Flexible Virtual Capacitance Control Of DC Microgrids Considering Multi-index Constraints

DC microgrid has the advantages of less line loss and high penetration of distributed generation.And there are no complex stability problems such as frequency and phase synchronization in DC microgrid.However,the DC bus voltage is vulnerable to the impact of intermittent new energy sources and loads due to the low inertia.The problem can be solved by adopting virtual capacitance control for converter at the control level.At this time,the converter can simulate the charging and discharging characteristics of the capacitor to improve the voltage quality of the DC micro-grid.And the virtual capacitance value can be adjusted,which enhances the flexible controllability of the control method.However,the flexible adjustment of virtual capacitance under this control strategy needs to satisfy a variety of constraints to ensure the stable operation of the system,such as the stability,dynamic response characteristics,actual operation requirements and realizability.Therefore,the flexible virtual capacitor control strategy of DC microgrid considering multi-index constraints based on voltage source converter is studied in this paper.The main research results are as follows:(1)The typical six terminal DC microgrid topology and hierarchical control system are studied in this paper.On this basis,the reasons of DC bus voltage sudden change and poor voltage quality are analyzed.In order to improve the output power quality of DC microgrid,a flexible virtual capacitor control method which can suppress DC voltage mutation and provide "inertia" support for DC microgrid is designed.(2)In order to measure the inertia support ability of virtual capacitor control to DC microgrid,the mechanism of transient power distribution at each terminal when the system is disturbed is studied in this paper.And the paper confirmed that the transient power at each terminal of DC microgrid is distributed according to the ratio of capacitance value at the initial stage of disturbance.Based on virtual capacitance control,the equivalent capacitance of DC microgrid is defined and evaluated.Compared with the traditional droop control,the virtual capacitance control system can better suppress the DC voltage mutation,and the port parallel capacitance can achieve the same effect.(3)Aiming at the engineering problem of flexible virtual capacitor control,the key indexes that limit the virtual capacitance value are analyzed,and a small signal model of six terminal DC microgrid is established in this paper.The stability,dynamic response characteristics,actual operation requirements and realizability is considered as the stable operation constraint set,and the flexible virtual capacitor control considering multiple index constraints is proposed.The stable operation index set satisfying multiple constraints under the virtual capacitance control is established,and its stable operation boundary is calculated in this paper.(4)In addition,a controller level hardware in the loop test platform is established to verify the effectiveness of flexible virtual capacitor control considering multiple index constraints,and the correctness of the calculation method of multiple constraint stable operation boundary proposed in this paper is verified.