Isolation monitoring using a test signal of variable frequency

Patent metadata

Field	Value
Publication number	US-9720025-B2
Application number	US-201314085903-A
Country	US
Kind code	B2
Filing date	Nov 21, 2013
Priority date	May 24, 2011
Publication date	Aug 1, 2017
Grant date	Aug 1, 2017

Abstract

In monitoring an isolation of an ungrounded power grid an AC voltage source is connected to the power grid via at least one test resistor. A test signal with a periodic continuous voltage course with regard to ground and with a frequency is applied to the power grid by means of the AC voltage source. A leakage current flowing due to the test signal is measured; and an ohmic isolation resistance is determined from the leakage current. The frequency of the test signal is varied such that an active current part of the leakage current keeps a predetermined recommended value at varying leakage capacitances of the power grid. This provides for a desired level of accuracy at maximum speed of isolation or ground fault detection.

First claim

The invention claimed is:

1. A method of monitoring an isolation of an ungrounded power grid, comprising:
connecting an AC voltage source via at least one test resistor to the power grid,
applying a test signal comprising a periodic continuous voltage course with regard to ground and a frequency to the power grid by means of the AC voltage source, where the test signal is a sine signal,
measuring a leakage current flowing due to the test signal, and
determining an ohmic isolation resistance from the measured leakage current,
wherein the frequency of the test signal is varied such that an active current part of the leakage current keeps a predetermined recommended value at varying leakage capacitances of the power grid.

2. The method of claim 1 , wherein the frequency of the test signal is varied according to a tracking method.

3. The method of claim 1 , wherein the frequency of the test signal is repeatedly increased and/or decreased over an extended range of frequencies.

4. The method of claim 1 , wherein the frequency of the test signal is continuously modulated.

5. The method of claim 1 , wherein the amplitude of the test signal is modulated.

6. The method of claim 5 , wherein the amplitude of the test signal is modulated such that:
the electric power coupled in by the AC voltage source, or
an absolute effective value of the active current part of the leakage current, or
an absolute active power value of the electric power coupled in by the AC voltage source, or
a relative active power value of the electric power coupled in by the AC voltage source and related to the entire electric value coupled in by the AC voltage source is kept constant.

7. The method of claim 1 , further comprising determining a cross-correlation between the test signal and the leakage current.

8. The method of claim 1 , further comprising:
multiplying the leakage current by a reference function which is in phase with the test signal to form a product, and averaging the product over an integer number of periods of the reference function for determining the active current part of the leakage current, and/or
multiplying the leakage current with a further reference function which has a phase offset with regard to the test signal of π/2 to form another product, and averaging the another product over an integer number of periods of the further reference function for determining a reactive current part of the leakage current.

9. The method of claim 1 , further comprising indicating an isolation fault if the active current part of the leakage current exceeds a threshold value and/or if the leakage current and/or the active current part of the leakage current display a steplike change.

10. The method of claim 1 , further comprising measuring and evaluating partial leakage currents from individual lines of the power grid, that are isolated with regard to each other, with regard to a location of a detected isolation fault.

11. The method of claim 10 , wherein the partial leakage currents are directly measured between the AC voltage source and the individual lines of the power grid or indirectly by means of electric voltages of the individual lines with regard to ground.

12. The method of claim 1 , wherein the AC voltage source is connected to the power grid on a DC current side of an inverter.

13. The method of claim 12 , wherein the leakage current and/or partial leakage currents from individual lines of the power grid:
are measured on the DC voltage side of an inverter,
while the inverter is in operation, and
while the inverter is not in operation, wherein one switch of a number of switches arranged in the lines is opened, or where the one switch is closed,

and evaluated with regard to the location of a detected isolation fault.

14. An apparatus for monitoring an isolation of an ungrounded power grid, comprising:
an AC voltage source configured to be connected to the power grid via at least one test resistor and generate a test signal having a periodic continuous voltage course with regard to ground, and having a frequency, where the test signal is a sine signal,
one or more measurement devices configured to measure a leakage current flowing due to the test signal, and
an evaluation device configured to determine an ohmic isolation resistance from the leakage current,
wherein the AC voltage source is configured to vary the frequency of the test signal in such a way that an active current part of the leakage current keeps a predetermined recommended value at varying leakage capacitances of the power grid.

15. The apparatus of claim 14 , wherein the AC voltage source is configured to vary the frequency of the test signal according to a tracking method.

16. The apparatus according to claim 14 , wherein the AC voltage source is configured to repeatedly increase or decrease the frequency of the test signal over a range of frequencies.

17. The apparatus of claim 14 , wherein the AC voltage source is configured to continuously modulate the frequency of the test signal.

18. The apparatus according to claim 14 , wherein the AC voltage source is configured to modulate the amplitude of the test signal.

19. The apparatus according to claim 17 , wherein the evaluation device is configured to determine a cross-correlation between the test signal and the leakage current.

20. The apparatus according to claim 14 , wherein the evaluation device is configured to:
multiply the leakage current by a reference function in phase with the test signal and average the product over an integer number of periods of the reference function for determining an active current part of the leakage current, and/or
multiply the leakage current by a further reference function having a phase offset of π/2 with regard to the test signal and average the product over an integer number of periods of the further reference function for determining a reactive current part of the leakage current.

21. The apparatus according to claim 14 , wherein the evaluation device is configured to indicate an isolation fault when the active current part of the leakage current exceeds a threshold value and/or when the leakage current and/or the active current part of the leakage current displays a steplike change.

22. The apparatus according to claim 14 , wherein the one or more measurement devices are configured to measure partial currents flowing from the individual lines of the power grid which are isolated with regard to each other, and wherein the evaluation device evaluates the partial currents with regard to the location of a detected ground fault.

23. The apparatus according to claim 22 , wherein the one or more measurement devices are configured to measure the partial currents directly between the AC voltage source and the individual lines of the power grid or indirectly by means of electric voltages of the individual lines with regard to ground.

Assignees

• Sma Solar Technology Ag

Inventors

• Hermeling Dirk
• Lehmann Reinhard

Classifications

• G01R27/025 Primary

  Measuring very high resistances, e.g. isolation resistances, i.e. megohm-meters · CPC title

• G01R27/18

  Measuring resistance to earth {, i.e. line to ground} · CPC title

• G01R31/025 Primary

  Physics · mapped topic

• G01R31/52

  Testing for short-circuits, leakage current or ground faults · CPC title