We consider power systems for which the amount
of power produced by their individual power plants is small
with respect to the total generation of the system, and analyze
how the transient instability mechanisms of these systems change
qualitatively when their size or the dispersion of their generators
increases. Simulation results show that loss of synchronism will
propagate more slowly and even stop propagating. Given the
evolution of power systems towards more dispersed generation
and geographically larger interconnections, we conclude that
research in transient stability should focus more on the propagation of the loss of synchronism over longer time periods, so
as to assess what happens to the overall system subsequently to
the loss of synchronism of the first generators. We also argue
that such studies might be very useful in order to provide
guidelines for setting up power system control schemes to contain
the propagation of instabilities, and we discuss some ideas for
designing islanding based emergency control schemes for this.