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Which one is better, one big UPS or multiple smaller ones in parallel?

Discussion in Server Room & Data Center Design Best Practices started by Petko , 5/24/2009 8:07 PM
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Which one is better, one big UPS or multiple smaller ones in parallel?

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  • petko


    Anyone can briefly explain to me what may be the benefits and the disadvantages of using multiple smaller ups units compared to using a single big one. Im not talking about redundancy, just the difference between 1+0 or N+0 configurations. For example using one 60kVA unit, or using 2 30kVA units in parallel producing the same power.
    Im asking about the differencies in cost, efficiency, reliability, practicality or anything else you can think about.

    Message was edited by: petko

    Message was edited by: petko

  • trinhkhai79


    My opinion is 3 units in a group parallel.

    1. Limmited of number parallel unit. Better you can use maxium is 03 unit in parallel.
    Some company (MGE, APC) have limited of the parallel unit (ie 4 unit, 6unit or 8 unit in parallel). If you exceed this limited, your engineer should caculation, some time need - centralinze bypass ....Do you know why they have this limited ? It's due to UPS become very hard in parallel synchronize operation for big system.

    Electrical system should oversize cable .... 6time in case you want upgrade to 6 parallel unit.
    Should use backfeed protection for each UPS.

    2. Difference type of version firmware.
    You should buy the same unit with the same firmware (control board, syschronize ...) to make sure unit run stable. I experience some case drop load due to difference firmware in Silcon and MGE Galaxy before.

    But the unit in 2004 maybe difference in 2007 and future. So don't use to big number parallel system.

    3. Poor quality performance in big number parallel system.
    As I mention before. Each UPS must have their own AVR system. So in parallel mode, they must folow the master unit and synchronize with the master unit. Depend on the PWM (20MHz or more) they can synchronize all the UPS output in some few miliseconds but it's will limited, and they also need to synchronize with the incoming main for the bypass safety.

    Some big system, they need the centralize bypass, in calse can't synchronize, UPS will tranfer "TEMP-Bypass" and "Re-synchronize" the system or drop load :(

  • ipicKedawinna

    here is some info i got...

    1.) 60kva (if its a sycf60k80f) would provide multiple points on the plus side:
    single footprint vs multiple foot prints (saves space)
    can be added to, to increase capability without the need for downtime
    individual components can be replaced without need for downtime
    high efficiency as the unit uses only the number of power modules it needs (50kva load would = N+1 for redundancy)

    2.) 30kva units depending on type/manufacturer:
    larger overall footprint due to multiple units + batteries
    If one unit fails and they are not paralleled then the load is exposed or lost
    greater number of failure points (if mains to one is lost its likely both will have to run on battery)

    generally a unit that can be scaled to meet the users needs would be better than 2 units that could increase costs due to maintenance, repair down time, etc

    a single unit would also decrease costs by needing only one feeder vs two separate feeders for 2 smaller units.

  • redun_it


    We are noticing a trend is building up in requirements for parallel capability in smaller units (single phase) with long backups (e.g. 2 hrs), not for its redundancy but only for its capacity increase property, this seems to be the case in sites where the load increase is not very clear in time (paticularly we are seeing this requirement in bank branches, financial institution offices, ...).
    Since some brands are offering this spec. with 0.9 power factor on their single phase units like 8-10 kVA then in tenders we need a strong arguement for APC solutions. Pls can you guide me in more details about documenting this strong argument with also taking into account 0.9 power factor too. Is there any APC sales guide or white paper with regards to this subject?


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