Research On Operation Characteristics And Control System Of Microgrid Based On Droop Control Of Inverters

Energy sources are power force of society and economy to develop. When the problems of environment and the gradual exhaustion of conventional energy stand out deeply, it is necessary to search a sustainable development way for power industry. Microgrid is an important part of the future power system for its intelligent and sustainable characteristics. Microgrid can be operated in both grid-connected and autonomous modes. In the autonomous mode, the control could achieve accurate power sharing between Distributed Generators(DG) to keep power supply high quality. The droop controlled microgrid could reduce the dependency on the communication, increase its reliability and flexibility, and is the object to research in this paper. The operation characteristics of islanded microgrid, and the regulating schemes of the voltage and frequency are studied in the paper. The modified droop control is proposed. Finally, the function and duty to implement or complete for MicroGrid Energy Management System(MG-EMS) is analyzed. The detailed research is as follows.(1) The basic concept, component of microgrid, and it’s significance in China, are summarized. The domestic and overseas situation and hot issues at present are analyzed. It is concluded that the droop controlled microgrid has a favourable picture and the research is useful and theoretical.(2) The output impedance of inverter is highly inductive. The voltage amplitude V is positively correlated with its active output P, while the phase angleΦ is positively correlated with its reactive output Q, which are both primary. So the decoupling control is adopted, the droop relationships between Q, V and P, ω introduced artificially, which are Q-V and P-ω droop control. The paper analyzes in theory the voltage and frequency regulation characteristics of microgrid based on droop controlled inverter. The results are that the microgrid can regulate the voltage and frequency automatically, and keeping the P-co droop gain small properly can improve the primary frequency control ability.(3) The structure of the droop controlled DG inveter, and the function of every part are briefly introduced. A systematic approach to modeling the dynamic characteristics of an islanded microgrid is presented, which captures the dynamic behavior of inverter, network and load and the influence of each others. An islanded microgrid case based on droop controlled DG inverter is studied by eigenvalue calculation and Matlab/Simulink. The results show that the droop gains affect the stability of microgrid greatly. The microgrid can regulate the output of DG automatically, which is easy to implement to plug and play for DG and loads. The DG near the disturbance point undertakes most of the unbalanced power in transient process. Besides, voltage deviation and frequency offset are aroused by the disturbance. Regulating the control parameters of the inverters can adjust the load power distribution and realize errorless regulation of the frequency and voltage.(4) To solve the dynamic problems existing in conventional droop controlled microgrid, such as circulating current, slow response, a modified droop control method is proposed in the paper. Both active power and reactive power signals are included in each equation; droop factor is added in the item whose correlation is higher; and the differential signal of the item is added. The small-signal model of the inverter based on modified droop control is presented. The calculation and analysis show that the phase angle, impedance angle and droop gains all affect the stability in different degree. Using Matlab/Simulink, the conventional and modified droop control methods are analyzed, which verify that the proposed droop control can improve dynamic performance of microgrid.(5) Finally, the task and the realization mode of MG-EMS is analyzed in theory in the paper. The strategies of microgrid’s economic operation and optimized scheduling are introduced, which are presented in related literatures. And the method of the particle swarm optimization is presented, which is of good performance and widely used.