How do you close a switch on a high-voltage network?
Firstly, the process (simplified): -



Picture taken from this very good article and I would recommend reading it for more detail. It is a simple line diagram and there are components not shown that will alleviate the connection should any small amount of non-synchronous voltage be present.

I'm no expert on this but I would consider the following to be relevant to achieve synchronization with as little disturbance as possible: -


The timing error might be 0.5% (as per the original question) so to minimize the disturbance, "connecting" would (or should) occur close to the peaks of the sinewaves of the two voltages.
0.5% timing error is a phase error of 1.8 degrees and that could produce a voltage error of \$\sqrt2\$ x 7200 x (Sin (90) - Sin(88.2 deg)) = 5.02  volts.
This of course relies on switching happening near the peak of the sinewave and the switching/connection device to be quick (or compensated for)
Reactors (inductors) placed in series with the connection will buffer the connection transient and quite possibly these can be bypassed pretty soon after a connection is verified.


I'm not expert so maybe someone else can shed more detail.