Corner Grounded Delta Secondary How to ground the frame of transformer?

Questions3000

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Chicago
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Lets say I have a transformer with a 3 phase 208 wye primary and a 3 phase corner grounded (lets say B phase) Delta secondary. As I understand it, the panel being fed by the delta secondary would look very similar to a single phase 240 wye, wherein which your B phase takes the place of the neutral bar. The disconnect would have fuses for A and C phase and a solid connection for B phase.

My main concern in this example is that if the frame of the transformer is grounded via grounding bushing/conduit from the 3phase 208 wye primary, then how exactly is the secondary 3 phase delta grounded? Wouldn't the ground from the secondary conduit and grounding bushing create a ground fault on the B phase back to the primary?

My guess is since there's no potential difference from the secondary grounded B phase to the primary ground, then a ground fault shouldn't occur?
 
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winnie

Senior Member
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Springfield, MA, USA
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Electric motor research
My guess is since there's no potential difference from the secondary grounded B phase to the primary ground, then a ground fault shouldn't occur?

This is basically correct.

Current is always trying to get back to its source, and the transformer secondary is an isolated source. (Code lingo: separately derived system.)

When you have two conductors with no electrical path between them, then there is no well defined potential between them. You could in theory measure a voltage, but this would be something like a static charge, leakage, capacitive coupling, or other 'high impedance' voltage that would vanish as soon as you made an intentional solid connection.

When you ground the B leg you create that solid electrical path. Every separate system can have 1, but only 1 'grounded conductor'.

-Jon
 

Questions3000

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Electrician
This is basically correct.

Current is always trying to get back to its source, and the transformer secondary is an isolated source. (Code lingo: separately derived system.)

When you have two conductors with no electrical path between them, then there is no well defined potential between them. You could in theory measure a voltage, but this would be something like a static charge, leakage, capacitive coupling, or other 'high impedance' voltage that would vanish as soon as you made an intentional solid connection.

When you ground the B leg you create that solid electrical path. Every separate system can have 1, but only 1 'grounded conductor'.

-Jon

Thanks for the reply.

So just to be clear, in this situation I would Bond the secondary (let's call it H1) H1 phase to the grounding bus (which has the electrode grounding wire running to it)?

Attaching one of the phases directly to the ground just seems counterintuitive.
 

texie

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Fort Collins, Colorado
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Electrician, Contractor, Inspector
Thanks for the reply.

So just to be clear, in this situation I would Bond the secondary (let's call it H1) H1 phase to the grounding bus (which has the electrode grounding wire running to it)?

Attaching one of the phases directly to the ground just seems counterintuitive.
You must also bond the phase to the case with a jumper sized to 250.102(C)(1). Also be aware that your gear will have to be listed for use with corner grounded delta and some loads are not acceptable for use on a corner grounded supply.
In most cases a corner grounded delta is a very poor choice of supply as it has a number of down sides. In this day and age a grounded Y is usually a much better choice.
 

MD Automation

Senior Member
Location
Maryland
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Engineer
When you say you have a Wye Primary and Delta Secondary, it makes me think you might be back feeding a standard step down transformer to step up 208 to something else like 240 or 480??

If that's the case I just wanted to mention that your Wye Primary will likely have an XO connection (the mid point of the Wye) and it's important that you do not connect anything to that - if back feeding the Wye as your primary. Don't land a 208 VAC source neutral to it - and if there was a bonding wire or strap from that XO to the transformer case (very common), you need to remove that too. The XO should float if the Wye windings are used as a back fed primary.

1000 pardons if you knew this already - just thought to mention it.

As Texie mentions above - for numerous reasons, a proper Delta to Wye transformer (with it's grounded Wye secondary) is typically a much better choice for an SDS than a corner grounded Delta.
 

Questions3000

Member
Location
Chicago
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Electrician
When you say you have a Wye Primary and Delta Secondary, it makes me think you might be back feeding a standard step down transformer to step up 208 to something else like 240 or 480??

If that's the case I just wanted to mention that your Wye Primary will likely have an XO connection (the mid point of the Wye) and it's important that you do not connect anything to that - if back feeding the Wye as your primary. Don't land a 208 VAC source neutral to it - and if there was a bonding wire or strap from that XO to the transformer case (very common), you need to remove that too. The XO should float if the Wye windings are used as a back fed primary.

1000 pardons if you knew this already - just thought to mention it.

As Texie mentions above - for numerous reasons, a proper Delta to Wye transformer (with it's grounded Wye secondary) is typically a much better choice for an SDS than a corner grounded Delta.

In this back feed case, would providing a neutral conductor to the XO create a ground fault path from the backfed secondary 480V back to the 208V backfed primary?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
In this back feed case, would providing a neutral conductor to the XO create a ground fault path from the backfed secondary 480V back to the 208V backfed primary?

Nope. You would still have an isolated 'system' (the 480V delta) and would still be able to ground any _one_ terminal of that system.

The problem with bringing a neutral to the primary of a wye:delta transformer is that if there is any imbalance in the supply voltage you can get large circulating currents in the system. The wye primary 'derives' a neutral from the phases, and if this derived neutral is connected to the supply neutral large currents can flow as the two neutrals try to equalize to the same voltage.

Jon
 

MD Automation

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Location
Maryland
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Engineer
In this back feed case, would providing a neutral conductor to the XO create a ground fault path from the backfed secondary 480V back to the 208V backfed primary?
No, that's not the worry here.

If there is any imbalance of the voltages delivered to the transformer on the 208 Wye - and there always will be some, however slight - and if there is any imbalance in the windings of the Wye coils - and there always will be some, however slight, then there will be a small potential difference in the voltage present at the supply midpoint (which is grounded) and this new tranformer XO midpoint (which you DON'T want to ground). If you did bond it to ground (or run a neutral to it), then current will flow to try and equalize that potential. Best case, it's a small current and only creates extra heat in the windings and conductors. Worse case, it will overheat and burn up something.

Think of the 3 Wye windings in the transformer like 3 springs. They each point outwards, 120 degrees apart in a 2D graph. If you pull on each spring equally, and each spring is perfectly like the other 2, then the midpoint stays right at the "zero" point in an X,Y graph. But each spring's "resistance" is never perfectly like the other 2 (each is wound ever so slightly different) - and the pulling force on each spring (your 3 supply voltages on the 208 supply) are never quite equal. So where the 3 springs are joined, the transformer XO midpoint, wanders slightly from the 0,0 point. But you don't care about that at all for the primary, all you care about is energizing the 3 coils, not whether the center point is spot on zero. But if you ground that point, then you are trying to "pull" it back to 0,0 and that's a bad thing. You must not pin the center of the 3 springs to zero, it must be free to float.

Edit - What Winnie said! ;)
 
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tortuga

Code Historian
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Oregon
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Electrical Design
for numerous reasons, a proper Delta to Wye transformer (with it's grounded Wye secondary) is typically a much better choice for an SDS than a corner grounded Delta.
Care to elaborate on that, I dont think I have seen a corner grounded delta, at least not in the last 20 years.
if its set up like
similar to a single phase 240 ... wherein which your B phase takes the place of the neutral bar.
Would you would have 2 pole breakers and disconnects instead of 3 pole for motor and 3 phase loads?
and single pole breakers for 240 lighting, heating, computers and other single phase 240 loads?
Cheers
 

MD Automation

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Location
Maryland
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Engineer
Care to elaborate on that, I dont think I have seen a corner grounded delta, at least not in the last 20 years.

Maybe my sentence you quoted was misleading??? I meant to say that, for an SDS, a center grounded Wye secondary is a better choice than a corner grounded Delta secondary.

An important reason for that, at least for me, is that much of my work is with Servo and VFD controlled motors. Most manufacturers of these expect the power supply section to be fed from a center grounded Wye. If you have a floating or corner grounded Delta for incoming Mains, some devices will have you remove a jumper inside the VFD or power supply. Some manufacturers will say no-go. Many of these devices have MOVs to protect the rectifier front end from surges/spikes and the MOVs are typically wired in Wye, expecting equal voltage from each phase to ground.

In addition - a corner grounded Delta will have higher phase to ground voltages than a center grounded Wye. That in turn dictates some different (more expensive) breakers, from what I understand.
 

tortuga

Code Historian
Location
Oregon
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Electrical Design
Right thanks MD
That's really interesting.
I have read about corner grounded delta in VFD manuals and stuff but never really seen one in the wild
Nor though it thru, how it would look like a single phase panel.

I hope to run across one one day.
Cheers
 

Osowers

New User
I have used a transformer in this very application: The wye side as primary and the delta as secondary. I am well aware that this is not typically done for multiple reasons. In my application it was a temporary feed for a refer unit that was supposed to be 208V but they delivered a 480 volt unit. The easiest thing to do was to set a temporary transformer but the only one I had in stock was a Square D 480-Delta to 208-Wye. In order to make it work safely, I let the XO float and grounded the H1 on the transformer. The nameplate showed a dashed line on the H1 (Delta Side) and on the XO (Wye Side). These dashed lines mean you can ground either the wye or the delta but not both as MD Automation explains in the post above. You end up with a grounded delta as the secondary but the refer unit and the transformer worked great with no problems. You cannot let the delta output of the transformer float with the primary wye grounded because that would be an ungrounded service and violate NEC as well as cause multiple unsafe issues. Hats off to MD Automation for his explanation of the springs and coils. Thanks to all.
 

MD Automation

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Maryland
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Engineer
Hats off to MD Automation for his explanation of the springs and coils. Thanks to all.

You are most welcome, glad you enjoyed it!

Funnily enough I have come to realize, in the small company I work at, that some of my engineering explanations tend to be geared towards mechanical engineers - since we have way more of them than EEs. So if somebody asks why are you taping off the white wire and not hooking it to XO and you start talking about circulating currents and equalizing voltages, the MEs tend to nod and their eyes glaze over. But if you make it so they can visualize 3 springs with a pin in the center that can locked up to float or be forced down to fit in a hole at 0,0 (at the risk of physical strain on the system) an ME gets that in a heartbeat.
 

brian john

Senior Member
Location
Leesburg, VA
I have used a transformer in this very application: The wye side as primary and the delta as secondary. I am well aware that this is not typically done for multiple reasons. In my application it was a temporary feed for a refer unit that was supposed to be 208V but they delivered a 480 volt unit. The easiest thing to do was to set a temporary transformer but the only one I had in stock was a Square D 480-Delta to 208-Wye. In order to make it work safely, I let the XO float and grounded the H1 on the transformer. The nameplate showed a dashed line on the H1 (Delta Side) and on the XO (Wye Side). These dashed lines mean you can ground either the wye or the delta but not both as MD Automation explains in the post above. You end up with a grounded delta as the secondary but the refer unit and the transformer worked great with no problems. You cannot let the delta output of the transformer float with the primary wye grounded because that would be an ungrounded service and violate NEC as well as cause multiple unsafe issues. Hats off to MD Automation for his explanation of the springs and coils. Thanks to all.

XO is really not floating, the system this transformer was fed from (the primary side) is I assume fully grounded at the main service? If so the primary is not floating.
 

infinity

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New Jersey
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XO is really not floating, the system this transformer was fed from (the primary side) is I assume fully grounded at the main service? If so the primary is not floating.
I think that he meant it is "floating" because it had nothing connected to it. A reverse wired step down Delta-Wye would have only three conductors on the Wye side so there is no connection to the X0. I do see your point because in a grounded system the neutral is grounded at somewhere.
 

don_resqcapt19

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Illinois
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retired electrician
...

Would you would have 2 pole breakers and disconnects instead of 3 pole for motor and 3 phase loads?
and single pole breakers for 240 lighting, heating, computers and other single phase 240 loads?
Cheers
You can do that with a corner grounded 240 volt system. No matter what the breakers have to be straight rated at 240. 120/240 volt breakers are not suitable for this application. In general, it is permitted to be wired just like a 120/240 volt single phase system, including the color of the grounded conductor.
 

kwired

Electron manager
Location
NE Nebraska
Maybe my sentence you quoted was misleading??? I meant to say that, for an SDS, a center grounded Wye secondary is a better choice than a corner grounded Delta secondary.

An important reason for that, at least for me, is that much of my work is with Servo and VFD controlled motors. Most manufacturers of these expect the power supply section to be fed from a center grounded Wye. If you have a floating or corner grounded Delta for incoming Mains, some devices will have you remove a jumper inside the VFD or power supply. Some manufacturers will say no-go. Many of these devices have MOVs to protect the rectifier front end from surges/spikes and the MOVs are typically wired in Wye, expecting equal voltage from each phase to ground.

In addition - a corner grounded Delta will have higher phase to ground voltages than a center grounded Wye. That in turn dictates some different (more expensive) breakers, from what I understand.
If supplying equipment designed around center grounded wye then it is not only the best source but the only source you can supply them with. Often it is surge protection that is designed around it being center grounded and connecting to corner ground or high leg ground will put too much voltage on a portion of the surbe protection components.

Things like motors often will not care what or if anything is grounded and will still function fine.
 

brian john

Senior Member
Location
Leesburg, VA
I think that he meant it is "floating" because it had nothing connected to it. A reverse wired step down Delta-Wye would have only three conductors on the Wye side so there is no connection to the X0. I do see your point because in a grounded system the neutral is grounded at somewhere.
But the system is solidly grounded, the phase have a reference to ground.
 
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