## 7 Conclusions  
The main achievements of this study are as follows: A frequency control support technique based on an adaptable power reference comprising a WPG and a battery is implemented. The battery and WPG were assigned different functions in the frequency  
Figure 20: Jensen wake model schematic representation [38; 40].  
control service to accomplish this purpose. The proposed frequency control technique incorporates a supplementary control loop used in combination with the MPPT control loop which relies on the frequency difference. The speed of the rotor and the frequency determine the gain to increase the minimum frequency and ensure that the WPG operates in a stable condition. In the low-rotor-speed region, the battery improves the operational stability of the WPG. This is accomplished by determining that the power of the battery operates depending on its state-of-charge (SOC) and the WPG's rotor speed. An IEEE 14-bus system with two wind power plants (WPPs), doubly fed induction generators (DFIGs), a battery with a 5 MWh capacity, five synchronous generators, and specified loads using the EMTP-RV simulator were used for the analysis of the suggested technique. The scenario study results present that the suggested technique can decrease the frequency drop by releasing more energy which is stored in the rotating masses of the WPP and battery than the typical conventional techniques.  
This study has some main limitations. Firstly, an ideal battery model system was employed for this study; however, in actual operating conditions, the non-linear battery model characteristics with varying systems and environmental conditions need to be considered. Additionally, a qualitative feasibility analysis on the wind farm where the wake effect is often experienced was performed to determine the future adaptability of the proposed approach within the system. This analysis does not provide a clear relation with the application of the proposed technique in frequency regulation and wake effect scenarios and thus needs further investigation. In addition, to further verify the effective application of the proposed technique, a sensitivity analysis of the proposed control technique was not presented considering various operating conditions.  
Hence, based on the above limitations of the study, the following future research needs to be performed. An actual battery system takes into consideration its non-linear characteristics and various operating conditions as well as investigating the battery states, which include the battery's state-of-health, ageing, and remaining useful life, to ensure an efficient battery system. Additionally, a detailed wake effect analysis needs to be further studied on the wind farm taking into account the frequency regulation control employing the battery system for an optimized wind farm. Furthermore, a sensitivity analysis study needs to be performed based on the proposed control technique considering various operating conditions.  
Conceptualization, H.L. and R.K.T.; methodology, H.L.; software, H.L.; validation, R.K.T., H.L. and S.H.; formal analysis, R.K.T.; investigation, R.K.T.; resources, H.L.; data curation, S.H.; writing--original draft preparation, R.K.T.; writing--review and editing, J.K.; visualization, R.K.T.; supervision, J.K.; project administration, J.K.; funding acquisition, J.K. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Korea Electric Power Corporation (R21X001-3) and Korea Institute of Energy Technology Evaluation and Planning (KETEP), as well as the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (no. 20210501010020).  
Not applicable.  
The data that support the findings of this study are available from the corresponding author upon reasonable request.  
The authors declare no conflict of interest.