With the continuous improvement of the penetration rate of new energy power generation systems such as wind power and photovoltaics, the power grid presents the characteristics of a weak grid or even a very weak grid, which brings serious challenges to the stable and efficient operation of the new energy grid-connected power generation system. The dual-mode control strategy based on grid impedance adaptation combines the advantages of current source mode and voltage source mode in stability, and is more suitable for occasions where grid impedance fluctuates greatly under high permeability. Finally, the above-mentioned typical control strategies are summarized and future research directions are discussed.
In order to solve the problem of environmental pollution caused by traditional fossil energy, new energy power generation technologies such as photovoltaic and wind power have received continuous attention and application. At present, China is the country with the fastest development of new energy power generation and the largest installed capacity in the world. In 2020, the newly installed capacity of wind power and photovoltaic in China will be 71.67 GW and 48.2 GW respectively. During the 75th United Nations General Assembly, President Xi Jinping proposed that China strive to achieve a peak in carbon dioxide emissions by 2030, and strive to achieve the goal of “carbon neutrality” by 2060. To achieve this goal, vigorously developing new energy power generation has become a major strategic demand in China.
High penetration rate of new energy power generation has become an inevitable trend. At present, the proportion of new energy installed capacity in China’s Eastern Mongolia, Gansu and other regions has exceeded that of developed countries such as Denmark and Portugal. However, the high penetration rate also leads to a series of problems such as abandoned wind, abandoned light and frequent grid failures, which brings many challenges to the safe and stable operation of the grid and new energy power generation systems. For example, in December 2012, there was a high-compensation series-compensated transmission line near the grid connection point of the Guyuan Wind Farm in Hebei Province, which caused subsynchronous oscillation. Due to the excessive current, some wind turbines were disconnected from the grid; in July 2015, the Santang in Hami, Xinjiang In September 2015, a bipolar blocking accident occurred in the Jinsu UHVDC system in East China, resulting in an overvoltage fault in the sending end grid, and a severe frequency of 0.41 Hz due to insufficient inertia of the power system. fall. In fact, the occurrence of the above accidents is closely related to the high penetration rate of new energy grid connection. High-penetration new energy power generation often has intermittent, random and output fluctuations, and the equivalent grid impedance of the grid connection point often fluctuates greatly, which makes the power grid show weak or even very weak grid characteristics. System stability and efficient operation bring serious challenges. Therefore, as the core equipment of new energy power generation, the stability of grid-connected inverters when the grid impedance fluctuates greatly has attracted the attention and research of a large number of experts and scholars.
It can be seen that in order to realize a stable new energy grid-connected system, the control design problem of the grid-connected inverter should be fully solved. At present, grid-connected inverters mainly have two basic control modes: current control and voltage control, or current source mode and voltage source mode [1-3]. At present, grid-connected inverters mostly use current source mode. A large number of literatures have analyzed the stability control strategies of new energy power generation units in current source mode, such as virtual impedance control, adjusting feed forward voltage control and adjusting phase-locked loop bandwidth control. However, this type of current source mode control method has many limitations in terms of system voltage and frequency adjustment, and it is difficult to adapt to the scenario where a high proportion of new energy power generation grid-connected inverters are connected. Therefore, some literatures put forward the voltage source mode control of the grid-connected inverter of new energy power generation. It takes the voltage as the control target, so that the external characteristics of the grid-connected inverter can be expressed as a controlled voltage source, which can provide support for voltage and frequency. , more suitable for high-penetration new energy power generation grid-connected occasions.
It can be seen that in order to realize a stable new energy grid-connected system, the control design problem of the grid-connected inverter should be fully solved. At present, grid-connected inverters mainly have two basic control modes: current control type and voltage control type, or called current source mode and voltage source mode. At present, grid-connected inverters mostly use current source mode. A large number of literatures have analyzed the stability control strategies of new energy power generation units in current source mode, such as virtual impedance control, adjusting feedforward voltage control and adjusting phase-locked loop bandwidth control. However, this type of current source mode control method has many limitations in terms of system voltage and frequency adjustment, and it is difficult to adapt to the scenario where a high proportion of new energy power generation grid-connected inverters are connected. Therefore, some literatures put forward the voltage source mode control of the grid-connected inverter of new energy power generation. It takes the voltage as the control target, so that the external characteristics of the grid-connected inverter can be expressed as a controlled voltage source, which can provide support for voltage and frequency. , more suitable for high-penetration new energy power generation grid-connected occasions.
In summary, this paper will summarize and analyze the existing control problems of high-permeability new energy grid-connected inverters, and respectively introduce two typical grid-connected inverter control modes of current source and voltage source, and the development of them. The resulting dual-mode control strategy.
With the advancement of science and technology, the high-penetration new energy grid-connected system will realize the following research plans and strategies with the support of information processing technology, big data technology, artificial intelligence and other technologies.
1) An online penetration estimation technique suitable for multi-inverter systems. In view of the poor timeliness and low accuracy of traditional passive identification of power grid impedance, and the problem of large disturbance to the power grid in active identification, it is necessary to study impedance quasi-passive identification technology based on high precision and low delay. Spectral characteristics and injection timing, effectively improving the interference of equipment identification technology to the power grid are also issues that need to be studied in the future.
2) Form a voltage source mode grid-connected control scheme that is more conducive to practical application. Research the coordinated control method of voltage source mode taking into account the characteristics of MPPT and the state of charge of the energy storage system, charging and discharging mode and response time, etc., and the DC voltage regulation technology to smooth the fluctuation of grid-connected power and maintain the balance of power between the front and rear stages.
3) Research on the hybrid mode control technology of multi-inverter system based on permeability index. The current source mode and the voltage source mode have complementary characteristics in stability. How to reasonably configure two different types of grid-connected inverters in the power grid so that the system can achieve the best dynamic and steady state characteristics is an urgent problem to be solved. Further in-depth research is also required to explore the mixed mode control strategy of voltage source and current source based on the permeability index, and to realize the optimal voltage source mode minimum capacity ratio based on the stability margin and the utilization rate of new energy.
4) Research on collaborative control of hybrid mode multi-grid converter system based on artificial intelligence optimization algorithm. Research the collaborative control strategy of hybrid mode grid-connected converter system based on artificial intelligence algorithm, including the placement of two different control structures in the power grid and the rational design of grid planning in current source mode and voltage source mode, so as to reduce the system Instability risk is an important research content in the future.
5) Energy interconnection technologies such as interconnection and data sharing among various devices and platforms with high penetration rate of new energy grid-connected. Based on 5G and other data communication technologies, it integrates the functions of data collection, scheduling and coordination of new energy grid-connected inverter equipment, connecting power generation, transmission and users, and realizing a data sharing system between platforms.
