## 2 Coupling Coordination Framework
### Benefit Analysis and Implementation Measures  
Compared to the similar concepts of a BRTGS and VPP, a CGS represents a unique approach that integrates multiple power sources with distinctive characteristics. A BRTGS mainly concentrates on the direct transmission of RESs with TPUs across different regions through high-voltage AC/DC transmission lines. The transmission power in BRTGSs is relatively stable, with infrequent variations. Additionally, they do not prioritize the geographical proximity of the power sources. On the other hand, a VPP emphasizes coordinating Distributed Energy Resources (DERs) like energy storage, controllable loads, and electric vehicles on the demand side to facilitate flexible regulation. A VPP's focus is predominantly on managing demand rather than supply, and the connected DERs are usually distributed across the grid. The distinctions between CGSs, BRTGSs, and VPPs are shown in Table 1; the table highlights the distinct objectives, coordination methods, and functional roles of CGSs, BRTGSs, and VPPs. Compared with the independent operation of RESs and TPUs, a CGS unifies geographically proximate power sources within the power grid. Through efficient internal coordination, the CGS operates as an integrated singular unit, supplying power to the main grid. The key advantages of CGSs include  
* Enhanced accommodation of REs fluctuations: A CGS allows for more effective handling of the inherent fluctuations in RES generation through its internal retrofitted TPUs without the need to consider the intricacies of the power flow or rely on coordination between operators across different regions. * Seamless integration into existing infrastructure: Qualified power sources can be effortlessly aggregated as CGSs use the existing power grid structure. This facilitates a broad adoption with minimal investment requirements for power-source operators. For instance, in the power grid of Liaoning, China, the capacity of RESs and TPUs that can be interconnected at the same PCC exceeds 42% [22]. * Simplified dispatch within the main power grid: Acting as a unified dispatch entity, CGSs can integrate smoothly with the prevailing main power grid. They simplify the scheduling process for the main power grid operator by eliminating the need for the comprehensive modeling of each power source within the CGS. Furthermore, dispatching instructions from the main power grid can be determined without extensive consideration for accommodating fluctuations from RESs. * Decentralized scheduling with high efficiency: The power source scheduling can be divided between the main power grid and CGS levels, significantly easing the modeling and computational complexity. * Enhanced revenue generation from the auxiliary service market: With precise control mechanisms, the TPUs within the CGS can more effectively participate in the DPR \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **CGS** & **BRTGS** & **VPP** \\ \hline Objective & Integration of geographically proximate large-scale RES and TPUs on the generation side & Transmission of large-scale RES and TPUs across the power grid & Management and coordination of distributed DERs or generation sources \\ Coordination Method & Utilization of local PCC & Deployment through the high-voltage transmission line & Primarily via the distribution network \\ Function & Mitigation of local RES fluctuations and provision of flexible support & Point-to-point stable power & Integration of regulatory resources for flexible control \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between CGS, BRTGS, and VPP. auxiliary service. This participation not only fully exploits the operational flexibility of retrofitted TPUs but also boosts the overall economic benefits of the whole CGS. The integration of power sources into the CGS in practical engineering applications can be smoothly accomplished. It necessitates the creation of a coordinated control and optimization management center.