Digital ac monitor

• US 4,672,555 A
• Filed: 10/18/1984
• Issued: 06/09/1987
• Est. Priority Date: 10/18/1984
• Status: Expired due to Term

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Kern, Edward C. Jr., Hart, George W.
• Primary Examiner(s): Chin, Gary
• Assistant Examiner(s): Juffernbruch, Daniel W.
• Application Number:
  US06/662,392
• Time in Patent Office: 964 Days
• Field of Search: 364/481, 364/483, 364/484, 364/575, 324/73 AT, 324/142, 324/77 A
• US Class Current: 700/276
• CPC Class Codes: G01R 19/25 using digital measurement techniques
• CPC Class Codes: G01R 21/133 by using digital technique
• CPC Class Codes: G01R 31/42 AC power supplies

Abstract

An apparatus and method is provided for monitoring a plurality of analog ac circuits by sampling the voltage and current waveform in each circuit at predetermined intervals, converting the analog current and voltage samples to digital format, storing the digitized current and voltage samples and using the stored digitized current and voltage samples to calculate a variety of electrical parameters; some of which are derived from the stored samples.

The non-derived quantities are repeatedly calculated and stored over many separate cycles then averaged. The derived quantities are then calculated at the end of an averaging period. This produces a more accurate reading, especially when averaging over a period in which the power varies over a wide dynamic range.

Frequency is measured by timing three cycles of the voltage waveform using the upward zero crossover point as a starting point for a digital timer.

Claims

1. Apparatus for determining the value of a plurality of ac power parameters of a plurality of ac analog circuits each of which have separate analog electrical waveforms propagating in the circuit comprising:

(a) sampling means for periodically sampling, during a first period of time for each circuit in succession, the analog current and voltage of the waveforms of each circuit a plurality of times during each cycle of the waveforms and over at least several cycles of the waveforms;

(b) converting means for converting, during said first period of time, the ac current and voltage samples to digital signals proportional to the current or voltage value of the samples;

(c) storage means for storing such digital signals during said first period of time;

(d) computing means for calculating and averaging certain non-derived values of power parameters of the circuits directly from such digital signals during intervals between taking of samples and storing such values, and, during a second later variable time period for each circuit deriving the values of additional power parameters from such stored values and, for adding the calculated values into running sums and storing said sums during a third later time period for each circuit, and during a fourth later time period, averaging the stored sums selected for outputting; and

(e) output means for interfacing said computing means with a data communications system whereby certain values can be selected for calculation and outputted.

2. The apparatus of claim 1 including frequency measurement means for calculating the frequency of the analog current and voltage waveforms of each circuit, during a fifth time period prior to said third time period, by timing several cycles of the waveforms using the zero crossover point of the waveform as the starting point and dividing by the number of cycles timed.

3. The apparatus of claim 1 in which one of the non-derived values is the RMS voltage and the computing means includes means for calculating the RMS voltage voltage by sampling the waveform and computing RMS voltage, as follows:

##EQU9## wherein n=number of samples taken per cycle and V_i =voltage value of each i-th sample.

4. The appratus of claim 1 in which one of the non-derived values is ac power and the computer means includes means for calculating ac power by integrating and averaging the product of each current and voltage sample.

5. The apparatus of claim 4 wherein the current used in the calculation is the average of the current sampled before and after the voltage sample and the means for calculating ac power includes means for averaging said sampled current.

6. The apparatus of claim 4 wherein the voltage used in the calculation is the average of the voltage sampled before and after the current sample and the means for calculating ac power includes means for averaging said sampled voltage.

7. The apparatus of claim 1 in which the derived values are one or more of the following group and means are provided for calculating said derived values as follows:

space="preserve" listing-type="equation">Apparent power=AP=IR×

VR

space="preserve" listing-type="equation">Power Factor=PF=PR/(IR×

VR)

space="preserve" listing-type="equation">Resistance=PR=RR/IR.sup.2

space="preserve" listing-type="equation">Reactance=ZX=RP/IR.sup.2

space="preserve" listing-type="equation">Conductance=YG=PR/VR.sup.2

space="preserve" listing-type="equation">Susceptance=YB=-RP/VR.sup.2

8. The apparatus of claim 7 in which the derived values are calculated based on the averaged values of RMS voltage VR, RMS current IR, real power PR, and reactive power RP and means are provided for calculating said average values and said derived values, as follows:

space="preserve" listing-type="equation">AP=IR×

VR

space="preserve" listing-type="equation">PF=PR/(IR×

VR)

space="preserve" listing-type="equation">ZR=PR/IR.sup.2

space="preserve" listing-type="equation">ZX=RP/IR.sup.2

space="preserve" listing-type="equation">YG=PR/VR.sup.2

space="preserve" listing-type="equation">YB=-RP/VR.sup.2

9. The apparatus of claim 1 in hwich the sampling means includes means for sampling the analog current and voltage samples alternately and coupling each circuit successively to the analog to digital means.

10. The apparatus of claim 9 including means for alternately sampling the current and voltage values.

11. The apparatus of claim 1 includng means for determining the rate at which the sampling means samples the waveforms of each circuit and time when the averaged stored sums are outputted in one of two alternate modes comprising:

(i) an automatic mode in which the output is automatically supplied at fixed time intervals;

or

(ii) a polled mode in which the output is only supplied upon demand.

13. The apparatus of claim 1 or 12 in which one of the non-derived values is total harmonic distortion (THD) for a current or voltage waveform and means are provided for calculating RMS voltage by computing the RMS value of the fundametal frequency of the waveform (A_F) using Fourier integrals;

substracting this RMS value from the RMS value of the waveform, A_RMS and normalizing the resultant value to the fundamental, and multiplying by 100 to yield a percentage as follows;

##EQU10## wherein;

N=number of samples taken per cycle

W_i =current or voltage value of ith sample.

12. Apparatus for determining the value of a plurality of ac power parameters of a plurality of ac analog circuits each of which have separate analog electrical waveforms propagating in the circuit comprising:

(a) sampling means for periodically sampling, during a first period of time for each circuit in succession, the analog current and voltage of the waveforms of each circuit a plurality of times during each cycle of the waveforms and over at least several cycles of the waveforms;

(b) converting means for converting the ac current and voltage samples to digital signals proportional to the current or voltage value of the samples;

(c) storage means for storing such digital signals;

(d) computing means for calculating and averaging certain non-derived values of power parameters of the circuits directly from such digital signals during intervals between the taking of samples and storing such values, and, for adding the calculated values into running sums and storing said sums during a later time period for each circuit, and during a later period of time, averaging the stored sums selected for outputting; and

(e) output means for interfacing said computing means with a data communications system whereby certain values can be selected for calculation and outputted.

14. The apparatus of claim 12 wherein one of the non-derived values is real power which is calculated as the sum of the current and voltage, and wherein the current value, at a time synchronous with any voltage value V_i, is approximated by averaging the current values immediately preceding the voltage measurement, I_i-1/2, and following the voltage measurement, I_i+1/2, so that real power PR is computed as a known constant times the average of the sum of V_i (I_i-1/2 +I_i+1/2).

15. The apparatus of claim 12 wherein one of the non-derived values is real power which is calculated as the sum of the current and voltage, and wherein the voltage value, at a time synchronous with any current value I_i, is approximated by averaging the voltage values immediately preceding the current measurement, V_i-1/2, and following the current measurement, V_i+1/2, so that real power PR is computed as a known constant times the average of the sum of I_i (V_i-1/2 +V_i+1/2).

16. The method of determining the value of a plurality of ac power parameters of a plurality of ac analog circuits each of which have separate analog electrical waveforms propagating in the circuit comprising the steps of:

(a) periodically sampling, during a first period of time for each circuit in succession, the analog current and voltage waveforms of each circuit a plurality of times during each cycle of the waveform and over at least several cycles of the waveforms;

(b) converting, during said first period of time, the ac current and voltage samples to digital signals proportional to the current or voltage value of the samples;

(c) storing such digital signals during said first period of time;

(d) calculating and averaging certain non-derived values of power parameters of the circuits directly from such digital signals during intervals between taking of samples and storing such values, and, during a second later variable time period for each circuit deriving the values of additional power parameters from such stored values and, for adding the calculated values into running sums and storing said sums during a third later time period for each circuit, and during a fourth later time period, averaging the stored sums selected for outputting.

17. The method of claim 16 including the step of, during a fifth period of time, measuring the frequency of the analog current and voltage waveforms by timing several cycles of the waveforms using the zero crossover point of the waveform as the starting point and dividing by the number of cycles timed.

18. The apparatus of claim 16 wherein the sampling of either the current or voltage value occurs before and after the sampling of the value.

19. The method of claim 16 wherein during step (d), a table of squares is generated for use in RMS calculations and such table is stored.

20. The method of claim 16 wherein the values to be calculated are selected in accordance with instructions stored in memory.

21. The apparatus of claim 16 wherein the sampling of the voltage value occurs about half-way between successive samples of current value.

22. The apparatus of claim 16 wherein the sampling of the current value occurs about half-way between successive samples of the voltage value.