The STS is a static transfer switch which accepts to Inputs and keeps the load powered by the primary source(UPS A) and will switch to secondary source(UPS B) if UPS a fails or is shut off. There is no interuption to the load during the transfer.
Also with this configuration you have an additional benefit. You can feed UPS A main 1 input with Utility 1 and they Bypass of UPS B with Utility 1 and vise versa with Utility 2. By doing this if Utility 1 is lost UPS A still has its bypass feed to power the load and and UPS B still has its main input to keep the UPS running in Online mode.
You would be able to gain the benefits of two utility sources and still accomplish the same thing as two SUVT's in parallel assuming that you were doing that for redundancy and not capacity.
By attaching a STS it would have limited the purpose of SUVTP's parallel capability. There's a need for both UPS in parallel to share loads.
Let's assume Parallel for Capacity is required and any other switching equipment on the output side is a definate NO.
Maybe if you don't mind just help me understand the Parallel operation abit
If both UPS (parallel configuration) has only 1 utility input and same source, what would happend if the following were to occur:
1. UPS-A somehow does not accept input and switch to battery mode. Does UPS-B operate in normal mode or being forced to batt. mode ?
2. UPS-A internal fault causing it to switch to static bypass mode. Does UPS-B operate in normal mode or being forced to bypass mode ?
From past experiences with UPS that doest not have parallel feature, strict rules apply when it comes to getting both output synchronized (VIA STS) whereby both UPS must tap from the same source and operate in the same mode in order to have zero transfer time.
If // for capacity is required and you are not planing on adding any output switching then you would have to have the same input to each SUVT because..
When both UPS are online each UPS output is synconized to each other as well as the bypass input which would also be the mains 1 input if single feed.
You wouldnt be able to get the two units to // with each other if they were being fed different phased sources.
Also if you were somehow able to get the units // and online when you went to transfer them to static bypass which is just utility flowing through the units, if they were fed by different utility sources then you would be // two seperatly derived sources of power together on the output bus which I'm sure you know is a bad thing.
Sync could be an issue with an ATS but I've seen it implemented various times without phase sync circuits installed. However it would be just easier to install two SUVT's in // to accomplish your redundancy because I can't ever think of a reason to buy two SUVT 30ks for capacity. There are to many good UPS which can handle 60+kva and be expandable that won't come with the disadvantages of having to // two units together to reach the desired KVA size.