Power Generation: Site Design, Installation, and Connection

[loveepg]

It is my understanding that for a typical battery UPS system, a back-up generator must be doubled in size of the UPS capacity.  IE: 100kva UPS requires a minimum 200kva generator.  Is there agreement on this and where would I find detailed reasoning.  Thanks for reviewing.

[Tech1]

I am looking at a white paper from Liebert on UPS sizing and the recommendation in this write up is

" if there is 1 ups then the gen should be rated at 3 times the KVA rating of the UPS ie... 9kva gen for a 3 kva ups."

"if there are 2 or more UPSs then the gen should be rated at 3 times the total UPS rating of the UPSs ie... a 2kv ups and a 3kvaups could share a standbye gen rated at 15 kva or higher"

"the gen shoud carry a kva rating equal to the sum of all the critical ups loads and the essential loads plus a reserve capacity of twice the kva rating of the critcal loads"

 Hope this info is of some use .

[rmcmahon]

There are UPS manufacturer's out there that say you can use their UPS systems on a 1:1 balance.  There are some that say 2 or 3 or 4 or more to 1 ratio when it comes to genset sizing.  As a certified trainer for Generac and a active Field Service Technician I have seen units at 1:1 and 2:1 work but the book says that the UPS should not be more than 20-30% of the load plan of the generator.  However even if you keep it to 20% then you have to take into account all the residual loads that are connected.  Pure 100% resistive loads will take  away from the immediate horsepower of the prime mover (in theory) and when the UPS goes into deep charge mode of it's onboard batteries there may not be enough bHP left to overcome the draw of the load......there are volumes of books on this subject.  In my opinion and when I spec systems containing UPS loads I try to keep the UPS load to 30% or less and if the UPS load is more than 20% I try to keep the total load to less than 60-70% of the Genset rating.  It is not perfect and there are ways to overcome the formula like isolation sensing, upsized alternators, isolation transformers, Line reactors, etc.... However it is about 33/33/33 science/luck/experience. 

[Wissam]

Dear Sirs,

A generator set has to be sized not only to operate the UPS, but also to supply the UPS load directly on bypass while the UPS is started and charging. The general rule is that the generator should have 1 kW of available capacity for every kVA of rated UPS capacity. Note that this is based on rated UPS capacity not the actual load on the UPS. For example, a 1000 kW generator set is the minimum generator set size that should be used with a 1000 kVA UPS when no other loads will be supplied by that generator set. This sizing provides sufficient generator capacity to supply nameplate UPS load, UPS losses and default or nearly default UPS recharging power (12 kW less than default for UPS 1000). It also allows sufficient capacity for normal UPS loads to operate on bypass while supply power to start the UPS. With loads that have unusually low power factor or high harmonic content, the generator set size may need to be increased to supply the high reactive or harmonic currents of the load.

I hope this information can help you,

With kindest regards,

Wissam HAYEK / Application Engineer

M. Ezzat Jallad & Fils / Caterpillar Dealer

6 Sigma Green Belt Certified

Tel :       (+961)-9-212077-78-79-80-81      Ext.: 242

Mobile : (+961)-3-817259

Fax :       (+961)-9-212073

P.O.BOX 110 208, Postal Code 1107 2020

Riad Solh, Beirut, Lebanon

E-mail : whayek@jallad.com.lb

www.jalladgroup.com

[Srivatsa]

Hi,

Recommend that you use the spec sizer to size for UPS loads.. The most important issue with UPS is non leniear loads and harmonics.. If other loads, especially inductive, are connected to the system they will absob some of the harmonics generated by the UPS .. The no of pulses of the rectifier also changes the the oversizing capacity

If we take an example of sizing using a 1000 KVA ups only with a 12 pulse rectifier we need to over size 1 : 1.4 (1000 KVA UPS needs 1400 KVA genset.) and a 6 pulse needs a 1800 KVA generator i.e ratio is 1: 1.8.. this is due to the greater harmonics the rectifier generaqtes when it is 6 pulse..

If we have other inductive loads on line , depending on how inductive it is, will absorb the harmonics.. If we use a 50 : 50 ratio of UPS with inductive loads with 0.85 pf the ratio drops to 1 : 1.2... Since there are so many variables it is difficult to establish any thumb rule for generator sizing and suggest the load connected and UPS are analysed with the specsizer to arrive at the sizing..

Hope this helps...

[carlos]

I think you probably have a little old Liebert catalog. Three times more genset sizing we had to use 10-15 years ago, when they used classic diode or thyristor bridge in the input UPS circuit. Now all UPS producers using IGBT rectifiers, and reccommended sizing of genset is 1,3.

The reason is in a relative big impedance of the genset. During commutation we can see the diode bridka as like short circuit, the voltage loop of generator try the voltage move to higher and the porcess is unstable. Solution is : bigger genset: more iron, more mass, smaller impedance....

carlos

[Janath]

Hi,

You got to be very careful running UPS - ie static drives on Generators, since the Aternators do not like the harmonics generated by the on-line UPS systems (On-Line means the UPS thyristors or IGBT's work all the time feeding the load with the DC link connected to the battery banks).

In the case with off-line UPS - you will not have much of a problem and the generator can be sized 20-30% morethan the total max demand.

With ON-Line UPS's you will need a choke (Filter) at the AC feeder to the UPS before connecting the UPS to generators, in this case you may be sizing the Generator 30-40% over the Max Demand.

Best Regards, Janath Chandra