supplying building with two different Power Types Wye and Delta

[izak]

got a plant that is currently supplied with a 3 wire 3 phase delta service. 240/480

installing backup generators

contractor wants to install 277/480y generators

aside from ALL KINDS of other obvious potential issues, right now i am concerned about any step-down transformers in the plant.

anything that would be going from 240/480 delta to 120/208 3 phase OR 240/480 delta to 120/240 single phase

what is going to happen if we feed these transformers that are on a Delta service with a Wye service during emergency backup conditions?

 

[augie47]

May be showing my dumbness here, but how do you get 240/480 from a 3 wire delta service ? Seems it would be one or the other (240 or 480).

If its an ungrounded delta and you have inadvertent grounds, supplying it with a 480/277 should not be a problem.
If there is an inadvertent ground you are going to open a OCP device when the generator supplies the building.

 

[Jraef]

May be showing my dumbness here, but how do you get 240/480 from a 3 wire delta service ? Seems it would be one or the other (240 or 480).

If its an ungrounded delta and you have inadvertent grounds, supplying it with a 480/277 should not be a problem.
If there is an inadvertent ground you are going to open a OCP device when the generator supplies the building.

I agree, a Delta servie is either 240 or 480. There is such a thing as a 240/120V 3 phase 4 wire Delta service, but that's a special case where the 120V is single phase. Never heard of that for a 480V system. They do that for motors so they can be configured to work on either voltage, but the distribution would not be set up that way, it's going to be one voltage or the other.

I also agree that the Wye generator power fed to a plant that is set up for delta is not a problem. There would potentially be a problem going the other way, i.e. a 4 wire plant system fed by a 3 wire delta generator.

 

[George Stolz]

If there are no line-neutral loads on the system (which I don't think there could be :? ) then I don't see a problem, but I wouldn't swear to it.

 

[broadgage]

A delta system could supply both 240 and 480 volts, but would need 4 wires and not three.

It would be exactly the same in principle as a 120/240 volt 4 wire delta service, at twice the voltage.

Three windings, each 480 volt, connected in delta, with a grounded center tap on one winding. I have not heard of this being done, but it could be.

 

[buddhakii]

I'm actually working at an old mine water treatment plant that has 480/240 volt center tapped delta. First one I've ever came across. The high leg is about 416 volts. There is a back up generator that is wired 480y. Just make sure if you are supplying the transfer switch with a neutral that you switch the neutral. You want your generator to be a seperately derived system. Other than that I don't see any issues.

 

[powerqualityworld]

A delta system could supply both 240 and 480 volts, but would need 4 wires and not three.

It would be exactly the same in principle as a 120/240 volt 4 wire delta service, at twice the voltage.

Three windings, each 480 volt, connected in delta, with a grounded center tap on one winding. I have not heard of this being done, but it could be.

WYE - delta (center-tapped grounded) is a very common configuration used by our utility. However, this connection is known to have voltage problems during single-phasing.

As for izak, there are no expected problems as long as you maintain the same voltage supply and phase rotation during emergency transfer. Practically, the loads will experience the same input, it's just that the source were changed.

If possible, kindly give feedback if any unforeseen event happens...

 

[izak]

ok. if this system actually HAD the 4th wire connected, (neutral, grounded conductor etc...) it would provide a 240/480v delta system with approx 420v to 480v high leg.

I have access to one of these systems that is installed with 4 wires, and the "neutral" is there only to provide protection for line to case faults.

if you take readings from all lines to case, you will get two at 240 volts, and one somewhere around 416 volts on the high leg.

the other system was installed without the "neutral" which I understand --EVERYONE--, is NOT needed for 'neutral' loads on a 240/480 system, but it IS NEEDED for line to case overcurrent protection.
otherwise, as one post clarified, there could be line to case faults that have existed for YEARS that will not make themselves evident until connection is made to the "neutral" on the pole.

all equipment grounds that are installed on this 3 wire system tie back to the ground buss in a motor control center, which in turn, is connected to a Chemical EARTH Electrode.

That will do NOTHING to clear a fault, it will only make all metal parts on premesis that are bonded together 'LIVE' in respect to the system CENTER or CORNER connected grounded conductor (neutral)


MY question is:
Is a Transformer that is wound Delta-Wye going to operate PROPERLY and efficently when connected to a WYE system part time.

My thinking is that if you want a transformer to provide 120/208WYE from a 240/480v DELTA feeder, then you install a DELTA-WYE transformer.

if you want a transformer to provide 120/208WYE from a 277/480WYE feeder, then you install a WYE-WYE transformer.

I need advice coming strictly from Transformer Theory and design.

my accuracy on correctly identifying the system and how it is connected is not the question

will the step down transformers work properly when fed with WYE or DELTA all other aspects being the same or not considered.

 

[GearMan]

Not getting to detailed regarding all the other potential issues, it would be very rare to have a specially wound Y x D dry type xfmr in your plant. Current ANSI standards are D x Y. Assuming all the existing xfmrs in your plant are wound with delta primary, no Neu should be brought to the primary and only the case should be grounded. The xfmr doesn't care if the system is Y or D. 480v on the primary you will have an induced 208y/120v secondary. Same thought goes for back feeding a D x Y xfmr, un-bond XO from the frame,feed it 3-wire and bingo...you tricked the xfmr in to thinking it's a delta primary. Yes, UL listed application.

 

[George Stolz]

Izak, I think you've got it under control. As long as your breakers are rated correctly (which they should be) then your 3 phase loads shouldn't tell the difference.

 

[jim dungar]

Winding configuration of the transformer primary is not dependent on the wiring of the source system. By far and away, the most common source voltage in US industrial facilities is 480Y/277, likewise the most common step down transformer is a 480V delta-208Y/120wye. The choice to make a transformer primary delta, wye, or even a T is usually made based on issues involving coil voltages and currents versus size and material costs (the same is usually true of motors as well).

 

[Hv&Lv]

If you are going to supply a service with a Y generator, make sure the existing 480 delta isn't corner grounded. Many engineers will allow a 480 volt delta to be center tapped at 240 which makes a lot of lineman happy. Tying a hot phase to ground and throwing in a fuse doesn't "feel" right to most lineman.