Harmonic measuring instrument for AC power systems with poly-phase threshold means

• US 5,298,885 A
• Filed: 08/21/1992
• Issued: 03/29/1994
• Est. Priority Date: 08/21/1992
• Status: Expired due to Fees

• Current Assignee: Basic Measuring Instruments Co.
• Original Assignee: Basic Measuring Instruments, Inc.
• Inventors: McEachern, Alexander, Nicholson, Jamie
• Primary Examiner(s): Peng, John K.
• Assistant Examiner(s): Johnson, Tim
• Application Number:
  US07/933,391
• Time in Patent Office: 585 Days
• Field of Search: 340/557-663, 340/664, 324/76.12, 324/76.24, 324/76, 324/42, 324/141, 324/142, 324/107, 324/133, 324/542, 324/543, 324/539, 364/483, 364/484, 361/78, 361/113
• US Class Current: 340/660
• CPC Class Codes: G01R 23/165 using filters

Abstract

An instrument for measuring voltage harmonics and current harmonics on AC (alternating current) power systems The instrument includes the ability to determine when threshold limits have been exceeded for harmonics. The instrument also includes timing thresholds whose inputs are activated when the harmonic thresholds are exceeded A single harmonic threshold is applied to measurements from multiple phases on poly-phase systems by comparing the square root of the sum of the squares of measurements from each phase to the harmonic threshold. Three indicators show when voltage harmonic limits are exceeded, current harmonics limits are exceeded, or both voltage and current harmonic limits are exceeded simultaneously.

Claims

1. An apparatus for measuring voltage harmonics and current harmonics on a poly-phase alternating current power system comprising:

a. means for sensing a plurality of harmonic voltage parameters on each phase coupled to the poly-phase alternating current power systems;

b. means for determining when one of the plurality of harmonic voltage parameters exceeds a predetermined voltage threshold coupled to the means for sensing a plurality of harmonic voltage parameters;

c. means for sensing a plurality of harmonic currents on each phase coupled to the poly-phase alternating current power system;

d. means for determining when one of the plurality of the harmonic currents exceeds a predetermined current threshold coupled to the means for sensing a plurality of harmonic currents;

e. means for indicating when either the predetermined voltage threshold is exceeded, the predetermined current threshold is exceeded or both the predetermined voltage threshold and the predetermined current threshold are exceeded simultaneously, the means for indicating coupled to the means for determining when one of the plurality of harmonic voltage parameters exceeds a predetermined voltage threshold and to the means for determining when one of the plurality of the harmonic currents exceeds a predetermined current threshold.

2. The apparatus as claimed in claim 1 wherein the poly-phase alternating current power system comprises a first phase conductor, a second phase conductor, a third phase conductor and a neutral phase conductor and wherein the means for sensing a plurality of harmonic voltage parameters comprises a first attenuator coupled to the first phase conductor, a second attenuator coupled to the second phase conductor and a third attenuator coupled to the third phase conductor for sensing a voltage difference between each phase conductor and the neutral conductor.

3. The apparatus as claimed in claim 2 wherein the means for sensing a plurality of harmonic currents comprises a first current transformer and a first burden resistor both coupled to the first phase conductor, a second current transformer and a second burden resistor both coupled to the second phase conductor and a third current transformer and a third burden resistor both coupled to the third phase conductor each for sensing the current flowing through each phase conductor.

4. The apparatus as claimed in claim 1 wherein the poly-phase alternating current power system comprises a plurality of phase conductors and a neutral conductor, wherein the means for sensing a plurality of harmonic voltage parameters comprises an attenuator coupled between each one of the plurality of phase conductors and the neutral conductor for sensing a voltage difference therebetween.

5. The apparatus as claimed in claim 4 wherein the means for sensing a plurality of harmonic currents comprises a plurality of current transformers and a plurality of burden resistors, one of the plurality of current transformers and one of the plurality of burden resistors coupled to a respective one of the plurality of each for sensing the current flowing through each phase conductor.

6. The apparatus as claimed in claim 1 further comprising a means for communicating when either the predetermined voltage threshold is exceeded, the predetermined current threshold is exceeded or both the predetermined voltage threshold and the predetermined current threshold are exceeded, the means for communicating coupled to the means for determining when one of the plurality of harmonic voltage parameters exceeds a predetermined voltage threshold and to the means for determining when one of the plurality of the harmonic currents exceeds a predetermined current threshold.

7. The apparatus as claimed in claim 1 wherein the means for indicating comprises:

a. a first indicator light for indicating when one of the harmonic voltage parameters exceeds the predetermined voltage threshold;

b. a second indicator light for indicating when one of the harmonic parameters current exceeds the predetermined current threshold; and

c. a third indicator light for indicating when both one of the harmonic voltage parameters exceeds the predetermined voltage threshold and one of the harmonic current parameters exceeds the predetermined current threshold.

8. The apparatus as claimed in claim 5 wherein the means for determining when one of the plurality of harmonic voltages exceeds a predetermined voltage threshold comprises a microprocessor subsystem which calculates the square root of the sum of the squares of the harmonic voltages on each phase and compares the result to the predetermined voltage threshold.

9. The apparatus as claimed in claim 8 wherein the means for determining when one of the plurality of harmonic currents exceeds a predetermined current threshold comprises the microprocessor subsystem which calculates the square root of the sum of the squares of the harmonic currents on each phase and compares the result to the predetermined current threshold.

10. An apparatus for measuring voltage harmonics and current harmonics on a poly-phase alternating current power system having a phase A conductor, a phase B conductor, a phase C conductor and a neutral conductor, said apparatus comprising:

a. a first means for sensing a harmonic voltage and a harmonic current coupled to the phase A conductor and the neutral conductor;

b. a second means for sensing a harmonic voltage and a harmonic current coupled to the phase B conductor and the neutral conductor;

c. a third means for sensing a harmonic voltage and a harmonic current coupled to the phase C conductor and the neutral conductor;

d. a first means for calculating the square root of the sum of the squares of the harmonic voltages on each phase and storing the result as a value in a first register, the first means for calculating coupled to the first, second and third means for sensing;

e. a second means for calculating the square root of the sum of the squares of the harmonic currents on each phase and storing the result as a value in a second register, the second means for calculating coupled to the first, second and third means for sensing;

f. a first means for determining when the value stored in the first register exceeds a predetermined voltage threshold, the first means for determining coupled to the first means for calculating and to the first register;

g. a second means for determining when the value stored in the second register exceeds a predetermined current threshold, the second means for determining coupled to the second means for calculating and to the second register; and

h. means for indicating when either the predetermined voltage threshold is exceeded, the predetermined current threshold is exceeded or both the predetermined voltage threshold and the predetermined current threshold are exceeded, the means for indicating coupled to the first and second means for determining.

11. The apparatus as claimed in claim 10 further comprising means for communicating when either the predetermined voltage threshold is exceeded, the predetermined current threshold is exceeded or both the predetermined voltage threshold and the predetermined current threshold are exceeded, the means for communicating coupled to the first and second means for determining.

12. The apparatus as claimed in claim 11 wherein the means for indicating comprises:

a. a first indicator light for indicating when the harmonic voltage exceeds the predetermined voltage threshold;

b. a second indicator light for indicating when the harmonic current exceeds the predetermined current threshold; and

c. a third indicator light for indicating when both the harmonic voltage exceeds the predetermined voltage threshold and the harmonic current exceeds the predetermined current threshold.

13. An apparatus for measuring voltage harmonics and current harmonics on a poly-phase alternating current power system having a phase A conductor, a phase B conductor, a phase C conductor and a neutral conductor, said apparatus comprising:

a. a first current transformer coupled to the phase A conductor;

b. a first burden resistor coupled to the first current transformer for producing a first voltage signal which represents a current flowing in the phase A conductor;

c. a first amplifier coupled to the first burden resistor for adjusting the first voltage signal;

d. a first filter coupled to the first amplifier for removing undesired frequencies from the first voltage signal;

e. a first attenuator coupled to the phase A conductor and the neutral conductor for receiving a second voltage signal which represents a voltage difference between the phase A conductor and the neutral conductor;

f. a second amplifier coupled to the first attenuator for adjusting the second voltage signal;

g. a second filter coupled to the second amplifier for removing undesired frequencies from the second voltage signal;

h. a second current transformer coupled to the phase B conductor;

i. a second burden resistor coupled to the second current transformer for producing a third voltage signal which represents a current flowing in the phase B conductor;

j. a third amplifier coupled to the second burden resistor for adjusting the third voltage signal;

k. a third filter coupled to the third amplifier for removing undesired frequencies from the third voltage signal;

l. a second attenuator coupled to the phase B conductor and the neutral conductor for receiving a fourth voltage signal which represents a voltage difference between the phase B conductor and the neutral conductor;

m. a fourth amplifier coupled to the second attenuator for adjusting the fourth voltage signal;

n. a fourth filter coupled to the fourth amplifier for removing undesired frequencies from the fourth voltage signal;

o. a third current transformer coupled to the phase C conductor;

p. a third burden resistor coupled to the third current transformer for producing a fifth voltage signal which represents a current flowing in the phase C conductor;

q. a fifth amplifier coupled to the third burden resistor for adjusting the fifth voltage signal;

r. a fifth filter coupled to the fifth amplifier for removing undesired frequencies from the fifth voltage signal;

s. a third attenuator coupled to the phase C conductor and the neutral conductor for receiving a sixth voltage signal which represents a voltage difference between the phase C conductor and the neutral conductor;

t. a sixth amplifier coupled to the third attenuator for adjusting the sixth voltage signal;

u. a sixth filter coupled to the sixth amplifier for removing undesired frequencies from the sixth voltage signal;

v. an analog multiplexer having a plurality of inputs and an output, said inputs coupled to the first filter, the second filter, the third filter, the fourth filter, the fifth filter and the sixth filter for outputting the first, second, third, fourth, fifth and sixth voltage signals one at a time, each in turn;

w. an analog-to-digital converter having an input and an output, the input coupled to the output of the analog multiplexer for receiving the first, second, third, fourth, fifth and sixth voltage signals each in turn and outputting them as a regularly sampled digital time-domain representation;

x. a digital signal processing circuit having an input and an output, the input coupled to the output of the analog-to-digital converter for converting the regularly sampled digital time-domain representation received from the analog-to-digital converter into a frequency-domain representation having a plurality of harmonics;

y. a microprocessor subsystem coupled to the output of the digital signal processing circuit for grouping the harmonics received from the digital signal processing circuit into a first group and a second group, the first group representing the voltage difference between each phase and the neutral conductor and the second group representing the current flowing in each phase conductor and for determining if any of the harmonics received from the digital signal processing circuit exceed a predetermined threshold;

z. a display coupled to the microprocessor subsystem for indicating if any of the frequency-domain representations do exceed a predetermined threshold.

14. The apparatus according to claim 13 wherein the digital signal processing circuit is a microprocessor.

15. The apparatus according to claim 13 wherein the digital signal processing circuit is the microprocessor.

16. The apparatus as claimed in claim 13 wherein the microprocessor subsystem determines if any of the harmonics received from the digital signal processing circuit exceed a predetermined threshold by calculating the square root of the sum of the squares of the harmonics in the first group and compares the result to a predetermined voltage threshold and by calculating the square root of the sum of the square of the harmonics in the second group and compares the result to a predetermined current threshold.

17. The apparatus as claimed in claim 16 wherein the display comprises:

a. a first indicator light for indicating when the predetermined voltage threshold has been exceeded;

b. a second indicator light for indicating when the predetermined current threshold has been exceeded; and

c. a third indicator light for indicating when both the predetermined voltage threshold and the predetermined current threshold have been exceeded.

18. The apparatus as claimed in claim 17 further comprising a serial communications port coupled to the microprocessor subsystem for communicating when any of the predetermined thresholds have been exceeded.