Generator sizing and use with UPS

[bartley]

Hi,

I am working with an existing generator. The unit is a 10kw 3600 RPM 100amp unit with automatic transfer switch. When the generator starts it powers the building however the power supplied will not work with the UPS units we have installed they work off of there own batteries while on generator power. I am tryting to find a generator to work with the UPS system. What I have found is that a generator with an electronic govenor is good but they also say to size this at 25% of the generator with that said I have 20 550w APC units 11000 I have 1 750w unit and 1 1500w unit so total UPS wattage is 13250 watts x 4 is 53000 watt unit this seems very excessive is there a solution to this without going so big.

Thanks
Bart

 

[charlie b]

. . . however the power supplied will not work with the UPS units we have installed they work off of there own batteries while on generator power.

I don't understand what you are saying here. In what sense do you mean that the UPS will not work on generator power? Once electricity is flowing down a wire, the loads downstream do not care what source is driving the current. How does the UPS know that it is on generator power? Is there a contact in the transfer switch that somehow shuts off the UPS?

Welcome to the forum.

 

[wireguru]

I don't understand what you are saying here. In what sense do you mean that the UPS will not work on generator power? Once electricity is flowing down a wire, the loads downstream do not care what source is driving the current. How does the UPS know that it is on generator power? Is there a contact in the transfer switch that somehow shuts off the UPS?

Welcome to the forum.

UPSs are finicky when it comes to their incoming power. Very frequency sensitive. As far as recommending oversizing the generator, would that be due to poor power factor load presented by the UPSs?

 

[rcwilson]

Some UPS systems monitor incoming normal power frequency and voltage. If the frequency or voltage goes out of the allowable limits, the UPS switches to battery power.

A small generator may not have precise frequency or voltage control so the UPS never connects to what it sees as being an unreliable power source.

If the UPS is an off-line style, its charger is not sized to carry the output load and recharge the battery at the same time. This type will connect to generator power but ends up dischaging the battery.

 

[ceb58]

The UPS systems we have in our communication buildings range from 12 to 18kw.
The UPS people will tell you the gen. set needs to be 1 1/2 to 2 times larger than the UPS. One of the UPS engineers that help design the style UPS we use told me that even with a 18kw. unit that only has a 5kw. load on it the UPS is still looking for 18kw worth of power. But I will say that we have some sites with a 18KW UPS and a 80 kw gen. the UPS still will not lock on to the gen. power. We have had to have factory reps come in to further expand the parameters on the UPS to accept the gen. power. Most do not like any thing over or under 2v or 2Hz

 

[bartley]

Are they correcting the UPS with software to make it work with the generator? How are they expanding the parameters of the UPS? Can you fill me in on any thing that was recommended to do or products to use. I do not mind spending the customers money to solve the problem as long as it will solve the problem so any input would be appreciated.

 

[weressl]

Hi,

I am working with an existing generator. The unit is a 10kw 3600 RPM 100amp unit with automatic transfer switch. When the generator starts it powers the building however the power supplied will not work with the UPS units we have installed they work off of there own batteries while on generator power. I am trying to find a generator to work with the UPS system. What I have found is that a generator with an electronic governor is good but they also say to size this at 25% of the generator with that said I have 20 550w APC units 11000 I have 1 750w unit and 1 1500w unit so total UPS wattage is 13250 watts x 4 is 53000 watt unit this seems very excessive is there a solution to this without going so big.

Thanks
Bart

Depends on the size and combination of UPS's it could be a nightmare. You are on the right track when suspecting the electronic 'governor' although is is a more complex problem than that as governors used to regulate the speed only, but the electronic controllers on generators control voltage, frequency as well. They are necessarily tolerant of non-linear loads, high harmonic content and large inrush currents, all of which are symptomatic of UPS packages. When you are using multiple small UPS's, this can become a random mixture of infinitely variable influences.

An single isolating transformer could be your solution, but you may even have to go for a ferroresonant unit to control those widl horses on the generator load side. Suggest that you consult a power disturbance specialist to fix your problem in a methodical and professional way. On the long run it may be the best solution.

Original design would call for a centralized, single UPS with a matched generator set. I personally hate those small UPS's, unless it is your home use or for a single or few computers. But hey, the old axiom: 'you get what you pay for, is alive and well.