Wye Primary Transformer Neutral 6

[nightfox1925]

I have a 750KVA dry type transformer to be installed in a system. Primary winding is 480Volts Wye and Secondary winding is 208/120v Wye. I understand that the secondary neutral is required to be bonded to ground in order to prevent floating neutral in case there are 120v 1phase loads .

However my question is;

In case the power source is coming from 480 volts 3phase 3 wire delta, Is it required or necessary to bond the transformer primary neutral to ground?

In case the power source is coming from a 480 volts 3phase 4wire Wye, is it required to connect the neutral of the power source to the neutral of the drry type transformer primary?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[davidbeach]

You will only have voltage stability if you:
1. Maintain perfectly balanced load on the secondary; or,
2. Connect the wye-point of the 480V windings to the NEUTRAL of the 480V system. It would be a NEC violation to connect that wye-point to ground.

 

[waross]

To expand on David's statements;
1> Floating the primary neutral on a wye-wye transformer of that size is very poor practice and is not done. In practice it is virtually impossible to maintain a load balance.
The common arrangement would be a delta-wye transformer.
If you must connect to a delta system the much preferred method is to use a delta wye transformer. If you are stuck with existing equipment that you must use, add a zig-zag transformer to the primary to generate the neutral.
2> You must connect the neutral to the system neutral. This may be an issue. The code does not require solid grounding of 480 volt wye system neutrals UNLESS the neutral is being used. If the existing system is ungrounded or impedance grounded and you must connect the neutral to the primary neutral of the new transformer, you will be required to solidly ground the neutral of the system feeding the new transformer.

If you go with the zig-zag transformer, and ground the neutral, it will now be grounding the supply system. If the zig-zag is located adjacent to the new transformer you may appeal to the AHJ for exemption on the basis of increased ground fault currents, and the sizing of the zig-zag.
But, as you said, the primary system is wye so this is a moot point.

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

 

[nightfox1925]

Thanks for the contribution. What is the bad effects of floating the Primary Wye Neutral in a Wye-Wye Transformer aside from voltage stability issues?

Davidbeach, do you mean that if the system is not having any neutral, then I have to place a grounding transformer to create a neutral point and bond this neutral to the Wye-wye transformer's primary wye neutral?

I understand that for a wye-neutral grounded primary winding of a wye-wye transformer, I will be creating a ground point for the system and at the same time supply ground fault current to the supply side in the a event of a L-G fault anywhere in the supply system and vice-versa...does this mean I have to put 67Ns or 51Gs on the primary wye grounded side?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[dpc]

http://www.geindustrial.com/publibrary/checkout/White Papers|GET-3388B|generic

This OLD GE paper might be of interest - The Why of the Wyes.

Gotta love it, just for the title.

 

[slavag]

Dave. I see you are really love GE .
Regards.
Slava

 

[dpc]

Hardly, Slava. I don't work for them and never have. Every time I call Multilin for tech support I end up wanting to jump out of the window. But maybe that's just me.

But they used to have a great group of technical engineers who put out useful publications like this. And I do appreciate the fact that they make this info available. My old copy was from an associate who WAS an ex-GE engineer. I was amazed to find this on the website.

Cheers,

Dave

 

[slavag]

Maybe, they forgot delete it;-).
GE, GE... Killed me 3 big projects with dumping prices
Slava

 

[gepman]

slavag
It wasn't dumping, it was that cheap American labor.

 

[slavag]

Hmm. Made in Spain

 

[ScottyUK]

Could be worse, could say 'made in UK' except then you would know it was a counterfeit from China, because we don't make anything here any more.

Thanks for the link dpc. I've saved it in case they follow Slava's idea and delete it from the web.

----------------------------------

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

 

[nightfox1925]

By the way waross, what do you mean by "moot point" and "AHJ"? I'm sorry but I have difficulty understanding some idioms. Can you elaborate what exemption you are refering to?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[waross]

You asked about delta systems even though I understood that your system is a wye system. By moot point, I meant that information about a delta system has no importance to you at this time, even though the information may be valuable sometime in the future on another system.
AHJ is the "Authority Having Jurisdiction". This is usually the local electrical inspection department.
I try to remember to spell out terms in full, capitalized, the first time I use them in a post, but I neglected to do it this time. Apologies offered.
But AHJ is commonly used on this site. You will see it used often.

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

 

[jghrist]

I understand that for a wye-neutral grounded primary winding of a wye-wye transformer, I will be creating a ground point for the system and at the same time supply ground fault current to the supply side in the a event of a L-G fault anywhere in the supply system and vice-versa...does this mean I have to put 67Ns or 51Gs on the primary wye grounded side?

How is the source of the 480V feed grounded? As you note, adding a grounding transformer to the primary can cause protection problems. If the source is grounded, you will be creating a second ground point. If it is ungrounded, you will be converting it to a grounded source.

The best solution would be to use a delta-wye transformer. I suppose you could leave the primary wye ungrounded, not connect the primary and secondary neutral points together, and add a grounding transformer to the secondary to create a grounded separately derived system.

 

[nightfox1925]

Thanks Jighrist.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[LionelHutz]

From your response I think you may still be confusing the issue. If your 480V system is ungrounded and does not have a neutral you can not just connect the Y of your new transformer to ground. The Y-Y transformer requires the proper voltage on each leg of the Y at all times to maintain the rated output voltages. On an ungrounded system the line-ground voltage measured on each phase can vary from 0VAC to 480VAC. The transformer primary expects 277VAC on each leg of the Y windings. So, your 120VAC output voltages could actually vary from 0VAC to 208VAC. This is not what you are looking for.

So, to answer your question
1. You can not install this transformer in a 3-phase, 3-wire delta system without converting the system into a 3-phase, 4-wire system.

2. You install the transformer into a 3-phase, 4-wire system as you posted. Connect the Y point of the transformer (probably H0) to the system neutral.

 

[waross]

Well put, LionelHutz.
LPS

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

 

[nightfox1925]

Well said LionelHutz!

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[jghrist]

Can you connect an ungrounded wye to grounded wye transformer between an ungrounded system and a grounded system (grounded with a grounding transformer)? If not, why not?

If the primary is a delta 480 volt system, could a grounded 120/208 volt system be established with a 480/277Y-208/120GrdY transformer and a zig-zag grounding transformer on the secondary? The grounded secondary neutral would establish stable phase-neutral voltages on the primary. I'll admit that this is a bit unconventional and requires two transformers where one delta-grdY transformer would do, but if you already have the Y-Y transformer, it might be the easiest way.

 

[LionelHutz]

Yes, I believe you could do that and it would work. I can see your reasoning. The zig-zag grounding transformer could be smaller only having to handle the fault currents available on the secondary of this Y-Y transformer instead of being required to handle the fault currents of the building power system.