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A steady‐state system frequency results when synchronous power generation matches the system load and the losses in power system supplying those loads. In case of a system disturbance, such as sudden loss of a major generation unit, the typical variation in system frequency and its subsequent restoration to the pre‐disturbance level is depicted in Figure 1.7 [8]. The frequency restoration is enabled through three sequential stages of frequency control – primary frequency control, secondary frequency control, and tertiary frequency control [8–13].

Assume that the power system is operating at steady state at t = 0⁻ and a large generation loss occurs at t = 0⁺. The kinetic energy of all the synchronous machines (generators, condensers, motors) is autonomously extracted to supply the load (inertial response), leading to a decline in the speed of generators and consequently the system frequency. The decline in frequency continues till additional power injection from synchronous generators balance out the load. The rate at which the frequency decreases is termed “Rate of Change of Frequency (ROCOF).” The lowest level at which the frequency is eventually arrested is known as “frequency nadir.” The time period from the onset of disturbance to reaching the frequency nadir is known as “arresting period.”

Primary frequency control (also referred as Frequency Containment Reserve [FCR]) is provided by synchronous generator turbine governors by injecting power from the generators during the arresting period and continuing thereafter. This causes the frequency to stabilize at the “settling frequency,” which is higher than the nadir but still lower than the steady‐state frequency before the disturbance. This time period until settling frequency is reached is termed “rebound period.”

Secondary frequency control through Automatic Generation Control (AGC) is then exercised to restore system frequency to its pre‐disturbance scheduled level. The period over which this secondary frequency control is provided is known as “recovery period,” which extends over 5–10 minutes (or more).

Tertiary frequency control is provided subsequently which involves restoration of the synchronous generator and other reserves which provided primary frequency control, to their preset levels, so that they can respond to any future loss‐of‐generation events. The tertiary control involves coordinated changes in the dispatch levels (outputs) of different generators. In this control, some generators are dispatched down to restore their reserve capability while some other generators are dispatched up by a corresponding amount, while maintaining the scheduled system frequency. Deployment of tertiary frequency control is the final stage of frequency restoration in the recovery period.

Figure 1.7 Sequential frequency controls after a sudden loss of generation and their impact on system frequency.

Source: Eto et al. [8]. Reprinted with permission from Lawrence Berkeley National Laboratory, Berkeley, CA, USA.

The focus of this book is on the arresting period, frequency nadir, and initial parts of the recovery period with and without the high penetration of inertia‐less solar PV systems. The key performance indicators involved in frequency response are explained below.