Generator Sizing for total 15kVA

[Anonymous user]

Hello to everyone, nice to meet you.

I would like your opinion to the following matter.

I Already own 2APC Smart UPS. The first one is the SRT5KXLI (Single Phase) and the second one is a SRT10KRMXLI (Three Phase). In our business we are running a small Data Centre and I would like to match the above products with a Stand-By Diesel Generator.

More specifically, the one that I have in mind is a Caterpillar 22KVA DE22.

The UPSs are running now at about the 45-60% of the load (each) and I also have a small Air Condition 18kBTU in the server room.

What do you think for the above mention Generator, is it going to work seamlessly with the configuration (the 2 UPSs & AC)?

Thank you very much

[Brad_C]

Honestly, backup power design and generator specification is a bit more complex than "Do you think this one will work?".

Both your UPS are power factor corrected, so no major issue there, and the 10KVa should balance across all three phases. The only real unknown is your AC. Singe or 3 phase supply? Any power factor correction? What is the start current on the compressor? Is it in inverter drive?

How do you intend to balance the load across the phases? Do you have a plan in place for maintaining the generator and sufficient constant load to prevent wet-stacking? How does the generator respond to a significant unbalanced load? Do you plan to run your lighting also (makes it awkward to see what you are doing when your battery emergency lights go flat and you have no lights).

Lots of variables to start with.

[Anonymous user]

Hello and thank you for your reply.

The AC has a three phase Power Supply with an Inverter Module, its a Carrier, but I don't know the Starting current at the moment.

As far as the lighting, yes, our back up lighting will be connected directly to the one phase of the Generator (later on).

The Generator will be activated for a few hours per month for maintaining purposes. In case that our loads (at the time speaking) are not sufficient and lets say they are 35-40% of the capability of the generator, do you have in mind an example of a dummy load we can connect to the Generator (for example some more 3-Phase AC or extra lighting) in order to prevent the problems you mentioned?

[Brad_C]

G'day George,

My experience with generator load banks is simply resistive heaters (some of them have been pretty huge). What you need to do is talk to the generator manufacturer to get recommendations on minimum load levels, minimum run times and maintenance recommendations.

With the AC being a 3 phase inverter, you'll find it's start current is actually pretty tame (unlike the old 3 phase DOL compressors), so you're probably pretty good there. You would do well to find out what its running power factor is as a bit of a heads up. Generator tests are a good time to crank up your redundant AC and verify it is performing also.

So really, your only issue is do you have enough load to reliably prevent the cylinder walls glazing while you run the unit. Diesel standbys are one of the rare exceptions to the "bigger is better" rule if you want them to live a long and useful life.

You then start getting into things like fuel rotation for shelf life and general diesel maintenance. 22KVA is not a big diesel in the scheme of things, but they're not cheap to purchase or maintain so you want to maximise your investment.

[Anonymous user · Accepted solution]

Thank you once again for your advice! I will come back to this thread to let you know about the progress of the project.