Apparatus and method for adaptive fault detection in MV distribution circuits

1. A method of detecting and analyzing faults in a three phase feeder system having a first cable and one or more other cables, the method being performed by at least one computer and comprising:

receiving a mode selection from a user through a user interface;

determining current magnitude in the first cable;

comparing the current magnitude to a threshold level;

when the current magnitude exceeds the threshold level, starting a counter of the at least one computer;

when the current magnitude falls below the threshold level, stopping the counter and determining from the counter how long the current magnitude stayed above the threshold level;

when the current magnitude is determined to have stayed above the threshold level for a duration that falls within a predetermined duration range having lower and upper levels, starting a timer;

after the timer measures a predetermined time interval, determining whether a fault is detected in the one or more other cables during the predetermined time interval; and

when a fault is not detected in the one or more other cables, determining that a single phase fault has occurred in the feeder system;

wherein when the mode selection is an adaptive mode, the threshold level is the greater of a fixed minimum value and an adaptive value calculated from prior current magnitudes, and

wherein when the mode selection is a fixed mode, the threshold level is a preset value received from a user through the user interface.

2. The method of claim 1 , wherein when a fault is detected in the one or more other cables, determining that a multi-phase fault has occurred in the cables.

3. The method of claim 1 , wherein the current magnitude is determined at a nominal frequency of the power.

4. The method of claim 3 , wherein the current magnitude is determined by performing a half-cycle Fourier transform of current measurements in a moving window from the first cable.

5. The method of claim 4 , further comprising receiving digital fault record data from a remotely located protective relay, the digital fault record data including the current measurements from the first cable.

6. The method of claim 1 , wherein the adaptive value is a multiple of the average value of prior current magnitudes over more than one prior power frequency cycle.

7. The method of claim 1 , wherein the lower level of the predetermined duration range is about a quarter cycle of a nominal frequency of the power and the upper level of the predetermined duration range is about five cycles of the nominal frequency.

8. The method of claim 7 , wherein the predetermined time interval is about one cycle of the nominal frequency.

9. The method of claim 1 , wherein the step of determining whether a fault is detected in the one or more other cables comprises for each other cable:

determining current magnitude in the other cable;

comparing the current magnitude in the other cable to another threshold level;

when the current magnitude in the other cable exceeds the other threshold level, measuring how long the current magnitude stays above the other threshold level; and

when the current magnitude stays above the other threshold level for a duration that falls within another predetermined duration range, determining that a fault has occurred.

10. A protective device for monitoring power conveyed by a first cable and one or more other cables, the protective device comprising a processor and memory storing a fault detection program, which when executed by the processor, performs a method of detecting and analyzing faults in a three phase feeder system having a first cable and one or more other cables, the method comprising:

receiving a mode selection from a user through a user interface;

determining current magnitude in the first cable;

comparing the current magnitude to a threshold level;

when the current magnitude exceeds the threshold level, starting a counter;

when the current magnitude falls below the threshold level, stopping the counter and determining from the counter how long the current magnitude stayed above the threshold level;

when the current magnitude is determined to have stayed above the threshold level for a duration that falls within a predetermined duration range having lower and upper levels, starting a timer;

after the timer measures a predetermined time interval, determining whether a fault is detected in the one or more other cables during the predetermined time interval; and

when a fault is not detected in the one or more other cables, determining that a single phase fault has occurred in the feeder system;

wherein when the mode selection is an adaptive mode, the threshold level is the greater of a fixed minimum value and an adaptive value calculated from prior current magnitudes; and

wherein when the mode selection is a fixed mode, the threshold level is a preset value received from a user through the user interface.

11. The protective device of claim 10 , wherein the current magnitude is determined by performing a Fourier transform of current measurements from the first cable.

12. The protective device of claim 10 , wherein the protective device further comprises a user interface, and wherein the threshold level is a preset value that is received from a user through a user interface.

13. The protective device of claim 10 , wherein the lower level of the predetermined duration range is about a quarter cycle of a nominal frequency of the power and the upper level of the predetermined duration range is about five cycles of the nominal frequency.

14. The protective device of claim 13 , wherein the predetermined time interval is about one cycle of the nominal frequency.

15. The protective device of claim 14 , wherein the protective device comprises a protective relay.