Method for detecting islanding operation of distributed power generator

1. A method of detecting islanding operation of a distributed power generator, wherein the method comprises:

determining a frequency of a grid to which the distributed power generator is connected;

measuring a rate of change of the frequency of the grid at an output of the distributed power generator;

determining a threshold value for an alteration speed;

comparing the rate of change of the frequency of the grid with the threshold value; and

detecting islanding operation when the measured rate of change of the frequency exceeds the threshold value for a certain time period,

wherein determining the threshold value for the rate of change of the frequency of the grid comprises:

forming an envelope curve from absolute values of a momentary rate of change of the frequency;

averaging the formed envelope curve; and

multiplying the averaged curve by a constant for obtaining the threshold value.

2. A method according to claim 1 , wherein the averaging of the formed envelope curve comprises:

low-pass filtering the formed envelope curve.

3. A method according to claim 1 , wherein the averaging of the formed envelope curve comprises:

calculating a moving average of the formed envelope curve.

4. A method according to claim 1 , wherein comparing the rate of change of the frequency with the threshold value comprises:

comparing the formed envelope curve with the threshold value.

5. A method according to claim 1 , comprising:

feeding reactive power by the distributed power generator to the grid, the reactive power having an amplitude that is proportional to active power produced by the distributed power generator.

6. A method according to claim 5 , wherein the reactive power fed to the grid has a square wave waveform.

7. A method according to claim 5 , wherein the reactive power fed to the grid has a waveform including changes that change the rate of change of frequency when an island network is formed.

8. A method according to claim 1 , comprising:

disconnecting the distributed power generator from the grid when islanding operation is detected.

9. An arrangement for detecting islanding operation of a distributed power generator, wherein the arrangement comprises:

means for determining a frequency of a grid upon connection of the grid to the distributed power generator;

means for measuring a rate of change of the frequency of the grid at an output of the distributed power generator;

means for determining a threshold value for an alteration speed;

means for comparing the rate of change of the frequency of the grid with the threshold value; and

means for detecting islanding operation when the measured rate of change of the frequency exceeds the threshold value for a certain time period,

wherein determining the threshold value for the rate of change of the frequency of the grid comprises:

forming an envelope curve from absolute values of a momentary rate of change of the frequency;

averaging the formed envelope curve; and

multiplying the averaged curve by a constant for obtaining the threshold value.

10. A method according to claim 2 , wherein comparing the rate of change of the frequency with the threshold value comprises:

comparing the formed envelope curve with the threshold value.

11. A method according to claim 3 , wherein comparing the rate of change of the frequency with the threshold value comprises:

comparing the formed envelope curve with the threshold value.

12. A method according to claim 4 , comprising:

feeding reactive power by the distributed power generator to the grid, the reactive power having an amplitude that is proportional to active power produced by the distributed power generator.

13. A method according to claim 12 , comprising:

disconnecting the distributed power generator from the grid when islanding operation is detected.

14. The arrangement of claim 1 , in combination with a power grid.