| With the depletion of fossil energy resources and the deterioration of the environment,the conflict between the development of human society and the traditional energy industry has been intensified,forcing the change of traditional power structure.In the past few decades,the development and utilization of renewable energy sources(RES)has received more and more attention.Although the traditional power system is reliable and controllable,due to the loss of long-distance transmission and the dissipation of thermal energy,the energy utilization rate is very low,and the exhaust gas emission is high.Smaller scale,higher efficiency,and lower discharge capacity has made renewable energy generation possible to improve energy use,especially the rapid development of wind power,photovoltaic,fuel cells,tidal power generation,and energy storage technologies,which have provided more and more feasible solutions for the development of Microgrids.Among the renewable energy sources,the development and utilization of wind power(WP)has received universal attention from all of the countries.WP has become one of the new energy sources with the highest level of development and utilization.Wind resource has obvious volatility and uncertainty,and the output power is greatly affected by region and weather,making the reliability of WP generation low.In order to increase the reliability of wind power generation,it is necessary to add a controllable generation unit or distributed energy storage complements wind power generation to form an combined power supply system.The combination of WP and MGT can ensure reliable power supply of the microgrid under various operating conditions.In this thesis,the working principle and equivalent mathematical model of wind turbine,MGT and battery were analyzed,and WP,MGT and Energy Storage System(ESS)simulation models were built on the PSCAD/EMTDC simulation platform according to the mathematical model of each part.Secondly,because each DG unit is connected to the AC bus via the inverter,there is a problem of coordination control,for example,power sharing among these power sources.Based on the multi-inverter parallel structure,the existing control methods of non-communication parallel inverter were compared and analyzed:the dynamic power sharing performance of Virtual Synchronous Generator(VSG)control method and Droop control.Draw the following conclusions:VSG control is an inverter control method that can simulate the steady state droop characteristics and simulate transient characteristics of synchronous generators.The moment of inertia and damping term of VSG control equations can increase the frequency stability of the system.The microgrid should have the capability of grid-connected and islanding operations,and can transfer seamlessly between the two modes.The respective advantages and disadvantages of the existing control strategies of seamless transfer were analyzed.A combined control strategy of direct current control and droop control method was proposed,which can guarantee the power supply quality of the load when switching from grid-connected mode to islanding mode,and can improve the reliability of the microgrid in islanding mode.A combined method of direct current control and droop control for multi-master and multi-slave architecture is proposed.When the grid is broken,the master DGs can transfer from direct current control to V/f control automatically and the quality of load voltage will not be affected.It also can improve the reliability of islanding mode because of the structure of multiple master DGs.Finally,based on the combined power supply system,the reasonable operation mechanism and coordinated control strategy are proposed.The load switching plan and the output of DG units were formulated according to wind speed and the SOC status of the energy storage system,thus can improve the overall system operating efficiency. |
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