Research On Current Control Strategy For Grid-connected Inverter In Weak Grid

With the reduction of traditional fossil energy consumption,renewable energy grid-connected generation with renewable energy such as wind energy and solar energy is rapidly developing.Distributed generation systems are the main ways to use renewable energy.The grid-connected inverter which is power conversion link between the new energy generation system and the public power grid system is particularly important.Distributed power generation systems for access to the power grid are distributed widely,generally faraway.Long-distance transmission routes,and power distribution transformers and other devices make the situation of the weak grid characteristics of the power grid more and more serious.Under weak grid condition,the wide-range time-varying grid impedance will change the transfer function of the grid-connected inverter system under the ideal condition.Continue to adopt the control strategy under ideal condition will lead to system instability and poor waveform quality.This dissertation is proposed the grid-connected inverter system under weak grid condition.The following aspects of the work for its current control strategy.Firstly,the mathematical model of the LCL type grid-connected inverter was established under the ideal grid condition.The high-frequency harmonics in the output voltage of the bridge arm were filtered by the LCL filter.The capacitive current feedback is used to dampen the resonance peaks which exists at the resonant frequency f_0.The capacitive current feedback and the current feedback control of network access made up a double-closed-loop current control strategy.The design method of the related control parameters of the grid-connected inverter was introduced in detail.Secondly,the mathematical model of the LCL grid-connected inverter under weak grid condition was established which resistance and inductance as the model at the point of common coupling.The influence of the grid impedance on the double closed-loop control system was analyzed.The passive damping control strategy was added to the double closed-loop current control strategy based on the active damping feedback.A new type of composite damping control strategy was proposed and the simulation experiments verified the effectiveness of the strategy.For the interference of the public power grid with harmonics to the grid current,this paper abandoned the widely used proportional feedforward control scheme,adopted the resonant feedforward control scheme,which improved the grid-connected power of the grid-connected system under weak grid condition and the tracking performance of the grid-connected system.According to the above theoretical analysis and simulation,experiment of grid-connected system were carried on to verify the effectiveness of the proposed control strategy.The experiment platform of 3KW single-phase inverter was designed and the experimental research on related content was carried out.The experimental results proved the practicability of the composite damping control strategy based on resonant feedforward.