Zig Zag Transformer??

[RatherBeFishing]

I work at a Government site with an existing 120/208v feed installed many years ago.....before my time.... in 1000' length of underground manholes. Its actually an old 4160v 3wire feed that was removed from service and then reused some years later to feed services to some remote buildings. Its three insulated 600mcm and a bare 600mcm uninsulated "neutral". We need to correct this circuit to add a new building and were looking at replacing all cables to add an insulated neutral. I'm wondering if any out their have run into this and used a zigzag transformer to create an insulated neutral conductor. Or maybe you have used another solution instead of pulling in new conductors. Currently the feeder serves two buildings and load is <100amps but their is an intermittent elevator load. We are adding a third 3000 sqft building with a 200amp main with light office loads and heat pumps. Thanks for any help you can lend.

 

[BAHTAH]

208V Feeder

208V Feeder

I work at a Government site with an existing 120/208v feed installed many years ago.....before my time.... in 1000' length of underground manholes. Its actually an old 4160v 3wire feed that was removed from service and then reused some years later to feed services to some remote buildings. Its three insulated 600mcm and a bare 600mcm uninsulated "neutral". We need to correct this circuit to add a new building and were looking at replacing all cables to add an insulated neutral. I'm wondering if any out their have run into this and used a zigzag transformer to create an insulated neutral conductor. Or maybe you have used another solution instead of pulling in new conductors. Currently the feeder serves two buildings and load is <100amps but their is an intermittent elevator load. We are adding a third 3000 sqft building with a 200amp main with light office loads and heat pumps. Thanks for any help you can lend.

I would determine if the now uninsulated neutral could be used as a grounding conductor then figure how the added loads will effect voltage drop. I would then use the three insulated conductors at a higher voltage and install transformers at each building to establish 208/120Y services.

 

[Mr. Bill]

How do you know the uninsulated conductor is a neutral and not a ground?

Why would a zigzag transformer create a neutral that's any different than a standard transformer?

Is the old wire being used at 120/208V or at 4160V again?

 

[winnie]

Mr. Bill,

I've seen two uses for the term 'zig-zag transformer', one being a type of secondary used on a transformer in place of the common wye secondary, the other being a type of autotransformer connection. The type of transformer produces a neutral, just like a wye secondary. I believe that the OP is asking about the second type of transformer.

The zig-zag autotransformer connection is used to _derive_ (create) a neutral from the three phases. The most common use is for grounding previously ungrounded delta systems; rather than replacing the supply transformer, a small zig-zag transformer is used to derive the neutral, and then the neutral of the zig-zag is grounded. If the neutral is only being used for grounding and not expected to supply loads during normal operation, a very small transformer can be used to ground a large system.

The _key_ feature is that the neutral created by the zig-zag is not 'separately derived' the way the neutral on a delta-wye transformer is. The zig-zag derived neutral is held in fixed voltage relation to the supply phases.

IMHO using a zig-zag to create a neutral at each building is a clever idea, but probably a bad idea in practise. Because the derived neutral is not 'separately derived', you couldn't bond it to ground. (If you did, it would be very similar to re-bonding a neutral to ground, except that rather than getting current flow on the parallel paths between ground and neutral, you would get some current flowing on the ground conductor, and some getting transformed by the zig-zag.) So you end up with a 'neutral' that is referenced to the phase conductors but _not_ locally referenced to bonded metal. I would expect the neutral-ground voltage to be quite variable and subject to supply faults.

A zig-zag transformer is also probably more difficult to source, although you can create one by using a bank of 3 single phase 1:1 transformers (eg. 120V:120V transformers).

IMHO Grant has the right approach. You end up with a standard, easily understood installation, and you probably reduce voltage drop as well.

-Jon

 

[LarryFine]

Why can't the bare wire be re-used as the neutral?

 

[benaround]

So, you are using this feeder right now for the 2 buildings and it is 120/208v and the

neutral is bare. Where does the zigzag fit in ( 450.5 ). ??

 

[benaround]

You can't use a zigzag on a system that is already grounded, ie; 120/208v