Open Delta used on Y spec equipment

[Sparkyrob]

I ran into an interesting install today, and wasn't sure what to think. I'm not a transformer expert, and I've learned a lot on this forum so I think I'm in the right place.

Building has 120/208 service coming in and is running some equipment using a step up 120/208 Y-480 Delta xformer. They added a new piece of equipment a while back ( I think he said about a year and a half) that was from Italy, and it required 440 Y. The EC that did it used two single transformers and looks to me to be feeding this equipment with an Open Delta configuration. The issue is that the main control transformer INSIDE the equipment cabinet burned up. The equipment is protected with a fused disconnect, and those fuses are good. Equipment doesn't require neutral, and original EC ran proper sized EGC. I've always matched my transformers to the equipment, so I couldn't say with certainty if this install was the culprit, or they just had a bad control transformer. Thanks for your input.

 

[templdl]

I ran into an interesting install today, and wasn't sure what to think. I'm not a transformer expert, and I've learned a lot on this forum so I think I'm in the right place.

Building has 120/208 service coming in and is running some equipment using a step up 120/208 Y-480 Delta xformer. They added a new piece of equipment a while back ( I think he said about a year and a half) that was from Italy, and it required 440 Y. The EC that did it used two single transformers and looks to me to be feeding this equipment with an Open Delta configuration. The issue is that the main control transformer INSIDE the equipment cabinet burned up. The equipment is protected with a fused disconnect, and those fuses are good. Equipment doesn't require neutral, and original EC ran propper sized EGC. I've always matched my transformers to the equipment, so I couldn't say with certainty if this install was the culprit, or they just had a bad control transformer. Thanks for your input.

I am to understand that you have a 208Y/120-480v delta step up transformer and someone is using (2) transformers to step back down to 440v 3ph from the 480v side. Take a look at the 2 transformers to see if they are buck boost transformers which can be done with (2) B-B transformers. You can usually get 32v buck out of them down to 448v. The 480v is usually less than 480 so the actual voltage you would end up with should be closer to 440v.

 

[Sparkyrob]

Yes Templdl, that's right. I snapped a picture of the bb transformers and according to mfctr's charts, will buck 480 to 436. Voltage readings at equipment were approx. 450 phase - phase between all 3 and 250v phase A to ground, 280 phase B to ground, 250 phase C to ground.

 

[broadgage]

For what voltage was the Italian equipment designed ? 440 volts NOMINAL or 440 volts absolute maximum.
The nominal voltage in Italy is generally 3 phase, 4 wire, at 230/400 volts.
I consider it possible that the equipment may be designed for 400 volts nominal, with a maximum of 440 (110% of 400) in which case even 441 volts is too much, and 450 is definatly too much.

It is of course possible that the equipment is designed for 440 nominal, in which case 450 volts actual would be fine, but 440 volts is not a standard nominal voltage in Italy.

 

[Sparkyrob]

Yes broadgage, the saw specs allow for 440 V +-10%, so 450 is well within requirements.

 

[Sparkyrob]

Oh, and you're right about the voltage needing to be 400 V, the secondary side of the control xformer inside the control cabinet is 400. I'll go back this afternoon and do some more sniffing around.

 

[kwired]

Yes Templdl, that's right. I snapped a picture of the bb transformers and according to mfctr's charts, will buck 480 to 436. Voltage readings at equipment were approx. 450 phase - phase between all 3 and 250v phase A to ground, 280 phase B to ground, 250 phase C to ground.

I don't understand why you get these readings. If corner grounded you will have full voltage to ground on two phases and zero to ground on the grounded phase. If you have a so called neutral or midpoint of a phase grounded you will have full voltage phase to phase, probably around 225 two phases to ground and somewhere 380 to ground on a wild leg. Autotransformers are going to mess with this some but you should still have fairly obvious corner ground or high leg system.

 

[Sparkyrob]

Is it possible the step up transformer being used has a floating ground? I didn't even check that...

 

[kwired]

Is it possible the step up transformer being used has a floating ground? I didn't even check that...

Is it a true step up transformer or a back fed step down transformer? A back fed step down will not have an H0 terminal, just H1 H2 H3. The only way to use this is to ground a phase or use as an ungrounded system with ground fault monitoring equipment.

If it is a true step up transformer then there is better chance of having a neutral terminal on the high voltage side.

 

[Sparkyrob]

Must not be a true step up, there's no HO terminal. I've never grounded a phase before, just come off H1 to ground? Sounds like a ground fault, haha. I've always had a delta primary, Wye secondary. I snapped a picture, I'll attach it. There's no XO terminal either...wouldn't there be an XO if it were a back fed step down?

 

[templdl]

Must not be a true step up, there's no HO terminal. I've never grounded a phase before, just come off H1 to ground? Sounds like a ground fault, haha. I've always had a delta primary, Wye secondary. I snapped a picture, I'll attach it. There's no XO terminal either...wouldn't there be an XO if it were a back fed step down? View attachment 8071

Nice picture. Whether or not it has an H0 or X0 terminal is of no consequence.
You seem to be skirting the issue of an actual actual picture of the name plate of the transformer and sare ending us into a circle chasing or tail.
Should you have taken a picture of the Name Plate which will have the HV and LV information as well as the actual wiring connection diagram that would settle a whole lot of questions.
It appears that you have a basic delta-delta transformer that was probably custom ordered because 480D-208D transformers are not normally a stocked item as a 408D-208Y/120 which a commonly used as a step up transformer by feeding the X1, X2 and X3 leaving te X0 unconnected.

 

[Fulthrotl]

I ran into an interesting install today, and wasn't sure what to think. I'm not a transformer expert, and I've learned a lot on this forum so I think I'm in the right place.

Building has 120/208 service coming in and is running some equipment using a step up 120/208 Y-480 Delta xformer. They added a new piece of equipment a while back ( I think he said about a year and a half) that was from Italy, and it required 440 Y. The EC that did it used two single transformers and looks to me to be feeding this equipment with an Open Delta configuration. The issue is that the main control transformer INSIDE the equipment cabinet burned up. The equipment is protected with a fused disconnect, and those fuses are good. Equipment doesn't require neutral, and original EC ran proper sized EGC. I've always matched my transformers to the equipment, so I couldn't say with certainty if this install was the culprit, or they just had a bad control transformer. Thanks for your input.

check what two phases the control transformier was across.

if it was across the open part of the open delta, that might be the culprit.
if it's across a winding of the open delta, i'd call it a bad control xfmr.
normally, you'll centertap A-C to get your 120, or take C to ground
if you are running straight three phase.

i've got a service that i've got to repull a feeder to that is open delta
480, and C phase is grounded... it's a way to get 3 phase cheap, and
then the utility can add a third can on the pole when the load, and
billing revenue warrants it.

 

[kwired]

Must not be a true step up, there's no HO terminal. I've never grounded a phase before, just come off H1 to ground? Sounds like a ground fault, haha. I've always had a delta primary, Wye secondary. I snapped a picture, I'll attach it. There's no XO terminal either...wouldn't there be an XO if it were a back fed step down? View attachment 8071

No X0 or H0 means the probability of it being delta-delta has just gone way up. Seeing that the H terminals connect to the taps for different voltage percentages likely means the H is the designed input side so this is likely a designed step down transformer.

You can ground any of the phases but you can only ground one phase otherwise there is a fault.

People get confused when they see something grounded that they are not accustomed to seeing grounded but think about basic theory for a moment. Even with a secondary with a neutral terminal that neutral still is not grounded until you physically connect it to ground. If you instead connect a phase conductor instead of the neutral to ground the transformer will operate just fine even with loads connected to it. Say it is 120/208 system and A phase is grounded instead of neutral, then everything still operates at same voltage to other system conductors, there is just a ground reference on phase A instead of N. Voltage of each system conductor to ground will be A=0, B=208, C=208, and N=120. There will still be 120 from N to A, B or C. Nothing will burn up, except maybe some supplied sensitive electronic equipment that would like to see 120 volts from phase conductors to ground, or very low voltage levels from N to ground.

Code tells us for most cases if there is a neutral conductor to a system that that is the conductor that should be grounded, this likely is to have lower line to ground voltages as much as possible more so than that grounding other conductors of the system will never work. With a delta system with no mid point taps, it does not matter which one you ground. For a 480 system you still have 480 to ground from the other two conductors no matter which one is grounded.