Electrical generation in power stations - how AC Hz and voltage fluctuates

The frequency and voltage can be controlled independently. Frequency in the UK is very accurate and synchronous clocks are rarely more than a few seconds out. Voltage can drop if the demand is too high and the DC connections to the continent cannot compensate.

Dropping the frequency can increase demand with inductive loads such as motors and fluorescent lights.

 

Frequency is controlled by varying the rpm's of the generating set. For steam turbines this means opening the governor valve to allow more steam to the turbine and more fuel to diesel set and so on. Alternator speed will vary with load so will be reflected in frequency change ie: as load increases speed (and frequency ) will drop so more steam(fuel) is needed to maintain frequency to strict limits.This is done by using a auto governor control on steam sets and some form of throttle control on internal combustion sets.
Voltage is also dependent on load and is varied by the excitation current of the rotor. An AVR(auto voltage regulator) is used to control rotor dc current (excitation) and so maintain voltage. A nice little balancing act all round!

 

One little point to add which I can't vouch for, but was recently told to me by an engineer who used to work for National Grid. As well as not letting frequency get too far out of synch, they have to maintain the cumulative number of cycles, so that synchronous clocks maintain long term accuracy. Although they can drift by several seconds during the day, they are required to be back on schedule at midnight every night - so that's the time to set your synchronous clock (which is why he was telling me this.)

 

Yes, I was told to set a synchronous clock at midnight on a Sunday when demand was low and the frequency and time would be set.