delta delta transformers

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mshields

mshields

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A colleague came accross a delta delta 13.8 to 480 transformer; happens to be a 2500KVA, VPI type from the 60's. Will want to switch this out for new project if only based on it's age but would also like to add problems associated with this configuration. Haven't seen one of these in some time. Don't remember all of the issues associated with them. Seems to me there is typically a problem with voltage regulation and then there is the question of what happens when there is a ground fault on the secondary side.

In any event, you're thought would be greatly appreciated.

Thanks,

Mike
 
Properly installed, a delta-delta can provide more reliability than a grounded wye secondary. Ground faults can be addressed thru ground detection, or intentionally grounding one phase.

I've worked with delta secondaries all of my adult life, and don't see any problem with them. Both wye and delta have benefits, it just depends on the application.

Jim T
 
grounding corner of a delta delta secondary

grounding corner of a delta delta secondary

If you do that (and it may well be that I have to go back to school here), what do you get when you put a voltage meter on that corner. I know that the answer must be zero but why?

I mean in a Y secondary, all current flows back to the center of the wye where all three phases vectorally cancel each other out and therefore will always be zero volts. But how does that work if you go back to a corner point of a delta?

Also, are there applications in your opinion where a delta wye is better than a delta delta and vice versa. i.e. when would you recommend one over the other. We always specify delta wye's.


Thanks,

Mike
 
I mean in a Y secondary, all current flows back to the center of the wye where all three phases vectorally cancel each other out and therefore will always be zero volts.
Click to expand...
The voltage bewteen earth and XO is only zero because you bonded it to earth. If you float XO and read to earth, the voltage will not always be 0. The same with the delta, you are just establishing a reference point to earth. Very little current will be needed to create the reference point. When you read from the grounded phase to earth, you are reading the voltage drop on the bonding system, because the current is very very close to zero, the voltage drop is also zero.
Don
 
mshields said:
If you do that (and it may well be that I have to go back to school here), what do you get when you put a voltage meter on that corner. I know that the answer must be zero but why?

I mean in a Y secondary, all current flows back to the center of the wye where all three phases vectorally cancel each other out and therefore will always be zero volts. But how does that work if you go back to a corner point of a delta?

Also, are there applications in your opinion where a delta wye is better than a delta delta and vice versa. i.e. when would you recommend one over the other. We always specify delta wye's.


Thanks,

Mike
Click to expand...

Mike

You can ground any part of the transformer once. There is nothing sacred about X0 as Don said. If you do a node equation at any terminal of a delta transformer the three currents will also add up to zero.

As I said in an earlier post, if you are serving three phase loads only, and reliability is a significant problem, a delta will be more reliable simply because the first grounded wire won't shut it down, it will take two grounds before an OCPD will trip.
Delta's are quite common in mining, sand and gravel pits, and oil field installations in New Mexico. The oilfield services are usually corner grounded, the mining and sand-gravel operations are often ungrounded and monitored.

Jim T
 
mshields said:
...

I mean in a Y secondary, all current flows back to the center of the wye where all three phases vectorally cancel each other out and therefore will always be zero volts. But how does that work if you go back to a corner point of a delta?

...
Click to expand...
The vectorial relationships do not change when a corner is grounded. The only difference is this grounded system now has a voltage reference on one of the line connections. Though it is not done for fairly obvious reasons, you could ground a "corner" of a wye system and leave the neutral X0 connection float. Again, the vectorial relationships remain the same: the neutral would still be a system neutral, but it would have voltage to earth.
 
2 faults to ground to trip

2 faults to ground to trip

But if you have a single fault to ground without tripping the breaker, doesn't that create an unsafe condition?
 
mshields said:
But if you have a single fault to ground without tripping the breaker, doesn't that create an unsafe condition?
Click to expand...

Well, the second fault is a doozie ! It does raise potential of the other legs when ref to ground and the shock hazard associated with those legs. You have just created a corner-grounded Delta! That is why you need indication, so the problem can be remedied - but the first fault typically does not damage anything - unless some surge devices are popped.
 
The other thing that could be considered if the L-G voltage in the event that there is a fault, 480v vs 277v with a wye.
Then there is the requirement the you must use a 480v rated device instead of a 480/277v device which may be a cost issue.
 
Mike,
But if you have a single fault to ground without tripping the breaker, doesn't that create an unsafe condition?
Click to expand...
Only a little more so that making an intentional grounding connection. The single ground fault is not a solid connection like an intentional grounding connection, so in the event of a second fault on an ungrounded system, it is likely that there will be arcing damage at both faults.
Don
 
jcormack said:
I prefer a zig-zag on a delta secondary to derive a neutral - or just let it float with P-G indication
Click to expand...

We second the vote for the zig zag if you don't want to float.

Consider also that the load on this transformer would be shifted from any delta/wyes that may be connected to the 13.8 - could actually mitigate some harmonics for you on the 13.8
 
You may use 300V rated insulation on a grounded delta-wye 480/277V system.

You MUST use 600V rated insulation on a delta-delta 480V system, since the line to ground voltage may be 480V if (when!) one corner becomes grounded.
 
never never never would I use 300V insulation on a 480/277 system, but then of course, I don't do lights ...

now I'm interested - does the code agree with 300V insulation as you claim?
 
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