Research On Grid Synchronization And Control Strategy Of Active Power Filter With LCTL-filter

With the continuous improvement of industrial modernization,the modern power system has made considerable progress,and the requirements of people for power quality in all aspects of the power system are also on the rise.Among many power quality problems,harmonic pollution caused by a large number of nonlinear loads in the power transmission and distribution process and the terminal of power consumption affects the safe and stable operation of the power grid,bringing about normal production and living great harm.Therefore,to improve power quality and restrain harmonic pollution of the power grid has become one of the most important issues that the power industry urgently needs to solve.Active power filter(APF)is an integrated power quality control device that can effectively restrain the harmonics of the power grid.APF has the advantages of fast dunamic response,high precision of steady-state compensation and flexible compensation mode.In this paper,some key technologies of shunt active power filter with LCTL-filter are discussed in depth.Based on the principle of analytical mechanics,the method and result of APF system analytical mechanics modeling based on Lagrange equation are given through the analogy between power electronics system and mechanical sys tem;then the inherent characteristics of APF system are analyzed in depth and summarized : the controllability and observability,the underactuated characteristics and the passive and dissipative properties.Based on the “ strong coupling,nonlinearity and underactuated ” characteristics of APF system,partial feedback linearization control design of APF system is carried out respectively from the per spective of current and power.This realizes the APF active current and reactive current decoupling control.Then combined with space vector pulse width modulation(SVPWM)technology to achieve accurate compensation of harmonic currents of the power grid.The current-loop uses repetitive control with high steady-state accuracy,and a frequency-adaptive current feed-forward compound repetitive control method is proposed,which effectively solves the problem that the repetitive control internal model is easily affected by the frequency fluctuation of the power grid.The traditional PI control is not conducive to guarantee the dynamic performance and stability of the object.To solve this problem,the ADRC is used for designing the voltage outer-loop.The article gives the design method of ADRC controller of APF DC voltage,and the linear method is used to optimize t he controller,which effectively improves the disturbance resistance of the DC voltage on the grid-side and the load-side,the system response speed and the control precision.As the harmonic control device,the requirement of APF for the switching frequency harmonic filter is more strict than the general power equipment s.Therefore,based on the analysis and summary of the advantages and disadvantages of the traditional LCL filter and its improved topology,this paper presents a LCTL filter and parameters design method.Compared with LCL and LLCL filters,the LCTL filter can effectively eliminate the switching frequency harmonics due to the presence of a trap branch.The LCTL filter can maintain a higher attenuation rate at the high frequency range.Meanwhile,it also reduces the system cost.Aiming at the problems that the selection range of passive damping parameters is usually too large and no clear theoretical support,this paper presents a exact selection method and basis of passive damping parameters of LCTL filter.Finally,this paper presents a complete LCTL filter design program for APF.In order to improve the harmonic detection speed and control efficiency of nonlinear strategy in APF system,accurate and rapid extraction of information such as frequency and phase of power grid is essential.Therefore,efficient and stable grid voltage synchronization method is important for high-quality operation of three-phase converter.In this paper,the grid voltage synchronization method under complex grid conditions,such as three-phase voltage unbalance,harmonic distortion,DC disturbance,voltage sag and frequency fluctuations,is studied.This paper presents a grid voltage synchronization method based on complex decoupling and adaptive filter(AF).The complex decoupling can quickly separate the positive and negative sequence components of the grid voltage and restrain the high-frequency harmonics of the grid.The disadvantage of the complex decoupling is that the decay capability is limited.The complex decoupling is easily affected by fluctuation of grid frequency at the same time.Adaptive filter can effectively realize the secondary separation of positive and negative sequence components of the power grid.At the same time,due to the existence of a notch filter,AF has a good attenuation of the DC component.The frequency lock-loop is design to realize the frequency adaptive control of the system and effectively reduce the frequency error accumulation problem existing in the traditional grid voltage synchronization methods.Aiming at the key techniques of APF during the theoretical analysis and engineering applications such as the application of high-precision nonlinear control strategy,the enhancement of anti-disturbance ability of the DC voltage,the design of switching frequency harmonic filter and efficient and stable grid voltage synchronization strategy,this paper gives a detailed theoretical analysis and specific solutions based on the underactuated theory and nonlinear control theory.These research results have certain reference value for the improvement of APF harmonic compensation performance and practical industrial application.