3 Phase Secondary From Single Phase Primary

[xptpcrewx]

See if this drawing helps.
Delta primary on distribution like your talking about, two transformers, three phases secondary with a neutral after the transformer.

figured the drawing would be easier than me trying to explain it..

View attachment 2557900

Needs a multi-grounded neutral to source.

 

[electrofelon]

The power company always has a grounded / neutral conductor on the primary side.

Often not. Some areas have extensive delta distribution. Usual it's a bit older, newer lines tend to be MGN (multi grounded neutral wye)

 

[MD Automation]

Needs a multi-grounded neutral to source.

I agree.

I've been wrong before, and surely will be again, but I don't see how the diagram in Hv&Lvs's post #19 works if it comes from a Delta primary distribution (w/o an neutral). Those 2 hot primary legs, w/o another leg to reference against, can only create a single phase voltage.

If it helps any, here is another quick pic, which I think is exactly the same setup that Hv&Lv posted ( 2 transformer Open Wye / Open Delta) except it also includes in the diagram the necessary neutral up top, coming from the Primary. Makes it just a tiny bit clearer that you need 3 connections from the source.



As other have mentioned, I believe you need that neutral from the supply - typically an MGN from a Wye source. Without that neutral you have only 2 hot legs from the primary which can only ever create a single phase sine wave - no way to develop 3 phases w/o the third leg coming in.

 

[electrofelon]

Needs a multi-grounded neutral to source.

If I may nitpick a bit, what does an MGN have to do with it? You need a neutral from a wye system

 

[LarryFine]

I've been wrong before, and surely will be again, but I don't see how the diagram in Hv&Lvs's post #19 works if it comes from a Delta primary distribution (w/o an neutral). Those 2 hot primary legs, w/o another leg to reference against, can only create a single phase voltage.

Look again. Both H2s connect to the secondary neutral below the transformers.

Primaries and secondaries often share the same neutral conductor on pole runs.

 

[xptpcrewx]

If I may nitpick a bit, what does an MGN have to do with it? You need a neutral from a wye system

The diagram in post #19 is valid for a MGN system. The diagram shows a separately derived secondary neutral only. This isn’t allowed as it would be relying on the earth as part of the primary circuit back to the substation. Anytime a conductor is used as a common neutral for primary and secondary circuits, a MGN system is required. You are correct the neutral from a wye system is needed.

 

[GoldDigger]

The diagram in post #19 is valid for a MGN system. The diagram shows a separately derived secondary neutral only. This isn’t allowed as it would be relying on the earth as part of the primary circuit back to the substation. Anytime a conductor is used as a common neutral for primary and secondary circuits, a MGN system is required. You are correct the neutral from a wye system is needed.

Because is you try to use a primary earth return to get the neutral you guarantee a high neutral to remote earth voltage on the seconday side. Which creates all sorts of problems for the customers when they bond POCO neutral to their GES.

 

[MD Automation]

Look again. Both H2s connect to the secondary neutral below the transformers.

Primaries and secondaries often share the same neutral conductor on pole runs.

Hi Larry - yes, saw that. But what maybe got me confused is the way I always looked at those types of diagrams, the wires up top indicate the primary connections (back to the source) and the wires below are the secondary connections (out to the load). And so I did not see a primary (wire) connection back to the POCO source for the neutral, only the derived secondary from below.

If the Neutral at the low voltage secondary connections (that crosses up to the H2 bushings) is understood to be shared by the Primary and Secondaries, and goes back to the source, then that works, and I just misunderstood the diagram.

Also, Hv&Lv mentioned a Delta Primary Distribution feeding that setup, which would not really have an effective neutral. So that confused me also.

But, again, I've been wrong before

 

[kwired]

Often not. Some areas have extensive delta distribution. Usual it's a bit older, newer lines tend to be MGN (multi grounded neutral wye)

In my area, and every place I’ve worked or seen, POCO a doesn’t have a neutral.

3 phase lines on a pole, period.

Then a transformer gets hung in the pole, or in some cases goes under ground to a transformer.

If the transformer is single phase, 2 of the distribution lines get tapped for the transformer primary, and off the secondary is two hots 180 degrees apart and a center tapped neutral.

If it’s a 3 phase transformer, all three go to the primary side. Still no neutral till the secondary.

So I’m not understanding where the POCO is getting this neutral from, to bring to the primary side of a xfrm.

You might have delta distribution like electrofelon mentioned. There won't be a neutral with those systems and you will need all three the ungrounded primary conductors to get three phase.

A majority of the local distribution is from wye secondaries at the substations. They almost always ground the neutral of the wye and often use just one bushing transformers for single phase transformers on the system. primary and secondary grounded conductors are bonded together.

When you only have two conductors for primary supply, regardless whether it is wye or delta fed you only have one phase angle between them and can only derive single phase by simple magnetic coupling like is done in transformers.