Method and apparatus for detecting ground fault

1. A ground fault detecting device, the device being configured:

to monitor a sum current of output phases of a frequency converter;

to detect a ground fault, if the sum current exceeds a set threshold value; and

to perform an initial setting of the threshold value by determining a sum current value, when the frequency converter starts up, and by setting the threshold value higher than the determined sum current value.

2. The device of claim 1 , wherein the device is configured to perform the initial setting of the threshold value when the frequency converter starts up for the first time.

3. The device of claim 1 , wherein the device is configured to perform the initial setting of the threshold value every time the frequency converter starts up.

4. The device of claim 1 , wherein the device is configured to set the threshold value in the initial setting of the threshold value to be 40 to 60% higher than the determined sum current value.

5. The device of claim 4 , wherein the device is configured to set the threshold value in the initial setting of the threshold value to be 50% higher than the determined sum current value.

6. The device of claim 1 , wherein the device is configured to detect a ground fault, if the value of the sum current determined in the initial setting of the threshold value exceeds a predetermined maximum value.

7. The device of claim 1 , wherein the device is configured to correct the set threshold value as a function of at least one of a switching frequency and a DC voltage of the frequency converter.

8. The device of claim 1 , wherein the device is configured to monitor the sum current of the output phases of the frequency converter by means of a quantity or quantities proportional thereto.

9. The device of claim 1 , wherein the device comprises a frequency converter.

10. The device of claim 9 , wherein the frequency converter is configured to supply one or more motors.

11. A method for detecting a ground fault, the method comprising:

monitoring a sum current of output phases of a frequency converter;

performing an initial setting of a threshold value, said initial setting including determining a sum current value, when the frequency converter is started, and setting the threshold value higher than the determined sum current value; and

detecting a ground fault, if the monitored sum current exceeds the set threshold value.

12. The method of claim 11 , wherein the initial setting of the threshold value is performed, when the frequency converter is started for the first time.

13. The method of claim 11 , wherein the initial setting of the threshold value is performed every time the frequency converter is started.

14. The method of claim 11 , wherein the threshold value in the initial setting of the threshold value is set to be 40 to 60% higher than the determined sum current value.

15. The method of claim 14 , wherein the threshold value in the initial setting of the threshold value is set to be 50% higher than the determined sum current value.

16. The method of claim 11 , wherein said sum current value is determined by measuring.

17. The method of claim 11 , wherein said sum current value is determined experimentally by searching for a threshold value, by which the detection of a ground fault takes place.

18. The method of claim 11 , wherein the initial setting of a threshold value comprises:

detecting a ground fault, if the determined sum current value exceeds the predetermined maximum value.

19. The method of claim 11 , the method comprising:

correcting the set threshold value as a function of at least one of a switching frequency and a DC voltage of the frequency converter.

20. The method of claim 11 , wherein the monitoring of the sum current of the output phases of the frequency converter is performed by means of a quantity or quantities proportional thereto.

21. A computer program product embodied on a non-transitory computer-readable medium comprising a computer program code, the execution of which on a computer makes the computer perform the steps of the method of claim 11 .

22. An apparatus for detecting a ground fault, the apparatus comprising:

means arranged for monitoring a sum current of output phases of a frequency converter;

means arranged for detecting a ground fault, if the sum current exceeds a set threshold value; and

means for performing an initial setting of the threshold value, which means are arranged for determining a sum current value, when the frequency converter starts up, and for setting the threshold value higher than the determined sum current value.