Apparatus and method for automatic ground fault location determination in high resistance grounded motor drive system

1. A method for identifying a suspected ground fault location in a multi-drive system having a plurality of drives, the method comprising:

using at least one processor, while a given drive is operating, determining whether an operating frequency of the given drive is within a range including a fault frequency corresponding to a detected ground fault condition in the system; and

using the at least one processor, selectively identifying the given drive as a suspected ground fault location if the operating frequency of the given drive is within the range including the fault frequency, wherein selectively identifying the given drive comprises:

using the at least one processor, identifying a set of drives in the multi-drive system operating at corresponding frequencies within the range including the fault frequency;

if the set includes a single drive, selectively identifying the single drive as a ground fault location in the system; and

if the set includes more than one drive, using the at least one processor:

(i) changing an operating frequency of a single selected drive in the set while any remaining drives in the set continue operation at their respective current operating frequencies,

(ii) selectively identifying the single selected drive as a ground fault location in the system if the fault frequency changes by more than a threshold amount in response to changing the operating frequency of the single selected drive,

(iii) selectively confirming the single selected drive is not a ground fault location if the fault frequency does not change by more than the threshold amount in response to changing the operating frequency of the single selected drive, and

(iv) selectively repeating steps (i)-(iii) for the remaining drives in the set.

2. The method of claim 1 , comprising:

measuring a neutral-ground voltage in the multi-drive system;

determining whether an integral of the neutral-ground voltage is greater than or equal to a threshold value; and

selectively identifying a detected ground fault condition in the system if the integral of the neutral-ground voltage is greater than or equal to the threshold value.

3. The method of claim 2 , comprising determining the fault frequency as a frequency of the neutral-ground voltage if the integral of the neutral-ground voltage is greater than or equal to the threshold value.

4. The method of claim 1 , wherein the at least one processor is integral to the given drive, the method comprising:

using the at least one processor, automatically obtaining a measured neutral-ground voltage associated with the given drive while the given drive is operating at a commanded frequency;

using the at least one processor, selectively identifying a detected ground fault condition in the given drive if an integral of the measured neutral-ground voltage is greater than or equal to a threshold value; and

using the at least one processor, determining the fault frequency as a frequency of the measured neutral-ground voltage if a detected ground fault condition has been identified in the given drive; and

using the at least one processor, selectively identifying the given drive as a suspected ground fault location if the operating frequency of the given drive is within the range including the fault frequency.

5. The method of claim 4 , further comprising, using the at least one processor, reporting a detected fault condition, an operating frequency of the given drive, and the fault frequency to a device in the multi-drive system.

6. The method of claim 5 , further comprising,

using the at least one processor, receiving a speed command change from the device in the multi-drive system;

using the at least one processor, operating the given drive at an adjusted operating frequency according to the received speed command change; and

using the at least one processor, determining an updated fault frequency as a frequency of the measured neutral-ground voltage while operating the given drive at the adjusted operating frequency; and

using the at least one processor, reporting the updated fault frequency to the device in the multi-drive system.

7. The method of claim 1 , wherein the fault frequency is a frequency associated with a neutral-ground signal in the system.

8. The method of claim 1 , comprising:

using the at least one processor, if the set includes no drives, or if the fault frequency does not change by more than the threshold amount in response to changing the operating frequency of any of the drives in the set, selectively identifying a system ground fault if the fault frequency is at or near a line frequency associated with the system.

9. The method of claim 8 , wherein the fault frequency is a frequency associated with a neutral-ground signal in the system.

10. The method of claim 1 , wherein the fault frequency is a frequency associated with a neutral-ground signal in the system.

11. A computer readable medium with computer executable instructions for automatically identifying a suspected ground fault location in a multi-drive system having a plurality of drives, the computer readable medium comprising computer executable instructions for:

determining whether an operating frequency of a given drive is within a range including a fault frequency corresponding to a detected ground fault condition in the system while the given drive is operating; and

selectively identifying the given drive as a suspected ground fault location if the operating frequency of the given drive is within the range including the fault frequency, wherein selectively identifying the given drive comprises:

using the at least one processor, identifying a set of drives in the multi-drive system operating at corresponding frequencies within the range including the fault frequency;

if the set includes a single drive, selectively identifying the single drive as a ground fault location in the system; and

if the set includes more than one drive, using the at least one processor:

(i) changing an operating frequency of a single selected drive in the set while any remaining drives in the set continue operation at their respective current operating frequencies,

(ii) selectively identifying the single selected drive as a ground fault location in the system if the fault frequency changes by more than a threshold amount in response to changing the operating frequency of the single selected drive,

(iii) selectively confirming the single selected drive is not a ground fault location if the fault frequency does not change by more than the threshold amount in response to changing the operating frequency of the single selected drive, and

(iv) selectively repeating steps (i)-(iii) for the remaining drives in the set.

12. A ground fault location system for identifying a suspected ground fault location in a multi-drive system having a plurality of drives, comprising:

an electronic memory;

a network interface operative to provide electronic communications between the system and a plurality of motor drives via a connected communications network; and

at least one processor electrically coupled to the electronic memory and to the network interface, the at least one processor being programmed to:

determine whether an operating frequency of a given drive is within a range including a fault frequency corresponding to a detected ground fault condition in the system while the given drive is operating, and

selectively identify the given drive as a suspected ground fault location if the operating frequency of the given drive is within the range including the fault frequency,

identify a set of drives in the multi-drive system operating at corresponding frequencies within the range including the fault frequency;

if the set includes a single drive, selectively identify the single drive as a ground fault location in the system; and

if the set includes more than one drive:

(i) change an operating of a single selected drive in the set while any remaining drives in the set continue operation at their respective current operating frequencies,

(ii) selectively identify the single selected drive as a ground fault location in the system if the fault frequency changes by more than a threshold amount in response to changing the operating frequency of the single selected drive,

(iii) selectively confirm the single selected drive is not a ground fault location if the fault frequency does not change by more than the threshold amount in response to changing the operating frequency of the single selected drive, and

(iv) selectively repeat steps (i)-(iii) for the remaining drives in the set.

13. The system of claim 12 , wherein the fault frequency is a frequency associated with a neutral-ground signal in the system.

14. The system of claim 12 , wherein the at least one processor is programmed to:

automatically obtain a measured neutral-ground voltage associated with the given drive while the given drive is operating at a commanded frequency;

selectively identify a detected ground fault condition in the given drive if an integral of the measured neutral-ground voltage is greater than or equal to a threshold value; and

determine the fault frequency as a frequency of the measured neutral-ground voltage if a detected ground fault condition has been identified in the given drive; and

selectively identify the given drive as a suspected ground fault location if the operating frequency of the given drive is within the range including the fault frequency.

15. The system of claim 14 , wherein the at least one processor is programmed to report a detected fault condition, an operating frequency of the given drive, and the fault frequency to a device in the multi-drive system.

16. The system of claim 15 , wherein the at least one processor is programmed to:

receive a speed command change from the device in the multi-drive system;

operate the given drive at an adjusted operating frequency according to the received speed command change;

determine an updated fault frequency as a frequency of the measured neutral-ground voltage while operating the given drive at the adjusted operating frequency; and

report the updated fault frequency to the device in the multi-drive system.

17. The system of claim 12 , wherein the at least one processor is programmed to:

if the set includes no drives, or if the fault frequency does not change by more than the threshold amount in response to changing the operating frequency of any of the drives in the set, selectively identify a system ground fault if the fault frequency is at or near a line frequency associated with the system.

18. The system of claim 17 , wherein the fault frequency is a frequency associated with a neutral-ground signal in the system.