Research On Key Technologies Of Grid-connected Inverters

In recent years, with the increasing environmental pollution and global shortage of fossil energy,renewable energy sources, such as solar energy, fuel cells, and wind energy, are desirable due to theirmerits of clean and safe, pollution-free, and renewable. The output of the solar cell or the fuel cell isdc voltage and that of the wind generator is ac voltage with variable frequency, while the grid is acvoltage with constant frequency. Therefore, grid-connected inverters are the important part ofdistributed generation systems.This dissertation is composed of two main parts. The first part is research on key technologies ofthree-phase grid-connected inverters, and the main contents are as follows.The reason for causing inrush current at start-up is analyzed. A soft-start control method for thethree-phase grid-connected inverter with space vector pulse width modulation (SVPWM) control isproposed. The detailed procedure and flow chart of the control strategy is given. The experimentalresults verify the theoretical analysis. The control method is easy to implement and is only related tothe d-axis component of the grid voltage under the balanced grid voltage condition.Stability analysis of digitally controlled LCL-filter-based three-phase SVPWM-controlledgrid-connected inverters without damping is presented. The z-domain model of the grid-connectedinverter without damping is deduced. The design procedure and example of the system is provided.An improved decoupled control strategy for three-phase grid-connected inverters with SVPWMcontrol is explored. The d-and q-axis components of the grid current in the decoupled components ofthe grid current controller are substituted by that of the reference grid current in the proposeddecoupled control method. The comparative analysis of the traditional and improved decoupledcontrol methods is illustrated. Simulation and experimental results show that the grid-connectedinverter is stable and that with improved decoupled control method has faster dynamic response andbetter waveform quality of the grid current compared with the traditional decoupled control method.Based on the improved decoupled control strategy, an improved direct power control using spacevector modulation (DPC-SVM) for three-phase grid-connected inverters is proposed. The comparativeanalysis of the traditional and improved DPC-SVM methods is illustrated. Simulation andexperimental results show that the grid-connected inverter with the improved DPC-SVM method hasfaster dynamic response compared with the traditional DPC-SVM method.The second part is research on key technologies of single-phase grid-connected inverters, and themain contents are as follows. A family of single-phase high-reliability grid-connected inverter topologies is proposed based onthe traditional bridge-type grid-connected inverters. As the shoot-through problem does not exist inthe proposed topologies, the reliability of the proposed topologies can be improved. The dead timebetween the switches need not be set, so the waveform quality of the grid current can be improved.The freewheeling current flows through the independent freewheeling diodes instead of the bodydiodes of the switches, so they can be designed optimally. The operating principles of the proposedtopologies are analyzed in detail. Simulation results verify the theoretical analysis. The two-switchdual-buck grid-connected inverter with hysteresis current control is taken as an example from theproposed topologies. The design guidelines and example, and stability analysis are provided.Experimental results verify the theoretical analysis. The comparisons among the proposedhigh-reliability and the traditional grid-connected inverters show that the dual-buck half-bridgegrid-connected inverter is suitable for110-120V grid-voltage applications and the full-bridge typehigh-reliability grid-connected inverters are promising in220-240V grid-voltage applications.A control method for single-phase grid-interactive inverters with nonlinear critical loads isproposed. The utility-connected inverter can operate at both stand-alone and grid-connected modes.The impact of the nonlinear load on the grid current is analyzed. The operating principle of theproposed control method is illustrated in detail. The stability of the grid-connected inverter isanalyzed. The filter inductor is chosen. Simulation and experimental results show that the grid currentin grid-connected mode and the output voltage in stand-alone mode have higher waveform qualitycompared with the conventional control method.