The Study On Control System With Its Stability Of Active Damping LCL Converter Considering Multi-parameters Effect

Due to the increasingly exhausted fossil fuels and serious pollution of human living environment,the world continues to optimize energy structure and actively to develop clean energy.And also,the sustainable development of economy and environment,with energy conservation and emissions reduction is advocated with strict controls on the ecological environment pollution.As power electronics technology is widely used in various fields,it plays an important role in maximizing energy efficiency,exploiting and utilizing renewable energy,and promoting national industrialization process with enormous economic and social benefits.More than 95% of the electricity is predicted to be processed by power electronic conversion technology before being used by the customers.However,the imbalance,electric surge and non-linear properties are introduced into the power grid by large amount installation of power electronic devices.They can result in serious distortion of grid voltage and current waveform,and place new challenges on the safety and reliability of the system operation.With the background of increasing prominent energy and environment crisis,the research on saving energy and improving power quality has extremely practical significance.In this paper,a three phase PWM converter based on active damping LCL topology is discussed,and several key problems are analyzed,which affect the control effect and power quality.And also,the in-depth study is conducted on control strategy,and system stability with multi-parameters effect is taken account.The main research work is as follows:1.This paper systematically set up the state equation of three-phase VSC based on active damping LCL control strategy under stationary ABC reference frame,rotary dq frame and stationary αβ reference frame respectively.And the average switching model for PWM converter is built under αβ plane,which can provide accurate and simple mathematical model of controlled object for controller’s design with system’s stability analysis.The inductance voltage vector of each sector is calculated based on current error,and the converter average voltage vector is composed by geometrical distribution model to establish optimal switching modulation rule.In this way,the reference vector calculation and sector selection is much easier than traditional SVPWM strategy.The outer loop control based on instantaneous power model and dual inner current loop control are presented.In order to satisfy THD limit,power factor requirements,additional damping loss constraint and the system stability,the mathematical model and its accurate calculation method of LCL topology are elaborated,which provides important theoretical basis for the design and optimization of three-phase VSC.2.To replace trigonometric function calculation,Park transformation,real-time detection of Phase Locked Loop and feed-forward decoupling under traditional dq frames,the quasi PR controller is introduced with infinite gain and zero steady state error tracking at specific frequency under αβ frames in order to improve the power quality.But 3-order active damping LCL and 2-order quasi PR controller cause control loop into high-order system.The influence of active damping gain,control time delay factor,LCL topology element and controller’s parameter configuration on system’s trajectory and robustness can be analyzed by the means of Bode diagram,zero-pole point trajectory and Routh-Hurwitz stability criterion in order to configure optimal parameter and discuss system’s stability rule.The preward Tustin with deviation compensation is proposed for quasi PR controller’s discretization,and it is with well approximation effect as continuous domain.The simulation model and experimental prototype of three-phase VSC system based on above mechanism analysis are built.The results show that theoretical analysis is consistent with the experimental data,and the proposed control system with the main circuit meets dynamic response and stability margin requirement.Unity power factor under different power load is achieved with low current harmonic distortion,and the reliable and stable operation of VSC is ensured.3.The IGBT modules in VSC system with high switching frequency,and high di/dt and parasitic inductance of the DC bus will lead to serious interelectrode voltage peak shock accompanied by oscillation phenomenon in IGBT transient switch process.These will worsen IGBT’s electrical stress,switching loss and EMI,causing bad power quality with voltage and current waveform distortion by a large number of high frequency harmonic injection.In order to ensure the safety and reliability of the VSC,the study on IGBT switch mechanism of transient process considering the influence of stray parameters is carried out,the accurate extraction of VSC parasitic parameters and preventive measures are taken to verify the feasibility and validity of theoretical analysis through multi-pulse test platform.4.The PWM waveform of VSC propagating through long distance transmission line is with high di/dt and dv/dt caused by IGBT transient switching process,and therefore the distribution parameters of transmission line will lead to PWM voltage pulse wave reflection phenomenon and common-mode conduction coupling interference,which may result in overvoltage and high frequency damping oscillation at load end.This paper discusses the mechanism of traveling wave transmission reflection,and then by the means of PEEC theory,vector fitting strategy and mirror configuration theory,the broadband equivalent circuit model is established to accurately estimate the overvoltage level.In order to verify and solve the problem of traveling wave reflection overvoltage,simulation and experiment are conducted on VSC with filter in the inverter mode,all results show that the above model and theoretical analysis are highly accurate.