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.
 
don_resqcapt19 said:
Have you thought about a closed transition transfer switch?
Click to expand...
No, the ATS' are set up for open transition.
Utility power as normal, any loss or interruption of power sends a start signal to gens, once gens reach voltage and frequency, the ATS transfers to emergency power.
 
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.
 
xtronik said:
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.
Click to expand...
So do you want that VFD to be able to ride through a short time power loss? The motor it drives is going down in such power loss as is now, even if the controller stays powered. I'm assuming this is for a combustion air blower on the boiler and if it is down the burner is going down with it which leaves having the rest of controls on a UPS a little pointless unless there is something there that they do not want to lose power for some reason even though boiler would be down anyway.
 
IMHO trying to size a UPS to supply AC power to a large VFD is going to be a nightmare. You would need a UPS that can supply a large rectifier load, and the UPS output would essentially be a very large capacity inverter with impressive output filtering, used for a very short period of time to provide ridethrough during these transitions.

As calculated in the OP, the total energy storage required is comparatively tiny, because of the short ride through duration. Where you will get hit is in the power handing capacity. You need less than a kWh of energy storage, but several hundred kW of power handling capacity. I don't think you will find this combination in a battery.

I'd suggest looking at a supplier of DC UPS systems for VFDs, rather than using a conventional AC UPS. For example:

Bonitron | Solutions for AC Drives - Home

Bonitron specializes in cutting-edge industrial electronics and VFD (Variable Frequency Drive) solutions designed to optimize system performance and improve energy efficiency. Our products, including overvoltage solutions, braking systems, and power quality modules, are trusted across industries...
bonitron.com bonitron.com

-Jonathan
 
You must log in or register to reply here.
Top