## 3 Control Strategy in Fast Frequency Response
### Novel Coordinated Control Strategy (NCCS)  
FFR using a wind turbine achieves a fast response using a converter; in the case of a control method that generates a temporary output according to the frequency deadband, the maximum output of the IBRs is activated, so the frequency nadir is boosted dramatically and the initial RoCoF is reduced dramatically. As a result of the quick response and the period required for the recovery of the wind turbine, a second frequency dip may occur. Therefore, in this study, we proposed a coordinated control method that could improve the lowest frequency by preventing the secondary frequency drop during FFR control and by efficiently utilizing resources. In general FFR control logic, when the frequency deadband is reached, additional output of the wind turbine and BESS output are simultaneously activated, which is highly likely to cause a second frequency dip owing to the recovery period of the wind turbine. If the FFR resources are activated at the same time, the secondary frequency dip due to wind turbine recovery is likely to be lower than the first frequency nadir. By activating the ESS near the first frequency nadir point, the lowest frequency point can be improved. When the BESS is activated near the first frequency nadir, the value of the first nadir is lower than that of the existing method, which increases the droop response of the synchronous generator and improves the second frequency dip. Figure 5 illustrates the concept used to improve the frequency nadir. In the conventional method, when an event occurs at time \(t_{a}\), the FFR resources of the wind power generator and ESS are activated at \(t_{b}\), which is the time when the frequency deadband is reached. However, when the FFR of the ESS is activated at \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Resources**} & **Wind** & **Lithium** & **Flow** & **Lead-** & **Cid** & **Flywheel** & **Capaci-** \\  & **Turbine** & **Batteries** & **Batteries** & **Batteries** & **Batteries** & **tor** & **HVDC** \\ \hline \
\begin{tabular}{c} Activate \\ (fully \\ activate) \\ \end{tabular} &
\begin{tabular}{c} 40 ms \\ (500 ms) \\ \end{tabular} & 10–20 ms & 10–20 ms & 40 ms & \(\leq\)4 ms & 10–20 ms & 50–500 ms \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Activation time for various resources. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Detection Type**} & **Measure \& Identify** & **Signal** \\ \hline \multirow{2}{*}{Frequency direct detection} & \(\leq\)2–3 cycles & \(\stackrel{{\text{\tiny{\textbox{1}}}}}{{\text{\tiny{\textbox{1}}}}}\) cycle \\  & (33–50 ms) & (8 ms) \\ \multirow{2}{*}{RoCoF detection} & \(\stackrel{{\text{\tiny{\textbox{1}}}}}{{\text{\tiny{\textbox{2}}}}}\)–2–3 cycles & \(\stackrel{{\text{\tiny{\textbox{1}}}}}{{\text{\tiny{\textbox{1}}}}}\)cycle \\  & (33–50 ms) & (8 ms) \\ \multirow{2}{*}{Local RoCoF/frequency measurement} & \(\geq\)5 cycles & - \\ \multirow{2}{*}{} & (83 ms) & \multirow{2}{*}{-} \\  & & & \\ \end{tabular}
\end{table} 
Table 2: Frequency/RoCoF detection and signaling time. close to the first frequency nadir point, the droop response of the synchronous generator increases compared to the conventional method and the second frequency is improved. IBRs are capable of quick response through converter control, so outputting near the nadir point is the most efficient way to improve the lowest frequency. Even if the first frequency nadir is lowered, the lower second frequency nadir can be increased, so it is efficient to output the BESS output near the nadir point to improve the overall lowest frequency. From the perspective of RoCoF, after the inertial response of the synchronous generator, the FFR of the wind turbine is activated, and the RoCoF increases with a gentle slope. In this study, we proposed the RoCoF constant \(\alpha\) to inject the BESS output before the RoCoF reaches zero, that is, before reaching the frequency nadir. \(\alpha\) was determined by the system inertia and the amount of FFR that could be output.