Research On Control Strategy Of Interlinking Converter In Hybrid AC/DC Microgrid

With the increase of DC loads and distributed power generation units,hybrid AC/DC microgrid has attracted wide attention since its advantages of multiple power supply forms and the ability to integrate multiple distributed generations.As a device connecting AC and DC sub-grids,the interlinking converter can realize the mutual support of two sub-grids and maintain the stable operation of hybrid AC/DC microgrid system by controlling bidirectional power flow.However,in the case of no communication,the control of interlinking converters has problems that the actual transmitted power does not match the required power of sub-grid,and power transmission cannot be realized accurately due to line impedance.In view of the foregoing problems,this dissertation will carry out research on control strategy of interlinking converter in hybrid AC/DC microgrid in island mode.Aiming at the problem of traditional normalized droop control that causes the increase of transmitted power and the deviation of bus voltage in sub-grids,this dissertation analyzes the operating states of sub-grids in normalized droop control and introduces the initial voltage value to divide the operating states of sub-grids.By deriving the relationship between the initial voltage and the output power of sub-grids,the power transmission objectives of interlinking converters in different sections are established,so that different control methods can be selected to avoid the transmitted power of interlinking converter from exceeding actual needs of sub-grids.To eliminate the power deviation caused by line impedance in low-voltage placement,a control strategy of interlinking converter based on line impedance compensation is proposed.Considering the characteristic of large line impedance ratio(R/X)in low-voltage situation,the relationship between line resistance and transmitted power of interlinking converter is deduced,and a method to detect line resistance by injecting harmonics is proposed.Besides,the impact of ignoring line reactance on the proposed method is analyzed.Based on line resistance detection,the line voltage drop can be compensated by local information of interlinking converter,so as to realize the accurate power transmission between sub-grids without communication.Based on theoretical analysis,simulation software is used for simulation verification,and hardware-in-the-loop platform is used to conduct experiment.The result of simulation and experiment prove that the normalized droop control using partition selection can reduce unnecessary power flow and improve the voltage accuracy of the sub-grid;Moreover,when line impedance is considered,accurate power transmission between sub-grids can still be achieved after control strategy of interlinking converter based on line impedance is implemented.