ground for an open delta
ground for an open delta
(OP)
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
How is the grounding usually done for an open delta supply? There is no neutral.
Alan






RE: ground for an open delta
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
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"Why not the best?"
Jimmy Carter
RE: ground for an open delta
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?
RE: ground for an open delta
RE: ground for an open delta
RE: ground for an open delta
It may be that he has two single phase 600V services.
RE: ground for an open delta
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.
RE: ground for an open delta
Thanks a lot for your detailed response. A "Winnipeg" connection huh? I've never heard of it, and I'm from Winnipeg.
Alan
RE: ground for an open delta
Ah, OK. Thanks. Alan better have the primary neutral firmly grounded all the time.
RE: ground for an open delta
Are you running a primary neutral conductor with your phase conductors over the mountain range?
RE: ground for an open delta
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."
RE: ground for an open delta
RE: ground for an open delta
Yes, there is a primary neutral.
RE: ground for an open delta
RE: ground for an open delta
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
RE: ground for an open delta
Mid-point is used in the US, mainly on 120/240 leg to get 120V easily.