What connection (delta wye) would be best

[guilio2010]

I have conflicting opinions on this project. We are looking to need 3 phase 480v service mostly for process heating (225kW). We have other items such as lights, heat tracing, etc...which has me thinking Wye (multiple voltage requirements). Another individual in the this project said they want delta coming in. Would a delta connection be more advantage in this scenario? In researching just to check my thoughts, I can't see delta in this scenario as I would have to have a neutral for lower voltage equipment which is going to give me a high leg I can't see any use for. I get the reliability is higher for delta, but I can't see this being an issue as if for some reason we lost a phase (mostly for process heating), low temp shutdowns in the controller will shut down the system and we would analyze the fault. I'm not fluent in the wye vs delta, but in research, it seems that delta is used more for motor applications also.

Just want to see if I'm missing something from my end and get other opinions.

Thanks,
Guilio

 

[Mbrooke]

Go wye, just don't use the neutral. Delta means more expensive circuit breakers, grounding detectors, a supply VFDs may not like and the fact the POCO may no longer supply delta down the road.

 

[GroovyGuy]

Hi,
I would recommend a delta-wye transformer with a 480/277V 3P4W secondary.
The secondary should be equipped with a neutral-ground resistor continuously rated 2A @ 277V.
Use a Startco type SE-330 relay to monitor the NGR.
Your supply should then be 480V 3P3W. (ie with a NGR, you will not have access to the (277V) line-to-neutral voltages.)
Any heat tracing or lighting circuits should be supplied from step-down / isolation transformers (ie rated 480V-208/120V or 480V-480/277V).
This is standard scheme used in industrial plants.
GG

ps A delta 480V secondary is old school, and is not recommended today.

pps Who is supplying the transformer (ie the Utility of the Owner)? If it is the Utility, you might have issues with them installing a NGR onto their neutral point. If this is the case then you will need to bring the neutral into your incoming MCC. This will be a significantly reduced neutral (ie #8awg), and the MCC does not need to carry the neutral.(ie the MCC should be rated 480V 3P3W.)

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[waross]

Wye! The reliability issue with delta is many generations past is "Best Before" date.
About 100 years ago, most distribution was delta:delta.
Most transformers were three transformer banks.
The insulation in the transformers was prone to failure.
On the loss of a transformer, it could be disconnected and service maintained as an open delta, but at 58% capacity.
Around 50 years ago, give or take a few decades, the trend was to increase system capacity by running a ground/neutral cable and switching to wye:delta connections.
Then followed some years of the problems and issues unique to the four wire wye:delta connection.
There are few advantages left to using a delta service.
Three phase motors use a three wire connection and the motor doesn't know if the supply is delta or wye
Most three phase induction motors in North America are internally connected wye. A few motors are internally connected delta but the system doesn't know and doesn't care.
With single three phase transformers, the ability to remove a failed transformer and run open delta is gone.
The trend today is delta:wye three phase transformers and wye:wye transformer banks of three single phase transformers.
I see a lot of 480/277 Volt heat trace controllers these days.
You can run a lot of your lighting on 480/277 Volts. (Subject to some restrictions.)
For lighting use 480:208/120 Volt delta:wye transformers. That is the most common connection.
The four wire delta connection will give you 480/240 Volts, not much good for lighting.
You would still need 480:240/120 or 480: 208/120 Volt transformers for lighting.
Bottom line, Use wye, either solidly grounded or impedance grounded.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

@Waross: I agree and well said. Even today when the need exists to operate with a grounded phase (continuity of service) you would still use a wye secondary, just grounded through an impedance such as a resistor selected to pass more current then the typical phase to ground capacitance passes. Reason for this is that a totally ungrounded system is susceptible to over voltages from arcing ground faults, but a properly sized impedance prevents that.