Paralleling two UPS

[davva]

Has anyone ever paralleled two UPS on the ac output? I realise it could be done quite easily if converted to dc but I was wondering if anyone has ever implemented two independent UPS in parallel synchronised on the ac output?

Thanks in advance for any replies.

 

[ruggedscot]

You are a brave man to tell a customer that "you will be covered for all eventualities" with the new UPS. If you commit that to writing then you are plain daft because the lawyers will have a field day when it goes wrong (and it will).

The whole n+1 ideal negates this by providing a means to protect and support the load. As soon as there is a fault that brings you on to n operation without the redundancy then the client would be told that unless they do something now they are pretty vulnerable. The system has supported the load and not allowed the fault to ripple through to the end user. Its not about being plain daft its about providing a solution to a problem and ensuring that the system doesnt fall over with the first hurdle.

A UPS installation that Im familure with comprises of four UPS units, these UPS units are configured as ((n+1)+1).

The load has items that are dual fed, each item being able to run from any supply provided. Normally both supplies are energised. fail one the other picks up and supplies power. Now each line to the equipment is configured with n+1 in terms of UPS capacity. Thus any failure will not result in the equipment being affected. The likelyhood or probability of having two UPS units go down at the same time is immesurable. Now the interesting bit is when you have a UPS taken out of service for maintenance, then you are vulnerable, you are one down out of four but the way the system is configured you could in theory with one UPS out for maintenance take another hit on the system. So you would survive but you would be telling the kind gentleman from the UPS company doing the servicing to pull his finger out and get the unit up and onto the bar asap.

How critical the process is means that you spend money and try to provide a solution to the problem in such a way as to limit any opportunity for a problem to impact the process. Four UPS units and any two can be taken out of service and the system will operate. Another failure triggers the system to go into bypass but you would never have more than two failures at the one time.... If you did then you would have picked the really short straw.