UPS kVA/kW Rating

[tomad]

Some UPS manufacturers label UPS with both apparent power (kVA) and active power (kW).
Ex. 225 kVA, power factor 0.8. (182.4 kW)
Is this indicated power factor minimal or maximal ?
Can this UPS be loaded up at 225 kW ?

 

[rbulsara]

Both are maximum limits. Neither rated kW nor rated kVA should be exceeded.

At rated pf (.8) both limits are reached simultaneously. For a pf better than rated, kW limit is reached first. For a pf poorer than rated, kVA limit is reached first.

 

[rbulsara]

Real ratings are kW and kVA. The "rated" p.f. is simply defined by its relationship. p.f.=kW/kVA. Either values can be different than rated but kW and kVA should not be exceeded.

The pf can be anything as defined by kW/kVA as long as they both are within their ratings.

 

[sibeen]

You need to be very careful with the kVA rating as for many UPSs the specified power factor of the machine is not symetrical. They can provide an inductive power factor, but if you tried to have the UPS provide a load with a leading power factor of 0.8 you will be overloading the UPS.

This is because some manufactures used to derate the output transformer and then add a bulk capacitor on the output to act as kVAR compensation for the expected lagging load.

For example, a 100 kVA UPS may only have an output transformer rated at 85 kVA.

If the load instead of being lagging was then moved to leading (capacitive) the inverter of the UPS would need to supply the full output current, which it was never designed to do, plus the current required by the bulk compensating capacitor on its output.

Quite a few data centres ran into trouble with their UPS systems when computer power supplies improved and the power factor moved close to unity and sometimes to a leading power factor.

 

[tomad]

In this case there is no transformer as integral part of the UPS, since both input and output voltage is 480V.
My understanding is that the UPS is sized to supply the rated output current, which is based on the kVA rating, thus everything (wires, connections, etc) is sized to be able to carry that current. If the (voltage)regulation capacity of the UPS gets worst with a power factor approaching 1 (ohmic load), the same phenomenon should be observed at lower loads (for instance at loads under 180 kW in our case, providing the corresponding power factor is 1, which is not the case.
So if we say the "kW rating is exceeded" what exactly are the effected on the UPS components ?

 

[rbulsara]

Kw=Heat

UPS components, fans, heat sinks and such are only rated for cetain kW output. Exceeding that will overheat the unit.

Battery backup time also will get adversely afftected. The reduction in batery time may not be linear. So at some point, the battery time takes a nose dive.

 

[tomad]

I agree that if some components are sized for a certain current, which through I^2R create heat, and if the respective conductor(component) is not sized properly, it leads to overheating it and further to overheating the whole unit.
But in this case all elements are sized to accomodate the current corresponding to the 225 kVA (i.e. 270A, which is the same for any power factor, including 1), so I think nothing should be overheated (with all its consequences: battery time reduction, etc).
The unit in question is MGE and when contacting their technical info address (which by the way is the same as for APC) I have been told that on some units any reference to a power factor has been deleted, so the only limitation is the ouput current, irrespective of the power factor.

 

[ScottyUK]

The kVA rating applies to the AC section of the UPS. The battery, charger / rectifier, DC link etc don't know about the power factor because PF is meaningless in a DC circuit. It is wholly possible to have a UPS where the kW is limited by the DC section and the kVA is limited by the AC section. It would be possible to size the DC section to support the maximum capability of the AC section at unity PF but this would lead to over-sizing of the DC section under almost all real-world load conditions.

Almost all UPS spec sheets have some degree of liberty with the truth. Read carefully and question anything which is not explicitly stated. Writing spec sheets which are not blatantly wrong but skirt around the edges of what is fact has become an artform to some UPS vendors. I'm not having a go at MGE particularly, just a general observation.


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If we learn from our mistakes I'm getting a great education!

 

[rbulsara]

tomad:

Read my first post again and what Scotty said. DC component are rated for kW only.

At unity power factor maximum KVA you can push is equal to its kW rating. Think why did the mfr not rate the unit as 225kW/225kVA? Because it cannot sustain more than 180kW.

In fact, if the applicaiton is for computer loads, for all practical purposes you should only consider kW rating of an UPS. KVA rating is of no use, unless you have primarily inductiove loads.

 

[tomad]

rbulsara,

When you say "it cannot sustain more than 180 kVA" what do you mean by it ? Will it overload any component and as a consequence will it shut down ?
I do not see any reason why the unit shall not be able to carry a load up to its rated value ( 225 kVA).
Some generating units have a minimum power factor listed, but never a maximum power factor.
In case of gen-sets there is a limitation of the active power that can be supplied by the generator (kW rating), based on the maximum output power of the driving engine (kW or HP)at that speed, which is not the case for UPS systems.

 

[davidbeach]

The DC link is good for only 182.4kW as you stated in your original post. That's it. By 183kW or so the controls will be limiting power to avoid overloading the electronics. If it has a 182.4kW rating there is now way of expecting that you can get 225kW out of it. Plain and simple.

 

[rbulsara]

tomad:

I have nothing more to add. You are free to do what you see fit.

Good Luck.

 

[sibeen]

Tomad, when the UPS is designed the DC section of the UPS (input transformer/chokes, rectifier, boost converter etc) are only designed to handle the rated kW of the unit.

The DC current into the inverter is only to provide the kW load on the inverter. Any kVAR current that is in the output load is a current that is not provided by this DC link. In a standard PWM inverter, any KVAR currents will take a different path through the inverter than an ‘real’ current. The output switches, normally IGBT, have an anti-parallel diode across the IGBT and it is through this device that any kVAR current will flow and not through the IGBT itself. The kVAR load does not interact with the DC section of the UPS as this current is just flowing between the inverter output and the laod and back again.

You mentioned APC in one of your posts. You need to be careful here as APC uses a UPS topology called ‘Delta Conversion’, which they bought from a company called Silcon about 10 years ago. This UPS design is not strictly a double conversion on-line UPS as most of the industry understands it (let the argument begin). This topology does have the advantage that the output kVA + kW ratings are the same.

 

[ScottyUK]

In case of gen-sets there is a limitation of the active power that can be supplied by the generator (kW rating), based on the maximum output power of the driving engine (kW or HP)at that speed, which is not the case for UPS systems.

Tomad,

That absolutely is the case: substitute the AC output stage of the UPS in lieu of the generator and the rectifier / charger / battery in lieu of the engine. You can not get more kVA out of the UPS than the output stage can handle, and you can not get more kW out than the DC section can handle.

You mis-quoted rbulsara in your previous post: go back and read what he said and others have said. Your opinion is yours to hold as you will, but the facts have been put in front of you. Ignore them if you wish: that's your choice.


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If we learn from our mistakes I'm getting a great education!