Research on building a new power system framework with new energy as the main body

I. Introduction

The proposal of the goal of carbon peak and carbon neutrality is a major strategic decision of the state, which bears on the sustainable development of the Chinese nation and the building of a community with a shared future for mankind. In the process of clean and low-carbon energy consumption, electricity occupies the dominant position of the energy system, and the development of the power system is facing a difficult task. Considering the endowment of various types of non-fossil energy resources in China and the technical economy of development and utilization, vigorously developing new energy is an inevitable choice. Building a new type of power system with new energy as the main body is not only an inevitable requirement of energy and power transformation, but also an important way to achieve the goal of carbon peak and carbon neutrality.

Energy and power industry technology capital intensive, there is a high degree of path dependence, technical route trial and error cost is very high. The construction of a new power system with new energy as the main body is a complex systematic project, which should be carried out in advance, a comprehensive analysis of the changes and challenges brought by the change of power production structure to the power system, an in-depth study of the path of low-carbon power transformation and major issues in the transformation process, and strive to form a broad consensus on key issues such as technical form and technical direction.

In view of this, this paper starts from the aspects of primary energy, power supply, network, load, balance mode, etc., studies the profound changes that will take place in the material and technical basis of the power system, and discusses the direct challenges faced by the future development of the power system from the aspects of reliable power supply, new energy consumption, and safe operation of the grid. This paper expounds the connotation, construction principles and ideas of the new power system, divides the development stages of the new power system, and puts forward strategic development suggestions in order to provide basic reference for the medium and long term development of the power industry.

2. Changes brought about by power system transformation

The process of achieving the goal of carbon peak and carbon neutrality in the power system is accompanied by the transformation and upgrading of the traditional power system to the new power system with new energy as the main body, and the relevant material base and technological base continue to undergo profound changes.

The first is a change in energy characteristics. The main body of primary energy in the power system changes from fossil energy that can be stored and transported to wind and solar energy resources that cannot be stored or transported and are related to meteorological environment, and the primary energy supply faces high uncertainty.

The second is the power supply layout and function changes. According to the distribution of wind and solar energy resources in China, the development of new energy will be centralized and decentralized, and the overall access location of the power supply will become more remote and deeper. In the future, as the main power source, new energy will not only be the main provider of electric power, but also have a considerable degree of active support, regulation and fault crossing and other "network" capabilities; Conventional power functions are gradually shifting to regulation and support.

The third is the change of network scale and form. The overall pattern of load center transmission in the eastern central region of the large clean energy base in the western and northern regions remains unchanged, and the scale of the grid will be further expanded in the near future. The form of power grid has changed from AC-DC hybrid power grid to microgrid and soft and direct power grid.

The fourth is the change of load structure and characteristics. Energy consumption is highly electrified and demand for electricity continues to grow. Distribution network is active, multi-energy flexible conversion, "production and consumption" widely exist, load from a single power to power generation/power integration direction change, regulation support capacity is enhanced.

The fifth is the change of power grid balance mode. Both the supply and demand sides of the new power system are faced with greater uncertainty, and the power balance mode has shifted from the power generation/power consumption balance of "source with load" to the balance in a larger space and a larger time scale in which energy storage and multi-energy conversion participate in the buffer.

The sixth is the change of power system technology foundation. The power grid connection technology has changed from AC synchronization to power electronics, and the synchronous operation mechanism of AC power system has changed from physical characteristics to human control algorithm. The microsecond switching process is introduced in power electronic devices to analyze and recognize the transition from electromechanical transient to electromagnetic transient. Operation control changes from centralized continuous control of large capacity homogenized units to discrete control of wide area massive heterogeneous resources. The fault defense system is transformed from the independent "three lines of defense" to the active and comprehensive defense system that extensively mobilizes the controllable resources of source network, load and storage.

3. Problems and challenges faced by the power system

(1) Ensuring the supply of electricity

First, the basic theory of ensuring adequate supply is facing challenges. Under the background of global climate change and large-scale development of renewable energy, the renewable energy resource endowment will change significantly in the long-term evolution process. The planning decision of power supply and network is faced with double uncertainty of resource endowment and operation and has obvious path dependence. The above characteristics bring great challenges to the traditional resource endowment assessment and planning theory.

Second, it is difficult to ensure the supply of new energy when it is small. With the rapid development of new energy power generation, the proportion of controllable power sources has declined, the characteristics of new energy "large installed capacity, small power" have become prominent, and the difficulty of ensuring power supply when wind and solar power are small has increased. In the carbon neutral stage, the proportion of thermal power will further decline, the installed scale of new energy will continue to increase, while the load will still maintain a certain growth, real-time power supply and medium - and long-term power supply security difficulties are more prominent.

Third, it is more difficult to ensure supply under rare sky events and extreme weather. Solar eclipse and other rare astronomical phenomena will significantly affect the output of new energy; With the intensification of global warming and climate anomalies, extreme weather events such as hurricanes, snowstorms and freezing, and extremely hot and windless weather are increasing and increasing, exceeding the existing cognition. Rare sky events and extreme weather have the characteristics of small probability, high risk and great harm, which have great influence in the scenario of high proportion of new energy, pushing up the cost of power supply guarantee.

(2) System balance adjustment

First, the basic theory of supply and demand balance is facing challenges. With the continuous increase of the proportion of new energy, both the supply and demand sides and the system regulation resources present a high degree of uncertainty, and the system balance mechanism has changed from "uncertain power generation tracking uncertain load" to "two-way matching between uncertain power generation and uncertain load". The operation characteristics of both sides of supply and demand are highly dependent on external conditions such as climate, so the theory of supply and demand balance established for traditional power systems needs to be developed and perfected.

Second, intra-day adjustment is facing greater difficulties. The random fluctuation of new energy output needs to be offset by the deep adjustment capacity of controllable power supply. The existing adjustment capacity of the power system has been basically tapped out, and more adjustment capacity is still needed in the near future to meet the consumption demand of new energy. After the long-term new energy has become the main power supply, it is difficult to meet the daily consumption demand by relying on the conventional power supply with a declining proportion and the limited load side adjustment capacity.

Third, the demand for long-term seasonal adjustment has increased. There is a seasonal mismatch between new energy power generation and electricity consumption, and the new energy output at the peak of summer and winter power consumption is lower than the average level, while the electricity consumption level at the spring and autumn new energy is at the lowest level throughout the year. The existing energy storage technology can only meet the daily adjustment demand, and the seasonal consumption contradiction will be more prominent under the scenario of a high proportion of new energy.

3. Safe and stable operation

First, the basic theory of stability is facing challenges. The new energy time-varying output leads to the rapid migration of system operating points, and the traditional Lyapunov stability theory based on a given equilibrium point has inadaptability. The synchronous mechanism and dynamic characteristics of new energy power generation are different from those of conventional units, so the classical definition of transient power Angle stability is no longer applicable. The high proportion of power electronic equipment leads to the system dynamic characteristics of multi-time scale interweaving, control strategy dominance, switching and discretization, which makes the corresponding transition process analysis theory and the basic theory of coordination with non-power frequency stability analysis need to be improved.

Second, the basic theory of control needs to be innovated. The control resources of traditional power system are mainly homogenous large capacity equipment such as synchronous generator. In the new power system, a large number of new energy and power electronic devices are connected from various voltage levels, and the control resources are fragmented, heterogeneous, black-box and time-varying, making the traditional model-driven centralized control difficult to adapt, and new basic control theories are needed to effectively implement the aggregation and regulation of various resources.

Third, traditional security problems persist for a long time. In the future for a long time, the power system will still be dominated by AC synchronous network. However, with a large number of new energy sources replacing conventional power supplies, the fundamental elements of maintaining the security and stability of the AC power system are weakened, and the stability problem of the traditional AC power grid is aggravated. For example, rotating equipment is replaced by stationary equipment, system inertia no longer increases with scale or even decreases, and the frequency control of power grid is more difficult; The voltage regulation ability is decreased, the voltage control in the high proportion of new energy access areas is difficult, and the dynamic reactive power support capacity in the high proportion of power receiving areas is insufficient. The electromagnetic transient process of power electronic equipment has a profound effect on the rotor motion of synchronous motor, and the power Angle stability problem is more complicated.

Fourth, the power system with a high proportion of power electronics and a high proportion of new energy (" double high ") faces new problems. In the near future, new energy units have the common vulnerability of power electronic equipment. In the face of frequency and voltage fluctuations, it is easy to go off-grid, and the fault evolution process is more complicated, which is superimposed with the further expansion of long-distance transmission scale, resulting in an increase in the risk of large-scale power outage. The decrease in the proportion of synchronous power supply and the lack of support capacity of power electronic equipment lead to new stability problems such as broadband oscillation, and the power system presents complex characteristics of multi-instability mode coupling. In the long term, a higher proportion of new energy or even all-power electronic systems will bring new stability problems.

(4) Overall power supply cost

New energy parity online is not equal to parity utilization. In addition to the cost of the new energy station, the cost of new energy utilization also includes system costs such as flexible power supply investment, system adjustment operation cost, large power grid expansion and reinforcement investment, network connection and distribution network investment. Research at home and abroad shows that after the penetration rate of new energy power exceeds 10%~15%, the system cost will enter the critical point of rapid growth, and the future cost reduction of new energy stations is difficult to fully offset the system cost increase of new energy. With the gradual increase of the penetration rate of new energy generation, the system cost increases significantly and the dredging is difficult, which will inevitably affect the power supply cost of the whole society.

Fourth, the connotation, construction principles and ideas of the new power system

To cope with the problems and challenges faced by the power system, a new type of power system should be constructed scientifically to ensure the implementation of the national energy transformation strategy. To build a new type of power system with new energy as the main body, we must adhere to the system thinking, follow the technical characteristics and objective laws of the power system, make full use of mature technologies, stock systems and further tap the potential, "open and inclusive" support the development of new technologies, and actively, steadily and step by step to realize the transformation.

(1) The connotation of the new power system

The new power system takes new energy as the main supply, meets the growing demand for clean electricity, and has a high degree of safety, openness and adaptability.

In terms of security, the coordinated development of power grids at all levels in the new power system, the integration of various power grid technologies, and the optimization of wide-area resource allocation capabilities have been significantly improved; The power grid security and stability level is controllable, controllable and in control, effectively carrying a high proportion of new energy, DC and other power electronic equipment access, to meet the needs of national energy security, reliable power supply, and safe operation of the power grid.

In terms of openness, the grid of the new power system is highly diverse, open and inclusive, compatible with all kinds of new power technologies, and supports the convenient access needs of all kinds of new equipment. It supports the interactive transformation of various energy sources and the two-way interaction of new loads, and becomes a hub for the organic interconnection of various energy networks.

In terms of adaptability, the various links of the source network, load and storage of the new power system are closely connected and coordinated, and through the application of advanced technology and the expansion of the control resource pool, strong flexible adjustment ability and highly intelligent operation control ability are achieved, and the application scenario of extensive access and intensive interaction of massive heterogeneous resources is adapted to.

(2) New power system construction principles

Adhere to the problem-oriented, goal-oriented and scientific development principles to build a new power system, and actively and steadily promote the transformation.

Problem-oriented, that is, seize the main contradictions in the process of new energy development, take into account current difficulties and long-term challenges, and break through the bottleneck of new energy development through system reconstruction, technology and institutional innovation.

Goal-oriented, that is, to achieve the target of carbon peak and carbon neutrality on schedule as the mission, choose the appropriate technical route, "reverse" development path, and take into account energy and power security in the transformation process.

Scientific development, that is, fully consider the energy and power industry assets, capital, technology intensive, path dependence is strong, effectively reflect the technical characteristics of the power system and the law of development, to maintain gradual transition transformation and development.

(3) New power system construction ideas

Based on the above construction principles, research and judgment of the future new power system technology form, shape the network form and balance mode to adapt to the new power production structure, match the power supply and demand in space and time, and design a technically feasible and cost appropriate development path accordingly.

1. Technical form

In the future for a long time, the power system will still be dominated by AC technology, the main reasons are: first, the current national power system asset scale of more than 16 trillion yuan, 90% of the installed capacity of coal power in operation less than 20 a, the huge stock system is still based on AC technology, it is impossible to "brake" "sharp turn"; Second, the installed capacity and the proportion of power generation of synchronous power sources such as thermal power, hydropower and nuclear power are declining in the future, but they still occupy a considerable proportion (see Figure 1). For example, by 2060, synchronous power sources are expected to still occupy 25% of installed capacity and 44% of power generation, mainly operating in a "large start-up, small output" mode (the output proportion can reach 79%). Provide necessary regulation and support for the power system. Therefore, the future power system will develop in the inheritance, and maintain the technical form based on alternating current for a long time, and the basic principles and technical requirements will not be fundamentally changed; Ac power grid is still the grid basis of power system, all kinds of power directly or indirectly through AC technology into the grid.

2. Network configuration

First, AC/DC interconnection as a large power grid backbone. The basic national conditions of the inverse distribution of energy resources and demand in China, the randomness of new energy output, and the strong temporal and spatial correlation all determine that the recent AC-DC interconnection grid still needs to expand in order to meet the needs of long-distance large-scale transmission and cross-provincial/cross-regional consumption balance of new energy.

Second, multiple networking modes coexist. Ac power system needs the support of synchronous power supply, and it is difficult to adapt to local scenarios such as centralized development of new energy, offshore wind power, and large-scale distributed new energy access. We should encourage the development of distributed microgrids, pure DC power systems and other networking technologies, and choose technical routes according to local conditions.

3. Balance your form

Strive to use energy storage as a medium to gradually realize the development of electrolytic coupling. The real-time balance of the current power system depends on the conventional power supply with adjustable output, while the new power system will take the new energy power generation with unadjustable output as the main body, and the adjustment capacity of the generation side will be significantly reduced. It is necessary to fully tap the adjustment capacity of the load side by means of demand response and multi-energy complementary, synchronously develop secondary energy (energy storage) that can efficiently and bidirectional conversion with electric energy and can be stored in large quantities and long-term, so that the real-time balance of "power-use" becomes the real-time balance of "power-storage-use".

4. Development path

Build a new power system step by step. The energy and power industry is technology-intensive, and the huge stock assets that have been formed cannot be "knocked down and started over", and it is suitable to adopt a gradual and transitional development mode. In the near future, the demand for the rapid development of new energy is more urgent, and there is an urgent need for mature, economic and effective technology and product solutions to meet the corresponding challenges. In the long term, the current material and technical basis of the power system is difficult to match the needs of the new power system, and breakthroughs should be made in large-scale energy storage, efficient electrohydrogen conversion, CCUS (carbon capture, utilization and storage), pure DC networking and other disruptive technologies as soon as possible. Different technologies will lead to different power system forms, and there is great uncertainty in the future development path. To this end, in the near future, we should focus on tapping the potential of mature technologies, supporting the rapid development of new energy, and simultaneously carrying out disruptive technology research; In the long term, after the breakthrough of disruptive technology, the power system will be gradually transformed to adapt to the new form of disruptive technology.