VFD-Ground Fault Detection
VFD-Ground Fault Detection
(OP)
We have LV 600V rated VFDs for some of our plant motors. The LV system is resistance grounded. These VFDs do not have any ground fault detection feature. Can anybody give possible options..cheap ones will be appreciated.





RE: VFD-Ground Fault Detection
RE: VFD-Ground Fault Detection
If, however, the resistance is high enough that you can run with one leg grounded and not release the current limiting devices (sometimes called high resistance grounding), then you have additional issues beyond just not having ground fault detection.
First, if the drive has a CE mark, then it has a noise suppression network that expects to see a balanced AC supply. An out-of-ballance condition causes damaging currents to flow into the drive thru this network with drive failure the result. Some drive manufacturers provide for disconnecting this network and, if this applies to you, you should find that information in the instruction manual and follow it. Also, usually, if the drive has output ground fault protection, it must be turned off in the parameters to avoid fals faulting every time the input AC shifts its balance. When you do that or if the drive has no ground fault protection, unfortunately, the drive becomes dependent upon its output short circuit detection circuitry to protect itself in the event of a hard ground fault. If the drive has short circuit detection on all three output phases, the drive is safe. But, as in many drives, if only two phases are monitored for short circuit current, a hard ground on the unmonitored phase will damage the drive.
No, I don't like floatind or unbalanced AC sources and the above is one reason why!
RE: VFD-Ground Fault Detection
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RE: VFD-Ground Fault Detection
RE: VFD-Ground Fault Detection
RE: VFD-Ground Fault Detection
CT's are designed to see sine wave power and there is no sine wave on the motor leads. Most CT's will not measure even close to actual amps due to the high frequency components which dominate on the motor leads.
You didn't mention it but I would at least consider solving the basic problem of floating and unbalanced power with a drive isolation transformer. The secondary would be configured as a wye with the center point grounded. Now the drive works right and you get a bonus of reduced input harmonics and transient protection too.
If your hp's are too high, that would be my recommendation. Beyond that, if you have VFD's clustered together, consider a single transformer for all VFD's within range. That would be less expensive, too.
RE: VFD-Ground Fault Detection