ground for an open delta 1

[alangh]

I am working on a project at a remote site that has only two phases supplied via an overhead line across a mountain range. Adding the third phase is very expensive. So there is an open wye/open delta 600V power source created from the two 12kv lines.

How is the grounding usually done for an open delta supply? There is no neutral.

Alan

 

[waross]

I would use 480 volt transformers (240/480 V) and ground the enter tap of one transformer (either directly or impedance grounded.) Now you have a grounded supply at 480 volts. Use a pair of auto-transformers in open delta configuration to develop your 600 volts. When BCHPA was changing their supplied voltage from 480 Volts to 600 Volts, we saw a number of open delta auto-transformer connections dropping 600 Volts down to 480 Volts. In some instances, such as when a small factory relocated to a new building, the whole service would be dropped from 600 Volts to 480 Volts to supply legacy equipment. During the confusion of the change-over (which lasted for a number of years) we often had to use the open delta auto-transformer connection to match 600 Volt equipment to 480 Volts and 480 Volt equipment to 600 Volts.
Your other options are corner grounding or a grounding transformer (zig-zag), the same as would be used on a closed delta. You may also use four 347 volt transformers in a "Winnipeg" connection to develop 347:600 Volts wye.
Connect transformer #1 and transformer #2 to phases "A:and "B" so as to develop two phases of your wye. Now connect transformers #3 and #4 to phases "A" and "B", but reverse the secondary connections. Now connect transformers #3 and #4 in open delta. A vector sketch will show that the resultant voltage will supply phase "C" of your wye.
You will still have the neutral offset mentioned below.
Be aware that your neutral current will be approximately equal to your phase currents, and the resulting neutral voltage drop will be giving you phase displacements on your two phases. I suggest oversizing all motors 50% or more to allow for the extra heating caused by the phase displacement.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

How can you have a 3 phase system from two phases? In a open delta system though only two "transformers" are used but you still need all three phases.

You need to provide more detail of your configuration. Your transformer primary voltage rating etc.

What is the source configuration of 12kV lines, back at the substation? Is it a grounded wye or delta?

What you describe sounds like you have two single phase transformers, each fed from a phase and other end of the transformer are joined together?? or something else?

 

[davidbeach]

Two phases and a neutral is all that is necessary for the open wye-open delta connection. waross seems to have answered the connection question.

 

[stevenal]

" There is no neutral." Are you referring to the primary? If your primary system has no neutral, you have single phase. You will need a converter of some sort. Perhaps a single to three phase VFD.

 

[rbulsara]

david?? you cannot produce a third phase out of two phases.

It may be that he has two single phase 600V services.

 

[magoo2]

Yes, you can.

The attachment is from an ABB Distribution Transformer Guide. I made 2 copies by mistake.

This is a fairly common connection for small three-phase motor loads.

 

[alangh]

Bill,

Thanks a lot for your detailed response. A "Winnipeg" connection huh? I've never heard of it, and I'm from Winnipeg.


Alan

 

[rbulsara]

magoo:

Ah, OK. Thanks. Alan better have the primary neutral firmly grounded all the time.

 

[stevenal]

Alan,

Are you running a primary neutral conductor with your phase conductors over the mountain range?

 

[busbar]

According to NEMA C84.1-2006 figure A1, the grounded conductor [not technically a “neutral”] is connected to mid-coil of one of the two transformer windings on the open-delta side.

I’ve heard that one slang term for primary-side 2-phases-and-a-neutral is “V-phase.”

 

[alehman]

I don't think earth only as a ground would be a good option, and probably would not be permitted by NESC.

 

[alangh]

Stevenal,

Yes, there is a primary neutral.

 

[alehman]

Ah, then you should be set.

 

[waross]

A couple of FYIs
600 Volts is more common in Canada so the NEC may not apply.
600 Volt systems typically use 600 V transformers (delta) or 347 V transformers (wye). Neither transformer has a "midpoint" available for grounding. Hence the suggestion to use 480 V transformers so as to have an accessible mid point on one of the transformers for grounding and then boosting to 600 V.

North America; Europe;
Open Delta = Vee connection.
Broken Delta = Open Delta

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Does it have to be a mid-point of the xfmr for grounding a delta system? Would a corner grounding not work?

Mid-point is used in the US, mainly on 120/240 leg to get 120V easily.