Sizing UPS for Boiler 480-volt / 198.3-amps

X

xtronik

Member
Location
Hanover, PA
Occupation
Electrician, Project Manager
Hello,

I'm sizing a UPS to maintain power during power blips from monthly generator load tests.
The power blip shuts down the Boiler which requires manual intervention to bring it back online.
Boiler is fed by a VFD (480v / 3-phase) that's fed from 400-amp breaker.
The Boiler spec states minimum circuit ampacity is 198.3a.
Using 1.2 - 1.5 as the multiplier for surges: 480 x 198.3 x 1.73 = 164,668kVA (x 1.2 = 197,602kva) (x 1.5 = 247,002kva)
Using the feeder breaker size: 480 x 400 x 1.73 = 332,160kva (x 1.2 = 398,592kva) (x 1.5 = 498,240kva)
The UPS will only need to carry load for 1-2 seconds for the ATS to switch to generator power, with 10-seconds mandatory, a 15-second target would be a good buffer if possible. I'm not sure of the minimum length of battery backup time a UPS can be set up for, except for watt hours and amp hours.
To get 15-seconds of WH (with the highest kVA), I did 1/60 x 1/4 x 498,240 = 2,076wh
To get 15-seconds of AH, I did 1/60 x 1/4 x 198.3 = 0.83ah

Any input on correct calculation?

Where in the code would be a good source for this?

Thanks
 
xtronik said:
Boiler is fed by a VFD (480v / 3-phase) that's fed from 400-amp breaker.
The Boiler spec states minimum circuit ampacity is 198.3a.
Using 1.2 - 1.5 as the multiplier for surges: 480 x 198.3 x 1.73 = 164,668kVA (x 1.2 = 197,602kva) (x 1.5 = 247,002kva)
Using the feeder breaker size: 480 x 400 x 1.73 = 332,160kva (x 1.2 = 398,592kva) (x 1.5 = 498,240kva)
The UPS will only need to carry load for 1-2 seconds for the ATS to switch to generator power, with 10-seconds mandatory, a 15-second target would be a good buffer if possible. I'm not sure of the minimum length of battery backup time a UPS can be set up for, except for watt hours and amp hours.
To get 15-seconds of WH (with the highest kVA), I did 1/60 x 1/4 x 498,240 = 2,076wh
Click to expand...
Just commenting on your calculation, presuming your UPS is going to be somewhat custom and its only going to run for seconds, and it wont be cold started, I'd wager it could be sized to the nameplate:
Rounding to 164.9 kVA * 1/60 * 1/4 = 688wh
 
retirede said:
I would get with the manufacturer to see if there’s a way to power just the controls with a UPS. If it can be done, it will be a far better solution than powering the entire boiler.
Click to expand...
Part of the issue with this would be the motor losing speed when the 480V disappears, the VFD would need to be able to 'catch a spinning load' and return it to the proper speed.
 
jim dungar said:
Part of the issue with this would be the motor losing speed when the 480V disappears, the VFD would need to be able to 'catch a spinning load' and return it to the proper speed.
Click to expand...
Along with possibly other gotchas we don’t know about. The designers of the system would definitely have to be involved in ant such solution.
 
retirede said:
I would get with the manufacturer to see if there’s a way to power just the controls with a UPS. If it can be done, it will be a far better solution than powering the entire boiler.
Click to expand...
The 120v control panel does have a UPS within it and does not lose power during the power blip. It displays a fault condition which I believe is an overcurrent fault from the blower fan.
 
jim dungar said:
Part of the issue with this would be the motor losing speed when the 480V disappears, the VFD would need to be able to 'catch a spinning load' and return it to the proper speed.
Click to expand...
Yes, that is what we believe is happening with the blower fan.
I'm interested in knowing if the VFD can be programmed to correct this, we have not looked into that yet.
 
tortuga said:
Just commenting on your calculation, presuming your UPS is going to be somewhat custom and its only going to run for seconds, and it wont be cold started, I'd wager it could be sized to the nameplate:
Rounding to 164.9 kVA * 1/60 * 1/4 = 688wh
Click to expand...
My only concern is if there are any initial surges after restarting.
 
xtronik said:
Boiler is fed by a VFD (480v / 3-phase) that's fed from 400-amp breaker.
Click to expand...
Please clarify what this means.

A VFD should only be driving a motor or in some cases multiple motors that operate together as in they accelerate/decelerate simultaneously.

I can see a VFD being the controller for a motor(s) that are part of the boiler but there likely is other loads that are part of the boiler that are not controlled by that same drive.

It could be possible for the entire machine to be powered through an inverter - if you have that it should be one the simpler ways to utilize a UPS concept as all you would need to do, from the basics perspective, is have another DC source with necessary capacity in parallel with the DC bus of the inverter. That concept can be applied if you do have a VFD that you want to be able to ride through a normal power interruption as well.
 
kwired said:
Please clarify what this means.

A VFD should only be driving a motor or in some cases multiple motors that operate together as in they accelerate/decelerate simultaneously.

I can see a VFD being the controller for a motor(s) that are part of the boiler but there likely is other loads that are part of the boiler that are not controlled by that same drive.

It could be possible for the entire machine to be powered through an inverter - if you have that it should be one the simpler ways to utilize a UPS concept as all you would need to do, from the basics perspective, is have another DC source with necessary capacity in parallel with the DC bus of the inverter. That concept can be applied if you do have a VFD that you want to be able to ride through a normal power interruption as well.
Click to expand...
From the schematics, the 400a breaker feed lands on the line side terminals of the VFD, within the VFD the 480-v branches off via xfmr down to 120-v which is piped over to feed the Control Box/PLC which has an internal UPS.
So from my understanding, the VFD is only controlling the motor, but the 480-v power feeding the VFD also powers the controls box.
 
Most decent VFDs on the market now have a “flying restart” capability in programming. It’s for exactly this situation. During the transition, the spinning motor has residual magnetism that makes it regenerate for a moment, which means when the VFD re-initializes and energizes into it, you have two “power sources” out of synch and a massive spike in current. The flying restart has the drive look for the back-emf on the motor, track its frequency and synch up its output frequency to it.

Back in the days before flying restart existed, the other alternative was to just delay the restart of the VFD for long enough to allow the residual magnetism to decay. The problem with that is that some motors retain it longer than others, getting worse as motor size increases. Some even have a small amount that is relatively permanent, depending on the type of steel used in the core. That was why they came up with flying restart back in the 80s.
 
As I understand this, the problem is really that the controls may not like the power blip and the blower motor turns drive doesn't restart when power is lost/applied (not surprising). Any heating elements won't care at all.
Do the controls have sufficient UPS to ride through the interruption? If not, that's the first item.
Does the blower VFD (and any others) have flying restart and is that configured? (#2)
Are there any other components that are affected? Those might need to be on battery/UPS also. (#3)

The simplest way of dealing with this might just be to manually restart the boiler when those monthly tests happen.
 
You must log in or register to reply here.
Top