By varying FIELD excitation we can control the output of DG.
Practically by changing AVR setting we can control the 
output of DG.
Conditions for synchronization:
1. Phase sequence should be the same
2. Terminal voltage should be the same
3. Speed/frequency should be the same
4. Load impedance should be the same.

By varying the turbine speed. voltage, frequency & speed are
to be consider for synchronization.

by contoling the fuel we can contol the output voltage of 
generator.

condition for syncronization.
1-phase sequence shuld be same.
2-frequency shuld be same.
3- terminal voltage shuld be same

generator out put voltage is controlled by varying 
excitatoin.

for syncronising of generator :
1 phase sequence should be  same
2. voltage and frequency should be  same and one most 
important
3. phase angle should be same

to be very clear, in commercial generator, nobody using 
separated excitation system. self excitation is the best 
method unless some specific reasons.

For controlling output voltage, a device called AVR, 
automatic voltage regulator to be used, and it sense the 
output voltage at each instance, and adjusts accordingly.

these are the condition for syncronization.
1.phase sequence of both generators should be same.
2-frequency shuld be same.
3-terminal voltage shuld be same.
4.both generators should have nearly same percentage     
impedence.

By varying FIELD excitation we can control the output of DG.
Practically but commertial d.g. sets are controlled by self 
excitation  and avr controls the output voltage output of 
DG.
Conditions for synchronization:
1. Phase sequence should be the same
2. Terminal voltage should be the same
3. frequency should be the same
4. percentage impedance should be the same.