isolation transformer secondary grounding for 12 pulses drive
isolation transformer secondary grounding for 12 pulses drive
(OP)
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
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





RE: isolation transformer secondary grounding for 12 pulses drive
RE: isolation transformer secondary grounding for 12 pulses drive
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: isolation transformer secondary grounding for 12 pulses drive
RE: isolation transformer secondary grounding for 12 pulses drive
RE: isolation transformer secondary grounding for 12 pulses drive
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: isolation transformer secondary grounding for 12 pulses drive
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
RE: isolation transformer secondary grounding for 12 pulses drive
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: isolation transformer secondary grounding for 12 pulses drive
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: isolation transformer secondary grounding for 12 pulses drive
RE: isolation transformer secondary grounding for 12 pulses drive
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.