A power system can be referred to as an islanding system when it is disconnected from other systems and does not exchange power through tie-lines.
Due to the fact that frequency is the same within the entire system, frequency control in an islanding system can be achieved in a relatively simple way. The turbine speed governors should be provided with supplementary elements that change the settings according to frequency variations.
After a load change, the frequency reaches a new steady-state level, different from the initial value. The frequency (or rotational speed) error causes an additional integral term to generate a signal which modifies the value of the power setting of a generating unit. With a sufficiently large number of generating units supplied with control systems of this type, the power system yields such a change in the generated power that frequency returns to its initial value.
This concept of frequency control can be referred to as decentralized since it is performed by regulators in power stations situated at various locations within the system.
Such a solution cannot be applied to cooperating power systems. In interconnected systems the goal of frequency control is to provide a power control also in the tie-lines. The frequency and power exchange control can only be achieved by means of centralized control.
The central control system also allows for other goals, such as automation of optimal load flow.