Method and apparatus of detecting and compensating for DC residual fault currents on electrical systems

1. A method of disconnecting an electric supply to an electrical installation upon detection of a predetermined fault current between the line and neutral conductors of said electric supply, said line and neutral conductors inductively coupled as a primary winding of a current transformer, the method comprising the steps of:

monitoring a fault current waveform from the output of a secondary winding inductively coupled to said current transformer and responsive to said fault current on said electrical installation;

digitally encoding said fault current waveform outputted from said secondary winding;

detecting on or more parameters of said encoded fault current waveform and generating a compensation factor which compensates for specific pulsed DC fault components present in said encoded fault current waveform;

weighting said encoded fault current waveform in accordance with said compensation factor; and

tripping a trip mechanism if said weighted fault current waveform exceeds said predetermined fault current.

2. The method as claimed in claim 1 , wherein the step of detecting one or more parameters of said encoded fault current waveform are selected from: duty cycle, ratio of RMS values of the positive and negative portions of waveform, positive and negative peak values, number of zero crossings, form factor and crest factor.

3. The method as claimed in claim 1 , wherein the step of detecting one or more parameters of said encoded fault current waveform are obtained from a measure of symmetry comparing the mean squared values of negative and positive sections of said encoded fault current waveform.

4. The method as claimed in claim 3 , wherein the step of detecting one or more parameters of said encoded fault current waveform are obtained from a measure of symmetry comparing the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform.

5. The method as claimed in claim 4 , wherein the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform further comprises the step of separately taking an infinite impulse response-averaged mean square of all positive values and negative values.

6. The method as claimed in claim 5 , wherein the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform is then calculated and the reciprocal taken if the result is less than one.

7. The method as claimed in claim 6 , wherein various bands of the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform are obtained, and said compensation factor is determined from a look-up table stored in a processing means.

8. The method as claimed in claim 4 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.00 to 1.22, then a sinusoidal AC fault current waveform is detected and a compensation factor of around 1.00 is generated.

9. The method as claimed in claim 4 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.22 to 1.51, then a full wave rectified DC fault current waveform is detected and a compensation factor of around 1.68 is generated.

10. The method as claimed in claim 4 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.51 to 1.67, then a ‘M’ wave rectified DC fault current waveform is detected and a compensation factor of around 5.28 is generated.

11. The method as claimed in claim 4 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.67 to 3.00, then a half wave rectified DC fault current waveform is detected and a compensation factor of around 3.66 is generated.

12. The method as claimed in claim 4 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 3.00 to 7.12, then a 90° switched sine fault current waveform is detected and a compensation factor of around 1.25 is generated.

13. The method as claimed in claim 4 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 7.12 to ∞, then a 135° switched sine fault current waveform is detected and a compensation factor of around 1.10 is generated.

14. The method as claimed in claim 1 , wherein the step of detecting one or more parameters of said encoded fault current waveform is obtained from more complex types of waveshape analysis, including template matching or spectral analysis.

15. An apparatus for disconnecting an electric supply to an electrical installation upon detection of a predetermined fault current between the line and neutral conductors of said electric supply, said line and neutral conductors inductively coupled as a primary winding of a current transformer, the method comprising the steps of:

monitoring means for monitoring a fault current waveform from the output of a secondary winding inductively coupled to said current transformer and responsive to said fault current on said electrical installation;

encoding means for digitally encoding said fault current waveform outputted from said secondary winding;

detection means for detecting one or more parameters of said encoded fault current waveform and generating a compensation factor which compensates for specified pulsed DC fault components present in said encoded fault current waveform;

weighting means for weighting said encoded fault current waveform in accordance with said compensation factor; and

tripping means for tripping a trip mechanism if said weighted fault current waveform exceeds said predetermined fault current.

16. The apparatus as claimed in claim 15 , wherein the encoding means, detection means, and weighting means are implemented in a processing means which may be a microprocessor or digital signal processor.

17. The apparatus as claimed in claim 15 , wherein said processing means is programmable and includes other ancillary features, including undervoltage release or warning indication.

18. The apparatus as claimed in claim 15 , wherein said secondary winding is a single or multi-turn winding.

19. The apparatus as claimed in claim 15 , wherein said detection means for detecting one or more parameters of said encoded fault current waveform are selected from: duty cycle, ratio of RMS values of the positive and negative portions of waveform, positive and negative peak values, number of zero crossings, form factor and crest factor.

20. The apparatus as claimed in claim 15 , wherein said detection means for detecting one or more parameters of said encoded fault current waveform are obtained from a measure of symmetry comparing the mean squared values of negative and positive sections of said encoded fault current waveform.

21. The apparatus as claimed in claim 20 , wherein said detection means for detecting one or more parameters of said encoded fault current waveform are obtained from a measure of symmetry comparing the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform.

22. The apparatus as claimed in claim 21 , wherein the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform further comprises the step of separately taking an infinite impulse response-averaged mean square of all positive values and negative values.

23. The apparatus as claimed in claim 21 , wherein the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform is then calculated and the reciprocal taken if the result is less than one.

24. The apparatus as claimed in claim 23 , wherein various bands of the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform are obtained, and said compensation factor is determined from a look-up table stored in said processing means.

25. The apparatus as claimed in claim 21 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.00 to 1.22, then a sinusoidal AC fault current waveform is detected and a compensation factor of around 1.00 is generated.

26. The apparatus as claimed in claim 21 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.22 to 1.51, then a full wave rectified DC fault current waveform is detected and a compensation factor of around 1.68 is generated.

27. The apparatus as claimed in claim 21 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.51 to 1.67, then a ‘M’ wave rectified DC fault current waveform is detected and a compensation factor of around 5.28 is generated.

28. The apparatus as claimed in claim 21 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.67 to 3.00, then a half wave rectified DC fault current waveform is detected and a compensation factor of around 3.66 is generated.

29. The apparatus as claimed in claim 21 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 3.00 to 7.12, then a 90° switched sine fault current waveform is detected and a compensation factor of around 1.25 is generated.

30. The apparatus as claimed in claim 21 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 7.12 to ∞, then a 135° switched sine fault current waveform is detected and a compensation factor of around 1.10 is generated.

31. The method as claimed in claim 1 , wherein said detection means for detecting one or more parameters of said encoded fault current waveform is obtained from more complex types of waveshape analysis, including template matching or spectral analysis.

32. A computer program product on a computer readable storage medium operatively installed on a computer for disconnecting an electric supply to an electrical installation upon detection of a predetermined fault current between the line and neutral conductors of said electric supply, said line and neutral conductors inductively coupled as a primary winding of a current transformer, the computer program executing on the computer for performing the method comprising the steps of:

monitoring a fault current waveform from the output of a secondary winding inductively coupled to said current transformer and responsive to said fault current on said electrical installation;

digitally encoding said fault current waveform outputted from said secondary winding;

detecting one or more parameters of said encoded fault current waveform and generating a compensation factor which compensates for specific pulsed DC fault components present in said encoded fault current waveform;

weighting said encoded fault current waveform in accordance with said compensation factor; and

tripping a trip mechanism if said weighted fault current waveform exceeds said predetermined fault current.

33. The computer program product as claimed in claim 32 , wherein said computer program means for detecting one or more parameters of said encoded fault current waveform are selected from: duty cycle, ratio of RMS values of the positive and negative portions of waveform, positive and negative peak values, number of zero crossings, form factor and crest factor.

34. The computer program product as claimed in claim 32 , wherein said computer program means for detecting one or more parameters of said encoded fault current waveform are obtained from a measure of symmetry comparing the mean squared values of negative and positive sections of said encoded fault current waveform.

35. The computer program product as claimed in claim 34 , wherein said computer program means for detecting one or more parameters of said encoded fault current waveform are obtained from a measure of symmetry comparing the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform.

36. The computer program product as claimed in claim 35 , wherein the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform further comprises the step of separately taking an infinite impulse response-averaged mean square of all positive values and negative values.

37. The computer program product as claimed in claim 35 , wherein the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform is then calculated and the reciprocal taken if the result is less than one.

38. The computer program product as claimed in claim 37 , wherein various bands of the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform are obtained, and said compensation factor is determined from a look-up table stored in a processing means.

39. The computer program product as claimed in claim 35 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.00 to 1.22, then a sinusoidal AC fault current waveform is detected and a compensation factor of around 1.00 is generated.

40. The computer program product as claimed in claim 35 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.22 to 1.51, then a full wave rectified DC fault current waveform is detected and a compensation factor or around 1.68 is generated.

41. The computer program product as claimed in claim 35 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.51 to 1.67, then a ‘M’ wave rectified DC fault current waveform is detected and a compensation factor of around 5.28 is generated.

42. The computer program product as claimed in claim 35 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 1.67 to 3.00, then a half wave rectified DC fault current waveform is detected and a compensation factor of around 3.66 is generated.

43. The computer program product as claimed in claim 35 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 3.00 to 7.12, then a 90° switched sine fault current waveform is detected and a compensation factor or around 1.25 is generated.

44. The computer program product as claimed in claim 35 , wherein if the ratio of the mean squared sum of the positive and negative halves of said encoded fault current waveform lies within the range 7.12 to ∞, then a 135° switched sine fault current waveform is detected and a compensation factor of around 1.10 is generated.

45. The computer program product as claimed in claim 32 , wherein said computer program means for detecting one or more parameters of said encoded fault current waveform is obtained from more complex types of waveshape analysis, including template matching or spectral analysis.