Adaptability Of LCL Type Grid-connected Inverter Under Weak Current Network

The stable operation of the grid-connected inverter is directly related to the quality and stability of the grid-connected current of the system,which is the key bridge of the distributed power generation system such as solar energy and wind energy.Due to the characteristics of geographical location,distributed generation systems are often located in areas far away from the main power grid.At this time,the actual power grid can no longer be equivalent to a strong power grid,but a weak power grid.The coupling between the weak power grid and the grid-connected inverter seriously affects the stability of the grid-connected inverter system,and also reduces the quality of the output current.In this paper,the grid-connected inverter with LCL filter is selected for analysis,and the adaptability of weak power grid and its improved control method are studied.The optimization measures of grid voltage feedforward and the compensation strategy of digital control delay are analyzed and studied respectively.Firstly,the filter design basis and steps of LCL type grid-connected inverter under strong current network are deduced.One of the key problems in the design of LCL grid-connected inverter lies in the processing of its inherent resonance characteristic.This paper analyzes the realization principle of passive damping and the problem of extra power loss.Based on this,the active damping method is obtained by equivalent transformation of control block diagram.Through the comparison of the transfer function of different active damping modes,it is concluded that the proportional feedback of capacitor current introduces the least variation and the damping effect is the best.After the filter resonant damping is effective,another key problem in the design of LCL grid-connected inverter is the design of grid-connected current loop.In this paper,the control characteristics of the common current regulator are analyzed and the requirements of the steady state error and stability margin of the system are derived.Based on this,the parameters of the capacitor current feedback loop and the grid-connected current feedback loop are designed.The validity of the above double-loop control method is verified by simulation,which provides a foundation for the system analysis and design after considering the power grid impedance.Then,the influence of the weak grid on the grid voltage feedforward of the grid-connected inverter and its improvement strategy are analyzed.Point of common coupling(PCC)is far from the main grid,the grid impedance,weak power grid characteristics,and with a rich background harmonic voltage,the voltage harmonic component will cause the inverter output current interference,serious when even will make the output current cannot satisfy the standard of interconnection,and therefore often adopt the method of join grid voltage feed-forward inhibition.In this paper,the Norton equivalent model of grid-connected inverter system under weak current is established,and the reason that the traditional grid voltage feedforward harmonic suppression method may cause the system instability under weak current is that the traditional grid voltage feedforward method will cause the phase margin of the output impedance of the grid-connected inverter system to be worse under weak current.In this paper,a feedforward strategy of subfrequency network voltage for virtual impedance correction in different frequency bands is studied.This method takes into account both the high quality of grid-connected current and the adaptability to the network impedance.The effectiveness of the proposed method is verified by simulation and experiment.Finally,the instability caused by digital control delay in weak current network and its improvement strategy are analyzed.The inherent delay of digital control will change the phase characteristics of the grid-connected inverter system,which will change the dynamic performance of the system.At the same time,the digital control delay will also change the original pole distribution,so as to make a complex impedance parallel on the filter capacitor and change the damping characteristics of the system.In the case of weak grid,the inductive grid impedance leads to the increase of the equivalent inductance on the grid side and the decrease of the actual resonance frequency of the system,which leads to the risk of crossing the positive and negative demarcation frequency of the resistance,which may cause the system to become unstable.Therefore,a delay compensation method for resonant damping inner loop and grid-connected current outer loop is studied,which enlarges the positive and negative demarcation frequencies of active damping equivalent resistors and takes into account the dynamic performance of the system.The effectiveness of the proposed method is verified by simulation and experiment.There are 48 figures,5 tables and 80 references in this paper.