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Connecting Parallel Uninterruptible Power Supply Systems
Parallel uninterruptible power supplies can be installed with a single or dual ac input. It is imperative, however, that the input supply is correctly rated and cabling sized appropriately. The overall rating for a parallel system must be adequate to supply the chosen method of parallel operation, be it parallel-capacity UPS or parallel-redundant UPS systems.
In the case of a parallel-capacity configuration, the ac supply must be rated to the maximum input current of each uninterruptible power supply module and multiplied by the total number of modules used.
In the case of a parallel-redundant system, there is a bit more choice; the overall supply rating can be either the same as that used for a parallel-capacity design, which is the preferred method, or equal to the total current required by each of the individual UPS modules, plus 10 percent for redundant modules to cover charging and system losses.
Cable Sizing
Sizing cables is not necessarily complicated but it needs to be done correctly so as to avoid costly and disruptive downtime once the installation has been commissioned. Input cables must be sized in accordance with the rating of the supply of each UPS they are connected to. Output cables must be rated to either the supply or the maximum output power available from each UPS module (whichever of these is the greater). Input and output cables should be equal in length to ensure an even power distribution, should the UPS need to transfer loads to bypass.
Input Supply Isolators and Circuit Breakers
The use of isolators on the input of each of the uninterruptible power supply modules provides a means of isolating any one for service, removal or decommissioning. Circuit breakers are preferred as they can also provide further protection, specifically from over-currents (discrimination).
On the output of an uninterruptible power supply, isolators are preferred instead of circuit breakers as they prevent automatic disconnection of any one UPS module, which could otherwise disrupt operation of the parallel system during use. System resilience is increased by interlocking output isolators. This can also prohibit unauthorised operation. Future capacity expansion of the system is assured, relatively easily and inexpensively, through the installation of a spare switchgear set.
Dual Input Connection
It is common practice when connecting critical applications to increase resilience of an uninterruptible power supply by attaching it to a dual input ac power source rather than relying on a single one. This allows the bypass and rectifier to be supplied from separately derived sources, which could be different sub-stations, to mitigate a single-point-of-failure.
With transformer-based UPS this is made easier as they are usually supplied with a dual input option as standard. The rectifier and bypass supplies of this type of uninterruptible power supply are independent of each other and can be powered from two separate ac sources.
In the case of transformerless UPS, the rectifier and bypass supplies require a common neutral connection to be made within the UPS itself, which makes it a little trickier to utilise with a dual input supply without it being installed as a factory fit option.
Parallel uninterruptible power supply connection need not be any more complicated than any other type of power protection system as long as a few simple steps are followed and the system is correctly sized and rated.
About the Author
Robin Koffler is the General Manager for Riello UPS Ltd the UK subsidiary of Riello UPS (RPS S.p.A) a leading European manufacturer of Uninterruptible Power Supplies and a co-author of The Power Protection Guide(ISBN 978-0-9554428-0-3)- available from Amazon.com
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