Research On Current Control Technology Of LCL-type Grid-connected Inverter In A Weak Grid

Distributed generation technology has gradually become one of the main ways of large-scale development and utilization of renewable energy because of its green and environmental advantages.Grid-connected inverter,as the energy interface between distributed generation units and power grid,plays the important role of transferring energy between distributed energy and power grid.The stability and high-quality operation of grid-connected inverter is of great significance to the distributed generation grid-connected system.However,with the large-scale interconnection of distributed generation system,the actual grid cannot be regarded as a strong grid,but as a weak grid.Non-ideal factors in a weak grid make the dynamic interaction between grid-connected inverter and weak grid invisible,which seriously threats the safe and efficient operation of distributed generation grid-connected system.For this reason,this paper takes single-phase LCL-type grid-connected inverter as the research object and deeply studies the current control technologies of grid-connected inverter in a weak grid,so as to effectively improve the stability and power quality of grid-connected inverter,and thus ensure the reliable operation of distributed generation grid-connected system.Weighted-average-current control has been widely used because of its loop order-reduction characteristics.For the traditional weighted average current control method,the weighted-average current of the inverter-side current and the grid current is usually controlled so that the control system of grid-connected inverter can be reduced from the third order to the first order and the adaptability of inverter to weak grid is dramatically enhanced.However,the impact of digital control delay on grid-inverter system has not been fully considered in the traditional weighted-average-current control methods.The analysis in this paper shows that the phase lag introduced by control delay makes the traditional weighted-average-current control method no longer have the characteristic of loop order-reduction,and leads to a reverse resonance peak in loop gain varying with the grid impedance,which puts the grid-inverter system at risk of instability.Focusing on the issue,a high-robustness weighted-average-current control strategy based on lead compensator is proposed and the design of parameters is also presented.The proposal compensates the phase-lag within the frequency range of instability by introducing a lead compensator into the voltage feed-forward channel,so as to reduce the order of the current-loop and ensure the stability of the grid-inverter system.A prototype of 3 kW single-phase LCL-type grid-connected inverter is built for the experimental tests.The comparative analysis and experimental results confirm the validity of the proposal.Current dual-loop control is a kind of precise current control strategy with grid current as the direct control target.In the grid-inverter system based on current dual-loop control structure,PCC voltage feed-forward strategy is usually applied to suppress the grid current distortion caused by PCC voltage harmonic,so as to meet the harmonic standard.However,in a weak grid,due to the influence of grid impedance,the positive feedback loop introduced by PCC voltage feed-forward would greatly reduce the robustness of grid-inverter system,resulting in resonance or instability of the system.Focusing on this problem,this paper proposes a novel frequency-division virtual impedance shaping control strategy,which shapes the output impedance of grid-connected inverter with different gains in different frequency bands respectively.With the proposal,the grid-connected inverter can be equipped with strong adaptability to a wide range of grid impedance and suppression ability to grid current harmonics.Its basic principle,parameters design and application range are also presented in detail.Experimental results fully verifies the effectiveness of the proposal.Delay introduced by digital control method would change the control characteristics of grid-connected inverter and reduce the robustness of the system.In addition,phase lag is lead into by control delay,which limits the bandwidth and low-frequency gain of the current-loop and affects the control performance of the grid-inverter system.For this issue,this paper proposes a novel control delay compensation strategy based on shadow-loading and pulse-width equivalence.By modifying PWM mode,the proposal can effectively compensate the control delay and allows half switching period of control algorithm execution time,while avoiding the problems of poor reliability and limited application scope.Moreover,the proposal improves the duty-ratio limitation of traditional compensation strategy and has better transient performance.Compared with traditional strategy,the proposal is easier to be applied in practical engineering.Performance comparison analysis and experimental results confirm the effectiveness of the proposed delay compensation strategy.