Method and machine for metering electric parameters

• US 4,581,705 A
• Filed: 01/28/1983
• Issued: 04/08/1986
• Est. Priority Date: 01/28/1983
• Status: Expired due to Fees

• Current Assignee: Mcgraw-Edison Company (Eaton Corporation PLC.)
• Original Assignee: Cooper Industries, Inc. (Eaton Corporation PLC.)
• Inventors: Gilker, Clyde, Nohria, Naresh K.
• Primary Examiner(s): Wise, Edward J.
• Application Number:
  US06/461,751
• Time in Patent Office: 1,166 Days
• Field of Search: 364/492, 364/483, 364/487, 324/76 R, 324/103 R, 340/646, 323/257
• US Class Current: 702/60
• CPC Class Codes: G01R 19/02 Measuring effective values, i.e. root-mean-square values
• CPC Class Codes: G01R 21/00 Arrangements for measuring electric power or power factor G01R7/12 takes precedence
• CPC Class Codes: G01R 21/133 by using digital technique

Abstract

The electric parameter metering machine comprises electric parameter pickup circuitry for sensing the instantaneous analog amplitude value of a electric parameter waveform in at least one phase in an electric power distribution system at selected time intervals during each full cycle, an A/D converter for converting the analog amplitude values to digital signals and signal processing circuitry including a memory for storing the digital signals. The signal processing circuitry includes a microprocessor operable to apply a conventional waveform analysis technique to the digital signal information collected with respect to a selected number of cycles of the electric parameter waveform to obtain an instantaneous RMS value of the electric parameter and operable to process a sequence of instantaneous RMS electric parameter values at discrete time intervals to obtain time averaged or demand electric parameter values. A visual display device is coupled to the signal processing circuitry and a pushbutton control panel is coupled to the signal processing circuitry for enabling an operator to operate pushbuttons to provide a digital readout of either instantaneous RMS, demand, or peak demand values on the visual display device.

Claims

1. A method for metering electric parameters comprising the steps of:

sensing the instantaneous analog amplitude value of an electric parameter waveform in at least one phase in a electric power distribution system at selected time intervals during each full cycle;

converting the analog amplitude values to digital signals;

storing the digital signals in a memory in, or associated with, signal processing circuitry to which the signals are supplied;

applying a conventional waveform analysis technique to the digital signal information collected with respect to a selected number of cycles of the electric parameter waveform to obtain an instantaneous RMS value of the electric parameter;

processing with a selected algorithm a sequence of instantaneous RMS electric parameter values at discrete time intervals to obtain a time averaged or demand electric parameter value;

storing the instantaneous RMS and demand values of the electric parameter; and

providing visual display means and control means coupled to the processing circuitry for enabling an operator to operate the control means to provide a readout of either the instantaneous RMS or demand electric parameter value.

2. The method of claim 1 including the steps of:

continually sensing analog amplitudes of an electric parameter in two or more phases;

multiplexing the analog amplitudes of the electric parameter;

sequentially digitizing the multiplexed amplitudes of the electric parameter by sampling and holding the multiplexed amplitudes for a sufficient period for digitizing to be completed;

storing the digitized amplitude values for each phase in signal processing memory for said selected number of cycles; and

performing the remaining steps on each phase electric parameter to obtain readouts of the electric parameter for each phase.

3. The method of claim 1 further including the steps of:

storing the demand electric parameter value in a peak area of memory;

comparing the next value of the demand electric parameter with that stored in the peak area; and

storing the larger of the two values of the demand electric parameter in the peak area of memory; and

further enhancing said visual display means and control means to enable an operator to obtain a readout of a peak demand electric parameter value stored in the peak area of memory.

4. The method of claim 3 wherein the demand and peak demand electric parameter values are updated about every 10 seconds.

5. The method of claim 1 wherein said conventional waveform analysis technique is a discrete Fourier analysis which is utilized to determine the coefficients of the power line frequency fundamental sinusoidal wave of the electric parameter waveform for the electric parameter being measured.

6. The method of claim 1 wherein each digital amplitude signal associated with a selected time interval is summed into a selected area of said memory for said selected number of cycles;

and divided by said selected number of cycles to obtain an averaged digital amplitude signal on a per cycle basis, before applying a conventional waveform analysis technique of a Fourier Transform variety, to obtain said instantaneous RMS electric parameter values.

7. The method of claim 1 wherein said processing of the instantaneous RMS electric parameter values at discrete time intervals includes the application of a pseudo-exponential algorithm to instantaneous RMS electric parameter values to obtain a demand values of the electric parameter.

8. The method of claim 7 wherein said pseudo-exponential algorithm is defined as follows:

space="preserve" listing-type="equation">P.sub.DO +(P.sub.i -P.sub.DO)1/N→

P.sub.D

where

P_i =instantaneous RMS electric parameter value

P_DO =the stored demand electric parameter value

P_D =the new demand electric parameter value

N=a divisor related to averaging time which is a power of 2

→

=a programming symbol meaning the number determined to the left of the arrow is placed in or assigned to the symbol to the right of the arrow.

9. The method of claim 8 including the steps of:

continuously sensing analog amplitudes of current and voltage in at least one phase;

multiplexing the analog amplitudes of current and voltage;

sequentially digitizing the multiplexed amplitudes of current and voltage by sampling and holding the multiplexed amplitudes for a sufficient period for digitizing to be completed;

storing the digitized amplitude values for current and voltage in signal processing memory for said selected number of cycles;

applying a conventional waveform analysis technique to the digitized amplitude values to obtain instantaneous RMS values of current and voltage and their phase angle relationship;

forming a power product of the instantaneous RMS current and voltage to obtain a instantaneous RMS power; and

applying the remaining steps to the instantaneous RMS power to obtain a readout of the demand power.

10. The method of claim 8 wherein said discrete time interval is about 10 seconds and N is about 32.

11. The method of claim 7 wherein the pseudo-exponential algorithm is called up on regular intervals by a timer, associated with, or situated in, said processing circuitry, and a new demand electric parameter value is calculated by subtracting the stored demand electric parameter value from the instantaneous RMS electric parameter value and dividing by N to find the change in the demand electric parameter values and adding that change in demand electric parameter values to the stored demand electric parameter value to obtain the new demand electric parameter value.

12. The method of claim 1 wherein said processing of the instantaneous RMS electric parameter values at discrete time intervals includes the application of an exponential algorithm to the instantaneous RMS electric parameter values to obtain a demand values of the electric parameter.

13. The method of claim 12 wherein said exponential algorithm is defined as follows:

space="preserve" listing-type="equation">P.sub.i +(P.sub.DO -P.sub.i)e.sup.-kt →

P.sub.D

where

P_i =instantaneous RMS electric parameter value at start of an exponential calculation period

P_DO =the stored electric parameter demand value at the start of an exponential calculation period

P_D =the new demand electric parameter value

e=2.72

k=a term related to the demand period

t=a term proportional to the time since the last significant change in P_i, which increases each time this algorithm is run for a given P_i

→

=a programming symbol meaning the number determined to the left of the arrow is placed in or assigned to the symbol to the right of the arrow.

14. The method of claim 1 wherein the elctric parameter is current.

15. The method of claim 1 wherein sixteen instantaneous analog amplitude values of a electric parameter are sensed per cycle.

16. The method of claim 15 wherein a conventional waveform analysis technique is applied to the average of the instantaneous analog amplitude values obtained for 32 cycles of the electric parameter waveform to obtain the instantaneous RMS electric parameter.

17. The method of claim 1 wherein the value of the instantaneous RMS electric parameter is updated and stored about every 10 seconds.

18. The method of claim 1 wherein the demand period at the end of which the demand electric parameter value reflects 90% of a step change in the instantaneous RMS value is a time period between 1 minute and 60 minutes.

19. The method of claim 18 wherein the demand period is 15 minutes.

20. An electric parameter metering machine comprising:

means for sensing the instantaneous analog amplitude value of an electric parameter waveform in at least one phase in an electric power distribution system at selected time intervals during each full cycle;

means for converting the analog amplitude values to digital signals;

signal processing circuitry including a memory for processing and storing the digital signals, said signal processing circuitry being operable to apply a conventional waveform analysis technique to the digital signal information collected with respect to a selected number of cycles of the electric parameter waveform to obtain an instantaneous RMS value of the electric parameter and being operable to process a sequence of instantaneous RMS electric parameter values at discrete time intervals to obtain time averaged or demand electric parameter values;

memory for storing the instantaneous RMS and demand electric parameter values; and

visual display means and control means coupled to said signal processing circuitry for enabling an operator to operate the control means to provide a digital readout of either the instantaneous RMS or the demand electric parameter value on said visual display.

21. The machine of claim 20 including means for obtaining instantaneous analog amplitude values of the electric parameter associated with two or more phases;

means for multiplexing the analog electric parameter values for each phase;

means for sampling and holding the multiplexed analog electric parameter values;

means for sequentially converting instantaneous analog amplitude values to digital amplitude values and supplying the digital amplitude values to said signal processing circuitry for temporary storage in said memory for processing by said signal processing circuitry on a phase basis to obtain a readout of either the instantaneous RMS or the demand electric parameter value for each phase on said display.

22. The machine of claim 21 wherein all three phases and a ground phase in a power distribution system are coupled to said multiplexer means by protective input circuits which are isolated from each other.

23. The machine of claim 22 wherein each protective input circuit supresses transients with filter means having a time constant of about 1 millisecond and further provides protection against long term over-voltages by a double ZENER diodes.

24. The machine of claim 20 wherein said processing circuitry has a trickle charged battery-powered reserve power supply for energizing said signal processing circuitry during a power outage.

25. The machine of claim 20 further including, means for storing the demand electric parameter value in a peak area of memory;

means for comparing the next value of the demand electric parameter with that stored in the peak area;

means for storing the larger of the two values of the demand electric parameter in the peak area of memory; and

means for recalling the stored value of demand electric parameter from the peak area of memory for display of a peak demand electric parameter value on said visual display.

26. The machine of claim 25 wherein said signal processing circuitry calculates and stores demand and peak demand electric parameter values about every every 10 seconds.

27. The machine of claim 25 wherein said control means include a control panel including pushbutton switches which are coupled to said processing circuitry and which include a button switch for each phase electric parameter to be measured and a button switch for causing either a instantaneous RMS demand or peak demand electric parameter value in a selected phase to be displayed on the visual display means.

28. The machine of claim 20 further including;

means for storing the instantaneous RMS electric parameter value in a peak area of memory;

means for comparing the next value of the instantaneous RMS electric parameter with that stored in the peak area;

means for storing the larger of the two values of the instantaneous RMS electric parameter in the peak area of memory; and

means for recalling the stored value of the instantaneous RMS electric parameter from the peak area of memory for display of a peak instantaneous RMS electric parameter value on said visual display.

29. The machine of claim 20 wherein said conventional waveform analysis technique is a discrete Fourier analysis which is applied by said signal processing circuitry to determine the coefficients of the power line frequency fundamental sinusoidal wave of the electric parameter waveform for the electric parameter being measured.

30. The machine of claim 20 further including a non volitile memory associated with said signal processing circuitry for storing process parameters and significant results of the process such as instantaneous RMS and demand values of the electric parameter.

31. The machine of claim 20 wherein said signal processing circuitry in processing the instantaneous RMS electric parameter values at discrete time intervals applies a pseudo-exponential algorithm to the instantaneous RMS electric parameter values to obtain demand values of the electric parameter.

32. The machine of claim 31 wherein said pseudo-exponential algorithm is defined as follows:

space="preserve" listing-type="equation">P.sub.DO +(P.sub.i -P.sub.DO)1/N→

P.sub.D

where

P_i =instantaneous RMS electric parameter value

P_DO =the stored demand electric parameter value

P_D =the new demand electric parameter value

N=a divisor related to averaging time which is a power of 2

→

=a programming symbol meaning the number determined to the left of the arrow is placed in or assigned to the symbol to the right of the arrow.

33. The machine of claim 32 including means for continuously sensing analog amplitudes of current and voltage in at least one phase;

means for multiplexing the analog values of current and voltage;

means for sequentially digitizing the multiplexed amplitudes of current and voltage by employing sample and hold means for sampling and holding the multiplexed amplitudes for a sufficient period for digitizing to be completed;

storage means for storing the digitized amplitude values of current and voltage for said selected number of cycles;

means for applying a conventional waveform analysis technique to the digitized amplitude values to obtain instantaneous RMS values of current and voltage and their phase angle relationship, and for forming a power product of the instantaneous RMS current and voltage values to obtain a instantaneous RMS power value for obtaining a readout of demand power values from said instantaneous RMS power values.

34. The machine of claim 32 wherein said discrete time interval is about 10 seconds and N is about 32.

35. The machine of claim 31 wherein the pseudo-exponential algorithm is called up on regular intervals by a timer, associated with, or situated in, said processing circuitry, and a new demand electric parameter value is calculated by subtracting the stored demand electric parameter value from the instantaneous RMS electric parameter value and dividing by N to find the change in the demand electric parameter values and adding that change in the demand electric parameter values to the stored demand electric parameter value to obtain the new demand electric parameter value.

36. The machine of claim 20 wherein said signal processing circuitry in processing the instantaneous RMS electric parameter values at discrete time intervals applies an exponential algorithm to the instantaneous RMS electric parameter values to obtain a demand value of the electric parameter.

37. The machine of claim 36 wherein said exponential algorithm is defined as follows:

space="preserve" listing-type="equation">P.sub.i +(P.sub.DO -P.sub.i)e.sup.-kt →

P.sub.D

where

P_i =the instantaneous current value at start of an exponential calculation period

P_DO =demand current value at start of an exponential calculation period

e=2.72

k=a term related to the demand period

t=a term proportional to the time since the last significant change in P_i, which increases each time this algorithm is run for a given P_i

→

=a programming symbol meaning the number determined to the left of the arrow is placed in or assigned to the symbol to the right of the arrow.

38. The machine of claim 20 wherein the electric parameters is current.

39. The machine of claim 20 wherein said signal processing circuitry is operable to sense about sixteen instantaneous analog amplitude values of a electric parameter per cycle.

40. The machine of claim 39 wherein said conventional waveform analysis technique is applied by said signal processing circuitry to the average of the instantaneous analog amplitude values obtained for about 32 cycles of the electric parameter waveform to obtain the instantaneous RMS electric parameter value.

41. The machine of claim 20 wherein said signal processing circuitry calculates and stores a new instantaneous RMS electric parameter value about every 10 seconds.

42. The machine of claim 20 wherein said signal processing circuitry calculates the demand electric parameter value so that the demand electric parameter value reflects 90% of a step change of instantaneous RMS electric parameter values in a time period of between 1 minute and 60 minutes.

43. The machine of claim 42 wherein said time period is 15 minutes.

44. The machine of claim 20 wherein said processing circuitry includes a microprocessor having A/D conversion means, random access memory means and read only memory means.

45. The machine of claim 20 wherein said visual display means include a light emitting diode display arrangement.

46. The machine of claim 20 wherein said visual display means includes a numeric 7 segment display for identifying the electric parameter.