Robustness Analysis And Optimal Control Of LCL-type Grid Connected Converters Under Complex Grid Conditions

LCL-type grid-connected power electronic converters are widely used in industrial fields and academic research due to their small size,low cost,and excellent performance.However,the new energy power generation units represented by photovoltaic and wind power in China are far from the main power grid and are at the end of the power grid.Long-distance transmission lines make the grid connection point weak and complicated,affecting the stable operation of grid-connected converters.The decentralized grid connection,the uncertainty of the line impedance,and the transformer leakage reactance make the grid connection point weakly sensitive.Non-linear loads and other power electronic loads make the grid connection voltage contain a large number of background harmonics.The switching of phase load and the use of unbalanced load make the grid voltage appear unbalanced.How to ensure the normal operation of the grid-connected converter under the above-mentioned complex power grid conditions is an urgent problem to be solved in the research of new energy grid connection,and it is also one of the key technologies.This topic is based on LCL grid-connected converters.Based on the objectivity of current complex power grid conditions,the research on the adaptability,robustness,stability,and optimal control of grid-connected conversion systems to non-ideal grid-connected voltages is conducted and explored,the specific work is as follows:Aiming at the problem of grid connected converter technology,the filtering principle,resonance mechanism,damping technology,delay effect and stability in digital control of LCL grid connected converter are studied.The relationship between the two basic control units(ICF,inverter current feedback,GCF,grid current feedback)and damping technology is clarified.At the same time,the delay of LCL converter caused by digital control or discrete domain model is studied.As one of the research contents,this paper re analyzes the instability and stability maintenance of LCL type converter,and comprehensively considers the objective factors such as delay effect,weakness and uncertainty of parallel nodes,time-varying of main circuit parameters,etc.,to explore and integrate the theory of LCL type grid connected converter.Based on the recognition of LCL grid connected converter,this paper makes a more in-depth study in the following chapters.Aiming at the problem of reduced robustness and adaptability of grid-connected converters under weak grid conditions,a method for improving and enhancing the adaptability and robustness of weak-grid grids during normal operation of grid-connected converters was studied.An additional unit delayed feedback robust current control that only samples the grid-connected current.An impedance model was established between traditional single-loop active damping strategies,and the limitations of the effective damping zone of the traditional strategy were analyzed.Subsequently,considering the digital delay and weak grid,a control strategy based on additional unit delay feedback was proposed.Which can alleviate the phase lag caused by the delay effect and expand the effective damping area,and can achieve active damping without additional sensors.In the case of distorted power grid,the power quality of grid connected converter is reduced.The method to improve the quality of grid connected converter current is studied.This strategy does not need additional sensors to collect the harmonic source current,reduces the hardware cost,simplifies the complexity of the control algorithm,so that the grid connected converter can still guarantee high-quality incoming current when the grid distortion is serious,and the stability and robustness of the converter are improved.Based on the impedance model of each loop of LCL topology under the condition of distorted power grid,the limitation of traditional capacitance current as harmonic source is pointed out by analyzing the frequency characteristics of each impedance model.Considering the digital control delay effect,the influence of capacitance current as control loop on the stability of the system is analyzed,and then the control strategy of this paper is proposed.In order to reduce the toughness of grid connected converter in unbalanced power grid,the method of improving the operation toughness of grid connected converter is studied.This paper analyzes the interaction of the AC and DC side harmonics under the unbalanced condition,considers the constraints of the converter grid current harmonics,power fluctuations,DC side voltage fluctuations and so on,and draws the toughness range of the converter operation under the constraints of the grid connection standard.According to the toughness range,the most advantages of the toughness under the constraints are selected,and then proposes a method to improve the active power output capacity of the converter optimization control strategy.In this paper,LCL-type grid-connected converters are used as research objects,and the problems of robustness and adaptability caused by weak grids,distorted grids or grids with background harmonics,and unbalanced grids are discussed.Through analysis and research,corresponding methods and strategies are proposed to alleviate the adverse effects of the complex grid environment on grid-connected converters,and improve the operating characteristics and grid-connected quality of grid-connected converters.The validity and feasibility of the research contents are verified by the built hardware platform.The research results of this topic conform to the development trend of new energy or renewable energy,improve the flexibility and reliability of grid-connected power generation units,and improve the auxiliary services of power electronic equipment in power systems.It has certain reference value in the research of power planning and integrated energy system.