Open delta grounded primary to Wye grounded secondary

[Question.meo]

Project involves a step up open delta 240V 3ph primary voltage to on a delta primary trans with a secondary wye 480V 3ph trans feeding a 3ph 480V machine. The trans came with a bonded sec wye connection. When energized (load in at sec. Turnrned off) the trans was extremely loud and vibrating. I ran across this many years ago on another project and had remove the sec. Neutral bonding jumper from the case of the trans. This solved the problem. However, I have a question on a possible fault current at the machine and it's path to to the primary 30deg. Phase shift main service grounded system.

 

[jim dungar]

Fault currents return to their source. Secondary side faults will not go to the primary side of an isolation transformer.

Delta-wye configurations probably make up at least 80% of customer, non POCO, owned transformers in the US. The NEC requires the transformer enclosure and the secondary wye point to be connected/bonded to the building grounding grid when there are line to neutral loads.

 

[synchro]

It sounds like the transformer secondary winding has an internal ground fault.

 

[retirede]

Are you sure the transformer isn’t a step down being used in reverse, putting the wye on the primary?

 

[augie47]

Double check your tap connections (if present)..be sure they are all the same.

 

[electrofelon]

Can you elaborate on what exactly you mean by "open Delta"? Is your supply two phases and a neutral? Or is the primary only two windings/transformers? Perhaps a sketch would help.

 

[Question.meo]

Fault currents return to their source. Secondary side faults will not go to the primary side of an isolation transformer.

Delta-wye configurations probably make up at least 80% of customer, non POCO, owned transformers in the US. The NEC requires the transformer enclosure and the lsecondary wye point to be connected/bonded to the building grounding grid when there are line to neutral loads.

There are no Line to neutral loads on the wye side of the transformer. Feeds a three ph machine. This was an open delta primary at 240 V 3ph bumping the secondary up to a wye secondary voltage to 480V 3ph to feed a 3 ph planner. With the center ground on the secondary bonded to the Tran cabinet the tran vibrated with very loud winding noise. We disconnected the center ground jumper and it returned to normal use and the machine ran great. There is 30 degree phase shift between the open delta primary and the wye secondary, Comments?

 

[electrofelon]

There are no Line to neutral loads on the wye side of the transformer. Feeds a three ph machine. This was an open delta primary at 240 V 3ph bumping the secondary up to a wye secondary voltage to 480V 3ph to feed a 3 ph planner. With the center ground on the secondary bonded to the Tran cabinet the tran vibrated with very loud winding noise. We disconnected the center ground jumper and it returned to normal use and the machine ran great. There is 30 degree phase shift between the open delta primary and the wye secondary, Comments?

I'm still confused about the open Delta. It's just really odd to have a <600 volt transformer with an open Delta. Is this a bank made up of individual units or an autotransformer?

 

[jim dungar]

The phase shift is of no concern.

It is possible that your open delta source is not stiff enough resulting in a fairly large 3-phase voltage regulation issue. By grounding the Wye point on the secondary you were forcing the 480V 3-phase to be perfectly balanced even though the primary was not. Once you floated the Wye point the secondary voltages were able to more align with the primary.

I assume you added a ground detection scheme as required by the NEC.

 

[tortuga]

Can a open delta make anything other than an open wye?

 

[synchro]

Project involves a step up open delta 240V 3ph primary voltage to on a delta primary trans with a secondary wye 480V 3ph trans feeding a 3ph 480V machine. The trans came with a bonded sec wye connection. When energized at (load in sec. Turned off) the trans was extremely loud and vibrating. I ran across this many years ago on another project and had remove the sec. Neutral bonding jumper from the case of the trans. This solved the problem.

If the load on the secondary was turned off, then the only connection to the secondary windings would've been the bonding jumper to the center point of the wye. It is not clear how that could cause currents to flow through the jumper and result in effects such as loud noise.

 

[jim dungar]

Can a open delta make anything other than an open wye?

Yes.

 

[TwoBlocked]

I think this arrangement results in a zero sequence current on the neutral. It's not something I understand, just that I have heard of it.

 

[electrofelon]

Can a open delta make anything other than an open wye?

Yes.

I think Tortuga is thinking of the case of a supply that is two phases plus neutral. A transformer connected to that system would be open wye secondary yes? Or can we connect a closed Delta bank to that?

 

[jim dungar]

I think Tortuga is thinking of the case of a supply that is two phases plus neutral. A transformer connected to that system would be open wye secondary yes? Or can we connect a closed Delta bank to that?

An open delta has all three line-line voltages, unlike an open wye.
It is also possible to have a high-leg open delta.

 

[electrofelon]

Pl

An open delta has all three line-line voltages, unlike an open wye.
It is also possible to have a high-leg open delta.

Jim, Please define "all line voltages". How does two phases and a neutral fit in.

 

[jim dungar]

Pl
Jim, Please define "all line voltages". How does two phases and a neutral fit in.

An open wye, provides (1) Line-Line voltage and (2) Line-Neutral voltages. There is a 120° angle between the two line-neutral voltages. In general open eyes are used to feed only loads that require a single line-line voltage.

An open delta provides (3) Line-Line voltages just like a closed delta. The problem with an open delta is typically the voltage regulation across the 'open' part of the delta, although this can be partially mitigated by oversizing the remaining transformer windings. In general an open delta can be used similar to any closed delta.

 

[tortuga]

Thats intersting thanks I like learning about odd transformer configurations. I have never herd of a open delta to wye step up transformer, but I am also not an industrial guy. What would be the advantage of such a setup over a standard delta - wye step up? I should add I have worked on a few open hi-leg delta services that had a plain old delta - wye transformer for a panel or load, but never a 'open delta'.

 

[jim dungar]

Thats intersting thanks I like learning about odd transformer configurations. I have never herd of a open delta to wye step up transformer, but I am also not an industrial guy. What would be the advantage of such a setup over a standard delta - wye step up? I should add I have worked on a few open hi-leg delta services that had a plain old delta - wye transformer for a panel or load, but never a 'open delta'.

Maybe i misunderstood one of the questions.

The transformer would have to be a standard three winding transformer arrangement in order to create a true wye output. It is only the source that can be open delta.
You cannot get (3) line-line voltages from an open wye, so it can not be used for 3-phase motors.

 

[Jpflex]

An open wye, provides (1) Line-Line voltage and (2) Line-Neutral voltages. There is a 120° angle between the two line-neutral voltages. In general open eyes are used to feed only loads that require a single line-line voltage.

An open delta provides (3) Line-Line voltages just like a closed delta. The problem with an open delta is typically the voltage regulation across the 'open' part of the delta, although this can be partially mitigated by oversizing the remaining transformer windings. In general an open delta can be used similar to any closed delta.

I have heard of open delta but only in engineering books I have read. In fact all deltas are to be opened and checked for voltage Or current between Legs to be closed and prior to closing them together.

If there is a voltage between points then legs are likely reversed and delta should not be closed until addressed. There was something also beneficial about them such as output but I forgot what it was