sampled values of the second alternating current waveform to correspond with the sampled values of the first alternating current waveform by interpolating between a corresponding one of the sampled values of the second alternating current waveform and a preceding sampled value of the second alternating current waveform, when the phase angle compensation factor is positive, or by interpolating between the preceding sampled value and a sampled value of the second alternating current waveform preceding the preceding sampled value, when the phase angle compensation factor is negative. A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: The present invention is described in association with meters for determining power and/or energy from a plurality of alternating current (AC) voltage and current signals, although the invention is applicable to a wide range of electrical apparatus and methods associated with two or more AC signals. Referring to First, at At Otherwise, for the predetermined phase angle compensation factor (CF) being less than zero, at From Equations 1 and 2, it will be appreciated that no compensation is employed if the predetermined phase angle compensation factor (CF) is zero. At The routine In this example, with the variables being initialized to zero at step In an AC power system (not shown), this is practical at rates as low as about 64 samples/cycle and at rates as high as desired. For example, with a specific implementation employing 512 samples per cycle, the acquisition sub-system (not shown) is expected to be accurate within about a few tenths of a degree, although one sample time is about 0.7 degree in this example. As a result, phase correction needs to be much less than one sample time. In order to correct the phase of the current waveform (I) by less than one sample time, the compensation routine For example, as shown in As another example, to retard the current phase by about 0.07 degree (i.e., 0.0703125 degree at 512 samples/cycle), use sample times “n−2” and “n−1,” in order to artificially create a digital sample at sample time “n−1.1” (not shown). In other words, linearly interpolate a tenth of the way between the digital values at sample times “n−1” and “n−2”. In practice, the actual phase error is preferably measured and the result is stored, as was discussed above in connection with As another example, if a sampling rate of 64 samples per cycle is employed, then the worst case error between the actual digital sample, if in phase, and the corrected digital sample is about 0.12%. This error decreases with increases in the sampling rate. Next, at - V
_{n }is the voltage digital value at sample n; and - V
_{n-1 }is the preceding voltage digital value at sample n-1, except for n=0, wherein - V
_{n-1}=0
If the test at At - I
_{n }is the current digital value at sample n; and - I
_{n-1 }is the preceding current digital value at sample n−1, except for n=0, wherein - I
_{n-1}=0.
If the test at Next, at - i is an integer between 1 and j; and
- ZeroCrossingCount is an integer count, j, of voltage or current zero crossings as determined at steps
**90**or**95**.
Then, at At At The processor The processor The routine Referring to After the values The processor The communication sub-system The disclosed phase compensation techniques provide digital precision for phase compensation without the hardware requirements of analog adjustment, relatively high-speed sampling and relatively complicated processing. This provides digital accuracy with relatively minimal processing. Although While for clarity of disclosure reference has been made herein to the exemplary display While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof. Referenced by
Classifications
Legal Events
Rotate |