## 2 State-of-the-Art and Challenges of China Power System  
### Current Dispatch and Control System  
From the perspective of scale and stability characteristics, the China state grid is becoming the largest and most complicated power system in the world. Up to the summer of 2020, the interprovincial transmission power of the state grid has reached 72 million kW. Moreover, the peak load of the power system in China has exceeded the load of most developed countries, and accounted for about 40% of the load of European Union. Hence, there are great difficulties in the dispatch and control of the power system in China. The dispatch system of the China state grid can be divided into the national, sub-national, and provincial levels. More specially, the dispatch system follows the component-based and service-oriented rule and is established with the aim of horizontal integration and vertical connection. The horizontal integration of the dispatch system can be demonstrated in Figure 1. The original supervisory control and data acquisition (SCADA), energy management system (EMS), hydropower automation system, electric energy metering system, wide-area measurement system (WAMS), dispatch schedule system, weather map system, lightning location system, dynamic security alarm system, dispatch management system, fault wave record information system, and so on more than 10 independent applications have been integrated into the dispatch system. Moreover, the bottom level of the dispatch system is the integrated supporting platform that contains three security regions. The real-time monitoring and alarming applications are deployed in the security area I. The applications of dispatch scheduling and security checking lie in the security area II. Furthermore, in the security area III, the applications of dispatch management are deployed. The vertical connection of the dispatch system can be shown in Figure 2. The national, sub-national, and provincial levels of the dispatch system should keep integrated operation. The virtual private network (VPN) is used to connect the different levels. More specially, the security area I of each level is connected via the real-time VPN of dispatch data network. In addition, the security area II of each level is connected by the non-real time VPN of dispatch data network. The VPN of the enterprise integrated data network is utilized to connect the security area III of each level. On the other hand, the power system is a large-scale, nonlinear, and time-varying system, which is depicted as differential-difference-logical-algebraic equations. Moreover, the severity and position of underlying disturbances are probabilistic. Thus, it is necessary to ensure the security and stability of power systems and counteract probabilistic contingencies. The security and stability control system of the state grid can be divided into three defense-lines [32], as shown in Figure 3. The first defense line is designed to deal with the  
Figure 1: Schematic diagram of horizontal integration of the dispatch system in China. Figure 2: Schematic diagram of vertical connection of the dispatch system in China. minor disturbances, and ensure the connection of power sources and loads, in which the preventative control measures are deployed. For example, the topology of power systems can be enhanced during the planning and construction periods. Under normal condition, the unit commitment can be carried out to increase the adjusting ability for active and reactive power. Moreover, the power system stabilizer (PSS), parallel and series capacitance compensation, dynamic reactive power compensation device (SVC), static reactive power compensator (STATCOM), and so on technologies can be utilized. After faults occurring, the relay protection devices isolate the faults from the grid promptly, which can expand the stability domain of power systems.