Self-Calibrating Voltage Sensor

• US 20080312856A1
• Filed: 06/16/2008
• Published: 12/18/2008
• Est. Priority Date: 06/15/2007
• Status: Active Grant

• Current Assignee: Schweitzer Engineering Laboratories, Incorporated
• Original Assignee: Schweitzer Engineering Laboratories, Incorporated
• Inventors: Feight, Laurence V., Swartzendruber, Ryan W., Fenske, Kurt J.
• Granted Patent: US 8,510,066 B2
• Time in Patent Office: Days
• US Class Current: 702/64
• CPC Class Codes: G01R 15/16 using capacitive devices
• CPC Class Codes: G01R 19/16547 voltage or current in AC supplies switching for protection H02H; circuits for emergency power supply H02J9/00
• CPC Class Codes: G01R 35/02 of auxiliary devices, e.g. of instrument transformers according to prescribed transformation ratio, phase angle, or wattage rating

Abstract

A voltage sensor consists of a number of capacitive voltage sensors coupled to test points in an electrical power system. The outputs of the voltage sensors are digitized and processed by a digital processor, which determines a normal value for the monitored AC voltages, and then determines if the monitored AC voltages are acceptable based on variations from the normal value.

Claims

1. A system for detecting when a monitored AC voltage falls outside of an acceptable range comprising:

i) at least one voltage acquisition circuit, the voltage acquisition circuit generating a digital signal representative of the monitored AC voltage;

ii) a processor coupled to the voltage acquisition circuit for executing a voltage monitoring algorithm and for generating a voltage out-of-range signal when the monitored AC voltage falls outside of an acceptable range, wherein

the processor (a) calculates a normal value for the monitored AC voltage, (b) calculates an acceptable range based on the normal value, and (c) generates a voltage-out-of-range signal if the monitored AC voltage exceeds the acceptable range.

2. The system of claim 1, wherein the voltage-out-of-range signal is generated if the monitored AC voltage varies from the normal value by a predetermined percentage.

3. The system of claim 1, wherein the normal value is calculated by:

i) accumulating a sum of a predetermined number of samples of the monitored voltage, wherein the samples are each acquired near a peak of a waveform of the monitored AC voltage;

ii) subtracting a previously calculated normal value from the sum;

iii) adding to the sum a present sample of the monitored voltage, wherein the sample is acquired near a peak of the waveform; and

iv) dividing the sum by the predetermined number.

4. The system of claim 1, further comprising a relay responsive to the voltage-out-range signal, wherein the relay transitions from a normal state to an abnormal state on generation of the voltage-out-of-range signal.

5. The system of claim 1, wherein the processor further (d) generates a drop-out voltage based on a predetermined percentage of the normal value, and wherein the voltage-out-of-range signal is generated if the monitored AC voltage drops below the drop-out voltage for a first predetermined time period.

6. The system of claim 1, wherein the processor further (e) generates a pull-in voltage based on a second predetermined percentage of the normal value, and (f) terminates the voltage-out-of-range signal after the voltage-out-of-range signal was generated, when the monitored AC voltage rises above the pull-in voltage for a second predetermined time period.

7. The system of claim 1, wherein the processor further maintains the voltage-out-of-range signal if the monitored AC voltage drops to near zero and returns to the acceptable range with a variation between the monitored AC voltage and the normal value for the monitored AC voltage.

8. A method for detecting when a monitored AC voltage falls outside of an acceptable range comprising the steps of:

i) calculating a normal value for the monitored AC voltage;

ii) calculating an acceptable range based on the normal value; and

iii) generating a voltage-out-of-range signal if the monitored AC voltage exceeds the acceptable range.

9. The method of claim 8, wherein the voltage-out-of-range signal is generated if the monitored AC voltage varies from the normal value by a predetermined percentage.

10. The method of claim 8, wherein the calculating a normal value step comprises the following steps:

i) accumulating a sum of a predetermined number of samples of the monitored AC voltage, wherein the samples are each acquired near a peak of a waveform of the monitored AC voltage;

ii) subtracting a previously calculated normal value from the sum;

iii) adding to the sum a present sample of the monitored AC voltage, wherein the sample is acquired near a peak of the waveform; and

iv) dividing the sum by the predetermined number.

11. The method of claim 8, further comprising the step of generating a drop-out voltage equal to a predetermined percentage of the normal value, and wherein the generating step comprises generating the voltage-out-of-range signal if the monitored AC voltage drops below the drop-out-voltage for a predetermined time period.

12. The method of claim 11, further comprising the steps of:

i) generating a pull-in voltage equal to a second predetermined percentage of the normal value; and

ii) terminating the voltage-out-of-range signal after the voltage-out-of-range signal was generated, when the monitored AC voltage rises above the pull-in voltage for a second predetermined time period.

13. The method of claim 8, further comprising the step of maintaining the voltage-out-of-range signal if the monitored AC voltage drops to near zero and returns to within the acceptable range with a variation between the monitored AC voltage and the normal value for the monitored AC voltage.

14. The method of claim 8, wherein the voltage-out-of-range signal is generated if the monitored AC voltage varies from the normal value by a percentage retrieved from a look-up table.

15. The method of claim 8, wherein the voltage-out-of-range signal is generated if the monitored AC voltage varies from the normal value by a calculated percentage.

16. The method of claim 8, further comprising the step of generating a drop-out voltage equal to a percentage of the normal value retrieved from a look-up table, and wherein the generating step comprises generating the voltage-out-of-range signal if the monitored AC voltage drops below the drop-out-voltage for a time period retrieved from a look-up table.

17. The method of claim 8, further comprising the step of generating a drop-out voltage equal to a calculated percentage of the normal value, and wherein the generating step comprises generating the voltage-out-of-range signal if the monitored AC voltage drops below the drop-out-voltage for a calculated time period.

18. The method of claim 16, further comprising the steps of:

i) generating a pull-in voltage equal to a second percentage of the normal value retrieved from a look-up table; and

ii) terminating the voltage-out-of-range signal after the voltage-out-of-range signal was generated, when the monitored AC voltage rises above the pull-in voltage for a second time period retrieved from a look-up table.

19. The method of claim 17, further comprising the steps of:

i) generating a pull-in voltage equal to a second calculated percentage of the normal value; and

ii) terminating the voltage-out-of-range signal after the voltage-out-of-range signal was generated, when the monitored AC voltage rises above the pull-in voltage for a second calculated time period.