  We consider the problem of synchronizing two electric power generators, one
of which (the leader) is serving a time-varying electrical load, so that they
can ultimately be connected to form a single power system. Each generator is
described by a second-order reduced state-space model. We assume that the
generator not serving an external load initially (the follower) has access to
measurements of the leader's phase angle, corrupted by some additive
disturbances. By using these measurements, and leveraging results on
reduced-order observers with ISS-type robustness, we propose a procedure that
drives (i) the angular velocity of the follower close enough to that of the
leader, and (ii) the phase angle of the follower close enough to that of the
point at which both systems will be electrically connected. An explicit bound
on the synchronization error in terms of the measurement disturbance and the
variations in the electrical load served by the leader is computed. We
illustrate the procedure via numerical simulations.
