Primary side neutral required for wye-wye transformer?

CelectricB

Member
Location
Amarillo, Tx
Occupation
MEP Designer
Hi all, I work at an MEP consultancy, and I'm not sure if I'm understanding an electrician correctly. I'll post the conversation below, any insight or simplification is much appreciated.

Engineer: Still confused by a neutral being needed on the primary side of utility transformers.

Electrician: The Company distribution system is wye connected, so they can connect pole or pad mount transformers with either a 3 wire delta primary or a 4 wire wye primary. Since a 3 phase pad mount transformer with a wye connected primary is less expensive to buy, Company normally buys those transformers wye connected, and usually the secondary is also wye connected. Only some older transformers are delta connected on the primary. Does that answer your question?

Engineer: I'm still confused by the 4th wire neutral being required on the primary wye side. Will the neutral go all the way back to the substation transformer? Or would they simply ground rod both the primary and secondary near the transformers? I've been thinking you wouldn't need the 4th neutral overhead unless you wanted to put single phase transformers up on the same overhead runs.

Electrician: The neutral goes all the way back to the substation. They don't recreate a ground somewhere down the feeder. The outgoing feeders from the sub are usually all 4 wire grounded wye. Then as the feeder gets extended, they may drop a phase (leaving 2 hots and a neutral) or can drop the neutral (3 hots, or 2 hots for single phase), or go down to one hot and the neutral. You can also connect the service transformer delta 3 wire, or either grounded wye or floating wye (that is, the primary is wye connected but the center point is not grounded to the distribution system). If the particular Company transformer has been ordered as grounded wye primary, you have to have a distribution neutral to connect to. If the transformer is ordered three phase ungrounded wye or delta, you don't need the primary neutral. In general, Company tends to install their single phase pole transformers connected phase to neutral, but they also have phase-to-phase single phase primary transformers available.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
The explanation sounds good, without being overly detailed

Is this for your general knowledge? You don't normally need to care what the utility is doing on the primary side of their transformers.
 

CelectricB

Member
Location
Amarillo, Tx
Occupation
MEP Designer
The explanation sounds good, without being overly detailed

Is this for your general knowledge? You don't normally need to care what the utility is doing on the primary side of their transformers.
We designed a small clinic to run on 120/208Y, but because there is no neutral on the primary side we are being told that we need a secondary delta configuration instead. Just looking for insight as to WHY the primary neutral is required for wye-wye.
 
We designed a small clinic to run on 120/208Y, but because there is no neutral on the primary side we are being told that we need a secondary delta configuration instead. Just looking for insight as to WHY the primary neutral is required for wye-wye.
Not required. As you say, you can use a Delta wye. Unless power company has some odd requirement that they won't use Delta wye for some reason, but I find that hard to imagine. Around here it's all Delta distribution.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
We designed a small clinic to run on 120/208Y, but because there is no neutral on the primary side we are being told that we need a secondary delta configuration instead. Just looking for insight as to WHY the primary neutral is required for wye-wye.
The transformer will be an isolation style, therefore whatever system is on the primary side does not affect what is on the secondary side.
One of the most common uses of transformer is 480V-208Y/120V, using a delta connected (no neutral) primary from a 480Y/277V system to a wye connected secondary.

Is this transformer being supplied by the utility, or is it customer owned?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I'm thinking the primary neutral should be connected for a wye-wye transformer, so each phase's voltage is maintained.

A wye-wye transformer behaves like three individual 1ph transformers, as does a delta-delta. Unless I'm missing something.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
I'm thinking the primary neutral should be connected for a wye-wye transformer, so each phase's voltage is maintained.

A wye-wye transformer behaves like three individual 1ph transformers, as does a delta-delta. Unless I'm missing something.
Maybe.

It really has more to do with the primary system (i.e. multi-grounded neutrals) rather than just the transformer construction.
 

Electromatic

Senior Member
Location
Virginia
Interesting...There is a recent thread in the Education forum regarding a wye primary, delta secondary that exhorts to not connect anything to the primary wye midpoint.

I know this thread is speaking of wye-wye. I'm with the OP in not understanding why one would need (or want) a primary neutral at all. In a wye-wye, would you only bond the secondary midpoint? Can someone expound a little more, please?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I'm with the OP in not understanding why one would need (or want) a primary neutral at all. In a wye-wye, would you only bond the secondary midpoint? Can someone expound a little more, please?
Without a neutral, the floating primary mid-point would stray from zero volts, resulting in unstable secondary voltages.
 

Electromatic

Senior Member
Location
Virginia
Would one bond both the primary and secondary neutrals within the transformer, then? (Of course, the primary neutral will be bonded upstream somewhere anyway.) It's only in a wye-delta that the objectionable circulating currents occur?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Without a neutral, the floating primary mid-point would stray from zero volts, resulting in unstable secondary voltages.
On the contrary. If you are not a MGN utility, you normally want the primary wye point to be able to move as it accommodates the secondary unbalanced voltages.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
On the contrary. If you are not a MGN utility, you normally want the primary wye point to be able to move as it accommodates the secondary unbalanced voltages.
In a wye-wye? Why, why? Why would you want unstable secondary voltages?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
An unconnected primary wye does not cause an unbalanced secondary, it simply allows one.
 
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