## 4 Case Study
### Actual Power Grid System in a Certain Region
#### 4.3.2 Analysis of Planning Scheme  
In this actual power grid system, node 23 is the grid connection node for wind power plants, and node 21 is the grid connection node for photovoltaic power stations. The grid connection scale for wind power plants and photovoltaic power stations is the same, with 1000 MW in the first stage, 1500 MW in the second stage, and 2500 MW in the third stage. Other parameters are the same as those in Section 4.3.1. When considering the single planning of transmission without considering the coordination between the transmission network and energy storage, the planning scheme and various costs are shown in Table 5. The temporal planning scheme for coordinated expansion planning of the transmission network and energy storage in the actual power system optimized under the given constraints is shown in Table 6.  
Figure 7: Simplified topology of the actual 23-Bus power system. From the data in Table 6, it can be seen that the nodes where energy storage is configured are mainly nodes 4 and 13. The addition of lines 14-15 is to meet the power plant dispatching needs at node 15, and the addition of lines 21-22 is to meet the new energy dispatching needs at node 22. The cost of solar curtailment in the three stages is 0, because the uncertainty of output of photovoltaic power stations is small, while due to the fluctuation and uncertainty of wind power output, the cost of wind curtailment is not zero. Furthermore, when the wind power plant grid-connected capacity reaches 3000 MW in the third stage, the cost of wind curtailment also reaches its maximum of 1.092 billion yuan. The comparison of data between Tables 5 and 6 shows that in this actual power system, the multi-stage coordinated expansion planning of the transmission network and energy storage greatly reduces the cost of wind curtailment and light compared to the single-stage multi-stage planning of the transmission alone. However, due to the fluctuation and uncertainty of wind power output, the cost of wind curtailment is still high. The cost of the transmission planning in the second stage in Table 6 is 0.08 billion less than that in Table 6, which is due to the replacement of the longer plan of lines 4-5 with the shorter plan of lines 14-15, demonstrating the synergistic effect between transmission expansion planning and energy storage configuration. cost will decrease, and the economic benefits of the multi-stage coordinated expansion planning of the transmission network and energy storage will be further enhanced. 2. The dynamic planning scheme has better economic benefits than the static planning scheme, with a total investment cost reduction of about 30%. The planning scheme of the transmission network and energy storage is also more reasonable, reducing the excessive investment of the power grid assets; the optimal configuration capacity of energy storage is sensitive to its unit cost; the less the transmission capacity of the system, the better the economic improvement effect of the multi-stage expansion coordinated expansion planning of the transmission network and energy storage compared with the single-stage planning of the transmission network. 3. The power capacity and energy capacity of energy storage in the coordinated expansion planning of the transmission network and energy storage are sensitive to its cost coefficient: the power capacity of energy storage increases with the decrease of its cost coefficient, and the role of delaying the upgrade of transmission lines and increasing the absorption of new energy is greater. 4. When the transmission capacity of the power system is reduced to 80% of its original level, the coordinated planning of transmission networks and energy storage saves approximately 9.85% in total investment costs compared to single transmission grid planning. This highlights that the economic benefits of deploying energy storage in-crease significantly in systems where grid capacity is more constrained. However, this study still has some limitations.