Design And Current Control Strategy Of LCL-filter Based On Static Var Generator

With the increase of power load,power quality problem increasingly prominent,static var generator is one of the key technologies to solve this problem.Static Var Generator(SVG)is a new type device for reactive power and harmonic dynamical compensation.L filter is used as output filter of SVG,it's used to restrain harmonic which is near switching frequency,but the attenuation ability is limited.Under the premise of not changing the total inductance,LCL filter for high frequency harmonics filtering effect is particularly evident.Through the design of the LCL filter parameters and control strategy can be well applied in SVG control system.This paper makes a research on the single L-filter of the static var generator.Based on the requirements of grid harmonic current for compensation device and VSC device voltage stress,it gives a design program of single L-filter.By the simulation of compensation characteristics it verifies the fact that the output filter designed by this method has a good compensation characteristics and it can effectively inhibit the high frequency harmonic.However,there are also some limitations.LCL filter with small volume and low power consumption to achieve good filtering performance.When the parameter design of LCL filters are not appropriate,will cause the resonance phenomenon,this phenomenon will directly influence the safety and stability of a static var generator.By introducing some new intermediate variables of inductance ratio,normalized resonant frequency and normalized harmonic currents,and comparatively analyzing the frequency characteristics of LCL-filter and L-filter,we find that the ratio of converter side inductance to the total inductance has a significant impact on the resonant frequency and the harmonic current of converter.In order to improve the robustness of the resonant frequency and reduce the inverter side current harmonics,to determine the optimal inductance ratio value.A reasonable resonant frequency range is proposed aiming to reduce grid-side current harmonic at the switching frequency and avoid grid harmonic amplification.On this basis,a concise design method of LCL parameters was presented.The simulation results and experimental results verify that by adopting the proposed method,the switching frequency harmonics is suppressed effectively and the system side total harmonic distortion is reduced.The resonant peaks of the LCL filter has put forward higher requirements for the design of control system,at the same time in order to avoid the phenomenon of coupling in the dq axis control,this paper proposes an control strategy based on the inverter side current closed-loop control and capacitor current feed-forward for quasi-PR.This control strategy take the low frequency filter capacitor branch current as the system damping to suppress the influence of the LCL filter and through quasi PR control to improve the reactive current tracking accuracy.On this basis for the design of the control parameters,design of a ± 10kVar/380V SVG experiment system and through hardware experiments verify the effectiveness of the proposed strategy in this paper.