## 1 Introduction
The implementation of TGC has significantly enhanced the attractiveness of investment in the renewable energy market, promoted the sustained growth of installed capacity for renewable energy generation, and facilitated the transition of electricity production from fossil fuels to renewable energy [3; 12; 13]. In addition, applying TGC has a significant effect on carbon reduction. Through the calculation of historical data, it was found that there is a positive correlation between the activity level of the TGC market and carbon emissions reduction. When the TGC trading volume increases by 1%, carbon emissions can be reduced by approximately 0.8%to 1.3% [14]. This result highlights the significant importance of TGC in promoting the long-term sustainable development of the electricity market at the economic level. However, the use of TGC also has certain limitations, as some companies report carbon emissions reductions by purchasing TGC and exaggerate the effectiveness of its widespread use in reducing carbon emissions [15; 16]. CET refers to the mechanism of market-based buying and selling of carbon emission quotas among emission entities under the premise of setting a carbon emission cap [17]. CET uses market price signals to guide enterprises in purchasing or selling carbon emission rights based on their needs, achieving overall emission reduction goals at the lowest cost [18]. CET has the dual advantages of cost savings and reduction of carbon emissions. It has been welcomed by developed countries and received attention from the academic community. In the past 20 years, the United States has paid more attention to CET policies than other energy-saving and carbon-reduction policies [19]. Australia also attaches great importance to CET policies and explains that they can improve performance by reducing carbon emissions and lowering costs [20]. Scholars have conducted extensive research on the effectiveness of CET, proving that CET policies have demonstrated their advantages in different pilot cities [21; 22]. Carbon emissions reduction can be improved by reducing the total quota of CET and increasing its trading price [23; 24]. However, the large-scale implementation of CET to reduce carbon emissions may trigger a series of negative economic impacts [25]. In the European Union, the increase in CET prices has led to issues such as rising consumer prices and rising unemployment rates [26]. Therefore, it is crucial to explore the optimal policy design for CET. With the increasing demand for carbon reduction in society, academia and policymakers are paying more and more attention to the interactive relationship between TGC and CET, exploring sustainability under the synergistic effect of the two. On the one hand, TGC increases the proportion of renewable energy generation through price signals, thereby reducing the unit carbon emission intensity of the electricity market. On the other hand, CET imposes economic constraints on enterprises by limiting their total carbon emissions, guiding them to optimize their energy structure and promote the low-carbon transformation of the entire society. The combination of TGC and CET can compensate for shortcomings in implementing a single policy, promote low-carbon transformation in the energy sector, and help alleviate fiscal pressure, effectively helping developing countries achieve their carbon reduction goals [2; 27]. As the world's largest carbon emitter, China's total carbon emissions in 2023 ranked first in the world, reaching 12.6 billion tons, an increase of 4.7% [1]. China's power industry accounts for over 40% of carbon emissions [27; 28]. To accelerate the sustainable development of the electricity market, the Chinese government established a TGC pilot in 2017 and proposed a trading mechanism for TGC in 2021, launching the CET market nationwide. Considering the relatively short implementation time of TGC and CET, their role and effectiveness in the sustainable development of China's electricity market are still unclear.