For a Electrical system to get synchronised, one of the 
condition to get satisfy is the system frequency.
If frequency to be changed , then it is to be considered 
right from power generation system.
While changing the frequency of the stationthe following 
factors to be considered.
Frequency = 12o*poles/speed of the machine.
If this frequency factor to get change.....then pole of the 
machine and the speed of the machine has to design 
accordingly, which is quite inefficient and many factors to 
be further considered.