Method of detecting instability in islanded electrical systems

• US 9,077,208 B2
• Filed: 12/30/2011
• Issued: 07/07/2015
• Est. Priority Date: 12/30/2011
• Status: Active Grant

• Current Assignee: Schneider Electric USA, Inc. (Schneider Electric SE)
• Original Assignee: Schneider Electric USA, Inc. (Schneider Electric SE)
• Inventors: Bickel, Jon A.
• Primary Examiner(s): Hollington, Jermele M
• Assistant Examiner(s): Astacio-Oquendo, Giovanni
• Application Number:
  US13/341,166
• Publication Number:
  US 20130169309A1
• Time in Patent Office: 1,285 Days
• Field of Search: 324/764.01, 324/500, 324/537, 340/506, 340/511, 361/1
• US Class Current: 1/1
• CPC Class Codes: H02J 3/241 The oscillation concerning frequency
• CPC Class Codes: H02J 3/46 Controlling of the sharing of output between the generators, converters, or transformers

Abstract

Systems and methods for evaluating the stability of an islanded electrical system (off-grid) using high-speed frequency measurements of the electrical signal supplied by an alternate power source in the islanded electrical system. Additional inputs may include status signals from an automatic transfer switch, a generator, and loads within the islanded electrical system. The high-speed frequency measurements have a resolution sufficient to enable analysis of any combination of the frequency magnitude (e.g., sudden increase), frequency slew rate (e.g., frequency rate of change), frequency rate of recovery (e.g., frequency recovery time), or frequency oscillations (e.g., frequency ringing around the nominal value before settling) to indicate the presence of an actual or impending instability of the islanded electrical system. The frequency referred to herein corresponds to the frequency at which an alternating current supplied by the alternate power source is cycling.

Claims

1. A method of automatically identifying a probable instability condition of an islanded electrical system, the method comprising:

receiving, using a controller, an indication that power to a plurality of loads has been transferred from a primary power source to an alternate power source electrically isolated from the primary power source to form an islanded electrical system that includes the alternate power source and the loads, the alternate power source providing electrical power to the loads;

responsive to receiving the power source change signal, receiving a load status change signal from a load in the islanded electrical system indicating that an operational status of the load has changed;

responsive to receiving the first load change signal, receiving frequency magnitude values, according to a first periodicity corresponding to at least one per 100 ms, of the alternating current or voltage measured by an intelligent electronic device (IED) coupled in the islanded electrical system to the alternate power source, and storing frequency data indicative of the measured frequency magnitude values and associated timestamp values corresponding to a time or cycle count each of the frequency magnitude values are measured;

based on the received frequency magnitude values, identifying a pickup time corresponding to a detected deviation from a nominal or fundamental frequency of the electrical power and a dropout time corresponding to a restoration of the frequency of the electrical power to the nominal or fundamental frequency;

based on at least the pickup time and the dropout time, calculating a rate of recovery for the detected frequency deviation from the nominal or fundamental frequency;

the controller determining whether an anomalous condition exists in the islanded electrical system based on at least whether the calculated rate of recovery exceeds a rate of recovery threshold; and

responsive to the rate of recovery exceeding the rate of recovery threshold, storing in a memory an indication that the anomalous condition exists.

2. The method of claim 1, wherein the rate of recovery is calculated using an absolute threshold by determining a time period between the pickup time and the dropout time or using a relative threshold by determining a percentage by which the time period between the pickup time and the dropout time exceeds a nominal time period indicating a normal rate of recovery, and wherein the alternate power source includes a generator having an engine.

3. The method of claim 2, wherein the rate of recovery threshold corresponds to the calculated time period or the calculated percentage plus a threshold tolerance value.

4. The method of claim 1, further comprising:

the controller calculating a slew rate of frequency based on the frequency data and the timestamp values, where the slew rate of frequency is the rate of frequency change with respect to time; and

the controller further determining whether the anomalous condition exists based on the slew rate by determining whether the slew rate exceeds a slew rate threshold.

5. The method of claim 4, further comprising, responsive to the controller determining that the anomalous condition exists based on the calculated rate of recovery, the controller using the calculated slew rate to confirm that the anomalous condition exists.

6. The method of claim 5, further comprising:

receiving a load change signal from a first of the loads that an operational status of the first load has changed;

responsive to receiving the first load change signal, receiving a further set of frequency magnitude values, at least one every 100 ms, of the alternating current or voltage measured by the IED and storing second frequency data indicative of the measured further set of frequency magnitude values and corresponding second timestamp values corresponding to a time each of the further set of frequency magnitude values are measured;

the controller calculating a second slew rate of frequency based on the second frequency data and the second timestamp values; and

responsive to the second slew rate exceeding a second slew rate threshold, storing a second indication that the islanded electrical system has a potential instability condition.

7. The method of claim 5, wherein the determining whether the anomalous condition is based further on the frequency magnitude values, and further includes the controller comparing the frequency magnitude values with a frequency magnitude threshold, wherein the storing the indication that the anomalous condition exists is further carried out responsive to any of the frequency magnitude values exceeding the frequency magnitude threshold.

8. The method of claim 5, further comprising receiving, using the controller, a status signal from the ATS indicating that power to the loads has been transferred, wherein the frequency magnitude values are received responsive to the receiving the power source change signal from the ATS.

9. The method of claim 5, further comprising:

receiving, using the controller, a load status change signal from a first of the loads indicating a change of status of the first load, wherein the frequency magnitude values are received responsive to the receiving the load status change signal from the first load.

10. The method of claim 5, further comprising:

changing the first periodicity at which the frequency magnitude values are measured to a second periodicity that is slower than the first periodicity when a steady-state condition is achieved in the islanded electrical system, where in the steady-state a frequency of the electrical current provided by the alternate power source is stable over an interval of time;

receiving measured data, according to the second periodicity, of a characteristic of the current or voltage measured by the IED until an anomalous change in the islanded electrical system is detected, where the anomalous change indicates a change in a characteristic of the electricity provided by the alternate power source, a change of a status of the alternate power source, a change in a characteristic of the alternate power source, or a change of a status of any of the loads in the islanded electrical system; and

responsive to detecting the anomalous change, changing the second periodicity back to the first periodicity and causing the IED to measure frequency magnitude values according to the first periodicity over a predetermined interval of time.

11. The method of claim 10, wherein:

the characteristic of the electricity includes current or voltage or a value derived from the current or the voltage,

the status of the alternate power source includes on or off,

the characteristic of the alternate power source includes a speed at which an engine of the alternate power source is rotating or a voltage output of the alternate power source, and

the status of any of the loads includes at least on or off.

12. The method of claim 5, wherein the alternate power source includes an uninterruptible power supply, the method further comprising:

receiving, using the controller, voltage data indicative of a voltage measured by the IED;

using the measured voltage data to verify whether the anomalous condition exists in the islanded electrical system by comparing whether changes in the voltage data track changes in the frequency data.

13. The method of claim 5, wherein the slew rate threshold includes a low threshold below the nominal or fundamental frequency and a high threshold above the low threshold and above the nominal or fundamental frequency, wherein the determining whether the slew rate exceeds the slew rate threshold includes determining whether the slew rate is above the high threshold or below the low threshold, the method further comprising:

responsive to the slew rate being above the high threshold, determining whether at least one of the loads is available to be turned on, and if so, causing the frequency of the alternating current to decrease by turning on the at least one load; and

responsive to the slew rate being below the low threshold, determining whether at least one of the loads is available to be turned off, and if so, causing the frequency of the alternating current to increase by turning off the at least one load.

14. The method of claim 13, further comprising:

determining, using the controller, an operational status of each of the at least some loads, the operational status including running in which the load is turned on or stopped in which the load is turned off; and

storing in the memory information indicating which of the loads was turned on or off in response to the determining whether the slew rate exceeds the slew rate threshold and when and by how much the frequency of the electrical power increased or decreased in response to turning the load on or off along with a timestamp corresponding to the time that the load was turned on or off.

15. The method of claim 5, wherein the alternate power source includes a generator, the method further comprising:

determining, using the controller, an operational status of the generator, the operational status including running indicating that the generator is running, on indicating that the generator is starting or on, or off indicating that the generator is stopping or off;

comparing using a statistical function the frequency data or the calculated slew rate with present or historical data associated with the islanded electrical system to project an instability trend or predict a future performance of the islanded electrical system; and

based on the comparing, storing in the memory a result indicative of the projected instability trend or the predicted future performance.

16. The method of claim 15, the method further comprising:

determining whether the operational status of the generator is starting, and if not;

comparing using a statistical function the calculated slew rate with a historical slew rate to project an instability trend of the islanded electrical system, or

comparing using a statistical function the frequency data with a present or historical voltage or current of the electrical power measured by the IED to project an instability trend of the islanded electrical system; and

if the operational status of the generator is starting;

determine a pick up time for the generator to pick up one of the loads and compare using a statistical function the pick up time with a historical pick up time for the generator to predict a future performance of the generator in the islanded electrical system, or

determine an actual loading of the generator while the generator is running and comparing the actual loading with a historical loading for the generator to predict a future performance of the generator in the islanded electrical system, or

calculate a speed of the generator as the generator is starting and a time elapsed for the generator to transition from the starting to the running operational status and compare using a statistical function the calculated speed and elapsed time with a historical speed and elapsed time for the generator to predict a future performance of the generator in the islanded electrical system, or

comparing using a statistical function the frequency data while the generator is transitioning from the starting to the running operational status with corresponding frequency data measured while the operational status of the generator is running to project an instability trend of the islanded electrical system.

17. A method of automatically identifying a probable instability condition of an islanded electrical system, the method comprising:

receiving, using a controller, an indication that power to a plurality of loads has been transferred from a primary power source to an alternate power source electrically isolated from the primary power source to form an islanded electrical system that includes the alternate power source and the loads, the alternate power source providing electrical power in the form of an alternating current to the loads;

responsive to receiving the indication that power to the loads has been transferred to the alternate power source, receiving a load status change signal indicating that an operational status of a load in the islanded electrical system has changed, causing a frequency deviation event in which a frequency of the alternating current deviates from a nominal or fundamental frequency of the alternating current;

responsive to receiving the load status change signal, receiving frequency magnitude values, according to a first periodicity corresponding to at least one per cycle of the alternating current, of the alternating current or voltage signal measured by an intelligent electronic device (IED) coupled in the islanded electrical system to the alternate power source, and storing frequency data indicative of the measured frequency magnitude values;

the controller analyzing the received frequency magnitude values to identify oscillations in the measured frequency magnitude values above and below a nominal or fundamental frequency for the alternating current during the frequency deviation event, the analyzing including quantifying a severity of the oscillations based on a number of times the measured frequency values exceed or drop below the nominal or fundamental frequency by more than a predetermined threshold;

the controller determining whether an anomalous condition exists in the islanded electrical system based on at least the severity of the oscillations during the frequency deviation event; and

responsive to the controller determining that the anomalous condition exists, storing in a memory an indication that the anomalous condition exists.

18. The method of claim 17, wherein the alternate power source includes a generator having an engine, and wherein the predetermined threshold is based on a pickup or dropout alarm associated with the engine of the generator.

19. The method of claim 17, further comprising:

receiving associated timestamp values corresponding to a time each of the frequency magnitude values are measured;

the controller calculating a slew rate of frequency based on the frequency data and the timestamp values, where the slew rate of frequency is the rate of frequency change with respect to time; and

the controller further determining whether the anomalous condition exists based on the slew rate by determining whether the slew rate exceeds a slew rate threshold.

20. The method of claim 17, further comprising:

based on the received frequency magnitude values, identifying a pickup time corresponding to a detected deviation from a nominal or fundamental frequency of the alternating current and a dropout time corresponding to a restoration of the frequency of the alternating current to the nominal or fundamental frequency;

based on the pickup time and the dropout time, calculating a rate of recovery for the detected frequency deviation from the nominal or fundamental frequency;

the controller further determining whether the anomalous condition exists in the islanded electrical system based on whether the calculated rate of recovery exceeds a rate of recovery threshold; and

responsive to the rate of recovery exceeding the rate of recovery threshold, the controller determining that the anomalous condition exists.