Study On Parallel Operation Strategy Of Active Power Filter

The operation strategy of multi-parallel Active Power Filter is the key technology to ensure the performance,stability and reliability of multi-machine system.It is also the key problem to control the user's compensation cost and the equipment manufacturer's production,operation and maintenance cost.Based on the single APF operation technology,this paper studies the parallel operation strategy for the purpose of optimizing the operating loss and stability of multi-machines system.According to the basic principle of APF,the paper analyzes the operating characteristics of different organization coordination modes between fractional,centralized harmonic detection and the current loop.Aiming at the APF multi-machines system,this paper analyzes the running characteristics of different coordination methods,and introduces the method of shunt and current limiting between machines.The APF mathematical model is established to analyze the mechanism of APF multi-unit circulation.For the current tracking control,the APF discrete time control model is established according to the inherent characteristics of the converter,and the model predictive control strategy(MPC)is used to control current loop.The DC side capacitor voltage is directly related to the tracking performance of APF.The higher DC-link voltage under the light load operation will bring significant ripple and loss,and lower DC-link voltage under heavy load condition leads to nonlinear modulation.In order to solve this problem,the paper analyzes the relationship between the DC-link voltage and the device loss and compensation performance from the theoretical level,and the feasibility of the selecting DC-link voltage adaptively.Then the real-time discrete observation model of DC-link voltage demand is eatablished,and its theoretical adjustment range is given.Combined with the MPC current loop,adaptive regulation of the DC-link voltage according to the harmonic load level,characteristics and grid voltage changes is realized,meetting the compensation performance while optimizing the loss.For heavy load condition,the DC-link voltage is limited,by using observation model the current limit factor is calculated to avoid over-modulation.Simulation and experimental results demonstrate the feasibility,effectiveness and superiority of optimized DC-link voltage control startegy.When APF multi-machines operate in the weak grid network,there are performance deterioration and capacity expansion problems.In order to solve this problem,the paper establishes the single phase equivalent model of the direct coupling APF system with the power source and load,analyzes the influence of the parallel number on the system stability,and compares it with the APF operating in the ideal strong grid.Based on the analysis of stability and DC-link voltage demand,the cascaded H-bridge is used to cancel the voltage disturbance of the common coupling point(PCC)and reduce the demand of the DC-link voltage of the full bridge APF modular and weaken the influence of the impedance in weak grid on the stability of the multi-parallel system.The steady state and dynamic performance of the coupling point isolated APF multi-machines system with H-bridge isolating voltage disturbance are verified by simulation,and the superiority of the scheme is verified by comparison with the traditional scheme.