## 1 Introduction
### Frequency Containment Reserves (FCR)  
#### 3.3.1 Proposed Definition and Its Justification  
FCRs are PPs operating, at nominal frequency, below their maximum capacity, thereby being able to provide additional power (or reduce their output) in case of frequency deviations. Their power-frequency response, called also _P-f_ droop, is usually stepwise linear (see Figure 10) while a frequency deadband is introduced, in order to avoid the continuous hunting of the frequency during the everyday operation of the power system. The time response of the FCR is of order of some seconds, depending on their governor and their primary source, while their target is to arrest the frequency nadir and stabilize the system frequency at a new steady-state, leaving its restoration to the FCRs. Therefore, FCR are the first to act for the frequency restoration, while the inertial response helps in deteriorating the ROCOF, thus, providing the required time for the FCR to be engaged.  
The increasing penetration of the DRES, particularly those with variable output leads to noticeable system frequency response degradation [60]. Moreover, several studies [64; 65; 66] agree that the demand for frequency reserves in several countries across Europe and the United States has increased nearly up to 10% of the additional RES capacity over the last few years. Although the opposite has been observed in Germany [67], where the commitment of reserves has been reduced despite the increased DRES penetration, this is attributed to change in the operation of the balancing market from static to dynamic and to the cooperation among the different TSOs that practically unified their control areas. Nevertheless, the increased DRES penetration and other physical reasons [68], e.g., excessive governor dead-bands, or generators operating in modes that do not offer PFR by blocking their governors, lead to the necessary participation of the DRES in the PFR.  
Properly controlled converter-interfaced DRES can act as FCR providing either upward or downward or both types of balancing capacity. Operation in downward mode is rather easy and is implemented by moving the DRES away from its MPP as function of the measured frequency at its terminals following a droop like \(S_{do}\) in Figure 10. Already, recent standards and codes at the transmission and distribution system levels [3; 11; 12], specify that \(S_{do}=2\%-12\%\) determined by the DSO together with the frequency deadband range. The slope of the droop \(S_{do}\) can be specified either with respect to the nominal DRES power in the case of dispatchable units (hydro or biomass) or with respect to the MPP power, \(P_{MPP}\), in the case of variable power DRES. It is noted that DRES like wind generators can limit their output up to a minimum power \(P_{\min}\) for stability reasons. In underfrequency  
Figure 10: Droop curve and operation points of DRES at a given time instant.  
events, the same Codes and standards [3,11,12], specify the reaction of DRES depending on whether they are assigned as FCR or not. In the former case, the frequency deadband \(\Delta f/2\) can be set by the DSO between \(\pm 0.2\)\(Hz\) and \(\pm 4\)\(Hz\) while the slope \(S_{du}=2\%-12\%\). In case the DRES is not assigned as FCR, it is specified that they should simply support the system by _\"limiting the reduction\"_ of their power. It is actually requested the DRES to _\"be resilient to the reduction of frequency at the point of connection while reducing the maximum active power as little as possible\"_[12]. Thus, a maximum allowed reduction rate of 10% of the maximum power per 1 Hz for frequencies below 49.5 Hz is specified for usual condition but a more stringent reduction of the order of 2% of the maximum power per 1 Hz for frequencies below 49 Hz may be requested by the DSO. It is evident that the Standards treat the frequency response of the DRES as system support functions.