Research On Droop Control Based Uniform Controller For Dual-mode Inverters In A Microgrid

It is already the consensus of the electric power industry domestic and abroadthat microgrid is one of the key technologies of the future smart grid. Microgrid canoperate in either grid-tied mode or off grid mode intentionally when the power gridfails. Voltage source microgrid inverter is critical equipment for microgrid seamlessswitching between grid-tied and off-grid modes. Wireless droop control method(WDCM) is one of the effective ways for microgrid inverter to realize dual modecontrol and seamless transfer. But output power control accuracy of parallelinverters adopting WDCM is greatly influence by the line impedance. It is difficultto realize accurate proportional share of the load power for parallel inverters inoff-grid mode, and difficult to realize accurate track of power command forinverters in grid-tied mode. These features make the study of the dual modeinverter based on droop control become a difficulty.In this paper, for the designing problem of microgrid inverter dual modecontroller supporting microgrid operation in both grid-tied and off-grid, deepresearch on droop control principle is carried out and the main factors impactingoutput power precise control of microgrid inverter based on droop control methodare analyzed under multi-inverters operating conditions of the microgrid system.Then dual-mode unified droop controller design method based on local sampling isproposed.The main contents of the specific research and innovative work are asfollows:（1）According to comprehensive analysis and compare of existing WDCMmethod, it is found that the droop controller based on the voltage amplitudefeedback has obvious advantages over other control methods with good featuresof anti-disturbance on the system parameters and accurate active and reactivepower control. However, the premise is that sampling point of the voltage requiredby the controller must be located at the PCC. When the inverter is far from thecommon bus, the long distance sampling signal line will not only introduceinterference, resulting in signal delay, but also is not benefit to the modular designof the microgrid inverters.（2）For the problems of the methods above, improved WDCM control methodbased on the local sampling is proposed in the thesis. The two-segment design method of virtual impedance is proposed to achieve the localization of the samplingpoint of voltage signal required by the controller. And power compensatingconductance and active power correction ratio is also proposed to achieve theprecise control of the output power and the regulation of voltage amplitude. Systemcontrol objectives can be achieved when microgrid in both grid-tied and off-gridmode, and thus it is not need to change the structure of the controller achieving thegoal of the design of the dual-mode controller.（3）The microgrid inverter voltage loop control is an important part of thedroop control of the inverter system. A detailed analysis and comparison of severalvoltage loop control methods commonly used for microgrid inverters and theperformances are carried out. The results show that the PR controller having goodstatic characteristics near the fundamental frequency, and it is easy to implementvirtual impedance. So it is more suitable for the droop control system of theinverter voltage loop control.（4）The microgrid simulation model consisting of two parallel inverters andload is established under MATLAB environment, wherein the inverters adopting theproposed dual mode controller. The correctness of this article’s analysis and theeffectiveness of the control strategy accordingly are proved by simulation results.