Acillary Services

The objective of primary frequency control is to maintain a balance between generation and consumption (demand) within the Synchronous Area. By the joint action of all interconnected parties/TSOs, primary frequency control aims to maintain the operational reliability of the power system of the Synchronous Area and stabilizes the system frequency at a stationary value after a disturbance or incident in the time-frame of seconds, but without restoring the system frequency and the power exchanges to their reference values.

Secondary frequency control is a centralized automatic control that adjusts the active power production Of the generating units to restore the frequency and the interchanges with other systems to their target values following an imbalance. In other words, while primary control limits and stops frequency excursions, secondary control brings the frequency back to its target value. Only the generating units that are located in the area where the imbalance originated should participate in this control as it is the responsibility of each area to maintain its load and generation in balance.

Tertiary frequency control is used to restore the primary and secondary frequency control reserves, to manage congestion in the transmission network, and to bring the frequency and interchanges back to their target value when the secondary frequency control is unable to perform this last task.

According to Article 3(2)(8) of the Network Code on System Operation replacement reserve (RR) means the active power reserves available to restore or support the required level of frequency restoration reserve (FRR) to be prepared for additional system imbalances, including generation reserves.

The objective is to serve as load following capacity that adjusts its output to balance the generation and the load within a specific region or area.

The objective is to maintain the frequency stability by helping to avoid load shedding in islands due to the feasible very prompt response of distributed energy storage systems (DESS).

The objective is to contribute to the process of recovering a power station to operation without relying on an external power network.

The objective is to maintain voltage by injecting or absorbing reactive power by means of synchronous or static compensation. Different kinds of voltage control are implemented by individual TSOs, based on their own policies:

​

• Primary voltage control is a local automatic control that maintains the voltage at a given bus at its set point.

• Secondary voltage control is a centralized automatic control that coordinates the actions of local regulators in order to manage the injection of reactive power within a regional voltage zone.

• Tertiary voltage control refers to the manual optimization of reactive power flows across the power system.