Ungrounded Isolation Transformer for VFD 4

[pittengineer]

Hello

I have a 4.16kV-480V Delta/Delta 550kVA isolation transformer used on a 400hp VFD. This transformer is new, and currently the secondary Delta winding is ungrounded. I am debating on weather or not to corner ground this transformer?

The primary of the transfomer is protected with type "E" fuses and the secondary of the transformer does not have a protective device located at the transformer. The feeder cables from the secondary of the transformer terminate into a molded case breaker located at the VFD (Breaker has ground fault settings)

If the transformer secondary is left ungronded will a ground fault on the VFD motor act the same as a regular ground fault on an ungrounded system, or does the presence of the VFD create another source, and ground fault current will return to vfd?

What about fault on cables between transformer and VFD? If secondary of transformer is left ungrounded, then we do not have any ground fault detetction monitor in place (required by code) to detect ground faults so we will have possible overpotential on equipment. If we do corner ground the secondary then for a fault on cables there is no breaker at secondary of transformer so we would be relying on primary fuses to clear a ground fault. Will these clear a secondary ground fault on a delta-delta transformer adequately?

With all this being said, should I leave the secondary ungrounded or go ahead and corner ground it?

 

[ozmosis]

I'm not sure on the reasons why your transformer would have been specified this way but I'm sure it was for a reason.
If using VFD's on an ungrounded network, then ensure the VFD has had the EMC Filter removed and all necessary laekage paths removed (there have been a number of discussions in the past on this).
You should consult with your VFD supplier but with our VFD's, especially the units >125Hp (for no reason other than the cost as a % of drive and the fact we have space in our enclosures), we supply the Bender units as an option on ungrounded supplies:

http://www.bender.org/Product_PDF/Bender IRDH275 Manual - 07-04.pdf

 

[pittengineer]

The transformer was specified with this winding configuratin to mitigate harmonics from the drive.

The more I think about it, I would absolutely prefer to have a corner grounded system. However my only concern is that for a ground fault on the secondary feeder cables, the only way of clearing this ground fault will be the fuses on the primary of the transformer? Should these fuses protect the transformer from a ground fault on the secondary?

 

[pittengineer]

I have questioned the engineering company who designed this sytem and specified these transformers but cannot seem to get a sensible answer from them as to why they specified the secondary of these units to be ungrounded. Unfortunately I dont think the people that I am talking to have a clue.

Does anyone see an advantage or a reason to run this configuration ungrounded.

What I did gather from talking to them was that these transformers were specifed as such for harmonics mitigation. We have three of these larger vfd's with these isolation transformers. (2) of these units are a Delta/Delta config, and the other is a Delta/wye isolation transformer. The explanation I recieved is that these transformers were given different configurations in order to phase shift the voltage across the units and essentially cancel the harmonics on the primarys. Most other transformers in the plant are Delta/wye. I guess all of these Delta/Wye transformers will phase shift by 30deg and will then cancel with those Delta/Delta units?

 

[pittengineer]

Thinking about this some more I can think of two other possible reasons why this system would be left ungrounded.

1) Corner grounding system will give everying in drive a 480V L-G reference for (2) of the phases. This could be damaging to some components in VFD?

2) Will corner grounding somehow take away from the effect that the secondary delta winding is having on the harmonics mitigation?

Since I cannot get a solid answer out of engineering, I have asked the VFD manufacturer for their recomendation.

By the way this is a 6-pulse drive and the transformer was designed as I described above to reduce 5th and 7th harmonics.

 

[busbar]

There can be some undesirable tradeoffs in using corner grounding. And, “floating” delta systems can have serious transient- and resonant-overvoltage conditions that will destroy insulation in short order. A simple and relatively economical solution may be to apply a high-resistance grounding assembly. [Warning: these are back-of-the-envelope calculations]

Connect three 1kVA 480:120V unfused machine-tool control-power transformers with primaries in grounded-wye and secondaries in broken delta. Connect a 50? 1kW resistor across the secondary “break”. By measuring voltage across the resistor, you can get an idea of insulation quality in the 480-volt circuit.

[small]www.microntransformers.com/pdf/B1K0BTWZ37XKH.pdf[/small]
[small]www.ohmite.com/catalog/pdf/tap1000_series.pdf[/small]

 

[xxjohnh]

It is normal to follow the drive vendor recommendation on this. The isolation tranformer is usually ungrounded for harmonic mitigation as I have seen on large up to 10MW ABB drives LCI type. But your drive is maybe a VSI type so harmonics are not the same problem as LCI.

 

[pittengineer]

xxjohnh

I know that VSI stands for Voltage Source Inversion technology but am not familiar with the LCI term and cannot seem to find an definition or explanation anywhere. What does the LCI refer to?

 

[LionelHutz]

Line commutated inverter

Typically used with a synchronous motor and similar to a current source inverter.

 

[xxjohnh]

LCI(load commutated inverters) inverters are the workhorse for large drives 10MW and up. But create large harmonic problems.Always large filter banks are required. VSI are used up to 10MW as a standard but now Siemens and Ansaldo have newer 10 to 30MW VSI drives using several drives paralled to a DC bus.
The foot print for VSI drives is a about a quarter of the SCI type and weighs about 20% of the VSI.

 

[MAGTiger]

You have a 6 pulse drive and a xfmr designed to "reduce" 5&7? Any transformer can reduce harmonics by a very small amount due to the increased impedance added to the circuit. But to get meaningful 5&7 reduction this would be a 12 pulse xfmr and 12 pulse drive or 2 six pulse drives of similar hp. Does the xfmr have 3 or 6 seconday terminals? Are there any capacitors in the xfmr enclosure?

Neil

 

[LionelHutz]

I would follow busbar's advice for a grounding scheme. Obviously, check the numbers for yourself first though.

MAGTiger - Yes, 3 drives total 2 with Y connection and 1 with delta connection should mean the delta connected one will cancel 5th and 7th harmonics with the 2 Y connected ones.

 

[pittengineer]

LionelHutz

There are three drives but two of them have a delta connection and only the one has a wye connection.

You mention that the delta connected ones will cancel the 5th and 7th harmonics from with the wye connected one. These drives are not on the same immediate bus, but I'm assuming this cancelation will take place on the point of common coupling which would be our main incoming plant bus?

Do these 5th and 7th harmonics cancel due to the fact that the wye connected transformer shifts the currents by 30deg compared to the detla connected units? The wye connected unit is a 660kVA unit and the two delta ones are 440kVA and 550kVA. I'm assuming then that at the main bus these phase shifted harmonics cancel out due to the nature and frequcney of the 5th and 7th harmonics.

What about when a delta/delta transformer is used on a drive but not in conjunction with any other transformers. Do any harmonics get trapped in the delta windings? I thought I heard this some where.

 

[LionelHutz]

Yes, the cancellation occurs at the point of common coupling.

A delta transformer connection will "trap" triplen harmonics. The delta/wye would trap them in the primary just as well a delta/delta would trap them in the secondary. However, in a perfect world, the VFD won't produce any triplen harmonics.