Study On Droop Control Scheme For Microgrid Interface Converters

With the traditional energy getting scarcer and environmental getting worse,various Distributed Generation(DG)technologies based on green renewable energy have been developed rapidly.But it has a lot of problems,for example: the fluctuation of DG makes it more difficult to control,and it has an adverse impact on the stable operation of grids.It can be applied in the microgrid to reduce the negative effects of DG for the grids.The electronic interface converters are an important component of micro-grid system between sources,grids and consumers,so the effective control of it directly affects the stable operation of the microgrid.Due to the requirement that the microgrid interface converters need to not only operate in both island and grid connected modes,but also transfer smoothly between these two modes,so the droop control strategy which can realize the above-mentioned ability and its application in the control of microgrid interface converters are studied in this paper.Firstly,this paper introduces the source of droop control-the droop characteristics of synchronous generator,and the traditional droop control strategy applied in microgrid interface converters.Due to the traditional droop control operated in island has the disadvantages of the deviation between the output voltage and the rated voltage,as well as between the output frequency and rated frequency,an improved droop control with adaptive droop coefficient and PI controller is proposed in this paper.Due to t he droop coefficient is related to the function about the actual output power,the droop coefficient can be changed according to the actual output power in real time with the advantage of decrease of the deviation of the voltage and frequency in the improved droop control.In addition,add voltage,frequency error feedback to the improved droop control,and through the PI controller to improve the performance of steady state and transient of voltage and frequency.At the same time,when the inverter is being operated in grid-connected,the improved droop control both with adaptive droop coefficient and PI controller can reduce the deviation between the output power and the reference power which compared to the droop control only with PI controller.And the pre-synchronization control strategies of three-phase converter based on ??,dq coordinate system and single-phase converter are proposed to achieve the smooth switching from island to grid-connected based on the mathematical modeling of three and single phase microgrid converters.At the same time,in order to match the characteristics of the line impedance,the virtual impedance is added to the voltage and current double closed-loops,and the equivalent output impedance of the converter is analyzed to determine the parameters value of the virtual impedance and double closed-loops.Finally,the simulation models of three-phase and single-phase microgrid interface converters are built,the operation conditions with linear and nonlinear loads are analyzed.The results verify the effectiveness and superiority of improved droop control strategy for the microgrid interface converters.And an experimental platform of microgrid interface converter based on DSP28335 is designed and built,the experimental results of the microgrid interface converter operated in island mode are presented,it preliminary verified the effectiveness and superiority of the improved droop control of microgrid interface converter operated in island mode.