240 corner grounded to 240/139 wye service

tortuga

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
This may be more of a question for the POCO folks on here; say a building has a old service 240 delta corner grounded B phase.
Three transformer bank on a pole; 240V 100kVA each.

Generally speaking can you just switch connections to make it 240/139 wye?

or would you need different transformers for some reason?
Would the CT metering need to be different?
 
You cant get 240Y/139 out of those transformers. You need an secondary voltage of 139 to get the 240Y/139 volt system.

If the transformers can internally be configured for as secondary voltage of 120 volts, then you could get a 208Y/120 volt system.

No idea about the metering.

What would a 240Y/139 system do that the corner grounded 240 volt system can't?
 
tortuga said:
This may be more of a question for the POCO folks on here; say a building has a old service 240 delta corner grounded B phase.
Three transformer bank on a pole; 240V 100kVA each.

Generally speaking can you just switch connections to make it 240/139 wye?

or would you need different transformers for some reason?
Would the CT metering need to be different?
Click to expand...
It would work fine to center tap the three transformers and make it 240/139Y.
Problem is, that’s not a utilization voltage.

You won’t be able to use the 139, so why?

As far as the CT metering, it should work fine if it’s something like a 9S configuration
 
Hv&Lv said:
It would work fine to center tap the three transformers and make it 240/139Y.
Problem is, that’s not a utilization voltage.

You won’t be able to use the 139, so why?

As far as the CT metering, it should work fine if it’s something like a 9S configuration
Click to expand...
I thought the secondary voltage of each transformer for a wye system has to equal the line to neutral voltage. How do you get 139 volts wit a center tap?
 
Hv&Lv said:
You won’t be able to use the 139, so why?
Click to expand...
Multi-voltage devices certainly would, granted that the connections might be interesting - shouldn't use a 5-15 device and shouldn't use a 6-15 device either.

One advantage of changing is to get rid of the corner ground, which confuses people, while maintaining a grounded system. Downsides, there are many unless you plan to wire the building from scratch.
 
Hv&Lv said:
It would work fine to center tap the three transformers and make it 240/139Y.
Problem is, that’s not a utilization voltage.

You won’t be able to use the 139, so why?

As far as the CT metering, it should work fine if it’s something like a 9S configuration
Click to expand...
Okay thanks good to know, the purpose would be for VFD pumps that like the same voltage to ground on each phase, and a possibly a simpler backup generator, the 139V would not be used for anything after the new service disconnects.
 
tortuga said:
Okay thanks good to know, the purpose would be for VFD pumps that like the same voltage to ground on each phase, and a possibly a simpler backup generator, the 139V would not be used for anything after the new service disconnects.
Click to expand...
Almost every drive manufacturer offers a 240Y/139 transformer for isolating drive, but these aren't used as commonly as they used to be.
 
tortuga said:
Okay thanks good to know, the purpose would be for VFD pumps that like the same voltage to ground on each phase, and a possibly a simpler backup generator, the 139V would not be used for anything after the new service disconnects.
Click to expand...

I know some VFDs can be reconfigured to accept 120/240 center tapped delta. So you might get the POCO to switch over to that configuration, I know I have seen it provided in Oregon.

However, if they can work with center tapped delta, I figure they could probably work with corner grounded delta also.

Any generator could physically do corner grounded delta, but the controls might not like it. Depends on the generator.

To get 139/240 delta, the existing tx would need to be set in parallel instead of series and then tapped down 15% on the primary. Normally they only have 5% so that is probably not going to work. The POCO is probably not going to want to give you 139 also.

It is my opinion that 120/240 center tapped delta would be the best choice, and you have to explicitly specify that to your VFD mfr.
 
jim dungar said:
Almost every drive manufacturer offers a 240Y/139 transformer for isolating drive, but these aren't used as commonly as they used to be.
Click to expand...
Yeah am familiar with them, it's obviously more efficient to not have an extra transformer and there are no loads that require a corner grounded delta but there are loads that require or prefer the wye configuration.
 
tortuga said:
This may be more of a question for the POCO folks on here; say a building has a old service 240 delta corner grounded B phase.
Three transformer bank on a pole; 240V 100kVA each.

Generally speaking can you just switch connections to make it 240/139 wye?

or would you need different transformers for some reason?
Would the CT metering need to be different?
Click to expand...
I'm sure you are aware of it, but you will need to add a fourth service conductor, unless they happened to install a spare in case it ever got converted to a wye system.
 
electrofelon said:
I'm not seeing it either. I don't think you can connect them in Delta and then connect all the center taps together, can you? 🤔
Click to expand...
Normally, internally the transformer would be re-jumpered for 120 volts output, X1+X3 and X2+X4, parallel. On big transformers it is external, they have 4 bushings on the output. Then you wire them wye to each other.
 
Birken Vogt said:
Normally, internally the transformer would be re-jumpered for 120 volts output, X1+X3 and X2+X4, parallel. On big transformers it is external, they have 4 bushings on the output. Then you wire them wye to each other.
Click to expand...
I don't know what is common, but if the transformers used for 480/277 banks would happen to be able to be re-jumpered in same manner then you would have a unit with ~139 volts at the output
 
kwired said:
I don't know what is common, but if the transformers used for 480/277 banks would happen to be able to be re-jumpered in same manner then you would have a unit with ~139 volts at the output
Click to expand...
True, but I have never seen a single phase 277 transformer as part of a bank. They occasionally use 3-bushing pole mounts but those are permanent wye, more commonly those will be padmounts anyway
 
Birken Vogt said:
True, but I have never seen a single phase 277 transformer as part of a bank. They occasionally use 3-bushing pole mounts but those are permanent wye, more commonly those will be padmounts anyway
Click to expand...
Come to where I am, they are everywhere supplying irrigation and other farm loads, and some smaller capacity 480 volt services in the towns, like maybe a well house or lift station. Not sure if they are able to be internally reconnected for different voltages or not. I'd have to look but seems there is usually three secondary bushings but they always just use two of them, kind of a sign to me that they may be able to be reconfigured internally.
 
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