US20050288884A1 - Meter apparatus and method for phase angle compensation employing linear interpolation of digital signals - Google Patents
Meter apparatus and method for phase angle compensation employing linear interpolation of digital signals Download PDFInfo
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- US20050288884A1 US20050288884A1 US10/865,476 US86547604A US2005288884A1 US 20050288884 A1 US20050288884 A1 US 20050288884A1 US 86547604 A US86547604 A US 86547604A US 2005288884 A1 US2005288884 A1 US 2005288884A1
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- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
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- 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.
- phase angle correction 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.
- 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.
- 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
- 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 crossing
- 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.
- 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,
- 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.
- 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.
- 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.
- FIG. 1 is a flowchart of a compensation routine for phase angle compensation in accordance with the present invention.
- FIG. 2 is a flowchart of a calibration routine for calculating the phase angle compensation factor of FIG. 1 .
- FIG. 3 is a plot of a portion of a current waveform showing application of the phase angle compensation factor of FIG. 1 .
- FIGS. 4A-4B form a flowchart of a routine for auto-calibrating a meter in accordance with an embodiment of the invention.
- FIGS. 5A-5B form a flowchart of a routine for calibrating a meter with an external calibration system in accordance with another embodiment of the invention.
- FIGS. 6-8 are block diagrams of meters including phase angle compensation in accordance with other embodiments of the invention.
- 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.
- AC alternating current
- phase angle compensation is applied to one of two time-varying voltage (V) and current (I) AC signals, although the invention is applicable to a wide range of signal types, to one or more phases (e.g., phase A, B and C) of current and voltage AC signals, and to application of phase angle compensation to the other of the two time-varying voltage (V) and current (I) AC signals.
- V time-varying voltage
- I current
- step 14 sequentially samples the values of each of the voltage and current waveforms from those channels at a rate of about 512 samples per AC cycle. Although an example sample rate is disclosed, a wide range of suitable smaller or larger sample rates may be employed.
- a predetermined direct current (DC) offset per channel may be applied to the samples.
- this corrects for any known DC offset errors in the acquisition circuit (not shown) for each of the channels.
- an integer, n is set to zero.
- CF phase angle compensation factor
- the first of the current samples, I[n] is saved in the temporary register, Temp.
- the temporary register, I n-1 is set equal to the temporary register, Temp.
- the phase angle compensation factor (CF) has an absolute value that is smaller than one.
- the first of the current samples, I[n] is saved in the temporary register, Temp.
- the temporary register, I n-2 is set equal to the temporary register, I n-1 .
- the temporary register, I n-1 is set equal to the temporary register, Temp.
- the temporary register, Temp is set equal to the first of the voltage samples, V[n].
- the first voltage sample, V[n] is set equal to the temporary register, V n-1 .
- the temporary register, V n-1 is set equal to the temporary register, Temp.
- the integer, n is incremented.
- execution resumes at 14 where a subsequent set of ten voltage and current samples is acquired. Otherwise, execution resumes at 20 for the next set of I[n] and V[n].
- the routine 10 adjusts the sampled values of the current AC waveform to correspond with the sampled values of the voltage AC waveform by interpolating, at 24 , between a corresponding one of the sampled values of the current AC waveform and a preceding sampled value of the current AC waveform, when the phase angle compensation factor (CF) is positive, or by interpolating, at 30 , between the preceding sampled value and a sampled value of the current AC waveform preceding the preceding sampled value, when the phase angle compensation factor (CF) is negative.
- this will cause an error in the first current sample if CF is positive, or an error in the first and second samples if CF is negative. This error is insignificant in meter applications and happens only on start up or power up. Alternatively, the first two current samples may be ignored.
- FIG. 2 shows a flowchart of a calibration routine 50 for calculating the phase angle compensation factor (CF) of FIG. 1 .
- 2400 sets of digital samples are acquired from the voltage and current channels (not shown) and are stored in a voltage array 53 and a current array 54 .
- the actual phase error between a voltage channel and the corresponding current channel is determined by preferably inputting pure in-phase sinusoidal signals into the voltage and current channels.
- a predetermined DC offset per channel may be applied to each of the samples in the arrays 53 , 54 .
- this corrects for any known DC offset errors in the acquisition circuit (not shown) for each of the channels.
- zero crossings are determined from the values in the voltage array 53 and, also, zero crossing sample times are calculated and those values are stored in a voltage time array 57 .
- zero crossings are determined from the values in the current array 54 and, also, zero crossing sample times are calculated and those values are stored in a current time array 59 .
- differences between the voltage and current crossing sample times from the arrays 57 , 59 are determined and stored in a time difference array 61 .
- the various time differences in the array 61 are averaged, in order to obtain the phase angle compensation factor (CF).
- This phase angle compensation factor (CF) is stored and employed, as was discussed above in connection with FIG. 1 , in order to determine how far to interpolate between the current digital samples in the array 54 .
- FIG. 3 shows an example plot 70 of a portion of a current AC waveform (D including application of the phase angle compensation factor (CF) of FIG. 1 .
- This compensates for the relative phase shift of one or more current channels (not shown) with respect to corresponding one or more voltage channels (not shown) associated with the determination of electrical power and/or energy.
- the effect of phase errors between the current and voltage channels is corrected by linearly interpolating a synthesized sample time between adjacent current digital samples, such as those at times n and n ⁇ 1, or between those at times n ⁇ 1 and n ⁇ 2.
- the compensation routine 10 of FIG. 1 linearly interpolates between adjacent digital samples. Specifically, three digital samples are employed, in order to provide a range of +/ ⁇ 1 sample time (i.e., about +/ ⁇ 0.7 degree at 512 samples/cycle). If the corresponding voltage waveform (not shown) is digitally sampled at sample time “n ⁇ 1”, then the three corresponding current digital samples are at sample times “n”, “n ⁇ 1” and “n ⁇ 2”.
- the actual phase error is preferably measured and the result is stored, as was discussed above in connection with FIG. 2 , and the stored result is employed in real time, as was discussed above in connection with FIG. 1 .
- 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.
- FIGS. 4A-4B show a flowchart of an auto-calibration routine 80 of a meter (not shown).
- standard voltage and current waveforms (not shown) are input to the meter.
- the standard voltage and current waveforms e.g., without limitation, 110 VAC at 60 Hz; 10 A at 60 Hz
- a command e.g., a suitable signal, such as, for example, a digital signal; a serial port signal; a data link signal; an input from a user interface
- the routine 80 acquires 2400 sets of current and voltage digital samples and applies DC offsets thereto.
- Equation 3 Equation 3 is determined to be either true or false, in order to find a voltage zero crossing: ( V n-1 ⁇ 0)AND( V n >0)OR( V n-1 >0)AND( V n ⁇ 0) (Eq. 3) wherein:
- Equation 4 Equation 4 is determined to be either true or false, in order to find a current zero crossing: ( I n-1 ⁇ 0)AND( I n >0)OR( I n-1 >0)AND( I n ⁇ 0) (Eq. 4) wherein:
- CF phase angle compensation factor
- the meter auto-calibration routine 80 sends the compensation factor (CF) 103 to the meter sub-system 104 , which saves and applies, at 106 , the compensation factor 103 to periodically acquired digital current samples (not shown).
- CF compensation factor
- FIGS. 5A-5B show a flowchart of an external calibration system routine 80 ′ for calibrating a meter 104 ′.
- the routine 80 ′ is similar to the routine 80 of FIGS. 4A-4B , except that the routine 80 ′ is executed by an external calibration system 108 , while the auto-calibration routine 80 is internal to the meter (not shown) associated with the meter sub-system 104 of FIGS. 4A-4B , and except as shown by different reference characters in FIGS. 5A-5B . For simplicity of disclosure, only those different reference characters are discussed with respect to FIGS. 5A-5B .
- a command (e.g., a suitable signal, such as, for example, a digital signal; a serial port signal; a data link signal) is output from the external calibration system 108 over a suitable port 110 (e.g., without limitation, a parallel port; a serial port; a data link; a communication network) to the meter 104 ′ to start the calibration.
- a suitable port 110 e.g., without limitation, a parallel port; a serial port; a data link; a communication network
- a meter routine 86 ′ acquires 2400 sets of current and voltage digital samples and applies DC offsets thereto.
- the external calibration system 108 receives the 2400 sets of digital samples from the meter 104 ′.
- the external calibration system 108 sends such compensation factor over the port 110 to the meter 104 ′, which saves and applies, at 106 ′, the compensation factor 103 ′ to the periodically acquired digital current samples (not shown).
- FIG. 6 shows a meter 120 including a processor 122 employing a phase angle compensation factor (CF) 124 .
- the meter 120 further includes one or more first inputs 125 adapted to receive one or more first AC waveforms 126 (e.g., without limitation, voltage waveforms), and one or more second inputs 127 adapted to receive one or more second AC waveforms 128 (e.g., without limitation, current waveforms).
- An analog to digital converter circuit (ADC) 130 is adapted to sequentially sample and convert received first AC waveforms 132 to a plurality of first digital values 134 and is adapted to sequentially sample and convert received second AC waveforms 136 to a plurality of second digital values 138 .
- ADC analog to digital converter circuit
- the processor 122 includes a routine 140 adapted to receive and process the first and second digital values 134 , 138 from the ADC 130 .
- the routine 140 cooperates with a phase compensation routine 142 (e.g., which may be the same as or similar to the compensation routine 10 of FIG. 1 ) having the phase angle compensation factor (CF) 124 , in order to compensate for phase differences between the sampled values 134 , 138 .
- the routine 140 and/or the ADC 130 may preferably include suitable DC offset and/or gain adjustments for the signals 134 , 138 .
- the processor 122 further includes a calibration routine 144 (e.g., which may be the same as or similar to the auto-calibration routine 80 of FIGS. 4A-4B ) adapted to calibrate the phase angle compensation factor (CF) 124 .
- the calibration routine 144 is executed at power up or start up (e.g., reset) and/or at any time responsive to a suitable command 146 received from port 148 .
- the routine 140 may determine power and/or energy values 150 for display on display 151 (e.g., a local or remote display).
- FIG. 7 another meter 120 ′ including a processor 122 ′ employing the phase angle compensation factor (CF) 124 is shown. Except as discussed, below, the meter 120 ′ and processor 122 ′ are the same as the respective meter 120 and processor 122 of FIG. 6 .
- the processor 122 ′ instead of the phase compensation routine 142 of FIG. 6 , the processor 122 ′ includes one or both of a first phase compensation routine 142 ′ and a second different phase compensation routine 142 ′′.
- the first compensation routine 142 ′ (e.g., similar to the compensation routine 10 of FIG.
- phase angle compensation factor 124 is positive and the second different compensation routine 142 ′′ (e.g., similar to the compensation routine 10 of FIG. 1 , but excluding steps 22 , 24 , 26 ) is employed when the phase angle compensation factor 124 is negative.
- FIG. 8 shows another meter 160 including processors 162 and 170 employing a phase angle compensation factor (CF) 164 .
- the meter 160 is adapted to cooperate with an external calibration system, such as the system 108 of FIG. 5A .
- the processor 162 includes an acquisition routine 166 adapted to receive and save a plurality of first and second digital calibration values 167 from an analog to digital converter 168 .
- the acquisition routine 166 is executed at power up or start up of the meter 160 or at any time with a proper command responsive to a command 172 from the external calibration system 108 as received by a communication sub-system 174 .
- the command 172 is communicated to the processor 170 through the processor circuit 162 .
- the processor 170 communicates those through the host processor circuit 162 to the communication sub-system 174 , which outputs those values in a message 176 to the external calibration system 108 of FIG. 5A .
- the external calibration system 108 determines an external phase angle compensation factor 178 , which is received by the communication sub-system 174 .
- the external compensation factor 178 is communicated to the processor 170 through the host processor 162 .
- the host processor 162 saves the externally determined compensation factor 178 as the local compensation factor (CF) 164 and sends the same to the processor 170 to compensate the signals.
- CF local compensation factor
- the processor 170 also includes a routine 180 employing the phase angle compensation factor (CF) 164 , in order to compensate for phase differences between sampled values 182 and 184 from respective first and second AC waveforms 126 and 128 .
- the routine 180 may be the same as or similar to the routines 140 , 142 of FIG. 6 .
- the communication sub-system 174 includes one or more suitable communication ports.
- 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.
- FIGS. 1 and 3 show phase compensation being applied to the current AC signals, such compensation may alternatively be applied to voltage AC signals.
Abstract
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:
-
FIG. 1 is a flowchart of a compensation routine for phase angle compensation in accordance with the present invention. -
FIG. 2 is a flowchart of a calibration routine for calculating the phase angle compensation factor ofFIG. 1 . -
FIG. 3 is a plot of a portion of a current waveform showing application of the phase angle compensation factor ofFIG. 1 . -
FIGS. 4A-4B form a flowchart of a routine for auto-calibrating a meter in accordance with an embodiment of the invention. -
FIGS. 5A-5B form a flowchart of a routine for calibrating a meter with an external calibration system in accordance with another embodiment of the invention. -
FIGS. 6-8 are block diagrams of meters including phase angle compensation in accordance with other embodiments of the invention. - 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
FIG. 1 , a flowchart of acompensation routine 10 is shown for phase angle compensation between sampled values of two AC waveforms. In this example, the phase angle compensation is applied to one of two time-varying voltage (V) and current (I) AC signals, although the invention is applicable to a wide range of signal types, to one or more phases (e.g., phase A, B and C) of current and voltage AC signals, and to application of phase angle compensation to the other of the two time-varying voltage (V) and current (I) AC signals. - First, at 12, temporary registers In-1, In-2, Vn-1 and Temp are initialized to zero. Next, at 14, ten sets of samples V[n] and I[n] from voltage and current channels (not shown), respectively, are acquired and saved for integer n ranging from 0 to 9. Although ten sets of samples are disclosed, one to nine, eleven or more sets of samples may be employed. Preferably,
step 14 sequentially samples the values of each of the voltage and current waveforms from those channels at a rate of about 512 samples per AC cycle. Although an example sample rate is disclosed, a wide range of suitable smaller or larger sample rates may be employed. At 16, a predetermined direct current (DC) offset per channel may be applied to the samples. Preferably, this corrects for any known DC offset errors in the acquisition circuit (not shown) for each of the channels. Next, at 18, an integer, n, is set to zero. Then, at 20, it is determined if a predetermined phase angle compensation factor (CF) is greater than zero. If so, thensteps steps steps - At 22, the first of the current samples, I[n], is saved in the temporary register, Temp. Next, at 24, the adjusted value of the first current sample, I[n], is determined from Equation 1:
I[n]=((I[n]−I n-1))*CF)+I n-1 (Eq. 1)
Then, at 26, the temporary register, In-1, is set equal to the temporary register, Temp. Typically, the phase angle compensation factor (CF) has an absolute value that is smaller than one. - Otherwise, for the predetermined phase angle compensation factor (CF) being less than zero, at 28, the first of the current samples, I[n], is saved in the temporary register, Temp. Next, at 30, the adjusted value of the first current sample, I[n], is determined from Equation 2:
I[n]=I n-1−((I n-1 −I n-2)*(1+CF)) (Eq. 2)
Then, at 32, the temporary register, In-2, is set equal to the temporary register, In-1. Next, at 34, the temporary register, In-1, is set equal to the temporary register, Temp. - From
Equations - At 36, the temporary register, Temp, is set equal to the first of the voltage samples, V[n]. Next, at 38, the first voltage sample, V[n], is set equal to the temporary register, Vn-1. Then, at 40, the temporary register, Vn-1, is set equal to the temporary register, Temp. Next, at 42, the integer, n, is incremented. Finally, at 44, if the integer, n, is equal to ten, then execution resumes at 14, where a subsequent set of ten voltage and current samples is acquired. Otherwise, execution resumes at 20 for the next set of I[n] and V[n].
- The routine 10 adjusts the sampled values of the current AC waveform to correspond with the sampled values of the voltage AC waveform by interpolating, at 24, between a corresponding one of the sampled values of the current AC waveform and a preceding sampled value of the current AC waveform, when the phase angle compensation factor (CF) is positive, or by interpolating, at 30, between the preceding sampled value and a sampled value of the current AC waveform preceding the preceding sampled value, when the phase angle compensation factor (CF) is negative.
- In this example, with the variables being initialized to zero at
step 12, this will cause an error in the first current sample if CF is positive, or an error in the first and second samples if CF is negative. This error is insignificant in meter applications and happens only on start up or power up. Alternatively, the first two current samples may be ignored. -
FIG. 2 shows a flowchart of acalibration routine 50 for calculating the phase angle compensation factor (CF) ofFIG. 1 . In this example, first, at 52, 2400 sets of digital samples are acquired from the voltage and current channels (not shown) and are stored in avoltage array 53 and acurrent array 54. Here, the actual phase error between a voltage channel and the corresponding current channel is determined by preferably inputting pure in-phase sinusoidal signals into the voltage and current channels. Although 2400 sets of samples is disclosed, a wide range of sample set counts may be employed. Next, at 55, a predetermined DC offset per channel may be applied to each of the samples in thearrays voltage array 53 and, also, zero crossing sample times are calculated and those values are stored in avoltage time array 57. Next, at 58, zero crossings are determined from the values in thecurrent array 54 and, also, zero crossing sample times are calculated and those values are stored in acurrent time array 59. Then, at 60, differences between the voltage and current crossing sample times from thearrays time difference array 61. Finally, at 62, the various time differences in thearray 61 are averaged, in order to obtain the phase angle compensation factor (CF). This phase angle compensation factor (CF) is stored and employed, as was discussed above in connection withFIG. 1 , in order to determine how far to interpolate between the current digital samples in thearray 54. -
FIG. 3 shows anexample plot 70 of a portion of a current AC waveform (D including application of the phase angle compensation factor (CF) ofFIG. 1 . This compensates for the relative phase shift of one or more current channels (not shown) with respect to corresponding one or more voltage channels (not shown) associated with the determination of electrical power and/or energy. The effect of phase errors between the current and voltage channels is corrected by linearly interpolating a synthesized sample time between adjacent current digital samples, such as those at times n and n−1, or between those at times n−1 and n−2. - 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 10 ofFIG. 1 linearly interpolates between adjacent digital samples. Specifically, three digital samples are employed, in order to provide a range of +/−1 sample time (i.e., about +/−0.7 degree at 512 samples/cycle). If the corresponding voltage waveform (not shown) is digitally sampled at sample time “n−1”, then the three corresponding current digital samples are at sample times “n”, “n−1” and “n−2”. - For example, as shown in
FIG. 3 , to advance the current phase by about 0.211 degree (i.e., 0.2109375 degree at 512 samples/cycle), use sample times “n−1” and “n,” in order to artificially create a digital sample at sample time “n−0.7”. In other words, linearly interpolate three tenths (i.e., CF=+0.3) of the way between the digital values at sample times “n−1” and “n”. - 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
FIG. 2 , and the stored result is employed in real time, as was discussed above in connection withFIG. 1 . - 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.
-
FIGS. 4A-4B show a flowchart of an auto-calibration routine 80 of a meter (not shown). First, at 82, standard voltage and current waveforms (not shown) are input to the meter. For example, the standard voltage and current waveforms (e.g., without limitation, 110 VAC at 60 Hz; 10 A at 60 Hz) are preferably pure in-phase sinusoidal signals, which are input into the voltage and current channels (not shown) of the meter. Next, at 84, a command (e.g., a suitable signal, such as, for example, a digital signal; a serial port signal; a data link signal; an input from a user interface) is input to the meter to start the auto-calibration. Then, at 86, as was discussed above in connection withsteps FIG. 2 , the routine 80 acquires 2400 sets of current and voltage digital samples and applies DC offsets thereto. - Next, at 87, the integer, n, is set to zero. Then, at 88, the logical expression of Equation 3 is determined to be either true or false, in order to find a voltage zero crossing:
(V n-1<0)AND(V n>0)OR(V n-1>0)AND(V n<0) (Eq. 3)
wherein: - Vn is the voltage digital value at sample n; and
- Vn-1 is the preceding voltage digital value at sample n-1, except for n=0, wherein
- Vn-1=0
- If the test at 88 is true, then at 90, the time of the voltage zero crossing, VoltageZeroCrossing[ ], is defined by Equation 4 with respect to the corresponding sample number:
VoltageZeroCrossing[ ]=(n−1)+V n-1/(V n-1 −V n) (Eq. 4)
Otherwise, or afterstep 90, it is determined if the integer, n, is equal to 2399. If so, then execution resumes atstep 93. Otherwise, the integer, n, is incremented at 92 beforestep 88 is repeated for the next sample. - At 93, after all voltage samples are considered, the integer, n, is set to zero. Then, at 94, the logical expression of Equation 4 is determined to be either true or false, in order to find a current zero crossing:
(I n-1<0)AND(I n>0)OR(I n-1>0)AND(I n<0) (Eq. 4)
wherein: - In is the current digital value at sample n; and
- In-1 is the preceding current digital value at sample n−1, except for n=0, wherein
- In-1=0.
- If the test at 94 is true, then at 95, the time of the current zero crossing, CurrentZeroCrossing[ ], is defined by Equation 5 with respect to the corresponding sample number:
CurrentZeroCrossing[ ]=(n−1)+I n-1/(I n-1 −I n) (Eq. 5)
Otherwise, or afterstep 95, it is determined if the integer, n, is equal to 2399. If so, then execution resumes atstep 100. Otherwise, the integer, n, is incremented at 98 beforestep 94 is repeated for the next sample. - Next, at 100, the phase angle compensation factor (CF) is determined from Equation 6:
wherein: - 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 - Then, at 102, the meter auto-
calibration routine 80 sends the compensation factor (CF) 103 to themeter sub-system 104, which saves and applies, at 106, thecompensation factor 103 to periodically acquired digital current samples (not shown). -
FIGS. 5A-5B show a flowchart of an external calibration system routine 80′ for calibrating ameter 104′. The routine 80′ is similar to the routine 80 ofFIGS. 4A-4B , except that the routine 80′ is executed by anexternal calibration system 108, while the auto-calibration routine 80 is internal to the meter (not shown) associated with themeter sub-system 104 ofFIGS. 4A-4B , and except as shown by different reference characters inFIGS. 5A-5B . For simplicity of disclosure, only those different reference characters are discussed with respect toFIGS. 5A-5B . - At 84′, a command (e.g., a suitable signal, such as, for example, a digital signal; a serial port signal; a data link signal) is output from the
external calibration system 108 over a suitable port 110 (e.g., without limitation, a parallel port; a serial port; a data link; a communication network) to themeter 104′ to start the calibration. Then, at 86′, as was discussed above in connection withsteps FIG. 2 , ameter routine 86′ acquires 2400 sets of current and voltage digital samples and applies DC offsets thereto. Next, at 86″, theexternal calibration system 108 receives the 2400 sets of digital samples from themeter 104′. - At 102′, after determining the
compensation factor 103′, theexternal calibration system 108 sends such compensation factor over theport 110 to themeter 104′, which saves and applies, at 106′, thecompensation factor 103′ to the periodically acquired digital current samples (not shown). -
FIG. 6 shows ameter 120 including aprocessor 122 employing a phase angle compensation factor (CF) 124. Themeter 120 further includes one or morefirst inputs 125 adapted to receive one or more first AC waveforms 126 (e.g., without limitation, voltage waveforms), and one or moresecond inputs 127 adapted to receive one or more second AC waveforms 128 (e.g., without limitation, current waveforms). An analog to digital converter circuit (ADC) 130 is adapted to sequentially sample and convert receivedfirst AC waveforms 132 to a plurality of firstdigital values 134 and is adapted to sequentially sample and convert receivedsecond AC waveforms 136 to a plurality of seconddigital values 138. - The
processor 122 includes a routine 140 adapted to receive and process the first and seconddigital values ADC 130. In accordance with an important aspect of the invention, the routine 140 cooperates with a phase compensation routine 142 (e.g., which may be the same as or similar to thecompensation routine 10 ofFIG. 1 ) having the phase angle compensation factor (CF) 124, in order to compensate for phase differences between the sampledvalues ADC 130 may preferably include suitable DC offset and/or gain adjustments for thesignals - The
processor 122 further includes a calibration routine 144 (e.g., which may be the same as or similar to the auto-calibration routine 80 ofFIGS. 4A-4B ) adapted to calibrate the phase angle compensation factor (CF) 124. Thecalibration routine 144 is executed at power up or start up (e.g., reset) and/or at any time responsive to asuitable command 146 received fromport 148. - The routine 140 may determine power and/or
energy values 150 for display on display 151 (e.g., a local or remote display). - Referring to
FIG. 7 , anothermeter 120′ including aprocessor 122′ employing the phase angle compensation factor (CF) 124 is shown. Except as discussed, below, themeter 120′ andprocessor 122′ are the same as therespective meter 120 andprocessor 122 ofFIG. 6 . Here, instead of thephase compensation routine 142 ofFIG. 6 , theprocessor 122′ includes one or both of a firstphase compensation routine 142′ and a second differentphase compensation routine 142″. Thefirst compensation routine 142′ (e.g., similar to thecompensation routine 10 ofFIG. 1 , but excludingsteps angle compensation factor 124 is positive and the seconddifferent compensation routine 142″ (e.g., similar to thecompensation routine 10 ofFIG. 1 , but excludingsteps angle compensation factor 124 is negative. -
FIG. 8 shows anothermeter 160 includingprocessors meter 160 is adapted to cooperate with an external calibration system, such as thesystem 108 ofFIG. 5A . Theprocessor 162 includes anacquisition routine 166 adapted to receive and save a plurality of first and second digital calibration values 167 from an analog todigital converter 168. Theacquisition routine 166 is executed at power up or start up of themeter 160 or at any time with a proper command responsive to acommand 172 from theexternal calibration system 108 as received by acommunication sub-system 174. In this example, thecommand 172 is communicated to theprocessor 170 through theprocessor circuit 162. - After the
values 167 are acquired, theprocessor 170 communicates those through thehost processor circuit 162 to thecommunication sub-system 174, which outputs those values in amessage 176 to theexternal calibration system 108 ofFIG. 5A . In turn, theexternal calibration system 108 determines an external phaseangle compensation factor 178, which is received by thecommunication sub-system 174. In this example, theexternal compensation factor 178 is communicated to theprocessor 170 through thehost processor 162. Thehost processor 162 saves the externallydetermined compensation factor 178 as the local compensation factor (CF) 164 and sends the same to theprocessor 170 to compensate the signals. - The
processor 170 also includes a routine 180 employing the phase angle compensation factor (CF) 164, in order to compensate for phase differences between sampledvalues second AC waveforms routines FIG. 6 . - The
communication sub-system 174 includes one or more suitable communication ports. - 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
FIGS. 1 and 3 show phase compensation being applied to the current AC signals, such compensation may alternatively be applied to voltage AC signals. - While for clarity of disclosure reference has been made herein to the
exemplary display 151 for displaying power and/or energy values, it will be appreciated that such values may be stored, printed on hard copy, be computer modified, be sent to a remote display, or be combined with other data. All such processing shall be deemed to fall within the terms “display” or “displaying” as employed herein. - 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.
Claims (22)
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US10/865,476 US6975951B1 (en) | 2004-06-10 | 2004-06-10 | Meter apparatus and method for phase angle compensation employing linear interpolation of digital signals |
CA002509473A CA2509473A1 (en) | 2004-06-10 | 2005-06-08 | Meter apparatus and method for phase angle compensation employing linear interpolation of digital signals |
EP05012551A EP1605275B1 (en) | 2004-06-10 | 2005-06-10 | Electronic meter digital phase compensation |
DE602005022652T DE602005022652D1 (en) | 2004-06-10 | 2005-06-10 | Digital phase compensation for an electronic meter |
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US10/865,476 US6975951B1 (en) | 2004-06-10 | 2004-06-10 | Meter apparatus and method for phase angle compensation employing linear interpolation of digital signals |
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US6975951B1 US6975951B1 (en) | 2005-12-13 |
US20050288884A1 true US20050288884A1 (en) | 2005-12-29 |
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US (1) | US6975951B1 (en) |
EP (1) | EP1605275B1 (en) |
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Cited By (3)
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US20110115423A1 (en) * | 2009-11-18 | 2011-05-19 | Kern Lynn R | Brushless, Three Phase Motor Drive |
US20160320217A1 (en) * | 2015-04-30 | 2016-11-03 | Goodrich Corporation | Self-calibrating linear voltage differential transformer demodulator |
US11320495B2 (en) * | 2019-08-30 | 2022-05-03 | Schweitzer Engineering Laboratories, Inc. | Current-based directional element in a power delivery system |
Families Citing this family (6)
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US8165832B1 (en) * | 2008-11-12 | 2012-04-24 | Ixys Ch Gmbh | Wall plug power monitor |
US9335352B2 (en) * | 2009-03-13 | 2016-05-10 | Veris Industries, Llc | Branch circuit monitor power measurement |
US8478550B2 (en) | 2010-07-23 | 2013-07-02 | Caterpillar Inc. | Generator set calibration controller |
US8942942B2 (en) | 2010-07-23 | 2015-01-27 | Caterpillar Inc. | Generator set calibration controller |
US8674713B2 (en) * | 2010-10-21 | 2014-03-18 | Tektronix, Inc. | Zero ampere level current data correction for a power device under test |
KR20220077804A (en) * | 2020-12-02 | 2022-06-09 | 현대모비스 주식회사 | Apparatus and method for compensation for offset in switching current sensing |
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US6043642A (en) * | 1996-08-01 | 2000-03-28 | Siemens Power Transmission & Distribution, Inc. | Watt-hour meter with communication on diagnostic error detection |
US6748344B2 (en) * | 2002-04-29 | 2004-06-08 | Eaton Corporation | Method and apparatus employing a scaling factor for measuring and displaying an electrical parameter of an electrical system |
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DE2630959C2 (en) * | 1976-07-07 | 1986-04-30 | Heliowatt Werke Elektrizitäts- Gesellschaft mbH, 1000 Berlin | Kilowatt hour meter with static measuring mechanism |
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US6735535B1 (en) * | 2000-05-05 | 2004-05-11 | Electro Industries/Gauge Tech. | Power meter having an auto-calibration feature and data acquisition capabilities |
US6636028B2 (en) * | 2001-06-01 | 2003-10-21 | General Electric Company | Electronic electricity meter configured to correct for transformer inaccuracies |
US6943714B2 (en) * | 2002-08-19 | 2005-09-13 | Tdk Semiconductor Corporation | Method and apparatus of obtaining power computation parameters |
-
2004
- 2004-06-10 US US10/865,476 patent/US6975951B1/en active Active
-
2005
- 2005-06-08 CA CA002509473A patent/CA2509473A1/en not_active Abandoned
- 2005-06-10 EP EP05012551A patent/EP1605275B1/en active Active
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US4996871A (en) * | 1989-06-02 | 1991-03-05 | Micro Motion, Inc. | Coriolis densimeter having substantially increased noise immunity |
US6043642A (en) * | 1996-08-01 | 2000-03-28 | Siemens Power Transmission & Distribution, Inc. | Watt-hour meter with communication on diagnostic error detection |
US6748344B2 (en) * | 2002-04-29 | 2004-06-08 | Eaton Corporation | Method and apparatus employing a scaling factor for measuring and displaying an electrical parameter of an electrical system |
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US20110115423A1 (en) * | 2009-11-18 | 2011-05-19 | Kern Lynn R | Brushless, Three Phase Motor Drive |
US8368334B2 (en) * | 2009-11-18 | 2013-02-05 | Standard Microsystems Corporation | Brushless, three phase motor drive |
US20160320217A1 (en) * | 2015-04-30 | 2016-11-03 | Goodrich Corporation | Self-calibrating linear voltage differential transformer demodulator |
US10132663B2 (en) * | 2015-04-30 | 2018-11-20 | Goodrich Corporation | Self-calibrating linear voltage differential transformer demodulator |
US11320495B2 (en) * | 2019-08-30 | 2022-05-03 | Schweitzer Engineering Laboratories, Inc. | Current-based directional element in a power delivery system |
Also Published As
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US6975951B1 (en) | 2005-12-13 |
DE602005022652D1 (en) | 2010-09-16 |
EP1605275A3 (en) | 2006-05-24 |
EP1605275A2 (en) | 2005-12-14 |
EP1605275B1 (en) | 2010-08-04 |
CA2509473A1 (en) | 2005-12-10 |
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