INSTANTANEOUS PHASOR DETERMINATION FOR POLY-PHASE ELECTRICAL GRIDS

• US 20130051498A1
• Filed: 08/31/2011
• Published: 02/28/2013
• Est. Priority Date: 08/31/2011
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Taft, Jeffrey D.
• Granted Patent: US 8,532,230 B2
• Time in Patent Office: Days
• US Class Current: 375/340
• CPC Class Codes: G01R 19/2513 Arrangements for monitoring electric power systems, e.g. power lines or loads; Logging
• CPC Class Codes: Y02E 40/70 Smart grids as climate change mitigation technology in the energy generation sector
• CPC Class Codes: Y02E 60/00 Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
• CPC Class Codes: Y04S 10/00 Systems supporting electrical power generation, transmission or distribution
• CPC Class Codes: Y04S 10/22 Flexible AC transmission systems [FACTS] or power factor or reactive power compensating or correcting units

Abstract

In one embodiment, three substantially simultaneous phase waveforms may be converted into a first quadrature signal and a zero sequence signal. For each phase waveform, a power system digital frequency may be determined through analysis of the first quadrature signal (e.g., and at least one additional prior quadrature signal) while eliminating waveform phase angles from the analysis. Subsequently, demodulation of the first quadrature signal and zero sequence signal based on the power system digital frequency results in a positive sequence phasor, a negative sequence phasor, and a zero sequence phasor.

Claims

1. A computer-implemented method, comprising:

converting three substantially simultaneous phase waveforms into a first quadrature signal and a zero sequence signal;

determining a power system digital frequency for each phase waveform through analysis of the first quadrature signal while eliminating waveform phase angles from the analysis; and

demodulating, based on the power system digital frequency, the first quadrature signal and zero sequence signal into a positive sequence phasor, a negative sequence phasor, and a zero sequence phasor.

2. The method as in claim 1, further comprising:

converting symmetrical components of the positive sequence phasor, negative sequence phasor, and zero sequence phasor into an unbalanced phasor set.

3. The method as in claim 1, wherein determining the power system digital frequency is through analysis of the sequence of the first quadrature signal and two additional prior quadrature signals.

4. The method as in claim 1, further comprising:

converting the power system digital frequency into an analog system angular frequency output.

5. The method as in claim 1, wherein demodulating comprises:

obtaining the positive sequence phasor by i) upshifting the first quadrature signal to move a negative sequence modulation frequency to zero and ii) removing a first double frequency component resulting from the upshifting to obtain the positive sequence phasor; and

obtaining the negative sequence phasor by i) downshifting the first quadrature signal to move a positive sequence modulation frequency to zero and ii) removing a second double frequency component resulting from the downshifting to obtain the negative sequence phasor.

6. The method as in claim 5, wherein demodulating further comprises:

obtaining the zero sequence phasor through a single demodulation that uses one of either a positive frequency or a negative frequency of the zero sequence signal.

7. The method as in claim 5, wherein removing the first and second double frequency components comprises:

utilizing a plurality of notch filters to remove the first and second double frequency components.

8. The method as in claim 7, wherein the plurality of notch filters are synthesized based on recursive feedback stabilization to provide sharply defined second order filtering.

9. The method as in claim 7, further comprising:

adapting one or more filter coefficients of the plurality of notch filters in response to a frequency deviation greater than a threshold between the power system digital frequency for the first quadrature signal and the at least one additional prior quadrature signal.

10. The method as in claim 1, further comprising:

determining that the three substantially simultaneous phase waveforms are balanced; and

wherein demodulating comprises, in response to determining that the three substantially simultaneous phase waveforms are balanced, demodulating, based on the power system digital frequency, the first quadrature signal into the positive sequence phasor only, wherein the negative sequence phasor and zero sequence phasor are exactly zero.

11. The method as in claim 10, wherein determining that the three substantially simultaneous phase waveforms are balanced comprises determining the balance based on the zero sequence signal prior to determining the power system digital frequency and demodulating.

12. The method as in claim 1, wherein determining the power system digital frequency is through analysis of the sequence of the first quadrature signal and at least one prior quadrature signal.

13. The method as in claim 1, further comprising:

utilizing a first complex coefficient notch filter between an output of the transformer and an input of the demodulator for the positive sequence phasor;

utilizing a second complex coefficient notch filter between the output of the transformer and an input of the demodulator for the zero sequence phasor; and

utilizing a complex subtractor between the output of the transformer and an input of the demodulator for the negative sequence phasor to subtract an output of the first complex coefficient notch filter from the output of the transformer for the input of the demodulator for the negative sequence phasor.

14. The method as in claim 1, wherein the positive sequence phasor, negative sequence phasor, and zero sequence phasor are synchrophasors.

15. An apparatus, comprising:

a Clarke transformer to convert three substantially simultaneous phase waveforms into a first quadrature signal and a zero sequence signal;

an instantaneous frequency evaluator to determine a power system digital frequency for each phase waveform through analysis of the first quadrature signal while eliminating waveform phase angles from the analysis; and

a complex frequency-adaptive synchrodyne demodulator to demodulate, based on the power system digital frequency, the first quadrature signal and zero sequence signal into a positive sequence phasor, a negative sequence phasor, and a zero sequence phasor.

16. The apparatus as in claim 15, further comprising:

an inverse Fortescue transformer to convert symmetrical components of the positive sequence phasor, negative sequence phasor, and zero sequence phasor into an unbalanced phasor set.

17. The apparatus as in claim 15, wherein the demodulator comprises:

a first portion to obtain the positive sequence phasor by i) an upshift of the first quadrature signal to move a negative sequence modulation frequency to zero and ii) a removal of a first double frequency component resulting from the upshift to obtain the positive sequence phasor;

a second portion to obtain the negative sequence phasor by i) a downshift of the first quadrature signal to move a positive sequence modulation frequency to zero and ii) a removal a second double frequency component resulting from the downshifting to obtain the negative sequence phasor; and

a third portion to obtain the zero sequence phasor through a single demodulation that uses one of either a positive frequency or a negative frequency of the zero sequence signal.

18. The apparatus as in claim 17, further comprising:

a plurality of notch filters to remove the first and second double frequency components.

19. The apparatus as in claim 15, wherein the demodulator is further configured to, in response to a determination that the three substantially simultaneous phase waveforms are balanced:

demodulate, based on the power system digital frequency, the first quadrature signal into the positive sequence phasor only, wherein the negative sequence phasor and the zero sequence phasor are exactly zero.

20. The apparatus as in claim 15, wherein determining the power system digital frequency is through analysis of the sequence of the first quadrature signal and at least one prior quadrature signal.

21. The apparatus as in claim 15, further comprising:

a first complex coefficient notch filter between an output of the transformer and an input of the demodulator for the positive sequence phasor;

a second complex coefficient notch filter between the output of the transformer and an input of the demodulator for the zero sequence phasor; and

a complex subtractor between the output of the transformer and an input of the demodulator for the negative sequence phasor to subtract an output of the first complex coefficient notch filter from the output of the transformer for the input of the demodulator for the negative sequence phasor.

24. The computer-readable media as in claim 21, wherein the software when executed is further operable to:

determine that the three substantially simultaneous phase waveforms are balanced based on the zero sequence signal prior to determining the power system digital frequency and demodulating; and

in response;

demodulate, based on the power system digital frequency, the first quadrature signal into the positive sequence phasor only, wherein the negative sequence phasor and the zero sequence phasor are exactly zero.

25. The computer-readable media as in claim 21, wherein the software when executed is further operable to:

implement a first complex coefficient notch filter between an output of the transformer and an input of the demodulator for the positive sequence phasor;

implement a second complex coefficient notch filter between the output of the transformer and an input of the demodulator for the zero sequence phasor; and

implement a complex subtractor between the output of the transformer and an input of the demodulator for the negative sequence phasor to subtract an output of the first complex coefficient notch filter from the output of the transformer for the input of the demodulator for the negative sequence phasor.

22. A tangible, non-transitory, computer-readable media having software encoded thereon, the software, when executed by a processor, operable to:

convert three substantially simultaneous phase waveforms into a first quadrature signal and a zero sequence signal;

determine a power system digital frequency for each phase waveform through analysis of the first quadrature signal while eliminating waveform phase angles from the analysis; and

demodulate, based on the power system digital frequency, the first quadrature signal and zero sequence signal into a positive sequence phasor, a negative sequence phasor, and a zero sequence phasor.

23. The computer-readable media as in claim 22, wherein the software when executed is further operable to:

convert symmetrical components of the positive sequence phasor, negative sequence phasor, and zero sequence phasor into an unbalanced phasor set.