Method and apparatus providing half-cycle digitization of AC signals by an analog-to-digital converter

• US 5,315,527 A
• Filed: 01/03/1992
• Issued: 05/24/1994
• Est. Priority Date: 01/03/1992
• Status: Expired due to Term

• Current Assignee: Robert W. Beckwith
• Original Assignee: Robert W. Beckwith
• Inventors: Beckwith, Robert W.
• Primary Examiner(s): Black, Thomas G.
• Assistant Examiner(s): ZANELLI, MICHAEL J
• Application Number:
  US07/816,242
• Time in Patent Office: 872 Days
• Field of Search: 364/481, 364/483, 364/492, 364/487, 324/76.39, 324/76.52, 324/76.55, 324/76.75, 324/76.77, 324/76.82, 307/236, 341/127, 341/131
• US Class Current: 702/72
• CPC Class Codes: G01R 19/25 using digital measurement techniques

Abstract

A circuit, and a processing and calculation method that makes use of the digital samples of either the positive or negative half-cycle of voltage or current signals to achieve a sixteen to one improvement in accuracy for controls connectable to an electrical utility system.

Claims

1. Apparatus for receiving and digitizing alternating current (AC) signals of the same frequency and for determining the phase angle between two received signals, each signal coming from a pair of terminals with the potential of one terminal being the ground reference for said signal and with the signal potential of the second terminal alternating about said reference potential, comprising in combination,

a) analog-to-digital converter means (ADC) having at least two analog inputs, having a low and a high reference input and having a range of operation starting with zero for an analog input equal to said low reference input voltage,

b) a high ADC reference voltage connected to said high reference input,

c) at least two of said second signal terminals being connected to said ADC analog inputs,

d) said ADC having said low reference input connected to each said AC signal ground terminal to set the change of signal polarity to said ADC zero,

e) said ADC providing periodic non-zero digital samples proportional to said AC signals whenever the polarity of said AC signal is the same as said high reference voltage and providing digital samples which are zeros whenever the polarity of said AC signal is not the same as said high reference voltage,

f) means for receiving and processing said digital samples and sensing transitions in said digital samples from zero to non-zero (znz) and non-zero to zero (nzz),

g) means for determining the phase angle between any two received AC signals after sensing (1) at least one pair of said znz transitions, the first from a first AC signal and the second sequentially from the second AC signal, and (2) at least one pair of said nzz transitions, the first from said first AC signal and the second sequentially from said second AC signal, and

h) means for averaging the phase angle result from said first pair of transitions with the results from said second pair of transitions to obtain the phase angle between the centers of the two signals.

2. Apparatus as in claim 1 for determining the phase angle between any two received AC signals when essentially the same number of samples are obtained for each of the two signals, said means for sensing said transitions senses only said znz transitions, the first from a first AC signal and the second sequentially from a second AC signal to determine said phase angle.

3. Apparatus as in claim 1 for determining the phase angle between any two received AC signals when essentially the same number of samples are obtained for each of the two signals, said means for sensing said transitions senses only said nzz transitions, the first from a first AC signal and the second sequentially from a second AC signal to determine said phase angle.

4. Apparatus as in claim 1 further including:

a) processor means,

b) means for entering data into said processor,

c) means for measuring said high ADC reference voltage and entering said measurement into said processor means,

d) means for providing a scaling factor for said AC signals, and

e) said processor means determining a correction to said scaling factor depending on said measurement.

5. Apparatus for digitizing alternating current (AC) signals of the same frequency, each signal coming from a pair of terminals with the potential of one terminal being the ground reference for said signal and with the signal potential of the second terminal alternating about said reference potential comprising, in combination,

a) analog-to-digital converter means (ADC) having at least one analog input, having a low and a high reference voltage input and having a range of operation starting with zero for an analog input equal to said low reference input voltage,

b) a high ADC reference voltage connected to said high reference input,

c) at least one said second signal terminal being connected to said ADC analog inputs resulting in signals on said second terminals having a selected polarity which is the same as the polarity of said ADC high voltage reference,

d) said ADC having said low reference input connected to each said AC signal ground terminal to set the change of signal polarity to said ADC zero,

e) said ADC providing periodic non-zero digital samples proportional to said AC signals whenever the polarity of said AC signal is the same as said high reference voltage and providing digital samples which are zero whenever said AC signals are not of the selected polarity, and

f) means for receiving and processing said non-zero digital samples to make desired measurements to obtain accurate parameters of said entire AC signals.

6. Apparatus for digitizing alternating current (AC) signals of the same electric power system frequency and wherein at least one said signal represents AC current having harmonics and produced by a current transformer with said signal coming from a pair of terminals with the potential of one terminal being the ground reference for said signal and with the signal potential of the second terminal alternating about said reference potential comprising, in combination,

a) analog-to-digital converter means (ADC) having at least one analog input representing said AC current, having a low and a high reference voltage input and having a range of operation starting with zero for an analog input equal to said low reference input voltage,

b) a high ADC reference voltage connected to said high reference input,

c) capacitive burden means connected from each said current signal second terminal to said current signal ground terminal thereby forming a low-pass filter with approximately 6 decibels per octave reduction in the amplitude of said signal harmonics,

d) each said second current signal terminals being connected to said ADC analog inputs resulting in signals on said second terminals having a selected polarity which is the same as the polarity of said ADC high reference voltage,

e) said ADC having said low reference input connected to each said AC signal ground terminal to set the change of signal polarity to said ADC zero,

f) said ADC providing periodic non-zero digital samples proportional to said AC signals whenever the polarity of said AC signal is the same as said high reference voltage and providing digital samples which are zeros whenever said AC signal is not of the selected polarity, and

g) means for receiving and processing said digital samples to make desired measurements of said signals using samples proportional to the selected polarity.

7. Apparatus for digitizing alternating current (AC) signals of the same frequency, each signal coming from a pair of terminals with the potential of one terminal being the ground reference for said signal and with the signal potential of the second terminal alternating about said reference potential comprising, in combination,

a) analog-to-digital converter means (ADC) using an all-capacitive charge-distribution means for conversions using capacitors having fixed tolerances, said ADC having at least one analog input, having a low and a high voltage reference input and having a range of operation starting with zero for an analog input equal to said low reference input voltage,

b) a high ADC reference voltage connected to said high reference input,

c) at least one second signal terminal being connected to said ADC analog input resulting in signals on said second terminals having a selected polarity which is the same as the polarity of said ADC high reference voltage,

d) said ADC having said low reference input connected to each said AC signal ground terminal to set a change of signal polarity to said ADC zero,

e) said ADC providing at least one hundred periodic non-zero digital samples proportional to said AC signals whenever the polarity of said AC signal is the same as said high reference voltage and providing digital samples which are zeros whenever said AC signal is not of the selected polarity, and

f) means for receiving and processing said digital samples to make desired measurements of said signal samples proportional to the selected polarity, to obtain accurate parameters of the entire signal.

8. In a method performed by an apparatus for providing selective measuring and controlling of electric power parameters, said apparatus including a processor and an analog to digital converter means (ADC) for receiving alternating current (AC) signals of the power frequency, said method consisting of the steps of:

a) obtaining periodic digital samples of said AC signals from said ADC and conveying said samples to said processor,

b) determining zero to non-zero (znz) transistors in the value of said samples and thereby selecting samples representing a selected polarity of said AC signals,

c) processing said samples to make a partial computation for a desire measurement using only samples of the selected polarity until a non-zero to zero (nzz) transition in the value of said samples is obtained, and

d) stopping said periodic sampling and completing said computations to obtain desired signal measuring and controlling parameters.

9. The method as in claim 8 wherein step c) consists of:

c) squaring each said sample, forming the sum of said squares, counting the number of squares,

and wherein step d) consists of;

d) dividing the sum of the squares by the number of samples, multiplying by the sampling period, taking the square root of the result and multiplying by two to obtain the rms amplitude of the entire signal.

10. The method as in claim 8 wherein step c) consists of:

c) forming the sum of said samples, counting the number of samples,

and wherein step d) consists of;

d) dividing the sum of the samples by the number of samples, multiplying by the sampling period and multiplying by two to obtain the average amplitude of the entire signal.

11. In a method performed by an apparatus for providing selective measuring and controlling of electric power parameters, said apparatus including a processor and an analog to digital converter means (ADC) for receiving alternating current (AC) signals of the power frequency, method consisting of the steps of:

a) obtaining from said ADC at least one hundred periodic digital samples of each said AC signal whenever the polarity of said signal is the same as the polarity of said high voltage reference and conveying said samples to said processor,

b) determining a zero to non-zero (znz) transition in the value of said samples and thereby selecting samples representing one selected polarity of said AC signals,

c) processing said samples to make a partial computation for a first desired measurement of a signal parameter until a non-zero to zero (nzz) transition in the value of said samples is obtained,

d) stopping said periodic sampling and completing said computations to obtain desired signal parameters of at least one bit greater resolution in the measurement of said parameter as compared to the maximum resolution of the ADC.

12. In a method performed by an apparatus for providing selective measuring and controlling of electric power parameters, said apparatus including a processor and an analog to digital converter means (ADC) for receiving alternating current (AC) signals of the power frequency and for determining the phase angle between two received signals, said method consisting of the steps of:

a) obtaining periodic digital samples of a first selected AC signal from said ADC and conveying said samples to said processor,

b) determining a first zero to non-zero (znz) transition in the value of said samples of said first selected signal,

c) determining a first non-zero to zero (nzz) transition in the value of said samples,

d) counting the number of samples between said first znz transition and said first nzz transition,

e) conveying samples of a second selected AC signal to said processor in place of said first selected AC signal,

f) determining a second znz transition in the values of said samples of said second selected AC signal,

g) determining a second nzz transition in the value of said samples of said second selected AC signal,

h) counting the number of samples between said second znz transition and said second nzz transition,

i) determining the average of the results of steps d) and h), and

j) using said average together with the period of said digital samples to determine the phase angle between the centers of said first selected signal and said second selected signal.

13. In a method performed by an apparatus for providing selective measuring and controlling of electric power parameters, said apparatus including a processor and an analog to digital converter means (ADC) for receiving alternating current (AC) signals of the power frequency, the method consisting of the steps of:

a) obtaining periodic digital samples of a first selected AC signal from said ADC and conveying said samples to said processor,

b) determining first zero to non-zero (znz) transitions in the value of said samples of said first selected signal and thereby selecting samples representing one selected polarity of said AC signal,

c) processing said samples to make a partial computation for a first desired measurement of the first selected AC signal using only samples of the selected polarity until a first non-zero to zero (nzz) transition in the value of said samples is obtained,

d) counting the number of samples between said first znz transition and said first nzz transition,

e) conveying samples of a second selected AC signal to said ADC in place of said first selected AC signal,

f) obtaining periodic digital samples of said second selected AC signal from said ADC and conveying said samples to said processor,

g) determining a second znz transition in the values of said samples of said second selected AC signal and thereby selecting samples representing said selected polarity of said AC signal,

h) processing said samples of said second selected signal to make a partial computation for a second desired measurement of said second selected AC signal using only samples of the selected polarity until a second nzz transition in the value of said samples is obtained,

i) counting the number of samples between said second znz transition and said second nzz transition,

j) stopping said periodic sampling,

k) determining the average of the results of steps d) and i),

l) using said average together with the period of said digital samples to determine the phase angle between said first selected signal and said second selected signal,

m) completing the computation of the first desired measurement to obtain parameters of the first selected signal, and

n) completing the computation of the second desired measurement to obtain parameters of the second selected signal.