Large Motor Loads

Status
Not open for further replies.
D

Dennis Miller

Member
We are planning to install a new service to supply what will be a large motor load. Would it be better to have a Delta secondary on the 480 volt/4000 amp service that is supplying a load of 90% motors? The transformer being suggested is a 13200/480 Delta with y secondary. We have no 277 volt loads.

Thanks'
Dennis
 
Dennis Miller said:
We are planning to install a new service to supply what will be a large motor load. Would it be better to have a Delta secondary on the 480 volt/4000 amp service that is supplying a load of 90% motors? The transformer being suggested is a 13200/480 Delta with y secondary. We have no 277 volt loads.

Thanks'
Dennis
Click to expand...

Is this one motor or multiple motors?

Thats a lot of juice.
 
If it is a delta it would have to be an ungrounded or a corner grounded system, both of which have some disadvantages when compared to a grounded wye system. Many times this size of system would be a wye with high impedance grounding.
Don
 
A 480Y/277 transformer is more common therefore from a potential emergency standpoint it will be easier to replace.
 
don_resqcapt19 said:
If it is a delta it would have to be an ungrounded or a corner grounded system, both of which have some disadvantages when compared to a grounded wye system. Don
Click to expand...

Don - what do you consider the disadvantages of delta versus wye?

I mostly see delta in factories where they do not want a single phase to ground fault to trip everything and wye where they don't know any better or don't care.
 
Bob,
The biggest one is the potential for serious over voltages on arcing ground faults under some conditions. In the event of two ground faults, there is often serious damage at both fault locations. Also many electricians have not worked on ungrounded systems and do not understand them.
I mostly see delta in factories where they do not want a single phase to ground fault to trip everything
Click to expand...
An impedance grounded system will also do that and at the same time limet the fault current to a point were there is little or no damage. The impedance on a high impedance grounded system is selected to limit the ground fault current to about 5 amps.
Don
 
don_resqcapt19 said:
Bob,
The biggest one is the potential for serious over voltages on arcing ground faults under some conditions. In the event of two ground faults, there is often serious damage at both fault locations. Also many electricians have not worked on ungrounded systems and do not understand them.

An impedance grounded system will also do that and at the same time limet the fault current to a point were there is little or no damage. The impedance on a high impedance grounded system is selected to limit the ground fault current to about 5 amps.
Don
Click to expand...
Don, does the impedance grounded system allow the system to maintain operation with one fault?
 
I pick grounded wye...

I pick grounded wye...

I grew up in the paper industry with multiple 1500 & 2500 kVA substations with 480V secondaries and they were all ungrounded delta. There was no need for 277 Volts. And in my 15 years at that plant site, there were no apparent instability problems on the secondaries of the transformers.

But logic tells me today that it's an inherently more stable and safer system if the secondary is a grounded wye, if the transformer has that ground reference.
There's no need to bring the neutral conductor past the switchgear, so the wiring to the 480V motors is the same as if it were a delta system.
And the conduits/grounds are referenced back to the transformer, so faults are provided with a positive path back to the transformer.. unlike the ungrounded delta.

Fault current will be high given the 2400 HP of motor loads. I think this installation needs a minimum 3000/3500 kVA transformer, and perhaps higher depending on the motor starting means. That translates to 85 kA rated equipment.

IMO, high-resistance-grounded systems are reserved for those installations where power continuity is critical, i.e. data centers, hospitals, etc. They require a significant detection system, as well as personnel experienced in troubleshooting that type of system.

I think I'd stay with the conventional grounded wye system, from a stability and safety standpoint - unless someone gave me a delta secondary transformer.

JM
 
Last edited:
wye/delta

wye/delta

To me it's more a case of how a fault will effect operations. Refering to what Bob said, if the goal is to not have a shut-down in the event of a fault, a delta system with good ground detection might be preferred. Like JM , my background is paper industry with 480v ungrounded systems. Other than perosnnel training, I never experienced a problem and avoided a lot of shut-down situations, but for some reason I do consider grounded system safer.
 
Status
Not open for further replies.
Top