VFD-Ground Fault Detection 1

[cherry2000]

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.

 

[MikiBg]

Maybe RCCB connected to logical input of VFD?

 

[DickDV]

If I understand correctly, your AC power source is configured as a wye (star) with the centerpoint grounded thru a resistor. If this resistor value is chosen to limit ground fault current but will still release the current limiting devices (sometimes called low resistance grounding), then your VFD issues are only as you mention.

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!

 

[jraef]

Nicely put DickDV.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[DickDV]

Thx, jraef! Today, I needed a boost! That was it!

 

[cherry2000]

Thanks for the inputs DickDV. We are thinking of retrofitting the units with a doughnut CT and a relay which can pick up on sensitive gf and then signal it. Throughout this plant, on LV, there is no intentional tripping on gf. I was wondering if there is any other solution.

 

[DickDV]

If the donut CT you mention is intended for use on the leads between the drive and the motor, I think you will not get the protection you seek.

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.

 

[PQ101]

On 3-phase straight delta connection you can have ground detection lights on your main 480V or 600V rated switchgear to detect ground fault inside the facility. As for emergency generators i.e. 13.8KV or 480V generators systems sometimes customers request to detect ground fault, and we installed CT (Device 50G) on grounded-neutral conductor going to Post Glover resistor box, anytime there was a phase-to-phase fault the fault would travel thru the neutral and the sensing CT would pick it up and cause an alarm in the control room, or locally.