Deriving Neutral for URD transformer

[supadupachalupa]

I'm currently working on a project for a military subdivision with URD transformers and noticed an odd setup. They currently have a 11.5 kV 3-wire overhead distribution leading up to a particular neighborhood. Not sure what happened but for some reason, a ground grid was placed at the outskirts of the neighborhood and a neutral connection was derived for the transformers. Instead of the transformers connected phase-to-phase, it is connected phase-to-derived neutral. Is this odd and doesn't this violate some kind of code or regulatory standard or is this pretty common?

Thanks,
Dennis

 

[charlie]

. . . They currently have a 11.5 kV 3-wire overhead distribution . . .

The voltage seems odd, are you sure or is it 12.47 kV? That is often called 12.5 kV but I have not heard of 11.5 kV. What is the voltage from the phase to the ground grid?

. . . a ground grid was placed . . . and a neutral connection was derived for the transformers. . .

This is a design that a lot of REMCs used so that they could feed the primary conductors into an area without pulling in a neutral. The downside is the large amount of earth currents that were set up. I do not believe any electric utilities use this system today. It is called single wire, earth return or SWER.

. . . Instead of the transformers connected phase-to-phase, it is connected phase-to-derived neutral. . .

Transformers are normally connected from the primary phase to the MGN or multi-grounded neutral.

. . . Is this odd and doesn't this violate some kind of code or regulatory standard or is this pretty common?

It is very odd and the NESC does not recognize this method. However, the REMCs and the government are not required to follow the NESC.

 

[bob]

supadupachalupa
Where are you located? I've noticed that some of the other countries have an 11 kv system with little load. The SWER might be able to work for a while.
It can not handle large loads.

 

[supadupachalupa]

Thanks for the replies guys. I'm located in Hawaii and there are some odd voltages systems that are located on a few military bases here. I just never cared to ask why and accepted it.

A lot of the subdivision design done here feeds the primaries of URD transformers with phase conductors. No neutral is distributed throughout the base.

All other transformers on the base are fed with 11.5 kV primary except for this one neighborhood of several hundred homes. I'm guessing 7200V transformers were ordered instead of 12kV ones and the base had to do something or eat a large sum of money.

Any other insight would be greatly appreciated!

 

[brian john]

supadupachalupa
Where are you located? I've noticed that some of the other countries have an 11 kv system with little load. The SWER might be able to work for a while.
It can not handle large loads.

Why?..........................

 

[charlie]

. . . I'm guessing 7200V transformers were ordered instead of 12kV . . .

7.2 kV is a standard voltage and works on a 12.47 kV wye primary, phase to neutral. Connecting phase to neutral is normal for 1? transformers.

Why?..........................

In the long term or in the case of high current, the earth will dry out and good contact is no longer made. In Hawaii, the earth will be kept pretty moist from a lot of rain if the location is on the correct side of the island. In other words, it may never present a problem in that location.

 

[don_resqcapt19]

In the long term or in the case of high current, the earth will dry out and good contact is no longer made. In Hawaii, the earth will be kept pretty moist from a lot of rain if the location is on the correct side of the island. In other words, it may never present a problem in that location.

When this method is used, is there not a large "step potential" around the grounding electrode?

 

[charlie]

When this method is used, is there not a large "step potential" around the grounding electrode?

It depends on how the ground grid is set up and the earth resistance. Really deep and numerous grounding electrodes that are spread out over a large area would not create a large enough step potential to be a problem to the average person with shoes. Kids running barefoot through the area could be problematic though. In my opinion, it is a dangerous option that is seldom used. I understand that it is still being installed in some developing countries.

 

[jghrist]

I'm currently working on a project for a military subdivision with URD transformers and noticed an odd setup. They currently have a 11.5 kV 3-wire overhead distribution leading up to a particular neighborhood.

Is it a grounded or ungrounded system? You might have to go back to the distribution substation to check the transformer connection.

Instead of the transformers connected phase-to-phase, it is connected phase-to-derived neutral.

If ungrounded, you would have to have a grounding transformer to establish a neutral.