## 7 Conclusions  
This paper proposes a coordinated planning model for unit retirement and source-grid that comprehensively considers system frequency security and node inertia constraints, achieving integrated planning of economic, reliability, and environmental protection in the power system. Under the premise of ensuring system frequency security, it optimally configures the low-carbon and green transition of a new power system by completing the retirement of coal-fired units and planning for new energy units. Additionally, a comparative analysis of the unit retirement and source-grid planning results before and after considering system frequency security and node inertia constraints was conducted using the IEEE 24-node case and a real-region case of eastern China. The results indicate that:  
1. The planning model for unit retirement and renewable energy, which considers frequency security, although it slows down the green transition of the new power system, ensures the security and stability of the system, making it more aligned with the current reality;
2. The planning model for unit retirement and coordinated source-grid that considers system frequency security can effectively reduce the rate of frequency change after power disturbances by properly scheduling the retirement of coal-fired units and the expansion of new energy units, thus limiting frequency deviations;
3. The planning model for unit retirement and coordinated source-grid that considers node inertia constraints can optimize the inertia levels of system nodes, ensure the inertia levels at key nodes, avoid excessively low inertia levels due to unit retirement, and enhance the system's frequency security characteristics.  
As the proportion of new energy and the retirement capacity of coal-fired units increase, system stability will further decline. How to address the frequency of security issues considering the uncertainty in the new power system requires further research based on this paper.  
**Author Contributions:** Methodology, software, investigation, and writing--original draft, Y.X. and C.S.; validation and supervision, L.L. and H.Z.; formal analysis and validation, W.L. and Z.T. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Key Technology Project of China Southern Power Grid Corporation, \"Key technologies and applications for coordinated planning of regulating power supply and large scale renewable energy\", grant number 067600 KK52222016.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors upon request.  
**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] are employees of Guizhou Power Grid Co., Ltd. The authors declare that this study received funding from China Southern Power Grid Corporation. The funder had no role in the design of the study; in the collection, analysis, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.