UPS & Mechanical on common transformer

[m sleem]

I am in a large project where one of the transformers feeds an 400 kva UPS in addition to a 450 kva mechanical load, I am thinking to split the common transformer into two transformers one for UPS & one for mech because of:

- Avoiding the high voltage surge imposed by mech which would affect the electronic devices of the UPS
- Avoiding the harmonic distortion imposed by UPS which would impact the motor operation

Am I right in splitting the transformer, or just SPD + harmonic filter on the main panel would be sufficient?

 

[m sleem]

Any feedback

 

[zbang]

I wouldn't bother-
A UPS is supposed to take dirty power and make clean power, if it can't handle some noise or surges on the input, well, it isn't much of a UPS IMHO.
Modern UPSs shouldn't generate much in the way of line-side harmonics (ask the mfg about this).
Even using two transformers, the surges/sags created on the secondary of one will work their way to the other (some of the spikes or noise may not).
The mech eqiupment may very well have VFDs, which could generate their own line-side noise/harmonics.

 

[jim dungar]

I have never considered either of your two concerns.
In large industrial plants harmonics from power electronics, like drives, regularly share transformers with motor loads.

I would look at using two transformers from a maintenance perspective. How often will you want your UPS to be down when the mechanical are being serviced.

 

[synchro]

Would the loads on the UPS have a need for a dedicated isolation transformer on your system when the UPS is in bypass mode and not "cleaning up" the delivered power? Bypass would likely be necessary during maintenance of the UPS. If such isolation is desired in bypass mode then that could be another reason for having two transformers, in addition to allowing maintenance of the mechanical side without shutting off power to the UPS as Jim has mentioned.

 

[m sleem]

I have never considered either of your two concerns.
In large industrial plants harmonics from power electronics, like drives, regularly share transformers with motor loads.

I would look at using two transformers from a maintenance perspective. How often will you want your UPS to be down when the mechanical are being serviced.

I think it's not an issue if we have drawable breakers, however all mech serving the emergency would be on standby power.

 

[m sleem]

Would the loads on the UPS have a need for a dedicated isolation transformer on your system when the UPS is in bypass mode and not "cleaning up" the delivered power? Bypass would likely be necessary during maintenance of the UPS. If such isolation is desired in bypass mode then that could be another reason for having two transformers, in addition to allowing maintenance of the mechanical side without shutting off power to the UPS as Jim has mentioned.

But normal MV/LV transformer is not an isolation transformer,

Also, who decides the presence of isolation transformer is mandatory?

 

[jim dungar]

I think it's not an issue if we have drawable breakers, however all mech serving the emergency would be on standby power.

Drawout breakers minimize the length of outages but they do not prevent them entirely. How will you service the switchgear and breaker compartments when they are still energized?