I am reading a book about transformers and I am confused by the following paragraph about synchronization check:
When the transformer trips or is taken out of service, the primary and secondary voltages decay because the supply voltage has been disconnected. However, the voltage on the load side of the transformer decays more slowly because of the back EMF induced by induction motors located downstream. Thus, the two sets of windings become out of synch. This occurs at approximately 40% of rated voltage, causing the synchronization check relay to activate and the transformer breaker to trip if it is not already open. This protects the transformer from electrical system upsets or loss of supply that cause a loss of synchronization while the transformer breaker is still closed.
What is the point of the relay opening, if the transformer breaker has already tripped? Also, what do they mean by "40% of rated voltage". Primary side or secondary?