isolation transformer secondary grounding for 12 pulses drive

[tem1234]

Hi,

To achieve a 12 pulses drive on a 600 volts systems, we have an isolation transformer with two secondary, one wye and one delta. There's one drive (ASC800 from ABB) on each secondary

We plan to ground solidly the neutral of the wye winding (the primary is also a wye with the neutral solidly grounded), but we are not sure what to do with the delta winding.

The solution that i think are:
1 - Leave it ungrouded and add a ground fault protection relay
2 - Achieve a corner delta grounded
3 - Add a grounding transformer with resistor, but we don't have space and time to install it.
4 - Any other options...

What is your thought about it? Is there some good practice to do that?

Thanks

 

[tem1234]

correction, the primary is also a delta winding

 

[Skogsgurra]

For very convincing reasons - leave the delta winding ungrounded.

If you ground it, you will create a short that trips the fuses and possibly also destroys the thyristors. That should be obvious if you draw the corresponding circuit diagram. Do that - and leave the delta as is.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LiteYear]

Is the DC bus on the two ASC800 drives joined together, or do they run separate buses? If they are separate, there is no short. In my neck of the woods the zigzag transformer is common. But if there's no room then I'd do a risk assessment on ground fault hazards, check with your local regulators if appropriate, and look to leave it ungrounded with a suitable ground fault protection relay. I'm not familiar enough with corner grounding to make comment on that approach.

 

[sibeen]

Why are you grounding the Wye?

 

[Skogsgurra]

I missed the ACS 800 part. Thought of a DC drive with a common DC output.
So, if there are two separate, independent drives with separate motors on each drive - then there's no risk for a short.

But then it won't be a twelve-pulse drive. Just a couple of drives that are fed with different phase angles and where the transformer's primary current will be genuinely 12-pulse only when the two drives have equal load.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[tem1234]

The 2 drive will not be connected. So it's not really a 12 pulse drive. But the load on the 2 drive will always be equal. We do that to reduce harmonic on a very weak system, with no utility, only generator.

Sibeen, if we don't ground the wye, by regulation, we have to had a ground fault protection relay. If there's advantage, we can, but i don't really like ungrounded system. The best would be to add a resistor to have a high resistance grounding, but for some reason i think we wouldn't be able to do it (space and time).

Thank for your help

 

[Skogsgurra]

So, why call it a twelve-pulse system, then?

That kind of loose terminology causes lots of unnecessary misunderstandings. And quite often, such misunderstandings develop into myths - which in quite a few quarters develop into engineering truths.

If Code says that you need to have an Earth fault protection - then you have to put it there. Quite simple, I think.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Three small lighting transformers connected in wye delta will provide an economical "off the shelf" grounding scheme. The primary wye point is grounded through a grounding resistor. The transformer primaries are sized to safely carry the current of the grounding resistor. Nothing is connected to the delta and the delta voltage is unimportant.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LiteYear]

Although very much dependant on the details, we've designed "12-pulse" systems in this fashion for conveyors. The motors tend to share the load quite well, so the 12-pulse effect is good. The last simulation I ran showed that it quickly dissolves to 6-pulse performance once the load on each motor differs by more than about 10%. Certainly it would be misleading to call two uncoupled 6-pulse drives a 12-pulse arrangement, but provided there's some sort of electrical, mechanical, or at a push control coupling, then as far as the supply transformer is concerned, it's a 12-pulse load.

 

[Drivesrock]

Apart from what your installation or local rules may or may not require for grounding the neutral point a very good reason is to help your VFDs. On a floating supply the DC links will also be floating with respect to the ground and that is not good for the IGBTs due to the stray capacitance in the drives to the frame and ground. I have plenty of experience of seeing blown up VFDs from many different manufacturers due to this scenario.
If you can't ground the transformer secondaries' neutral points then an alternative is to ground the centre point of the VFDs' DC links. This could be achieved by 2 resistors in series connected across the DC+ and DC- and grounding the centre 0V point either directly or via another resistor. Fit a suitable ground current detection relay in the wire to ground to detect for ground current - eg Bender RCMA 475.

The drives will appreciate this - well, the maintenance team will more - and you will have adequate ground fault paths and detection.