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 the peak of 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.
The grid-connected inverter mainly adopts the grid voltage orientation scheme based on phase-locked loop (PLL) control. Under this control method, the grid-connected inverter can be equivalent to a current source, called current source mode, PLL is a grid-connected synchronization unit with simple structure and stable performance under strong power grid. In addition to PLLs, grid-connected inverters in current source mode usually use grid voltage feedforward to improve the dynamic response performance of grid-connected inverters in the case of grid voltage disturbances, and suppress the grid voltage background harmonics. Grid current harmonics and other issues.
The grid voltage feedforward 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 feedforward 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.
For example, starting from the resonance damping characteristics of grid-connected inverters, the literature analyzes the effect of digital control delay on the resonance frequency, and concludes that the grid voltage feedforward control in a weak grid affects the system stability to a certain extent. The literature analyzes the stability and frequency response of the grid voltage full feedforward strategy under the weak grid. According to the additional feedforward error introduced by the grid impedance, a differential approximation controller is used to ensure that the grid-connected inverter uses the grid voltage. Robustness and dynamic-steady-state performance in feedforward. The literature proposes a scheme that combines proportional grid voltage feedforward control and repetitive control, while retaining the rapidity of proportional control and the steady-state performance of repetitive control, improving system robustness in weak grids.
In fact, the use of traditional voltage feedforward control will introduce a positive feedback channel related to the grid impedance, which will cause the inverter output grid-connected current to appear harmonic resonance, and even tend to become unstable. For this reason, some literatures propose to set different transfer functions on the positive feedback channel from this perspective 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, the literature also proposes a grid-connected point voltage feedforward adaptive control based on the superposition of multiple SOGIs, which ensures the ability to suppress the low-order background harmonics of the grid voltage by adaptively adjusting the number of superimposed SOGIs. Some literatures also consider introducing a weighted proportional coefficient in the grid voltage feedforward to weaken the positive feedback channel introduced by the feedforward, so that the grid-connected inverter has the advantages of partial grid voltage feedforward 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. In a virtual equivalent inductance is introduced into the voltage feedforward channel in series with the phase angle compensation function, which weakens the dynamic iThe grid voltage feedforward 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 feedforward 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.
For example, starting from the resonance damping characteristics of grid-connected inverters, the literature analyzes the effect of digital control delay on the resonance frequency, and concludes that the grid voltage feedforward control in a weak grid affects the system stability to a certain extent. The literature analyzes the stability and frequency response of the grid voltage full feedforward strategy under the weak grid. According to the additional feedforward error introduced by the grid impedance, a differential approximation controller is used to ensure that the grid-connected inverter uses the grid voltage. Robustness and dynamic-steady-state performance in feedforward. The literature proposes a scheme that combines proportional grid voltage feedforward control and repetitive control, while retaining the rapidity of proportional control and the steady-state performance of repetitive control, improving system robustness in weak grids.
Nteraction of proportional feedforward control and improves the adaptability of grid-connected inverters to changes in grid impedance.
In recent years, relevant literatures have been studied on the stability of voltage source mode grid-connected inverters. For example, a power synchronization control is proposed, which enables the grid-connected inverter terminal to achieve inherent synchronization with the grid. To the HVDC link of very weak AC grid systems, this method has proven to be a superior solution. The droop control-based voltage source mode full-power wind power converter’s DC side voltage stability control in weak grids is studied. However, this scheme does not consider the system stability when the grid impedance changes greatly, especially the need to analyze the voltage source in strong grids. System stability of mode grid-tied inverters. From the perspective of small-signal modeling based on sequence impedance, the stability of the voltage source mode grid-connected inverter based on VSG control in high grid impedance scenarios is analyzed. Offline is more stable. By constructing the sequence impedance model of the three-phase grid-connected inverter with droop control, the frequency coupling effect generated by the power synchronization loop is analyzed, and it is concluded that the droop control based on the voltage source mode in the weak grid has better grid-connected characteristics. Furthermore, based on the harmonic linearization modeling method, the overall sequence impedance model based on VSG control is obtained, and the reason for the VSG frequency coupling effect is obtained. By analyzing and comparing the current source mode grid-connected inverter and the voltage source mode grid-connected inverter, the literature finds that the sequence impedance of the former is negative in the middle frequency band, while the latter is inductive in the middle frequency band. The web is more adaptable.
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.
