## 5 Case Study Analysis
### Real Region Case  
To improve the reference value of this paper, a real region of the eastern China case is used for testing. The real region case network topology is shown in Figure 5. In the figure, the blue dots and line segments indicate the nodes and transmission lines, respectively; the red icon is for coal-fired power units, the green icon is for hydropower units, the yellow icon is for photovoltaic systems, the blue icon is for wind power systems, and the black icon is for the external power. The region contains 72 nodes, 134 established transmission lines, 60,014 MW for established conventional power source installed capacity, 15,100 MW for established new energy installed capacity, 45,200 MW for external power capacity, and 6690 MW for aging coal-fired power unit installed capacity. Additionally, there are 52 candidate transmission lines, 4820 MW for the candidate coal-fired power unit installed capacity, 32,600 MW for the candidate new energy installed capacity, and 1850 MW for the candidate energy storage capacity. The model-related parameters are shown in Table 3. The scenario setup for the wind-solar load is consistent with the 24-node case. The maximum load level in the case is 98,000 MW, the disturbed power is 4900 MW, and the upper limit of the frequency change rate is 0.24 Hz/s. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Total reserve ratio & 15\% & \begin{tabular}{c} Solar system \\ construction cost \\ \end{tabular} & 4,000,000 CNY/MW \\ \hline Rotating & 5\% & \begin{tabular}{c} Transmission line \\ construction cost \\ \end{tabular} & 6,000,000 CNY/km \\ \hline Maximum load & 98,000 MW & \
\begin{tabular}{c} New energy \\ operation and \\ maintenance cost \\ \end{tabular} & 80 CNY/MWh \\ \hline Coal-fired power & 3,500,000 CNY/MW & Cut load cost & 25,000 CNY/MWh \\ \hline Wind turbine & 5,000,000 CNY/MW &
\begin{tabular}{c} Energy \\ abandonment cost \\ \end{tabular} & 500 CNY/MWh \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Model-related parameters. Figure 5: Real region case network topology. In order to verify the validity and feasibility of the proposed model in the real area, the following three schemes are set up. Scheme I: a retirement and source-grid planning model that does not consider frequency security;  
Scheme II: a retirement and source-grid planning model that considers overall system frequency security but does not account for node inertia constraints;  
Scheme III: a retirement and source-grid planning model that considers both overall system frequency security and node inertia constraints. The real-region case planning results are shown in Table 4.  
All schemes aim at economic optimization, and the planning results are similar to the 24-node case. Scheme I is only required to meet the basic planning needs such as load growth and power transmission, with the largest capacity of 4000 MW of retired coal-fired power units and the smallest installed capacity of 1300 MW of new coal-fired power units. Scheme II requires a guaranteed level of system inertia and therefore reduces the capacity of retired coal-fired units to only 3800 MW and increases the installed capacity of new coal-fired units to 1840 MW. Similarly, Scheme III reduces the capacity of retired coal-fired power units to 3800 MW and increases the installed capacity of new coal-fired power units to 2020 MW. In terms of cost, Scheme I has the lowest total cost of 657 billion with an investment cost of 69 billion and an operation cost of 588 billion. Scheme II has an investment cost of 3 billion more than Scheme I and an operation cost of 5 billion more than Scheme I, for a total cost of 665 billion. Scheme III has the highest investment and operation costs of 73 billion and 595 billion, respectively, for a total cost of 668 billion.