[2024] All About Frequency control Explained in Detail

    Frequency control is the process of maintaining the frequency for the stability of the power system. Frequency variations in a power system occurs because of an imbalance between the generated power and connected load. When the frequency of a power system reaches the emergency condition then, the frequency control system is initiated. The frequency control system is divided into three layers which are listed below.

1. Primary frequency control
2. Secondary frequency control
3. Tertiary frequency control

    Each layers have its own features and purpose.

Primary Frequency Control

    The primary frequency control (PFC) or frequency response control is an automatic and fastest layer of frequency control as its response period is only few seconds. The PFC is the first regulation measure designed to respond to frequency disturbance just after disturbance occurs. PFC is exerted by speed governors i.e control action is based on generator speed.

    When an imbalance between the power supply and load occurs, the frequency of the power system changes. Suppose the load increases but the generated power does not immediately increases so energy required to compensate this load increase is fulfilled by the kinetic energy of the rotor connected to the generator that decreases the velocity of the rotor.

    After this, the governor of each generator acts to increase power generation in-order to recover the decreased speed and restore balance and vice-versa. PFC can limit frequency because of its droops behavior but unable to eliminate the steady state error in the frequency signal so, a secondary frequency control is needed.

Secondary Frequency Control

    After PFC, the value of frequency is differ from the nominal value and the reserve margin of each generator have been used or partially used so, it become necessary to restore frequency to it's nominal value and reserve of each generator used. This is the purpose of the secondary frequency control (SFC).

    There are some generators entitled to perform secondary control through dedicated available reserve power. This reserve depends on the requirement of TSO or a percentage of the maximum power available usually a predefined minimum value.

    If the frequency is lesser than nominal frequency, additional generators are started while frequency is higher than the nominal frequency, load is increased or some generator units are stopped. The SFC usually performed in automatic way by all the generators or through a specific set-point by a central controller.

Tertiary Frequency Control

    After SFC, the dedicated available reserve margin used for SFC must be restored too. This is the purpose of tertiary frequency control (TFC) which is last layer of the frequency control. This layer is not automatic and only executed after the request from the grid operator

Conclusion

    Hope, you have clearly understand about the frequency control, its layers - primary, secondary and tertiary frequency control. Make sure to leave your opinion and suggestion about this post in the comment section below.