Changing Wye to Delta

[Davids1964]

I have a situation were we need an isolated transfomer to inhance sound quality. We need a 200 amp 3 phase isolated trans. We already have 120/208 4 wire wye existing switch. I researched and found that a 120/208 WYE to 120/208 Wye Trans is not good for harmonics. A tranfomer manufacture advised me to drop the neutraul and you have Delta. Is this trure that if I only use the three ungrounded conductors I will have Delta?

 

[jim dungar]

I have a situation were we need an isolated transfomer to inhance sound quality. We need a 200 amp 3 phase isolated trans. We already have 120/208 4 wire wye existing switch. I researched and found that a 120/208 WYE to 120/208 Wye Trans is not good for harmonics. A tranfomer manufacture advised me to drop the neutraul and you have Delta. Is this trure that if I only use the three ungrounded conductors I will have Delta?

Kind of .
You won't have a delta, you will have an ungrounded wye, but the result will be the same - no intentionally grounded conductor.

What kind of installation is this? The NEC all but prohibits an ungrounded installation with an obtainable potential of 120V line to ground.

 

[winnie]

For most internal distribution uses, a delta-wye transformer is used. This means that the primary of the transformer is connected delta and the secondary of the transformer is connected wye.

A wye source can supply a delta load, simply by not connecting the neutral. This doesn't mean that the supply suddenly becomes 'delta'; because it still is a wye connected source, presumably with a grounded neutral. All it means is that the wye source will correctly supply power to a delta load. Consider the analogy of supplying a straight 240V single phase load from a 120/240V center tapped system; you don't need to bring the neutral to the load, but that doesn't change the system to a straight 240V system.

I don't know the details of your application, but it sounds to me as though you are being advised to connect your wye supply to the delta primary of a transformer, and then using that transformer to derive a new wye system. That is a common and generally correct approach.

-Jon

 

[augie47]

250.20(B) may throw a fly in the ointment. As shown below, there is some question as to not connecting to XO (when there is one)in this situation.

250.20 Alternating-Current Systems to Be Grounded.
(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:
(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts

 

[Davids1964]

This is for a theater that has shore power for productions. I have 3 600amp switches. All are 3 phase 4wire. Two of them tie back into a bus duct that originates at the switch gear. The other switch is isolated back to the same switch gear. The switch gear is 208/120 wye. We plan to purchase a mobile isolated 75 Kva transformer and use a cam lock system for temporary show audio power. This is not for permanent connections.

Thank you for your help.

 

[Davids1964]

Let me get this right. I will hook up only the ungrounded conductors from my wye system to the primary side of the isolated transformer. Then use the secondary side of my isolated transofmer to hook up to my load. Do I hook the secodary side ground to building ground or can I use that grounded wire that I'm not using on the primary side? What would or could happen?