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New energy equipment—Grid-connected inverter

Summary

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. By reviewing the achievements of typical new energy grid-connected inverter stabilization control strategies, the basic principles and research status of current source mode, voltage source mode and dual-mode control are summarized, and the current source based on phase-locked loop control is pointed out. Although the mode grid connection can ensure the system stability and the rapidity of power control under the strong power grid, but this type of control method has many limitations in stability, system voltage and frequency adjustment, etc., and it is difficult to adapt to the high proportion of new energy power generation units. scene. The voltage source mode control based on power self-synchronization control can provide support for voltage and frequency, and is more suitable for weak grid occasions where high-penetration new energy power generation is connected to the grid; the dual-mode control strategy based on grid impedance adaptation combines the current source at the same time. The advantages of the mode and the voltage source mode in terms of stability are more suitable for occasions where the grid impedance fluctuates greatly under high permeability. Finally, the above-mentioned typical control strategies are summarized and future research directions are discussed.

Power-system

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.

solar-energy-generation

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. 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.

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.

The grid voltage feed forward control can offset the influence of the background harmonics of the grid voltage on the grid-connected current. At the same time, it is widely used because of its advantages of suppressing the inrush current when the grid-connected inverter starts and reducing the steady-state error of the grid-connected current. . Under the weak power grid, the voltage feed forward link of the current source mode grid-connected inverter will reduce the system stability margin, causing problems such as harmonic resonance of the grid-connected current, and deteriorating the power quality.

Grid-connected-inverter

In fact, the use of traditional voltage feed forward control will introduce a positive feedback channel related to the grid impedance, which will cause harmonic resonance in the output grid-connected current of the inverter, and even tend to become unstable. For this reason, from this perspective, some literatures propose to set different transfer functions on the positive feedback channel to weaken the influence of the positive feedback and improve the stability of the grid-connected inverter. For example, it is proposed to add a second-order generalized integrator (SOGI) centered on the fundamental frequency on the grid voltage positive feedback channel. Since the phase margin of the grid-connected inverter is compensated by SOGI, the system stability is improved, but the suppression effect on the low-order background harmonics of the grid voltage is correspondingly weakened. Furthermore, a feed forward adaptive control of grid-connected point voltage based on the superposition of multiple SOGIs is proposed, and the ability to suppress low-order background harmonics of the grid voltage is ensured by adaptively adjusting the number of superimposed SOGIs. Some literatures also consider introducing a weighted proportional coefficient in the grid voltage feed forward to weaken the positive feedback channel introduced by the feed forward, so that the grid-connected inverter has the advantages of partial grid voltage feed forward and improves the system stability. The literature analyzes the weighted proportional coefficient scheme, and points out that the weighted proportional coefficient needs to set a certain value range when it satisfies the stability criterion and the minimum damping ratio. A compromise is required between dynamic performance. Then, the virtual equivalent inductance is introduced into the series phase angle compensation function in the voltage feed forward channel, which weakens the dynamic interaction of the proportional feed forward control and improves the adaptability of the grid-connected inverter to the change of grid impedance.

In the case of high-penetration new energy power generation, the stability problems such as the resonance of the new energy power generation unit are also becoming more and more significant. With the joint efforts of experts and scholars, a large number of theoretical scientific research results have been obtained. This paper focuses on the research status of current source mode, voltage source mode and dual mode of grid-connected inverter equipment, and discusses and summarizes possible future research directions and ideas. It is hoped that this paper can provide a useful reference for the future high-penetration new energy grid-connected inverter control technology, promote China’s new energy grid-connected converter technology innovation and sustainable energy development, and meet China’s major energy interconnection strategy.

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