## 2 State-of-the-Art and Challenges of China Power System
### Underlying Challenges  
As mentioned above, the dispatch and control system is useful for the economic and stable operation of the power system in China. However, with the fast development of RESs and DESs, the composition, structure, and operation characteristics of power systems are changing. The above dispatch and control system is facing great challenges, which hinder the accommodation of RESs and impair the security and stability of power systems. In summary, the underlying challenges can be listed as follows. 1. The fluctuant and uncertain characteristics of RESs. The wind and solar energies are the two main kinds of RESs, which have the inherent fluctuant and uncertain characteristics. With the increasing penetration of wind and solar energies, the power system should not only meet the fluctuant and uncertain loads but also take additional measures to deal with the output power of RESs. More and faster spinning reserves are needed to handle the double-side fluctuations and uncertainties. More specially, as analyzed in our earlier work [1,33], the hourly ramp rates of wind and solar energies can come up to 0.5393 pu/h and 0.3449 pu/h, respectively, and the corresponding spinning reserves should be deployed to smooth the net ramp of RESs and loads. Moreover, the complementarities between wind and solar energies can be used to reduce the fluctuations and uncertainties, but the rational utilization forms, planning  
Figure 3: Schematic diagram of three defense-lines. schemes, and control strategies should be considered thoroughly. The limited spinning reserves and adjustment capability restrict the accommodation of RESs and compel the power system close to the boundary of stability region. Thus, there is a conflict between the continuous accommodation of RESs and keeping the security and stability of power systems. Moreover, how to securely accommodate the fluctuant and uncertain RESs becomes a challenge. 2. The low- or no-inertia characteristics of non-synchronous RESs. As the power electronic technologies develop, the ratio of power sources with electronic interfaces in power system increases promptly. Although the power electronic devices have high controllability, the power sources usually operates in asynchronous mode. The output active power and reactive power of inverter-based sources are usually fixed values, considering the uncontrollable characteristics of their primary energy resources. There is no frequency or voltage response from these power sources, which leads to the decrease of equivalent inertia and control capability of the power system with high ratio of power electronic devices. Furthermore, the decrease of equivalent inertia brings about more drastic frequency dynamic response when the power system is subjected to the same disturbances, as shown in Figure 4.  
In the analysis, the generating capacity of thermal units is assumed to be replaced by the inverter-based power sources, so the equivalent inertia \(H\) of the studied system decreases. \(H_{0}\) is the initial inertia of the studied system without inverter-based power sources. From Figure 4, it is obvious that the magnitude of rate of change of frequency (ROCOF) increases and the frequency nadir decreases as the equivalent inertia \(H\) decreases. Moreover, because the control capability of the studied systems also decreases, the quasi-steady frequency deviation grows. To a certain degree, the integration of power electronic devices decreases the equivalent inertia, and aggravates frequency stability of power systems, and the RESs usually use the inverter-based integration form. Thus, how to adjust the equivalent inertia of power systems with high ratio of power electronic devices is also a challenge. 3. The low-observability and controllability characteristics of DESs. In the above dispatch and control system, the centralized RESs are monitored, dispatched, and controlled. However, there is low observability and controllability of DERs  
Figure 4: Frequency dynamic response with different equivalent inertia \(H\).