Research On Control Method Of T-type Three-level Mixed Conduction Mode Grid-connected Inverter

The grid-connected inverter has a wide range of applications.The efficiency,power density and control performance of grid-connected inverters are increasingly required in these fields.For grid-connected inverters,discontinuous conduction mode(DCM)can realize fixed switching frequency operation and requires a small size of inductor.And zero current switching(ZCS)can be realized without detection circuit.Trapezium conduction mode(TPCM)achieved by three-level circuit is a special kind of boundary conduction mode(BCM).Under TPCM,zero voltage switching(ZVS)can be realized at fixed switching frequency and has low current stress.This paper constructs the LCL grid-connected inverter based on the T-type three-level circuit topology,realizes the control of the grid-connected inverter through mixed conduction mode(MCM)combined with DCM and TPCM,and carries out in-depth research on the gridside current control and efficiency optimization.Currently,the modeling methods for LCL-filtered inverters in existing literatures are mostly based on continuous conduction mode(CCM).The state space average model established is a third-order system,which makes the design of controller complex and cannot realize effective control for DCM.To address this issue,this paper firstly establishes a secondorder linear model based on the charge quantities for the LCL-filtered inverter of any conduction mode and any inverter circuit.The model takes charge quantities output from the bridge arm as the control quantity and simplifies the design of controller greatly.Based on the charge quantity model,the direct charge control(DCC)method is proposed in this paper,and the control method combining PID regulator with grid voltage full-feedforward controller is realized.The design of the controller is completed and the excellent control performance is achieved as simulations and experimental results shown.Then,in order to solve the problem of high current stress caused by DCM operation,this paper uses T-type three-level inverter circuit to build an LCL-filtered grid-connected inverter.Based on the analysis of three switching modes of MCM grid-connected inverter,the mixed switching mode control strategy combined by DCM and TPCM operation to minimize current stress is proposed,and the modulation calculation process of charge quantity to duty ratios for swtiches in each switching mode is analyzed.On the basis of DCM/ZCS-TPCM,a ZVS-TPCM operation control scheme with fixed reversed resonant time(FRRT)is proposed for TPCM.Simulations and experiments show that the switching between three switching modes can be smooth and stable,and the efficiency can be improved significantly.In order to improve the utilization of DC side voltage and broaden the operating range of ZVS-TPCM mode based on FRRT,T-type five-level MCM inverter is further studied in this paper.Based on four switching modes of the five-level grid-connected inverter,the efficiency is further optimized on the basis of realizing the direct charge control.The simulation and experiment verify that the five-level circuit has good control performance and better optimization effect for current stress.Thirdly,the analysis and design of LCL filter are completed for the MCM grid-connected inverter.And the loss models for the inverter-side inductor and switches are established respectively.Based on the loss model,the loss characteristics of inverter-side inductor and switches are analyzed,and the losses are compared with that of conventional CCM operation.At the same time,the loss optimization effect of T-type three-level circuit and T-type five-level circuit is compared and analyzed.The results show that the DCM/TPCM operations based on multi-level circuit can significantly improve the efficiency and power density.Finally,the experimental prototype of LCL grid-connected inverter is built respectively based on the T-type three-level circuit with rated power of 800 W and the T-type five-level circuit with rated power of 1600 W.The experimental results show that: 1.The direct charge control method can realize the efficient control of grid-side current;2.The switching process between switching modes is smooth and stable,which can realize fast and stable control under the circumstance of sudden change of current;3.The efficiency of MCM inverter is higher than that of CCM mode under any working condition;4.The T-type five-level circuit can realize ZVS-TPCM in a wider range of working conditions.