Harmonic measuring instrument for AC power systems with a time-based threshold means

• US 5,300,924 A
• Filed: 08/21/1992
• Issued: 04/05/1994
• Est. Priority Date: 08/21/1992
• Status: Expired due to Fees

• Current Assignee: Basic Measuring Instruments, Inc.
• Original Assignee: Basic Measuring Instruments, Inc.
• Inventors: McEachern, Alexander, Nicholson, Jamie
• Primary Examiner(s): Hofsass, Jeffery A.
• Application Number:
  US07/934,355
• Time in Patent Office: 592 Days
• Field of Search: 340/664, 340/660, 340/661, 340/662, 340/658, 340/657, 364/483-484, 361/113, 324/623, 324/107-142
• 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 rot 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 an alternating current power system comprising:

a. means for sensing a predetermined harmonic voltage parameter and a predetermined harmonic current parameter coupled to the alternating current power system;

b. means for comparing the harmonic voltage parameter to a predetermined voltage threshold and for comparing the harmonic current parameter to a predetermined current threshold coupled to the means for sensing;

c. means for calculating a first time parameter during which the predetermined voltage threshold was exceeded by the harmonic voltage parameter and a second time parameter during which the predetermined current threshold was exceeded by the harmonic current parameter coupled to the means for comparing; and

d. means for indicating when either the first time parameter or the second time parameter exceed a predetermined time threshold.

2. The apparatus according to claim 1 further comprising:

a. means for sensing each harmonic voltage parameter and a each harmonic current parameter coupled to the alternating current power system;

b. means for comparing each of the harmonic voltage parameters to a respective predetermined voltage threshold and for comparing each of the harmonic current parameters to a respective predetermined current threshold coupled to the means for sensing;

c. means for calculating a selected first time parameter for any one of the harmonic voltage parameters during which the respective predetermined voltage threshold was exceeded by that harmonic voltage parameter and a selected second time parameter for any one of the harmonic current parameters during which the respective predetermined current threshold was exceeded by that harmonic current parameter coupled to the means for comparing; and

d. means for indicating when either the selected first time parameter or the selected second time parameter exceed a predetermined time threshold.

3. The apparatus according to claim 1 further comprising means for measuring total harmonic distortion.

4. The apparatus according to claim 2 further comprising means for measuring total harmonic distortion.

5. The apparatus as claimed in claim 1 wherein the alternating current power system is a poly-phase system and the means for sensing is coupled to each phase of the poly-phase system.

6. The apparatus as claimed in claim 1 further comprising a means for communicating when either the first time parameter or the second time parameter exceeds the predetermined time threshold.

7. The apparatus as claimed in claim 1 wherein the means for calculating comprises means for measuring a first percentage of time that the predetermined voltage threshold was exceeded and a second percentage of time that the predetermined current threshold was exceeded.

8. The apparatus as claimed in claim 1 wherein the means for calculating comprises means for measuring a first minimum continuous duration of time in which the predetermined voltage threshold was exceeded and a second minimum continuous duration of time in which the predetermined current threshold was exceeded.

9. The apparatus as claimed in claim 1 wherein the means for calculating comprises means for measuring a first rolling percentage of time over a predetermined time interval in which the predetermined voltage threshold was exceeded and a second rolling percentage of time over a predetermined time interval in which the predetermined current threshold was exceeded.

10. An apparatus for measuring voltage harmonic parameters and current harmonic parameters on an alternating current power system comprising:

a. a first means for sensing a plurality of harmonic voltage parameters coupled to the alternating current power system;

b. a second means for sensing a plurality of harmonic current parameters coupled to the alternating current power system;

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

d. a first means for calculating a first time parameter during which the predetermined voltage threshold was exceeded coupled to the first means for determining;

e. a second means for determining when one of the plurality of harmonic current parameters exceeds a predetermined current threshold coupled to the second means for sensing;

f. a second means for calculating a second time parameter during which the predetermined voltage threshold was exceeded coupled to the second means for determining;

g. a fifth means for determining when either the first time parameter or the second time parameter exceeds a predetermined time threshold coupled to the third means for determining and to the fourth means for determining; and

h. means for indicating when either the first time parameter or the second time parameter exceeds a predetermined time threshold coupled to the fifth means for determining.

11. The apparatus as claimed in claim 10 wherein the alternating current power system is a poly-phase system and the first means for sensing and the second means for sensing are both coupled to each phase.

12. The apparatus as claimed in claim 10 further comprising a means for communicating when either the first time parameter or the second time parameter exceed the predetermined time threshold.

13. The apparatus as claimed in claim 11 wherein the first means for determining when one of the plurality of harmonic voltage parameters exceeds a predetermined voltage threshold comprises a microprocessor subsystem which calculates the square root of the sum of the squares of the harmonic voltage parameters on each phase and compares the result to the predetermined voltage threshold.

14. The apparatus as claimed in claim 10 wherein the second means for determining when one of the plurality of harmonic current parameters exceeds a predetermined current threshold comprises the microprocessor subsystem which calculates the square root of the sum of the squares of the harmonic current parameters on each phase and compares the result to the predetermined current threshold.

15. The apparatus as claimed in claim 10 wherein the first means for calculating comprises means for measuring a percentage of time that the predetermined voltage threshold was exceeded.

16. The apparatus as claimed in claim 12 wherein the second means for calculating comprises means for measuring the percentage of time that the predetermined current threshold was exceeded.

17. The apparatus as claimed in claim 10 wherein the first means for calculating comprises means for measuring a minimum continuous duration of time in which the predetermined voltage threshold was exceeded.

18. The apparatus as claimed in claim 14 wherein the second means for calculating comprises means for measuring a minimum continuous duration of time in which the predetermined current threshold was exceeded.

19. The apparatus as claimed in claim 10 wherein the first means for calculating comprises means for measuring a rolling percentage of time over a predetermined time interval in which the predetermined voltage threshold was exceeded.

20. The apparatus as claimed in claim 16 wherein the second means for calculating comprises means for measuring a rolling percentage of time over the predetermined time interval in which the predetermined current threshold was exceeded.

21. An apparatus for measuring voltage and current harmonic parameters on an alternating current power system having a power line conductor and a power neutral conductor, comprising:

a. a current transformer coupled to the power line conductor;

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

c. a first amplifier coupled to the 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. an attenuator coupled to the power line conductor and the power neutral conductor for receiving a second voltage signal which represents a voltage difference between the power line conductor and the power neutral conductor;

f. a second amplifier coupled to the 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 first analog-to-digital converter coupled to the first filter for converting the analog first voltage signal to a first digital time-domain signal;

i. a second analog-to-digital converter coupled to the second filter for converting the analog second voltage signal to a second digital time-domain signal;

j. means for processing digital signals coupled to the first and the second analog-to-digital converters for converting the first time-domain signal to a first frequency-domain representation having a plurality of current harmonic parameters representing the current in the power line and for converting the second time-domain signal to a second frequency-domain representation having a plurality of voltage harmonic parameters representing the voltage difference between the power line conductor and the power neutral conductor;

k. a control subsystem coupled to the digital signal processing circuit for calculating a first time parameter during which the current harmonics exceed a predetermined current threshold and a second time parameter during which the voltage harmonics exceed a predetermined voltage threshold; and

l. a display coupled to the microprocessor subsystem for indicating when either the first time parameter or the second time parameter exceed a predetermined time threshold.

22. The apparatus according to claim 21 wherein the control subsystem is a microprocessor.

23. The apparatus according to claim 22 wherein the means for processing digital signals is performed by the microprocessor.

24. The apparatus according to claim 22 wherein the means for processing digital signals is performed by a digital signal processor.