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FFR is the power injected into (or absorbed from) the grid in response to changes in measured or observed frequency during the arresting phase of a frequency excursion event to improve the frequency nadir or initial ROCOF [9].

In systems dominated by synchronous machines, FFR is provided by the inertial response of synchronous machines and conventional turbine generator response. The different types of frequency response including inertial response, PFR and FFR, which can act in coordination, are illustrated in Figure 1.8 [9].

The beneficial impacts are imparted by both nonsustained inertial response and the sustained FFR and PFR. The sustained frequency response methods provide continuous injection of power until secondary frequency controls bring the frequency back to the scheduled level. It should be noted that the nonsustained response such as inertial response which ceases after a short time period does not adversely impact the frequency during the arresting period or the recovery period. Hence there is a need for inertial response to be coordinated with other mechanisms of FFR and PFR.

FFR can be initiated based on a knowledge of magnitude of frequency deviation, ROCOF, or other factors. There is, however, a delay involved in estimating these quantities which needs to be accounted for. FFR is provided by the following methods, either individually or in combination [9, 15]:

1 Active power injection which is proportional to the measured frequency deviation (proportional response)

2 Injection of fixed magnitude of active power as soon as the frequency reaches a prespecified trigger point (step response)

3 Active power injection that is proportional to the computed ROCOF (derivative response)Figure 1.8 Simultaneous contributions of inertial response, primary frequency response, and fast frequency response.Source: Reprinted with permission from North American Electric Reliability Corporation [9]. This information from the North American Electric Reliability Corporation's website is the property of the NERC and available on the Standards page (https://www.nerc.com/pa/Stand/Pages/default.aspx). This content may not be reproduced in whole or any part without the prior express written permission of the North American Electric Reliability Corporation.

4 Injection of fixed amount of active power as soon as a prespecified ROCOF is attained (step response)

5 Controlled decrease in load in proportion to measured frequency deviation or ROCOF (proportional or derivative response)

6 Controlled decrease of a fixed amount of load once a prespecified frequency or ROCOF is reached (step response)

It should be ensured that any of the step responses described above does not adversely impact system stability or system frequency.

It is also desirable that the FFR must be timely and sustained rather than injected for a short period and then withdrawn [16].

FFR and inertia are two distinct entities. Inertia is a characteristic of synchronous generators which provides autonomous response to slow down the ROCOF but cannot restore power system frequency. Inertial response, in fact, provides time for active power injection to correct the supply–demand imbalance. On the other hand, FFR results from control action which is tunable based on system needs. FFR injects active power into the system, helps correct the generation–load imbalance, and subsequently restores the power system frequency. Inertia and FFR are, therefore, two distinct services which cannot be interchanged. They are characterized by the following features [17]:

 Large power systems presently require a minimum level of inertia, below which no amount of FFR can be utilized to ensure a stable power system.

 The magnitude and type of FFR required to maintain grid reliability are related to the amount of existing system inertia.

 The relationship between needed FFR and system inertia is nonlinear and can be evaluated through detailed system modeling. It also depends upon several operational considerations.

 FFR through IBRs is controllable and can be designed to provide responses which are much wider than that provided by synchronous generators.