## 6 Conclusions  
With the continuous integration of RESs, critical challenges like varying inertia distribution characteristics and the decline of the inertia support capability of power systems have become increasingly prominent. This study focuses on analyzing the impact of generator sets on the inertia distribution characteristics in a power system. Moreover, based on the grid structure of the power system, the NFDC is proposed to evaluate the frequency  
Figure 11: Comparative evaluation results of inertia level in IEEE 9-Bus System.  
dynamic characteristics of each node. This proposed evaluation method can quantify the configuration requirements of a GFM-ESS. Aiming at the inertia distribution characteristics of power system, an optimization configuration method for both a GFL-ESS and GFM-ESS is proposed. The conclusions drawn in this study are as follows:  
The evaluation method based on the NFDC can describe the inertia distribution characteristics of a power system, which are determined by the distribution of generators and ESSs. The value of the NFDC is subject to the influence of multiple factors, including the inertia time constant of generators, the power output of RESs, and the grid structure of the power system. The elevated value of the NFDC at a certain bus signifies the degraded inertia response in the power system.  
The NFDC can be utilized to formulate the operational boundaries of the power system and clarify the requirement of inertia resources in the power system. By means of rational configuration of GFM-ESSs and GFL-ESSs, the inertia resource in a GFM-ESS can be fully harnessed, thereby enabling the ESS to exert its regulatory ability in stabilizing power fluctuations. However, compared to traditional methods, the physical meaning of the NFDC is less clear, and its applicability in extreme scenarios is relatively weaker.  
A GFM-ESS can effectively enhance the accommodation level of RESs and provide inertia support capability. The operational cost of the system and the configuration cost of ESSs can be reduced by employing the optimization configuration method proposed in this study. This method can also clarify the configuration requirements for both GFL-ESSs and GFM-ESSs. Furthermore, the upper and lower limits of the NFDC can be more clearly defined based on the operational standards of renewable energy power systems. Additionally, by incorporating certain extreme operating scenarios and using time-domain analysis methods, the weak nodes of the power system can be fully identified, enabling more reliable planning and configuration of ESSs.  
Conceptualization, K.Z. and L.W.; methodology, K.Z.; software, L.W.; validation, K.Z. and Z.T.; formal analysis, K.Z. and Z.T.; investigation, Z.T.; resources, Y.X. and L.W.; data curation, L.W.; writing--original draft preparation, K.Z. and Z.T.; writing--review and editing, L.W. and Z.T.; visualization, Y.X.; supervision, L.W.; project administration, Z.T. and Y.X. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data that support the findings of this study are available from the corresponding author upon reasonable request due to trade secrets.  
[PERSON] is acknowledged as having provided reliable renewable energy operation data and offered constructive feedback from an industry perspective on the writing of this study. His expertise and insights significantly enriched the content and helped improve the overall quality of this study.  
The author [PERSON] was employed by the company Datang North China Electric Power Test and Research Institute. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.