Primary side neutral required for wye-wye transformer?

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don_resqcapt19

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So the configuration would be 3 hots, neutral grounded conductor, plus the equipment grounding conductor right? 4W+G
There is no grounding conductor on the line side of the service disconnect. The neutral serves both as the grounded circuit conductor and the grounding conductor on the line side of a the service equipment.
 

Sahib

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With a wye primary and delta secondary there are voltage constraints on the delta side (closed vector loop) which are not necessarily satisifed on the primary side when the neutral is connected, leading to potentially enormous circultaing currents.
But still third harmonic circulating currents would flow in the delta winding if the primary neutral not grounded.
 

GoldDigger

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But still third harmonic circulating currents would flow in the delta winding if the primary neutral not grounded.
Which many consider a benefit of the delta secondary. It isolates the third harmonic load from the primary.

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GoldDigger

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So in practice the neutral of Wye delta transformer is grounded ie connected back to source.
Except for utility installations where voltage imbalance is carefully controlled, the wye point of the primary is not connected to either neutral or ground. It is left floating.
It is necessary to bring a grounding method to the transformer for earthing, but not a current carrying neutral.

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Sahib

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Which many consider a benefit of the delta secondary. It isolates the third harmonic load from the primary.

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In this case primary neutral grounding spares even the secondary delta winding from third harmonic circulating currents.😊
 

GoldDigger

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In this case primary neutral grounding spares even the secondary delta winding from third harmonic circulating currents.
But it allows even higher and more damaging circulating currents from phase voltage imbalance.
Given the choice I will prefer the former because the third harmonic currents are limited.

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Sahib

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But it allows even higher and more damaging circulating currents from phase voltage imbalance.
Given the choice I will prefer the former because the third harmonic currents are limited.

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Please furnish typical applications where primary neutral is not grounded in Wye delta transformers.
 

winnie

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A typical situation where a wye:delta transformer is used with the primary neutral point not connected is the common situation of a 480V:208V delta:wye transformer used in reverse.

This gets used when a facility has a grounded 208/120V wye service, a piece of equipment needs 480V, and someone is too cheap to buy the proper transformer.

In this case connecting a primary neutral conductor or worse grounding the wye primary is at best a bad idea.

Jon
 

MD Automation

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A typical situation where a wye:delta transformer is used with the primary neutral point not connected is the common situation of a 480V:208V delta:wye transformer used in reverse.

This gets used when a facility has a grounded 208/120V wye service, a piece of equipment needs 480V, and someone is too cheap to buy the proper transformer.

In this case connecting a primary neutral conductor or worse grounding the wye primary is at best a bad idea.

Jon

As both Jon and GoldDigger point out - the "classic" use of a Wye: Delta transformer is backfeeding a Delta: Wye when that's all you have. I consider this a mistake for the reasons they point out.

I still had a test transformer in the shop today and backfed it to see what current was on the neutral when connected to X0. This is a typical 30 kVA 480D to 208Y step down. You can see in the pic below that the neutral is drawing 18+ amps when I power on. There is no load present on the 480D windings - just the idle magnetizing current on the Wye side. I did measure the incoming 3 legs to N just to get a feel for what kind of balance the POCO was giving me right then. It was 123.5 /124.5 / 122.9.

1K8A6251_1.JPG

If the incoming voltage imbalance ever gets excessive you will have large neutral currents on that wire - which is typically NOT protected with an over-current device!

I recall seeing somewhere on the Mike Holt Forum a thread where a bond of the X0 to chassis ground (when backfed) caused enough current to flow in the ground path to melt the conduit or flex feeding the unit.

In my (admittedly) limited experience, I would think the bulk of grounded Wye Primary applications are on POCO distribution systems?
 
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As both Jon and GoldDigger point out - the "classic" use of a Wye: Delta transformer is backfeeding a Delta: Wye when that's all you have. I consider this a mistake for the reasons they point out.

I still had a test transformer in the shop today and backfed it to see what current was on the neutral when connected to X0. This is a typical 30 kVA 480D to 208Y step down. You can see in the pic below that the neutral is drawing 18+ amps when I power on. There is no load present on the 480D windings - just the idle magnetizing current on the Wye side. I did measure the incoming 3 legs to N just to get a feel for what kind of balance the POCO was giving me right then. It was 123.5 /124.5 / 122.9.

View attachment 2558204

If the incoming voltage imbalance ever gets excessive you will have large neutral currents on that wire - which is typically NOT protected with an over-current device!

I recall seeing somewhere on the Mike Holt Forum a thread where a bond of the X0 to chassis ground (when backfed) caused enough current to flow in the ground path to melt the conduit or flex feeding the unit.

In my (admittedly) limited experience, I would think the bulk of grounded Wye Primary applications are on POCO distribution systems?
A while back I did the same thing: measured the XO current with a completely unloaded backfed 240D - 208Y 45 KVA. XO current was 40 amps!
 

ActionDave

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As both Jon and GoldDigger point out - the "classic" use of a Wye: Delta transformer is backfeeding a Delta: Wye when that's all you have. I consider this a mistake for the reasons they point out.

I still had a test transformer in the shop today and backfed it to see what current was on the neutral when connected to X0. This is a typical 30 kVA 480D to 208Y step down. You can see in the pic below that the neutral is drawing 18+ amps when I power on. There is no load present on the 480D windings - just the idle magnetizing current on the Wye side. I did measure the incoming 3 legs to N just to get a feel for what kind of balance the POCO was giving me right then. It was 123.5 /124.5 / 122.9.

View attachment 2558204

If the incoming voltage imbalance ever gets excessive you will have large neutral currents on that wire - which is typically NOT protected with an over-current device!

I recall seeing somewhere on the Mike Holt Forum a thread where a bond of the X0 to chassis ground (when backfed) caused enough current to flow in the ground path to melt the conduit or flex feeding the unit.

In my (admittedly) limited experience, I would think the bulk of grounded Wye Primary applications are on POCO distribution systems?
Hey! Wait! Back the truck up. Since when do engineers have a shop, tools, equipment, and meters?
 

MTW

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If you think that 40A common wye current on an unloaded unit is bad on a 45KVA unit, you should try a 75KVA unit or larger on a unbalanced utility system. It can make a threating noise, to tell you to turn it off, right away.
 
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winnie

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Springfield, MA, USA
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Electric motor research
I would argue that the neutral point of a wye:delta transformer is a circuit conductor. Clearly circuit current flows through that node.

I was then going to argue that 250.24(A)(5) prohibits such grounding, but that only applies to the _grounded_ circuit conductor. (You are generally not permitted to 're-ground' the grounded circuit conductor.) While the neutral point of a wye:delta transformer supplied by a grounded system is at approximately ground potential, I think it really needs to be considered an ungrounded conductor. (Well, not a conductor, but a terminal that would connect to a conductor....)

I don't think that the NEC explicitly prohibits grounding an ungrounded conductor. :) :) In other words I believe that grounding the neutral of a wye primary transformer is a ground fault, but I don't think the NEC explicitly prohibits ground faults.

-Jon
 
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