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#202857 - 08/31/11 05:06 AM Xformer theory Q
sparky Offline
Member

Registered: 10/18/00
Posts: 5542
In a Y / Y (or star / star) Xformer setup, when given a nuetral on the H side, can it be a common connection on the X side?

I've looked in Satllcup/Ugly's/American handbook this a.m., no clarity

Looks like it's an SDS along with MBJ, GEC, , but why create a nuetral when given one?

Maybe i'm missin' the obvious & need more coffee?

~S~

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#202860 - 08/31/11 06:28 AM Re: Xformer theory Q [Re: sparky]
HotLine1 Offline

Member

Registered: 04/03/02
Posts: 6785
Loc: Brick, NJ USA
How about the possible voltage differentials btwn the primary (high) and secondary (low) when there is current flow present within the respectful system neutrals?

Sounds plausable, but I'm looking for more...coffee!

Just my first thought...
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#202863 - 08/31/11 09:10 AM Re: Xformer theory Q [Re: HotLine1]
Vindiceptor Offline
Member

Registered: 03/28/11
Posts: 98
Loc: San Diego

It is a common connection and a fault on one side can/will trip OC devices on the other, I have seen it happen.

Transformer manufacturers charge a premium to make one though and there is usually a phase shift, but I do not remember the details on that, though I'm sure someone will chime in.

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#202864 - 08/31/11 09:31 AM Re: Xformer theory Q [Re: sparky]
gfretwell Online   content

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
Maybe I misunderstand the question but aren't we talking about the grounded conductor on each side of the transformer?

Don't you usually bond all the grounding conductors together?

I am still on my first coffee too. wink
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Greg Fretwell

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#202867 - 08/31/11 01:19 PM Re: Xformer theory Q [Re: gfretwell]
Vindiceptor Offline
Member

Registered: 03/28/11
Posts: 98
Loc: San Diego

You do, but with a typical wye/delta, except for the ground the two sides are isolated from each other. With a wye/wye the neutral passes straight through.

If you're not using it on one side no big deal, but if you are then there are many issues to overcome or at least to be aware of.

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#202869 - 08/31/11 02:40 PM Re: Xformer theory Q [Re: sparky]
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
Vindi...

Delta/Wye transforms cause a 30 degree phase shift... with the Wye lagging the Delta... It's in Ugly's -- the fine print.

BTW, Delta and Wye are TENSOR diagrams. So you can visually see the math by placing the star inside the delta and note that the star tensors bisect the delta's wedge -- each wedge is 60 degrees -- and cutting it in half gives you the 30 degrees lagging shift.

But enough Vector Algebra...

Wye/Wye transformers use more wire -- to no good purpose.

The windings ratio in a 480 to 208Y120 is 4:1...

This reality is beyond many electricians, I've been on the end of many flaming posts because of such belief. Some j-man think that the ratio is 480/208 -- which is entirely false. For those disputing this... open up a transformer and trace the connections. ( Turn it off, first. )

The delta connection is H1-H2 : 480 volts

The wye connection is L1-N : 120 volts

Wye-Wye transforms also permit noise from either side to crossfeed.

There are occasions where you'll find wye-wye: massive DC power supplies used to drive electro-chemistry: aluminum reduction, chlor-alkalai plants and such. They use a mix of delta-wye and wye-wye to phase shift -- creating 12 phase power from 3 phase. At that point the AC is rectified with a massive low pass filter -- thence to the reduction line.




Edited by Tesla (08/31/11 02:48 PM)
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#202872 - 08/31/11 03:11 PM Re: Xformer theory Q [Re: Tesla]
Vindiceptor Offline
Member

Registered: 03/28/11
Posts: 98
Loc: San Diego

That's the big issue, the noise if you will.

This came up for me when using large step-up step-dn transformers on large scale PV projects, several 'other' engineers wanted to use a y/d-d/y configuration and it took forever to get across to them that there was no (easy/cheap) way to open a fault that way so they proposed a y/d-y/y configuration, which would work (noise was not an issue), but the additional cost and lead time was not worth it.

We standardized on a y/d-y/d configuration, but still wound up with one installation with a y/d-d/y configuration where we corner grounded the delta on the utility side of the run.

I still remember the jw and foreman calling me asking repeatedly, "this is going to work when I throw the switch right?" and the total surprise in their voice when they called back to say "damned if it doesn't work!".

Interestingly enough I discovered that a LOT of utilities use Wye/Wye transformers within their own distribution networks.

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#202875 - 08/31/11 03:26 PM Re: Xformer theory Q [Re: sparky]
gfretwell Online   content

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
Quote:
Interestingly enough I discovered that a LOT of utilities use Wye/Wye transformers within their own distribution networks.


FPL does it all the time.



The center tapped transformer in front of my house is fed from a wye. Pretty much the same issue with the neutral bonded on both sides.
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Greg Fretwell

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#202880 - 08/31/11 08:10 PM Re: Xformer theory Q [Re: sparky]
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Greg's Image above looks like a MGCN setup (Multi-Grounded Common Neutral).

The Primary -appears- to be connected as a Wye, but I am not 100% sure.

The Secondary must be Wye connected, as the Left-Side Bushing of the Two visible Pots has only a single lead with no interties; and the Right-Side Bushings have Bare Leads, which appear to be heading to the Star Point + MGCN termination.

Greg, do you have a Non-marked Up version of this Pole's Setup?

-- Scott
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Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#202886 - 08/31/11 09:16 PM Re: Xformer theory Q [Re: sparky]
gfretwell Online   content

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
Sorry about the big pictures but I want them to be easy to see.

You can see the common grounded wire going up the pole here



This is the original

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Greg Fretwell

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