Research On Digital Control Technologies Of Three-phase Grid-connected Inverter With LCLFilter

As the interface unit between the grid and the distributed energy resources(DERs)based on the renewable energy sources such as photovoltaic,wind energy,the grid-connected inverters(GCI)convert DC power into high-quality AC power and inject it into the grid.Thus,it plays a vital role in ensuring the operating status of GCI to meet the relevant grid standards for the entire grid's security,stability and high quality operation.The grid current transient response will produce overshoot and oscillation when the grid current command value has a step change due to actual demand or abnormality.The undamped oscillation and overshoot will cause the system to generate overcurrent,deteriorate the quality of power injected into the grid,and threaten the safety of the system and grid when it is serious.Instead of L filter for filtering high-frequency switching harmonics,LCL filter has the advantages of low inductance required and good filtering performance,but its inherent resonance characteristics easily lead to system stability problems.Therefore,aiming at the three-phase GCI with LCL Filter under digital control,this paper investigates control strategies about the improvement of transient characteristic and LCL filter resonance damped.In order to accurately model the system under digital control,this paper derives its s-domain average switch models on the abc stationary frame,ab stationary frame and dq synchronous rotating reference frame.Then,this paper analyzes the delay in digital control of the system,and establishes the models of digital pulse-width modulator and digital control process in s-domain.It is clear that this paper adopts the sampling period about the system delay of 1.5 times,which provides a theoretical basis for the study of discrete z-domain modeling of the system and subsequent control strategies.In order to improve the dynamic performance of the system when the grid-connected current command value has a step change due to actual demand or abnormality,and eliminate the overshoot and oscillation of grid current transient response.In this paper,two pseudo derivative feedback(PDF)controllers with different feedback terms are studied for the inverter current feedback(ICF)system and grid current feedback(GCF)system in dq synchronous rotating reference frame.For the ICF system,a PDF controller with a proportional feedback term is used.And the influence of the controller parameters on transient response is discussed in detail by the closed-loop zero-pole map of the system in z-domain.Compared with PI controller by reducing its own gain to reduce overshoot,PDF controller can achieve no overshoot easily over a wide range of controller parameters.For the GCF system,a PDF controller with a proportional and a second-order derivative feedback term is used,which achieves no-overshoot grid current control while damping LCL filter resonance.Based on the establishment of the discrete system model,the implementation of the controller and the design of parameters are deduced in detail.Aiming at the problem of system stability which caused by the inherent resonance characteristic of LCL filter,on the basis of considering the grid impedance and digital delay,this paper considers a grid-voltage-feedforward active damping control strategy.This strategy can achieve the damping of the resonant peak without adding additional high-precision detection circuit,and can reduce the impact of grid disturbance.This paper analyzes the active damping mechanism of the strategy in s-domain,and clarifies the physical meaning,at the same time,it explains the damping performance in different frequency ranges.Based on the establishment of the system model in z-domain,the stable range of the grid-voltage-feedforward coefficient is deduced according to the Julie criterion,which provides the basis for the design of controller parameters.Simulation and experiment verify the correctness of the above analyses and the effectiveness of the control strategies.