Reaserch On Multi-module Parallel Active Power Filter For Nonlinear Impact Loads

Over the recent decades,with the rapid growth of demand for electricity,a large number of nonlinear impact loads with harmonic and reactive impact widely exists in the power grid,such as the electrified railways,arc furnace,steel rolling and aluminum electrolyzing,which have a strong impact on reliability of the grid.Shunt active power filter(SAPF)has gradually become a preferred method to suppress harmonics for its real time compensation,fast dynamic response and no resonance with power grid.However,stand-alone device is limited by device capacity,switching frequency and heat dissipation.Based on the background above and harmonic characteristic,the technology of modularized multiple APF is researched,which includes optimal parallel-connected topology,high reliable current-limiting strategy,complex coupling mechanism and stability analysis,to minimize the impact of shock load and improve the power quality of power system.Firstly,in the aspects of optimal topology and coordinated control strategy,the traditional parallel modes appear to an open-loop architecture in view of the system control target which has an unexpected poor performance on steady-state accuracy.A closed-loop compensation topology is introduced by sampling the grid-side current to improving the precision of steady state.In consideration of the shortcomings of the two architectures and the load characteristics of typical low voltage and high current harmonic pollution sources,the coordination control strategy combining open-loop with closed-loop is proposed,which preserves the advantages of fast dynamic response of open-loop structure and high compensation precision of closed-loop structure,and combines the merits of high capacity utilization with respective harmonic compensation method and the briefness with current-share compensation.Secondly,the paper analyses the essential of the current-limiting demands for the comprehensive protection of SAPF,namely the limiting control objects: 1)the root mean square(RMS)of the compensation current(mainly for the overheat protection of the IGBT and inductor);2)the instantaneous wave of compensation current(mainly for the accurate current control and IGBT Icnom specification)and 3)the instantaneous wave of PWM-VSC modulation voltage(for the need of liner close-loop control).Then,the corresponding constraint conditions are obtained.On the basis of a comparative study of the three traditional currentlimiting strategies,an improved current-limiting scheme based on particle swarm optimization(PSO)is proposed to achieve the two optimization targets: 1)the minimization THD for the grid-side current;2)the maximization utilization ratio for the capacity of the SAPF.The main advantage lies on the optimum limiting ratios of each harmonic order are calculated in real time respectively to achieve the flexible and liner limiting control.Thirdly,in view of the complex coupling of multi-module parallel APF under weak grid condition,the equivalent admittance model is derived from the basic current control loop.In consideration of sampling CT is added to the current control loop,the equivalent admittance model of harmonic control loop and two-port NORTON equivalent circuit are established through simplifying the circuit,in addition,the current response expression of the parallel system is obtained.Thus,the paper reveals the intrinsic reason of instability in paralleled multimodules system.Finally,the main circuit based on three-phase four-leg inverter and the dual-core control based on DSP+FPGA are introduced,including the design of control system and main components.The experimental results further illustrate the effectiveness and rationality of the proposed methods.