Consider a single UPS system:
In nearly all UPS systems there will be an internal transformer on the output of the inverter. The UPS monitors the synchronisation after this transformer. Therefore the output of this transformer is synchronised to the bypass supply at this point. Therefore no phase shift with respect to the bypass supply.
If a transformer is introduced external to the UPS for stepping up/down voltage or for isolation then phase shift can then be introduced. This phase shift does not affect any synchronisation issues with the bypass supply as it is on the output of the UPS. The UPS does not really "see" this transformer. The phase shift will only be between the output of the UPS and the output of the transformer.
The problem would only raise its head when you then take the output of 2 UPS systems into an external static switch.
If both of the UPS systems have the same bypass supply then both outputs of the UPS will be in synch. If these outputs were connected directly to the static switch then you would have no problems.
If an output of one of the two UPS's goes to an external delta/wye transformer then to the static switch, a 30 degree phase shift will be introduced. Therefore the waveforms at the static switch will be unsychronised. The static switch is a usually "make before break" arrangement although it is completed without moving parts using parallel connected SCR's. This would therefore connect unsynchronised supplies in parallel causing very high circulating currents for a very short time (ms). This is why most static switches will see that the supplies are unsynchronised and insert a break (usually 5ms or more) between switching supplies.
IHOASH, to answer your questions:
1. When you say that the two supplies must be synchronized, what parameters are you refering to? (Voltage, Phase angle, frequency and rotation) and what would your typical settings be?
All of the parameters must match.
2. What do you mean by external transformer? Typically I see them on the output of the inverter which connects to either an internal or external maintenance by-pass switch. But regardless of the transformer locations there will be a phase shift of 30 degrees.
Hopefully I have explained this above.
3. How does an internal transformer differ?
The internal transformer is used to step up/down the inverter output voltage and also acts as part of the sinewave filter circuit.
Hope this helps!
UPS Service Manager
