The traditional power system is based on conventional energy sources such as fossil fuels. The power generation units on the power source side have a large capacity, limited number of units, and are centrally controllable. The main grid structure on the transmission side has a high voltage level and a large transmission scale. The distribution side is dominated by passive networks, and the load is mainly consumed by electricity. The power balance method, safe and stable operation, and control technology developed on the basis of traditional power systems have become relatively mature, and most power systems are in a determined and controllable safe state. With the large-scale integration of new energy sources such as wind and solar power into the power grid, the power supply of the power system has shifted from mainly installing controllable conventional power sources to mainly installing uncertain and weakly controllable new energy sources; Due to the development of distributed and other new energy sources, the distribution network has shifted from a passive network to an active network, greatly increasing the uncertainty on the demand side. The integration of a large number of power electronic components into the system brings about new safety and stability issues as well as power quality problems, posing significant challenges to the power system in both theory and practice.

(Source of article: WeChat official account: China Energy Media Research Institute Author: Jiang Debin (Statistics and Data Center of China Electricity Council))

1、 Challenge: New supply-demand balance and new system stability issues in the power system

The total amount of electricity supply during a certain period should meet the total demand for electricity, and the demand and supply of the real-time power system should always be balanced. In the real-time operation of the power system, it is constantly affected by various disturbance factors such as demand fluctuations, equipment operation and faults, external impacts, etc. The power system should be a stable and balanced system that can autonomously restore stability or transition to a new stable operating state under various interference influences, and can ensure the reliability and quality of power supply throughout the process. This is the problem of safe and stable operation of the power system. Whether it is a traditional power system or a new type of power system, ensuring the balance of electricity and ensuring the safety and stability of the power system are important issues that need to be addressed in the operation of the power system.

The energy transition has changed the nature and conditions of traditional electricity balance and power system security and stability.

In terms of power and energy balance, due to the large capacity and prominent characteristics of each power generation unit in traditional power systems, which are easy to control, adjusting and controlling the power generation units and their upstream in the system based on the idea of power generation following user changes can achieve accurate balance of power and energy at various time scales. The technology of system scheduling management and control is relatively mature. In the new power system, the capacity of new energy generation units is small but the quantity is huge. New energy such as wind and solar power have strong randomness, are difficult to predict, have large fluctuations, and are difficult to track and control. The output is intermittent in the time series, with no light during late peak hours and no wind during extreme heat, making it difficult for the system to adjust peak load. The system balance mechanism has shifted from deterministic power generation tracking uncertain loads to bidirectional matching between uncertain power generation and uncertain loads. This makes it very difficult to accurately ensure supply-demand balance at any time scale.

In terms of power system security and stability, traditional power systems mainly rely on AC for transportation and distribution of electrical energy. The more rotating devices directly integrated into the same system, the greater the system's moment of inertia, and the better the stability and balance characteristics. Small scale external shocks and internal system failures will not have a fatal impact on the power system. In the case of large-scale or continuous shocks, due to the inertia characteristics, it is also easier to take control measures to restore system stability. In the new power system, a large amount of wind and photovoltaic power can be connected to the system through converter devices. As the system capacity increases, the moment of inertia gradually decreases with the gradual withdrawal of traditional power sources, and the safety and stability performance of the power system decreases. The issues of power angle stability, voltage stability, and frequency stability have become more prominent due to the expansion of new energy sources. In addition, power electronic devices have strong nonlinearity, switching, and discreteness characteristics. The application of high proportion power electronic converter equipment in new power systems greatly increases the complexity of traditional power angle stability, frequency stability, and voltage stability mechanisms and operation control. It also brings new stability problems to the power system, such as new oscillation problems caused by the characteristics of power electronic components, voltage stability problems caused by multi fed DC at the load center, as well as power quality problems such as harmonic pollution and voltage flicker. In the process of constructing a new power system, it is not only necessary to technically solve the inertia stability problem caused by the decreasing proportion of traditional high inertia power facilities, but also to develop new stability theories to guide the solution of new stability problems caused by the integration of high proportion new energy and high proportion power electronic equipment.

In the process of transitioning from traditional power systems to new power systems, the issues of new supply-demand balance and new system stability are very prominent. If these problems cannot be well solved, the new power system will not be able to achieve the basic goal of safety and reliability.

2、 Approach: Application of energy storage in solving new supply-demand balance and new stability problems

For the new supply-demand balance problem, the current mainstream approach is to plan and construct sufficient flexible power sources in the system to meet the climbing demand, achieve multi energy complementarity as much as possible to reduce dependence on a single energy source, and strengthen inter grid mutual assistance capabilities, taking necessary measures on the demand side, etc; On the other hand, it is necessary to strengthen system operation management, improve scheduling management mechanisms, enhance prediction and warning levels, optimize scheduling strategies, and improve scheduling technology. These measures can increase the probability of adapting to power balance, but cannot guarantee accurate power balance at all time scales. At the same time, using coal-fired power as the main flexible resource not only incurs high costs, but also faces the risk of resource scarcity and depletion in the long run. The large-scale construction of lithium based short-term electrochemical energy storage also faces key resource constraints, energy density limitations, and insufficient safety and stability.

For the stability issues of new systems, in addition to traditional safety and stability control measures, active support technologies such as virtual inertia control, virtual synchronous machine, and droop control are utilized at the wind and solar power plant side to improve the dynamic support capability of the wind and solar power plant for the power system. On the dispatch side, a smart grid dispatch control system is equipped to integrate and share grid information, improve multi-level dispatch collaboration efficiency, conduct online analysis and evaluation of safety and stability levels, and enhance safety warning capabilities. On the power grid side, it is planned to build a stronger grid structure, construct pumped storage power stations and shared electrochemical energy storage power stations, build high inertia synchronous phase-shifting machines, energy storage synchronous phase-shifting machines, etc. that meet the stability needs of the new power system, or transform retired units into phase-shifting machines. While supporting system voltage, increase system inertia, participate in system frequency regulation, and improve system stability. On the demand side, various application scenarios such as multi energy complementarity and diversified aggregation are implemented to ensure the continuous and safe supply of electricity under high proportion of new energy access, while actively supporting large systems as much as possible.

At present, a complete theoretical system and mature solutions have not yet been formed for the stability issues related to the new power system. But it can be confirmed that the use of energy storage is indispensable for solving both new supply-demand balance problems and new system stability problems.

In terms of power and energy balance, energy storage is used as a flexible adjustment resource to smooth out the randomness, volatility, and intermittency of new energy, achieve peak shaving, cold and hot standby, market regulation, etc. The ability of energy storage to assist in energy time shift is utilized to fill or reduce the deviation (power) between new energy output and electricity load in real time, fill or reduce the deviation (energy) between new energy energy energy and electricity demand energy within a certain period of time, and maintain the real-time power balance and energy balance of the system within a certain period of time.

In terms of safe and stable operation, both active support on the station side and diversified aggregation on the demand side rely on the characteristics of energy storage assisting energy time shift, as well as the control characteristics of various energy storage types. Such as pumped storage and compressed air energy storage, which have large capacity and can provide rotational inertia for the system; Electrochemical energy storage has high conversion efficiency, fast reaction speed, and is easy to achieve rapid control; Molten salt energy storage has a large capacity, long supply time, and multiple utilization scenarios. By comprehensively utilizing various energy storage technologies and coordinating and optimizing control, multi-objective integration of energy storage application functions can be achieved. The system can participate in fast frequency regulation, automatic generation control (AGC), automatic voltage control (AVC), suppression of low-frequency oscillation, suppression of overvoltage, black start, etc.

It should still be noted that although energy storage, especially electrochemical energy storage, is highly valued in the construction of new power systems, some new types of energy storage will also bring more power electronic devices into the system when connected, thereby complicating the stability issues of the power system.

3、 Reflection: Dimensionality Reduction Approach for Dealing with New Balance and New Stability Problems

The issues of new balance and new system stability are essential checkpoints on the journey of building a new power system. The current mainstream approach in the industry is to develop new balance theories to solve new balance problems and new stability theories to solve new stability problems. These theories not only require in-depth work on the mechanisms, probabilities, massive data, complex calculations, and application verification of related issues, but also require accurate predictive analysis and seamless integration in various aspects such as climate and meteorological change, natural environment impact, socio-economic development, and resource element circulation. Even if it is theoretically believed that a complex monitoring system can comprehensively perceive changes in the power system and related influencing factors, and has sufficient online computing capability and speed, in the face of multiple complex systems, massive data and huge computing scales, uncertainty on both sides of supply and demand, as well as the possibility of failure probability of monitoring and control equipment itself, there may still be issues related to system balance and stability in specific engineering practices.

The path of technological development is from simple to complex, from low dimensional to high-dimensional, while the path to solving technical problems is exactly the opposite, which should be to decompose complex problems into simple problems and reduce high-dimensional problems to low dimensional problems. The "dual high" new power system is much more complex and high-dimensional than traditional power systems. To solve the problems of the new power system, it can be considered to transform the new power system into the traditional power system model, and then use the mature theories, technologies, and methods of the traditional power system to handle it, ultimately achieving power balance guarantee and safe and stable operation of the power system.

In this sense, the solution to the problems of new balance and new system stability can be expanded, that is, through active planning and design at the physical level, the new power system can be reduced to a traditional power system, thereby reducing the new balance and new system stability problems to traditional balance and traditional stability problems. Then, the balance theory and methods of traditional power systems, as well as the stability control methods of traditional power systems, can be utilized to achieve the balance of power and energy in the system and the safe and stable operation of the system. The key to this approach is to achieve dimensionality reduction in the new power system through proactive planning and design.

One method is to convert new energy sources such as wind and solar power into other energy sources that can be stored for a long time on site. For example, artificial natural gas (such as methane) can be used as the gas source for large-scale gas-fired power generation, and natural gas generators can be integrated into the power system. Thus, the artificial gas facilities and gas-fired power plant facilities connected to the new energy generation facilities form a long-term energy storage facility. Large scale supporting of such long-term energy storage facilities for new energy power generation bases can reduce the proportion of new energy directly connected to the power system, maintain a reasonable structure of fossil energy and new energy generation in the power system, and ensure the large-scale consumption of new energy while preserving the traditional characteristics of the power system.

The main advantages of this approach are that it simplifies complex problems, has clear concepts, simple technology, and is safe and reliable. The technical difficulty lies in artificial natural gas, which currently has multiple routes, but the cost is still high. If large-scale production can be achieved, the cost can be reduced to an affordable level. At the same time, it is necessary to dynamically study the reasonable proportion structure of traditional energy and new energy in the power system. Secondly, gas turbines can significantly increase the rotational inertia and flexibility adjustment capability of the system, thereby better enhancing the safety and stability of the system. Thirdly, it can significantly reduce the number of power electronic devices directly connected to the power system, effectively reducing the probability of new stability problems occurring. Fourthly, gas is easy to transport and store, and can be directly utilized in terminal energy consumption, which is beneficial for the construction of comprehensive energy systems. Fifth, it can effectively mitigate the risk of primary energy resource shortage, especially the huge risk that coal-fired power may bring as a single backup power source. The sixth is to make sufficient technological reserves for the development and utilization of deep-sea energy resources in the future. Of course, this approach can be parallel and well compatible with the current mainstream research ideas on new balance and new system stability issues.

4、 Scenario: Application concept of using artificial natural gas as a long-term energy storage carrier

At present, there is no unified definition for Long Term Energy Storage (LDES) both domestically and internationally. Some institutions consider energy storage technologies with a discharge duration exceeding 4 hours and a lifespan of not less than 20 years as long-term energy storage, while others define discharge duration as exceeding 8 hours or 10 hours. The author believes that using a discharge duration of more than 10 hours as the definition of long-term energy storage is more reasonable, as this time can basically cover the normal period of a day without light. From this definition, some physical energy storage facilities such as pumped storage and compressed air storage, as well as electrochemical energy storage such as hydrogen storage and molten salt storage, can be classified as long-term energy storage. Convert new energy electricity into hydrogen and further react with carbon dioxide to produce methane, which can then be used to generate electricity through natural gas, co combustion, or direct combustion, achieving an electric carbon cycle. This method is particularly suitable for future energy production models that construct energy bases in the deep sea and supply them back to land, as well as for onshore new energy power generation bases with sufficient water sources nearby.