There a lot of differents schematics of power stage of UPS but the correct one of the true line interactive UPS is this one.
There is no switching of the output, inverter and mains works in // with an inductor between them and inverter interact on line voltage to correct distortion of waveform.
There is one output transformer (T1) with magnetic shunts for coupling inverter and mains.
Greatest problem with this technology is that the inverter is highly overloaded when mains fails because it has to feed the load and the short circuit of the mains (current limited by inductance).
Line must be opened as fast as possible (by thyristors) to limit this current.
I have made a lot of experiences and tests of such a line interactive UPS using a 10KVA inverter with modifications for varying phase between inverter and mains and measuring resulting charging current.
I don't need to look at videos of Dave, I have 30 years of experience in power electronics....
With all the respect I have for Dave that I appreciate very much, Dave is not God ! (nor I...
)
NB: one of the avantages of line interactive UPS is that there is no switching, nor transfer of the load.
When mains fails, there should nothing occurre to the load as inverter is already working in // with the mains.
@hgg: you have a storage DSO, why not measure the transfer time of your UPS instead of looking if you computer is rebooting or not ?
Your approach of the problem is absolutely not profissional.
When you have such a problem, you have to know exactly what happen...What is the transfer time ? Is this always the same or not