Substation load distribution monitor system

• US 5,406,495 A
• Filed: 02/01/1993
• Issued: 04/11/1995
• Est. Priority Date: 02/01/1993
• Status: Expired due to Fees

• Current Assignee: Imperial Bank (Imperial Bancorp)
• Original Assignee: Systems Analysis & Integration, Inc.
• Inventors: Hill, Gregory O.
• Primary Examiner(s): Voeltz, Emanuel T.
• Assistant Examiner(s): Stamber, Eric W.
• Application Number:
  US08/012,280
• Time in Patent Office: 799 Days
• Field of Search: 364/550, 364/483, 364/492, 364/487, 324/142, 324/76.11, 324/76.21, 324/76.24, 324/141, 341/123, 323/205, 323/212
• US Class Current: 702/72
• CPC Class Codes: G01R 21/1331 Measuring real or reactive component, measuring apparent energy
• CPC Class Codes: H02J 13/00017 using optical fiber
• CPC Class Codes: H02J 13/00034 the elements or equipment being or involving an electric power substation
• CPC Class Codes: Y02E 60/00 Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
• CPC Class Codes: Y04S 40/124 using wired telecommunication networks or data transmission busses

Abstract

The monitoring and control system of the present invention provides a distributed intelligence, data acquisition and control system which collects and analyzes large amounts of data representing power usage from a power distribution substation. Using a Discrete Fourier Transform, the system provides accurate tracking of the primary frequency of the voltage and current waveforms in the power equipment, and determines the relative phase between the voltage and current waveforms. The system provides real time monitoring of power usage and real time control of various functions in the substation.

Claims

1. A method of monitoring power distribution equipment carrying voltage and current to distribute electrical power, said monitoring method comprising the steps of:

sampling periodic waveforms representative of said current and said voltage within said power equipment to obtain samples which are substantially equally spaced in time so that said samples have a sample period, said samples having magnitudes and wherein a value derived from said magnitudes of said samples is used as feedback to maintain a sample rate such that a period of the periodic waveforms is substantially an integer multiple of said sample period; and

analyzing said sample values by means of a discrete Fourier Transform to obtain a relative phase between the voltage and current waveforms.

2. A method of determining the relative phase between a periodic voltage waveform and a periodic current waveform carried in power equipment, said determining method comprising the steps of:

simultaneously sampling said periodic current waveform and said periodic voltage waveform at a sampling rate to obtain first and second sets of sample values, said first set of sample values comprising a representation of said periodic current waveform with discrete sample values substantially equally spaced in time to define a first sample period, said second set of sample values comprising a representation of said periodic voltage waveform with discrete sample values substantially equally spaced in time to define a second sample period and wherein a feedback value derived from said first and second sets of sample values is used as feedback to automatically adjust said sample rate to insure that a period of said periodic current waveform is substantially an integer multiple of said first sample period and a period of said periodic voltage waveform is substantially an integer multiple of said second sample period;

applying a discrete Fourier Transform to the first and second sets of sample values of said periodic voltage waveform and said periodic current waveform to obtain a phase of said periodic voltage waveform and a phase of said periodic current waveform; and

comparing the phase of said periodic voltage waveform and said periodic current waveform to obtain a relative phase between said periodic voltage waveform and said periodic current waveform.

3. The method of claim 2, wherein said step of sampling comprises digitally sampling said periodic current waveform and said periodic voltage waveform through an analog to digital convertor.

4. The method of claim 2, further comprising the steps of:

calculating an RMS voltage from said second set of sample values; and

calculating an RMS current from said first set of sample values.

5. The method of claim 4, further comprising the steps of:

multiplying the RMS voltage by the RMS current to obtain a volt-amps value.

6. The method of claim 5, further comprising the steps of:

multiplying said volt-amps value by the cosine of the relative phase to determine the watts.

7. The method of claim 5, further comprising the steps of:

multiplying said volt-amps value by the sine of the relative phase to determine the vars.

8. A method of taking an integer number of sample values per cycle of a periodic voltage or current waveform carried by power equipment, said method comprising the steps of:

sampling said periodic voltage or current waveform at a selected sampling rate corresponding to a first expected frequency to obtain a first set of samples equally spaced in time to define a first sample period and covering substantially a first cycle of said periodic voltage or current waveform;

transforming the samples from said first set of samples by means of a discrete Fourier Transform to obtain a first X transform value and a first Y transform value;

sampling said periodic voltage or current waveform at said selected sampling rate to obtain a second set of samples equally spaced in time to define a second sample period, said second set of samples covering at least a portion of a second cycle of said periodic voltage or current waveform;

transforming at least one of the samples from said second set of samples by means of a discrete Fourier Transform to obtain a second X transform value and a second Y transform value;

comparing said first Y transform value with said second X transform value; and

adjusting said selected sampling rate in response to said comparison to insure that a period of said periodic voltage or current waveform is substantially an integer multiple of said first and second sample periods.

9. The method of claim 8, wherein said step of adjusting said selected sampling rate comprises the steps of:

comparing said first Y transform value with said second X transform value, and if said first Y transform value does not equal the negative of said second X transform value, comparing said second Y transform value with zero and summing the second X transform value with said first Y transform value to generate a sum;

if said second Y transform value is less than zero, and the sum of said second X transform value and said first Y transform value is less than zero, decreasing said selected sampling rate;

if said second Y transform value is less than zero, and the sum of said second X transform value and said first Y transform value is greater than zero, increasing said selected sampling rate;

if said second Y transform value is greater than or equal to zero, and the sum of said second X transform and said first Y transform value is less than zero, increasing said selected sampling rate; and

if said second Y transform value is greater than or equal to zero, and the sum of said second X transform and said first Y transform value is greater than zero, decreasing said selected sampling rate.

10. The method of claim 8, wherein said step of adjusting said selected sampling rate comprises the steps of:

comparing the second X transform value, the first Y transform value, and the second Y transform to determine if said selected sampling rate is too slow to obtain a predetermined integer number of samples per cycle of said periodic voltage or current waveform; and

if said selected sampling rate is too slow, increasing said selected sampling rate of an analog to digital converter which obtains said first set of samples and said second set of samples.

11. The method of claim 8, wherein said step of adjusting said selected sampling rate comprises the steps of:

comparing the second X transform value, the first Y transform value, and the second Y transform to determine if said selected sampling rate is too fast to obtain a predetermined integer number of samples per cycle of said periodic voltage or current waveform; and

if said selected sampling rate is too fast, decreasing selected sampling rate of an analog to digital converter which obtains said first set of samples and said second set of samples.