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What you should know when specifying a Parallel Redundant UPS!

Posted by Christos Papakyriacou on

Assuming you want to ensure, that the electricity supply to your "mission critical" computer system is not interrupted, under any condition, you need to use Uninterruptible Power Supplies (UPS) in Parallel Redundant Configuration.


That essentially means using two or more identical UPS connected and running in "Parallel" with "Redundancy" in every critical element of the system!


Notice we are using two separate terms. "Parallel" and "Redundant", since each refers to a different benefit of the system.

A) To be specific, a simple Parallel system can have an output Automatic Transfer Switch, such as the diagram bellow:



Two UPS working in parallel but without redundancy since both supply a common ATS. If one UPS fails the other will continue feeding the load. But if the ATS fails, the load goes down! Even if the electricity supply is still present and even if both UPS are still working. Of course there is no load sharing and no communication line redundancy either!

OK then what is a better solution you may ask.

B) Another more advanced option would be a Parallel system with central controlling logic, in a separate cabinet.




UPS connected and working in Parallel with the first UPS incorporating the control logic, which controls the distribution of the load to all the UPS.

Ooops... do you see a problem here? If the "lead" UPS module fails, the remaining units are uncontrolled. The system may go to bypass, or may stop operating completely! Second issue is that, if the single communication line between the lead and the other UPS fails, the load may be interrupted, even without a power failure.


What is often not specified correctly in tenders is the type of the Paralleling method.

As we've seen, there are several ways manufacturers claim to "parallel" UPS. More than four in fact! Each with different costs, degree of protection, advantages and disadvantages.


We felt it would benefit potential users, to pass on our experience, for what we consider to be the most reliable and effective paralleling method, we have been using successfully in Cyprus.

Used for more than three decades and for mission critical applications, by organizations such as CYTA, Hellas Sat, The American Medical Center, Hellenic Bank, Alpha Bank, Columbia Ship Management and many others. Please visit our website at for more information.


ABB-GE's Redundant Paralleling Architecture or RPA provides complete Redundancy of all critical components with "no single point of failure" for the highest reliability.



In a correct Parallel Redundant configuration, the UPS share the load equally and each UPS is able to power the load, without any interruption or switching. Even if any of the UPS malfunctions, even if the control electronics in one UPS fails, even if one communication line (redundant) is cut between the UPS.


Some of the features and benefits of GE's RPA are:


  1. RPA Configuration provides complete redundancy of all critical components

  2. Modular design allows system upgrades without interruption to the load

  3. Easy to install and maintain

  4. Redundant high speed Data Bus for UPS communication

  5. Redundant electronics for high reliability

  6. Peer-to-Peer architecture ensuring no single point of failure

  7. Sequential soft start (during mains recovery), avoids over loading of input supply


A comparative overview of other "paralleling" methods, most not redundant, such as with automatic transfer switch, or with a paralleling cabinet or hot standby or with centralized logic can be found here:


Our SP/SG Series 3-Phase Transformer based UPS, from ABB-GE, available in power ratings of 10kVA to 600kVA, not only use RPA Architecture with Redundant Communication Lines and electronics, but also incorporate an inverter output isolation transformer inside the UPS cabinet. No external cabinets of any kind are required. UPS outputs are simply connected to the bus bar of the output distribution panel.


Finally to give an example, an 80kVA load can be powered by 2 x SG 100kVA UPS. Under normal operation each UPS will supply 50% of the load, in this case 40kVA. However if one UPS fails, the other can and will continue to supply the full load of 80kVA, since it is rated for 100kVA. Without any interruption to the load or switching.


So the next time you consider purchasing or specifying Parallel UPS protection for your "mission critical" application, please "clarify" the Paralleling method, for your benefit.


We invite you to visit our web site at or better still, call us at 22-765432 and we will be more than happy to offer free advice, for the best solution to your requirements. We've been doing it successfully for more than 35 years, so you have nothing to lose. We know our UPS technologies and we are here to stay. In fact we have been certified as "Critical Power Experts" by GE. Contact us today!

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