Under the dual carbon target, the load structure of the new power system is more diversified, the two-way interaction of users is deeper, and the load characteristics are more complex, which is bound to put forward higher requirements for the operation analysis and control methods of the traditional power system. Based on the trend of load change of new power system, this paper simply discusses the influence of load change of new power system on power system analysis, operation and control.
Trend of load change of new power system
The load structure is more diversified. Driven by the dual carbon goal, the load structure of the new power system will be more diversified, and the development strategies of "replacing oil with electricity" and "replacing coal with electricity" will be implemented successively. Electric power products represented by new energy vehicles and electric heating will gradually occupy the market of traditional high-emission products. According to the latest statistics of China Automobile Association, by the end of May 2021, the number of new energy vehicles in China reached 5.8 million, and the growth rate of production and sales of new energy vehicles is expected to remain above 40% in the next five years. On the other hand, the central financial support for clean heating reform policies such as "replacing coal with gas" and "replacing coal with electricity" continues to increase, breaking the traditional rule of "setting electricity by heat", promoting the consumption of new energy and making it possible for heat load to participate in the demand side response. The strong development of these energy substitute products is bound to affect the future load curve of the power system.
User two-way interaction is more in-depth. At present, China's energy consumption side of energy efficiency and electricity is relatively low, the interaction between users and energy system is insufficient. The new power system relies more on clean energy with strong randomness of output, and the flexible regulation ability of the power generation side is reduced. Therefore, energy storage construction needs to be vigorously developed and the regulation potential of the user side should be deeply explored. With the continuous emergence of new loads such as electric vehicles, the promotion and application of user-side distributed energy storage, and the continuous improvement of spot trading mechanism in the power market, improving the interaction level of power grid supply and demand is the objective requirement and necessary foundation to realize the efficient operation of new power system. Flexible and in-depth supply-demand interaction will change the load form of the new power system: the access of distributed energy storage will transform users from consumers to prosumers, and the load is no longer a single flow distribution, but a two-way energy interaction on the grid side.
Load characteristics are more complex. The high degree of electronization is one of the typical characteristics of the new power system, which is not only reflected in the change of the dynamic characteristics of the generation side power supply, but also presents more and more complex load characteristics of electronization. In order to meet users' higher requirements on reliability, convenience and efficiency, the user side will interact with the power grid side more and more, and the user interface will increasingly rely on power electronic equipment with higher auxiliary control performance, such as electric vehicle charging station, rail transit traction system, office building frequency conversion refrigeration system, etc. Similarly, in order to adapt to the rapid update and plug and play requirements of "source-network-load" equipment of the new power system, comprehensive solutions based on power electronics technology are more likely to be adopted in future distribution network infrastructure construction, such as DC distribution network, microgrid and cloud energy storage. These changes are bound to make the load side gradually move towards high electric power electronization and make the load characteristics of urban distribution network more complicated.
Discussion on the influence of new power system load change
Load modeling is complex. Load model is one of the basic tools to analyze the dynamic behavior of power system, which has different effects on transient stability, small disturbance stability and voltage stability. The load characteristics of the new power system are more complex, which is different from the traditional load, resulting in the decline of the description ability of the existing load model. The main performance is as follows: 1) The access of flexible loads, such as electric vehicles, brings greater load volatility, time variability and randomness, and increases the difficulty of selecting load model parameters; 2) The user load on the distribution network side has the inherent characteristics of scattered space-time distribution and small single regulating capacity, which is not conducive to the dynamic aggregation of large-scale load; 3) The traditional load model lacks consideration of the nonlinear characteristics of power electronic load, which affects the accuracy of voltage and frequency stability analysis results of power system. Therefore, under the trend of new power system load change, the traditional power system load modeling method needs to be improved and optimized.
Load forecasting is difficult. In the electric power market environment, load forecasting is the basis of generating plan, trading plan and dispatching plan, and plays an important role in the electric power system. With the great promotion of electric vehicles and other new loads in the future and the deepening of multi-user interaction, the randomness and uncertainty of loads increase and the difficulty of load prediction increases. Different load types require more targeted and refined load forecasting methods. The user's power consumption behavior is more complex, forming a multi-dimensional coupling relationship in environmental, social, economic and other aspects, and it is difficult to obtain accurate characteristics and distribution rules of user side power load. Therefore, in the context of new power system load, future load prediction can combine model-driven and data-driven methods, and give play to the mutual assistance of big data, cloud computing, artificial intelligence and other new technologies.
Ultra high harmonic injection. Different from conventional loads, power electronic loads have nonlinear impedance characteristics, which can easily cause the sine wave distortion of AC power grid, and affect the power quality of user side and even the stable operation of power grid side. Semiconductor technology is the key technology and realization basis of electric power electronization. With the improvement of switching frequency of semiconductor devices connected to power grid, the harmonics injected into power grid by converter extend to the direction of high frequency, and the power quality problems caused by ultra-high harmonics will be more and more. With the development of semiconductor technology, ultra-high order harmonics are rapidly becoming a new type of power quality problem, which needs to be paid more attention to, and its generation mechanism, propagation law, measurement methods and standards should be further studied.
Broadband oscillation problem. With the improvement of electric power electronization, electromagnetic oscillation caused by power electronic equipment becomes more and more prominent. The electric power electronization of the load in the distribution network enlarged the oscillation frequency range of the power system, which has the characteristic of "wide frequency". "Wide-band oscillation" refers to the continuous oscillations in a wide frequency range (from a few Hertz to thousands of Hertz) caused by the multi-time scale interaction of a large number of isomerized power electronics devices, starting with a small signal negatively damped instability. At present, the academic circle has not formed a unified understanding of broadband oscillation. In the context of the new power system, broadband oscillation will show new characteristics such as diversified forms, time-varying heterogeneity and wide area propagation. How to scientifically quantify and effectively manage it is an urgent topic to be explored in the future power system stability analysis.
Network protection challenges. The load side of the new power system will present large-scale "grid-user" supply-demand interaction, which not only maximizes the energy efficiency on the user side, but also provides more flexible resource allocation space for the grid side. However, large-scale bidirectional energy interaction will change the form of distribution network, and the flexible distribution network composed of microgrid and DC distribution network will bring new problems to the distribution network protection system, mainly manifested in: 1) the bidirectional flow of power flow on the load side of distribution network affects the sensitivity of relay protection devices; 2) The traditional centralized protection architecture is not conducive to the expansion of supply and demand interactive resources with multiple flexible distribution points and wide areas; 3) The complex power flow environment will make the distribution automation system more dependent on the reliability of communication network. Therefore, under the background of flexible interaction of load-side resources of new power system, distribution network protection system will face multiple challenges.
