Subject: Why does it take so long to restart power generators/electrical stations?

Can you explain why it takes so long to restart power generators/electrical 
power stations to restore the grid after an outage?

Many of these generators were supplying power at 13.6 kV and 10's of MWs/unit a 
few seconds before the wave of disconnects from the grid, and they keep 
spinning.  The steam power is unaffected.  Is it because they need external 
power for the exciters?   But getting a diesel generator up to excite the field 
coils should only take a few minutes.  There was spinning reserve before the 
grid collapse, so excess capacity over load does not seem to be the answer.

I understand that connecting to the grid requires careful attention to 
synchronization in phase and frequency, and that 3-phase power has to be 
balanced; this is routine.  But it would seem that isolating various "islands" 
where power can be restored without the grid interconnects should be easy and 
fast.  

Why isn't this so?  

Re: Why does it take so long to restart power generators/electrical stations?