VFD and HRG system

[electrolitic]

If I have a VFD in a HRG system the ground fault is difficult to be detected. It depends where the fault occurs. Can be at AC entry side, DC bus or drive output, sometimes running at very low output frequency. Expensive microprocessor based relay reading a CT at neutral resistor circuit can discriminate all these faults and can provide a good protection.The question is:can a true RMS voltage drop measurement at neutral resistor (using a series voltage divider to reduce the measured voltage level) get similar results? Thanks.

 

[jraef]

The VFD will be protecting the motor side. On the line side of the VFD you will need to look at the specifics of your drive to see what the mfr recommends for HRG systems. Some have very specific issues that you need to be aware of.

I have never heard of measuring the voltage drop at the NGR as a way of providing ground fault protection. But my first inclination would be that if it worked, a lot of people would be doing it that way to avoid the cost of a ZSCT and GFR...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[ScottyUK]

A core-balance CT can be circuit-specific, where the voltage developed across the neutral earthing resistor can only be used to detect total earth fault current. Neutral voltage displacement is certainly an accepted way of detecting earth fault current, although a voltage transformer is the normal way of making the connection, not a potential divider.


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If we learn from our mistakes I'm getting a great education!

 

[electrolitic]

Sir
A ground fault at VFD DC bus bar can cause an almost pure DC fault current. At VFD output (running at low speed) will cause a low frequency fault current. Conventional CTs, PTs and relays are not designed to deal correcty with them. A very sophisticated
relay as LittelfuseEL731 is designed to overcome all these problems, I know. The reason of the question is more a technical curiosity than a solution to be applied. Regards.

 

[electrolitic]

Mr Jraef
I am sure that voltage drop at neutral resistor it was already used to detect ground faults. GE, as an example. My question is related to TRUE RMS measurement to detect VFD ground faults. Regards.

 

[jraef]

In most quality* VFDs, ground faults on the load side are monitored and protected against in the VFD circuitry. Modern Low Voltage VFDs are Voltage Source Inverters so from an electrical perspective, the output of a VFD is almost like a separate source compared to the line supply. Then from that standpoint, a VFD is monitoring phase currents all the time anyway so monitoring for ground faults is just a matter of firmware in the VFD design, and most use an RCD method.

* On some very small very cheap VFDs they don't actually measure individual phase currents but on those, the risks of major GF events are very low.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electrolitic]

Mr Jraef
I am sorry because I'm returning to the same subject but "in many VFDs, the built-in ground fault
protection will trip only if current to ground reaches a
fixed amount such as 33 or 50 percent of full-load current.
However, in an HRG system, the ground-fault current is
limited to a small value (generally just a few amps) so the
drive can never trip on a ground fault".[extracted from Littelfuse tech catalog.]
The technology of drive transformerless connects the drives directly to the bus bar without transformers that provides galvanic insulation, harmonic compensation, etc. These drives need a lot of capacitors to compensate the lack of transformers (risks are obvious). So, ground faults need to be detected and cleared more than ever in a very precise and fast way. That's why I am interested in VFD fault detection in a system that applies HRG.Regards
(excuse my bad english)

 

[Drivesrock]

Hi Electrolitic

VFDs will really only detect output cables and motor ground faults if there is somewhere for the current to flow to.
I think there are 2 solutions - 1. Ground the feeding supply transformer neutral, solidly or via an HRG. 2. Ground the VFD's DC bus neutral point. This is done by connecting 2 resistors across the DC+ and - and grounding the centre point. Now there is a closed circuit.
Ah, ok, 3 solutions because I have a customer whose synchronous motor neutral point is grounded via resistors for the same reasons I explain below.

One grounding method though per system!

In a flux vector controlled motor on a floating system the drive may trip on motor control failures due to the change in the circuit connected to the drive output. Not guaranteed. From what I have seen of VFDs there is not a ground fault detection but rather a phase-current imbalance detection, perhaps 20% for a few seconds and the drive trips.

What seems to be most important for a VFD is that somewhere there is this grounding!

In IT networks (i.e. isolated neutral networks), the power circuit is totally floating. A capacitive potential divider can exist between the supply transformer primary, the motor and the system ground, with the power electronics of the DC link being the centre node. If switching transients are capacitively coupled from the primary of the transformer (I think it should be the rule to provide the shield between primary and secondary windings to prevent this and other problems), the DC link could move to a very high voltage with respect to ground. This can exceed the voltage rating of the insulation between the power circuit and the control electronics of the IGBTs and something may fail - probably catastrophically. I have seen this many times, but to tell the truth not exclusively. May be that customer is lucky or they have the right transformers?

With AFE drives without isolating transformers, 12, 18, 24 or more pulses for the harmonics reductions there must still be a ground reference somewhere. I am not aware of "extra" capacitors needed to be fitted for these drives - I presume you mean DC link caps?

As long as the AFE drive has its DC link referenced to ground I am aware of no more issues to the reliability of the system. In fact, I know they work very well.

Let the the standard ground fault detection circuits do their protecting or simply detect to alarm of a fault. The last thing needed is to trip machinery instantaneously on a system that has HRG. I don't think that is the point. Decent drives take care of themselves!