In case of a.c. systems frequency can be controlled by adjusting steam input to the prime movers. If the frequency is high, the steam input to the prime mover is decreased temporarily and the K.E. is drawn on to supply the required electrical output. On the other hand if the frequency is low, steam input to the prime movers in increased and the excess of mechanical power input over electrical power output goes into increase of kinetic energy which results into increased speed and frequency.
The frequency of the a.c. system can also be controlled if it is connected to a d.c. system which has its power rating comparable or slightly greater than that of a.c. system. The frequency can be controlled both at the receiving end of the system and/or the sending end of the system.
A frequency discriminator circuit is used at the terminal where the frequency is to be controlled. Its output signal is proportional to the frequency deviation from its normal value and is used as an error signal to advance or retard the firing of converters.
If the frequency is low and if it is an inverter circuit, the ignition of the thyristors should be advanced so that the received power is increased. On the other hand, if it is a rectifier circuit, the ignition should be delayed so that the power at the sending end is decreased. Hence a reversing switch is required on the output of the discriminator.
The same error signal in transmitted from the discriminator to the distant terminal. There is no change in the error signal at the rectifying converter but at the inverter a marginal signal is subtracted from it, so that even though the frequency is correct, it appears low regardless of whether the frequency controlled network is importing or exporting. In either case, the inverter advances its ignition angle in a vain bid to increase the frequency and as a result reaches the minimum allowable extinction angle and thus determines the direct voltage.
In case of low voltage at the rectifier, limiting the line voltage the inverter does take over the frequency control, even though there is then a small frequency error due to the marginal signal. The frequency control is analogous to the current control already discussed. The converter with lower voltage, controls the direct voltage of the line and the one with higher voltage controls the frequency.
