CelectricB
Member
- Location
- Texas Panhandle
- 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.
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.
