Do we need synchronization module between 2N UPS system supply STS? If don't have synchronization module, what happen?
It depends on what sources feed the bypasses of each ups and the type of STS. Both UPS need to have their outputs in sync for proper STS transfers. However some STS can transfer between two out of phase sources up to 180 degress out so you wouldn't need critical bus sync circuitry as a requirement. However its always ideal to keep both outputs synced if possible with a STS.
I would definitely install the sync module. In case of the STS switching over between two non synced phases, device PSUs could switch off. Also STS could 'refuse' to switch over at all.
As upsguy said this depends on the type of STS you have, but the sync box is a best practice.
In any case would recommend that you talk to your supplier / electrician.
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