## 1 Introduction  
With the rapid growth of global renewable energy generation capacity, wind and photovoltaic (PV) power generation pose significant challenges for grid PR and frequency regulation due to their inherent stochasticity [1, 2, 3]. In recent years, thermal power generation and renewable energy have been integrated into a CS entity, combining them both physically and in terms of their external outputs. Regions or power sources that have the conditions to form a CS are widely available. For example, in China's Liaoning power grid, the installed capacity of thermal power and renewable energy that can be integrated into a CS at the same grid connection point exceeds 42% [4]. This integrated approach, with the PCC, has become a key method for promoting renewable energy consumption and participating in deep peak regulation (DPR) [5]. As a novel application scenario in power systems, the CS has become increasingly prevalent in China. Therefore,focusing on the regulated market framework and operational dispatch methodology of China's CSs holds pioneering significance. However, high penetration of renewable energy increasingly requires flexible modifications of thermal power units. Thermal power units face difficulties in simultaneously achieving rapid load changes, a wide range of PR, and lower coal consumption and emissions [6; 7; 8]. On the other hand, the involvement of multiple stakeholders in scheduling complicates the modeling of the coordinated operation of the MCS in the large grid. At present, the allocation of ancillary service revenues in China is regulated and primarily managed by national authorities and the power grid in China. The issues related to the distribution of benefits and costs among the various stakeholders within the MCS emerge during the PR process. To further explore the coordinated operation capabilities of MCSs, studying their optimal operational scheduling within the context of flexible modifications to thermal power units is of practical significance. In promoting renewable energy integration, the flexible transformation of thermal power units forms the foundation for achieving rapid DPR, as well as frequency regulation, in CSs as a whole [9; 10; 11; 12]. Accurately describing the physical characteristics and mathematical models of the transformed thermal power units is crucial for the operation of the CS under high renewable energy penetration scenarios. Currently, the flexible transformation schemes for thermal power units primarily focus on three aspects: low-load stable combustion, rapid startup/shutdown, and fast ramping. Rapid startup/shutdown and fast ramping can adjust the coal-fired plant's output in real time [13]. The study [14] mainly relies on coupling with external devices to achieve improvements in minimum load, start-up time, and ramp rate. The study [6] modified the control scheme for steam temperature and fuel demand, and both flexibility and economic performance were improved when the revised control scheme was adopted. However, they increase the mechanical degradation of the unit and raise additional coal consumption and emissions [15]. In contrast, low-load stable combustion, the use of thermal energy storage in coal-fired plants, is widely adopted. According to [16; 17], transforming retrofitting with an MSTS system can significantly enhance the depth of load-following capability. The study [18] uses internal computer algorithms to simulate power boilers under fast thermal loads to achieve low-load stable combustion. The studies [17; 19] propose achieving internal thermal energy storage by throttling external thermal energy storage of high-/low-pressure heaters, feed water/condensate bypass, and condensate water throttling. However, due to the limitations of storage capacity, it is challenging to achieve RLVs over a wide range of operating conditions.