Electrical Engineering - need help with UPS sizing

Author

need help with UPS sizing

essenz

2007-09-20, 3:25 am

I am purchasing a Liebert Series 300 UPS. Input 480V, Output 208V. And
I believe the output is 208Y, so three hot 120V legs, 208 phase-phase.

Two questions.

First, from a sizing standpoint, my data is all 120V single phase. I
dont really know how that correlates to the input rating of the unit
which is done, 30KVA, 65KVA, etc.,.

What throughs me off is if that is per phase, or across all three
phase of the 480 input, etc.,.

To put things in perspective with a practical example...

I have a total of 500 amps of 120V equipment. So with phase balancing,
figure 165 amps on phase A, 165 amps on phase B, and 165 amps on phase
C of my UPS's 208Y output.

To support that, what size KVA UPS do I need? Using my ubdoubtly
incorrect knowlegde, I was figuring something around 60KVA (500amps x
120V / 1000).

The second question, when buying a UPS, everyone says I should get one
where my real usage is close to the full capacity of the UPS. If I
were to get a 60KVA UPS and only have 10KVA of load, I've been told it
will waste alot of power. I sthat true? If so, how do you get around
the inevitable situation where you will grow into the unit, and have
to start small? Or do you simply have to deal with inefficiencies at
low usage?

Thanks,
John

Charles Perry

2007-09-20, 1:25 pm

"essenz" <john@essenz.com> wrote in message
news:1190270605.783156.44700@w3g2000hsg.googlegroups.com...
>I am purchasing a Liebert Series 300 UPS. Input 480V, Output 208V. And
> I believe the output is 208Y, so three hot 120V legs, 208 phase-phase.
>
> Two questions.
>
> First, from a sizing standpoint, my data is all 120V single phase. I
> dont really know how that correlates to the input rating of the unit
> which is done, 30KVA, 65KVA, etc.,.
>
> What throughs me off is if that is per phase, or across all three
> phase of the 480 input, etc.,.
>
> To put things in perspective with a practical example...
>
> I have a total of 500 amps of 120V equipment. So with phase balancing,
> figure 165 amps on phase A, 165 amps on phase B, and 165 amps on phase
> C of my UPS's 208Y output.
>
> To support that, what size KVA UPS do I need? Using my ubdoubtly
> incorrect knowlegde, I was figuring something around 60KVA (500amps x
> 120V / 1000).

You have calculated correctly. However, do not assume you will be able to
balance the load that well.
>
>
> The second question, when buying a UPS, everyone says I should get one
> where my real usage is close to the full capacity of the UPS. If I
> were to get a 60KVA UPS and only have 10KVA of load, I've been told it
> will waste alot of power. I sthat true? If so, how do you get around
> the inevitable situation where you will grow into the unit, and have
> to start small? Or do you simply have to deal with inefficiencies at
> low usage?

Most large UPS are rated for a VA at a power factor. Usually it is
something like 60kVA at .8pf. This means the UPS can ONLY supply 48kW of
load. Even if you are at unity power factor, you will only get the kW that
is rated for. This often confuses people. Next, UPS don't really like to
run at their max rating. They often are more sensitive at full load and may
drop the protected load as a result of minor power disturbances. We have
tested many UPS from a few hundred VA to 150kVA and this seems to apply
across the board.

Underloading the UPS does make the efficiency go down. It is due to the
fact that for the most part, the UPS consumes its share of power regardless
of the loading. This can be quite large, 1kW or more in some cases. So you
are correct, the lower your load as a percentage of the UPS rating, the
lower the efficiency. However, the drop in efficiency is a small price to
pay compared to overloading the UPS and having it not work at all. Shoot
for 75-80% loading. Keep in mind the fact that a 60kVA UPS with NOT support
60kW even at unity power factor.
>
> Thanks,
> John
>

Charles Perry P.E.

essenz

2007-09-20, 1:25 pm

On Sep 20, 2:02 pm, "Charles Perry" <pipesandtoba...@hotmail.com>
wrote:
> "essenz" <j...@essenz.com> wrote in message
>
> news:1190270605.783156.44700@w3g2000hsg.googlegroups.com...
>
>
>
>
>
>
>
>
>
>
> You have calculated correctly. However, do not assume you will be able to
> balance the load that well.
>
>
>
>
> Most large UPS are rated for a VA at a power factor. Usually it is
> something like 60kVA at .8pf. This means the UPS can ONLY supply 48kW of
> load. Even if you are at unity power factor, you will only get the kW that
> is rated for. This often confuses people. Next, UPS don't really like to
> run at their max rating. They often are more sensitive at full load and may
> drop the protected load as a result of minor power disturbances. We have
> tested many UPS from a few hundred VA to 150kVA and this seems to apply
> across the board.
>
> Underloading the UPS does make the efficiency go down. It is due to the
> fact that for the most part, the UPS consumes its share of power regardless
> of the loading. This can be quite large, 1kW or more in some cases. So you
> are correct, the lower your load as a percentage of the UPS rating, the
> lower the efficiency. However, the drop in efficiency is a small price to
> pay compared to overloading the UPS and having it not work at all. Shoot
> for 75-80% loading. Keep in mind the fact that a 60kVA UPS with NOT support
> 60kW even at unity power factor.
>
>
>
>
> Charles Perry P.E.

Thanks. If I run at only 25% load, the efficiency will drop down, but
how significant do you think it will be from a price standpoint? I
dont mind spending an extra 5% for power for a UPS with 25% load
(which will eventually grow to 75%). You said it could cost me up to
1KW of wasted power, is that all together?

1KW of extra power being consumed for me to run at 25% load or
similiar doesnt sound like alot of money. 1KW 24x7 for the month is
only a $100.

The vendors I am talking too make it sound like it will cost a fortune
to run at such a low load, say 10KW on a 65KVA UPS. But I really want
to buy big and grow into it. Besides, the larger units are priced much
better. For 10% increase in cost I can get a UPS twice the size.

-John

Palindrome

2007-09-20, 5:25 pm

essenz wrote:
> On Sep 20, 2:02 pm, "Charles Perry" <pipesandtoba...@hotmail.com>
> wrote:
>
>
> Thanks. If I run at only 25% load, the efficiency will drop down, but
> how significant do you think it will be from a price standpoint? I
> dont mind spending an extra 5% for power for a UPS with 25% load
> (which will eventually grow to 75%). You said it could cost me up to
> 1KW of wasted power, is that all together?
>
> 1KW of extra power being consumed for me to run at 25% load or
> similiar doesnt sound like alot of money. 1KW 24x7 for the month is
> only a $100.
>
> The vendors I am talking too make it sound like it will cost a fortune
> to run at such a low load, say 10KW on a 65KVA UPS. But I really want
> to buy big and grow into it. Besides, the larger units are priced much
> better. For 10% increase in cost I can get a UPS twice the size.
>
You need to get detailed specifications, including load graphs, of
models that you are interested in.

Some large units actually use parallel inverter modules that only start
up when the load demands it - thus giving relatively high efficiency
over a wide range of loads. With some, you can simply buy and add
additional modules later, if your load increases.

Some actually do deliver the rated kVA as kW at unity pf, eg:
http://www.apcpower.co.uk/silcon.html

Some have far, far better battery management and can work well with
mixtures of battery types and ages.

etc. etc.

I'm not familiar with 120v units, so can't say what is available for
you. Most of the sites that I am familiar with are concerned about
efficiency but more because it gets dumped as heat for the air con to
remove (tricky during a prolonged power fail) than the effect on the
electric bill. But their main concerns are reliability, redundancy and
maintainability..

... Things may change as "Green" is rapidly becoming the new must have..

--
Sue

essenz

2007-09-20, 5:25 pm

On Sep 20, 2:40 pm, Palindrome <m...@privacy.net> wrote:
> essenz wrote:
>
>
>
>
>
>
>
>
>
> You need to get detailed specifications, including load graphs, of
> models that you are interested in.
>
> Some large units actually use parallel inverter modules that only start
> up when the load demands it - thus giving relatively high efficiency
> over a wide range of loads. With some, you can simply buy and add
> additional modules later, if your load increases.
>
> Some actually do deliver the rated kVA as kW at unity pf, eg:http://www.apcpower.co.uk/silcon.html
>
> Some have far, far better battery management and can work well with
> mixtures of battery types and ages.
>
> etc. etc.
>
> I'm not familiar with 120v units, so can't say what is available for
> you. Most of the sites that I am familiar with are concerned about
> efficiency but more because it gets dumped as heat for the air con to
> remove (tricky during a prolonged power fail) than the effect on the
> electric bill. But their main concerns are reliability, redundancy and
> maintainability..
>
> .. Things may change as "Green" is rapidly becoming the new must have..
>
> --
> Sue

I am looking at a Liebert Series 300. Searched the website for docs/
specs, but could find anything for power efficiency.

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