Systems and methods for asynchronous sampling data conversion

• US 8,373,309 B2
• Filed: 09/04/2009
• Issued: 02/12/2013
• Est. Priority Date: 08/19/2009
• Status: Active Grant

• Current Assignee: Schweitzer Engineering Laboratories, Incorporated
• Original Assignee: Schweitzer Engineering Laboratories, Incorporated
• Inventors: Qin, Jianchun
• Primary Examiner(s): Fureman, Jared
• Assistant Examiner(s): PEREZ BORROTO, ALFONSO
• Application Number:
  US12/554,057
• Publication Number:
  US 20110043045A1
• Time in Patent Office: 1,257 Days
• Field of Search: 307/85, 307/87, 363/16, 361/93, 341/144, 324/77
• US Class Current: 307/85
• CPC Class Codes: H02J 3/08 Synchronising of networks

Abstract

Disclosed herein are systems and methods for converting data samples representing alternating electric currents. The data samples may be obtained by sampling a first alternating electric current having a first frequency at a sampling frequency, sampling a second alternating electric current having a second frequency at the sampling frequency, and converting the data samples using a conversion algorithm to compensate for any discrepancy caused by the difference between the first frequency and the second frequency. The corrected data samples may be utilized to determine various characteristics of the alternating electric current, such as voltage magnitude, voltage phase angle, current magnitude, current phase angle, and other related attributes. In one application, the systems and methods disclosed herein may be utilized in connection with an intelligent electronic device used to couple a first electrical system, such as a power generator, to a second electrical system, such as a utility intertie.

Claims

1. A method for selectively connecting a first electrical system and a second electrical system, the method comprising:

sampling a first alternating electric current of the first electrical system including a generator and having a first frequency at a sampling frequency to obtain a first set of data samples, the sampling frequency corresponding to a system frequency of the first electrical system or the second electrical system multiplied by a sampling rate;

sampling a second alternating electric current of the second electrical system including an interne and having a second frequency at the sampling frequency to obtain a second set of data samples;

converting at least one of the first set of data samples and the second set of data samples to a converted set of data samples using a conversion algorithm to compensate for a frequency difference between the first frequency and the second frequency;

determining when the converted set of data samples and at least one of the first set of data samples and the second set of data samples satisfy a predetermined condition; and

actuating a breaker when the predetermined condition is satisfied to selectively connect together the first electrical system and the second electrical system.

2. The method of claim 1, wherein the sampling frequency is approximately equal to the product of the first frequency and the sampling rate.

3. The method of claim 2, wherein the second set of data samples are converted into the converted set of data samples and wherein the system frequency corresponds to the second frequency.

4. The method of claim 1, wherein the sampling frequency is approximately equal to the product of the second frequency and the sampling rate.

5. The method of claim 4, wherein the first set of data samples are converted into the converted set of data samples and wherein the system frequency corresponds to the first frequency.

6. The method of claim 1, wherein the system frequency is a predetermined frequency;

and wherein the first set of data samples and a second set of data samples are converted to the converted set of data samples.

7. The method of claim 1, wherein the first set of data samples and the second set of data samples each comprise at least a present data sample and a previous data sample.

8. The method of claim 1, wherein the conversion algorithm comprises a method of:

determining a difference between;

a previous data sample multiplied by a first periodic function of a sampling step, the first frequency and the second frequency; and

a present data sample multiplied by a second periodic function of the sampling step, the first frequency and the second frequency; and

dividing the difference by;

a third periodic function of the sampling step, the first frequency and the second frequency.

9. The method of claim 1, further comprising determining an attribute of at least one of the first alternating electric current and the second alternating electric current using two data samples separated by one quarter cycle from the converted set of data samples, the attribute selected from the group consisting of a voltage magnitude, a voltage phase angle, a current magnitude and a current phase angle;

and

using the attribute to determine if the predetermined condition is satisfied.

10. The method of claim 1, further comprising determining a real part of a phasor within the converted set of data samples using a cosine filter.

11. The method of claim 1, further comprising:

generating the first alternating electric current using the generator; and

carrying the second alternating electric current on the intertie.

12. An intelligent electronic device (IED) configured to selectively connect a first electrical system to a second electrical system, the IED comprising:

a first input configured to sample a first alternating electric current of the first electrical system including a generator and having a first frequency at a sampling frequency to obtain a first set of data samples, the sampling frequency corresponding to a system frequency multiplied by a sampling rate;

a second input configured to sample a second alternating electric current of the second electrical system including an intertie and having a second frequency at the sampling frequency to obtain a second set of data samples;

a processor;

a computer-readable storage medium in electrical communication with the processor, the computer-readable storage medium comprising;

a data conversion module executable on the processor and configured to compensate for a frequency difference between the first frequency of the first electrical system and the second frequency of the second electrical system, and to generate a converted set of data samples from at least one of the first set of data samples and the second set of data samples;

wherein the IED is configured to determine when the converted set of data samples and at least one of the first set of data samples and the second set of data samples satisfy a predetermined condition; and

wherein the IED is configured to actuate a breaker when the predetermined condition is satisfied to selectively connect together the first electrical system to the second electrical system.

13. The IED of claim 12, wherein the sampling frequency is approximately equal to the product of the first frequency and the sampling rate.

14. The IED of claim 13, wherein the second set of data samples are converted into the converted set of data samples and wherein the system frequency corresponds to the second frequency.

15. The IED of claim 12, wherein the system frequency is approximately equal to the product of the second frequency and the sampling rate.

16. The IED of claim 15, wherein the first set of data samples are converted into the converted set of data samples and wherein the system frequency corresponds to the first frequency.

17. The IED of claim 12, wherein the sampling frequency is a predetermined frequency;

and

wherein the data conversion module is further configured to convert the first set of data samples into a first converted set of data samples, to convert the second set of data samples into a second converted set of data samples, and to use the first converted set of data samples and the second set of data samples to determine when the predetermined condition is satisfied.

18. The IED of claim 12, wherein the first set of data samples and the second set of data samples each comprise at least a present data sample and a previous data sample.

19. The IED of claim 12, wherein the data conversion module is further configured:

to determine a difference between;

a previous data sample multiplied by a first periodic function of a sampling step, the frequency of the first alternating electric current, and the frequency of the second alternating electric current; and

a present data sample multiplied by a second periodic function of the sampling step, the frequency of the first alternating electric current and the frequency of the second alternating electric current; and

to divide the difference by;

a third periodic function of the sampling step, frequency of the first alternating electric current and the frequency of the second alternating electric current.

20. The IED of claim 12, wherein the data conversion module is configured to determine an attribute of at least one of the first alternating electric current and the second alternating electric current using two data samples separated by one quarter wavelength from the converted set of data samples, the attribute selected from the group consisting of a voltage magnitude, a voltage phase angle, a current magnitude and a current phase angle;

and

wherein the system is configured to use the attribute to determine if the predetermined condition is satisfied.

21. The IED of claim 12, wherein the data conversion module is further configured to utilize a cosine filter to determine a real part of a phasor within the converted set of data samples.

22. A system to selectively connect a first electrical system and a second electrical system, the system comprising:

a breaker configured to selectively connect the first electrical system to the second electrical system; and

an intelligent electronic device (IED) configured to sample the first alternating electric current of the first electrical system including a generator at a sampling frequency to obtain a first set of data samples, the sampling frequency corresponding to a system frequency multiplied by a sampling rate, and to sample the second alternating electric current of the second electrical system including an intertie at the sampling frequency to obtain a second set of data samples, the IED comprising;

a processor;

a computer-readable storage medium in electrical communication with the processor, the computer-readable storage medium comprising a data conversion module executable comprising computer instructions executable on the processor for performing a method of;

converting at least one of the first set of data samples and the second set of data samples to a converted set of data samples using a conversion algorithm to compensate for a frequency difference between the first frequency and the second frequency;

determining when the converted set of data samples and at least one of the first set of data samples and the second set of data samples satisfy a predetermined condition; and

wherein the IED actuates the breaker when the predetermined condition is satisfied to selectively connect together the first electrical system and the second electrical system.