Unbalanced Voltage Control Strategy For Bipolar DC Microgrid Based On Improved Droop Control

Compared with AC microgrid,DC microgrid has the advantages of simple structure,easy control and high power quality.Connecting distributed generation,energy storage device and DC load(such as electric vehicle)through DC microgrid can reduce the operation loss and additional cost caused by repeated energy conversion.At the same time,the development of power electronics technology makes DC conversion a low-cost,efficient and easy to realize link.Compared with unipolar DC microgrid,bipolar DC microgrid can provide more voltage level interfaces with higher reliability and security;however,due to the coupling between positive and negative poles,the change of load power will lead to the DC bus voltage deviation from the rated value,resulting in the unique voltage imbalance problem between poles of bipolar DC microgrid;this not only increases the network loss,but also may affect the network performance The stable operation of bipolar DC microgrid is affected.In order to improve the voltage quality of bipolar DC microgrid,based on the improved droop control,this paper studied the unbalanced voltage control of bipolar DC microgrid under the conditions of no communication and communication respectively.The main contents of this paper are as follows:(1)Aiming at the limitation of droop control in DC microgrid and its improvement methods,the hierarchical control structure of DC microgrid with droop control was firstly summarized.Then,based on the basic principle of droop control,the contradiction between the accuracy of shunt and output voltage control in droop control was expounded,and the improvement principle of droop control was analyzed.On this basis,the improved droop control schemes based on fuzzy algorithm and consensus algorithm were compared and analyzed,which laied a theoretical foundation for the application of improved droop control in unbalanced voltage control of bipolar DC microgrid.(2)Aiming at the voltage unbalance between poles of bipolar DC microgrid without communication,a coordinated control strategy of unbalanced voltage based on fuzzy algorithm was proposed.The control strategy only used the local information of each unit in the bipolar DC microgrid to achieve the effective control of unbalanced voltage.Firstly,the structural characteristics of bipolar DC microgrid were analyzed,and the quantitative relationship between the voltage unbalance factor and the positive and negative power supply voltage,the positive and negative load,and the line impedance was deduced,and the influence of power supply voltage and line parameter unbalance on the voltage unbalance factor was analyzed.On this basis,the fuzzy algorithm was used to construct the fuzzy controller,and the voltage unbalance factor and the positive and negative bus voltage were taken as the input of the controller to realize the dynamic regulation of the droop coefficient of the controller.(3)Aiming at the voltage unbalance between poles in bipolar DC microgrid with communication,a distributed cooperative control strategy based on consensus algorithm was proposed,which can achieve the consistency between the average bus voltage and the voltage unbalance factor and maintain at the rated value.Firstly,the voltage droop control was used as the primary control,and an unbalanced voltage observer and a voltage unbalance factor controller are established based on the consensus algorithm and the function relationship between voltage unbalance factor and the positive and negative voltage.On this basis,the secondary control integrated the consensus control of the bus average voltage and voltage unbalance factor was designed to coordinate different distributed generators to participate in the DC microgrid.The stability of the bipolar DC microgrid under the control strategy was analyzed...