Hierarchical Coordination Control For DC Microgrid Based On Lowbandwidth Communication

The shortage of world's energy and environmental crisis have provided an opportunity for the development of renewable energy power generation such as photovoltaic and wind power.Microgrid is one of the effective organizational forms of grid-connected distributed generation units in large-scale.The DC microgrid has better power supply form,which is compatible with more distributed renewable energy generation,energy storage,and increasing DC load,and has high transmission efficiency,operational reliability,and flexibility.Therefore,it has become one of the research focus in recent years.This article mainly focuses on the operational control of DC microgrids,and develops the idea of hierarchical control.Under the condition of plug-and-play of polymorphic resources,effective system stability,power sharing,and power quality optimization can be realized by adopting distributed multi-source coordinated control.Firstly,a DC microgrid primary control strategy based on DC bus voltage signal is proposed.Each distributed unit can work in cooperation according to preset rules under different operating conditions of the DC microgrid by setting the operating priority of each distributed unit reasonably.Coordinate work to maintain DC bus voltage stability and ensure real-time power balance.DC microgrid primary control based on DBS adopts the idea of voltage hierarchical control,divides the operation mode of the DC microgrid according to different working conditions of the DC microgrid,and associates each operating mode to different DC voltage sections by using the hierarchical droop control for reference.Different distributed units are responsible for adjusting the DC bus voltage and power balance in different voltage ranges.Each distributed unit distinguishes the operating mode according to the variation of DC bus voltage and autonomously adjusts the control to achieve flexible mode adaptive switching.Secondly,a optimization secondary control of DC microgrid based on lowbandwidth communication is proposed.DC microgrid's function and performance optimization is realized upper the basic of primary control by using the low-bandwidth communications technology.The voltage compensation is obtained by comparing the DC bus voltage with the rated voltage value.This voltage compensation acts on the distributed unit who adopts the droop control to shift its droop control curve to restore the dc bus voltage to the rated value,and the voltage compensation is transmitted to the other distributed units through low-bandwidth communication,and their control curves are shifted accordingly,so as not to affect the effect of underlying multi-source coordinated control.By using this voltage compensation again,the current compensation can be calculated.This current compensation can enhance the current sharing accuracy.Thirdly the PI parameters is optimized and the influence of parameters such as communication delay and current sharing coefficient on the stability of the DC microgrid are analyzed.The DC/DC converter under different levels of control is modeled.By analyzing the small-signal model of DC/DC converter in the primary control,the pole of the small-signal transfer function is analyzed when the PI parameter changes,the range of PI parameters which ensure the stability of the DC microgrid system can be obtained.By simplifying and modeling the DC/DC converters in secondary control,the dominant poles of the transfer function of the control system when the parameters such as communication delay are changed.Results shows that the parameters in normal range under study are not likely to destroy the system stability.Finally,on the basis of theoretical analysis and PSCAD/EMTDC simulation,the RT-LAB experimental platform is built based on the existing conditions of the laboratory,which verifies the effectiveness of the proposed hierarchical coordination control strategy for DC microgrid based on low-bandwidth communication.