Characteristic Analysis And Performance Optimization Of Digital Control For Grid-connected Inverter

Grid-connected inverters,as the critical power-electronic interface for the renewable energy conversion,widely apply the digital-control pattern,and their control performance influence the reliable operation of the grids and the power quality.Therefore,the procedure—such as designing and optimizing the control system,ensuring the high-efficient,secure and reliable operation of the devices—is based on the premise that we should adequate understand their digital-control law.To achieve the above targets,this paper focus on the digital-control characteristics of the grid-connected inverters,while applying the direct digital correction to optimize their control loop.The mainly tasks and innovations in this paper are embodied as follow:(1)Establish an average model of the grid-connected inverters and a small-signal model of the PLL;We will detail the origin of the digital control delay terms(including Sampling and calculation delay,PWM modulation delay and hardware inherent delay),and analysis the effect of the above delay terms and critical parameters on the performance of the digital control loop and the PLL,from the frequency-domain characteristic perspective.The research found that digital control delay reduces the stability margin,both to the control loop and phase locked loop.The foresaid phenomenon considered,we will design the parameters of both the controller and the PLL.(2)Analyze the effect of the seven discretization method on the performance of the both PR controller and the current control loop,by the frequency-domain method;then we will analyze the difference among the seven methods from the multiple perspectives,such as resonance peaks,phase lag and DSP operation time,in order to determine the optimum method for the controller.Moreover,this paper will propose a parameter design method,which is based on the direct-digital correction,for quasi-proportion resonant controllers;that method could optimize the performance of the inverters control loop.The correctness of the theoretical analysis is verified by the power-electronic simulation.(3)Detail the design processes for hardware circuits and software system with respect to the grid-connected inverter;by means of designing and debugging the algorithms,the designed inverter,which is convenient for the redevelopment in hardware and software,possesses the grid-connected function with unit power factor.Based on the inverter prototype,this paper verifies: 1)the influence of different discretization methods on the dynamic performance of the system;2)The extent to which the proposed optimization method improves the dynamic performance of the inverter.