Distributed Voltage-Current Cooperative Control Of Multi-Bus DC Microgrids

Microgrid is an effective means to integrate distributed renewable energy resources.Compared with AC microgrids,DC microgrids have the advantages of high efficiency,low cost and without frequency,phase and reactive power considerations for integrating distributed generations(DGs)and loads.In DC microgrids,DGs are connected with DC buses via DCDC converters,and the grid-connected points of DC microgrids are jointed to the main grid via inverters.DC microgrids can operated in grid-connected mode or islanded mode.In gridconnected mode,the main grid is responsible for the voltage stability of DC microgrids,and the loads in DC microgrids are supplied by the main grid and DGs.In islanded mode,DC microgrids operate separately from the main grid,and voltage stability and power balance of DC microgrids are supported by DGs.Compared with grid-connected mode,it is a more challenging work to guarantee normal operation of DC microgrids in islanded mode,and thus we will investigate the issues about operation of DC microgrids in islanded mode.To ensure stable operation of DC microgrids in islanded mode,DGs are required to cooperatively work for the control objectives of voltage regulation and reasonable sharing of load current among DGs.In order to achieve such control objectives,a hierarchical control scheme is generally adopted for the DC-DC converters of DGs.In primary control layer,a decentralized control method is usually used without communication to achieve voltage regulation and current sharing,such as droop control which has obvious disadvantages,i.e.,the accuracies of current sharing and voltage regulation is deteriorated when the impedances of power lines are nonnegligible.The secondary control layer is utilized to compensate the performance of the primary control layer to achieve accurate voltage regulation and current sharing.Besides,distributed secondary control based on multi-agent consensus algorithm has obvious advantages of avoiding failure of single point,less communication and computing bandwidth requirements,and plug and play for DGs.The tertiary control layer aims to realize power scheduling between main grid and DC microgrids and optimization within DC microgrids.From the perspective of stable operation of DC microgrids,tertiary control layer is not indispensable in islanded mode.This thesis focuses on how to realize stable operation of multi-bus DC microgrids based on distributed cooperative secondary control in islanded mode.Due to the physical characteristics of electrical circuit,there exists an inevitable conflict between accurate voltage regulation and accurate current sharing,which cannot be realized simultaneously.To address this issue,this thesis focuses on the distributed cooperative control method which takes into account voltage regulation and current sharing in multi-bus DC microgrids simultaneously.The major works of this thesis are as follows.Firstly,based on the simplified model of multi-bus DC microgrids,it is clarified that bus voltages cannot be arbitrarily regulated in multi-bus DC microgrids if accurate current sharing among DGs is achieved.Accordingly,the compromised control method is the only effective way to take both current sharing and voltage deviation regulation into consideration.In order to quantify the accuracies of current sharing and voltage regulation,a series of metrics are proposed,and then a new control problem is presented in which the accuracies of both current sharing and voltage regulation are restricted into reasonable ranges.To address this problem,a distributed compromised control law is designed for DC microgrids,which features a tradeoff factor to effectively adjust the control objectives from accurate current sharing to accurate reference voltage regulation.For this control law,the stability of the closed-loop system is analyzed in Lyapunov sense,and then it is clarified that the accuracies of current sharing and voltage regulation are determined by two special system matrices respectively,which is independent of the rated voltage of the DC microgrid.For given requirements of accuracies of current sharing and voltage regulation,the existing condition of the corresponding trade-off factor is given.The effectiveness of the proposed distributed compromised control method is verified by simulation examples.Secondly,considering a general multi-bus DC microgrid model,the conflict between accurate current sharing and accurate voltage regulation is analyzed,and the necessary and sufficient condition for the conflict is given.By using the metrics of accuracies of current sharing and voltage regulation,the concepts of accurate current sharing,accurate voltage consensus and voltage-current compromise are defined.Then,according to an existing distributed accurate current sharing control method,a distributed voltage-current compromised control method is proposed,which can adjust the trade-off factor to achieve control objective from accurate current distribution to accurate voltage consensus.Further,the steady-state characteristics of the closed-loop system are analyzed.According to the given voltage deviation requirements by consumers,the reasonable range of the trade-off factor is represented.The proposed distributed voltage-current conpromised control method is verified by simulations.Thirdly,in DC microgrids,the electrical nodes which equip DGs and loads are classified into two categories,critical nodes and ordinary nodes.Based on a common communication network,different control objectives are endowed for these two types of nodes.For critical nodes,it is required to achieve voltage balancing and voltage deviation restriction.For ordinary nodes,accurate current sharing is required.For achieving the above control objectives,Kron reduction which used to reduce the electrical topology of the power systems is applied to the communication network,and a voltage balancing control method among critical nodes is proposed.Then,a distributed cooperative control method is proposed to achieve the different voltage deviation requirements of critical nodes and ordinary nodes simultaneously.Based on the distributed control scheme,the steady states analysis of the closed-loop system is conducted.Furthermore,from control parameter adjusting point of view,a sufficient condition to satisfy the requirement of voltage deviation ratio restriction,and the reasonable range of the output currents of ordinary nodes are proposed,respectively.The proposed distributed cooperative control method is verified by simulations.