Studies On Effectiveness Of The Average Value Model And Its Application

With the development of modern power system,environmental pollution becomes more and more serious.Renewable energy has become the main goals of energy development all over the world.Microgrid and VSC-HVDC,as effective ways to accept distributed renewable energy and large-scale centralized renewable energy,have become a hot research topic.A large number of controllable power electronic devices are required for microgrid to process current transformation.Besides,modular multi-level converters(MMCs)in VSC-HVDC transmission system contain a large number of sub-modules.Simulation of the microgrid and VSC-HVDC by using their detailed models will result in huge memory requirement,slow simulation speed and large simulation time.For microgrid containing a large number of power electronic devices and VSC-HVDC transmission system with many MMCs,effectiveness assessment and application of the average value modeling method are carried out in this thesis.The main work of the thesis is summarized as follows.(1)The theory of the average value modeling method is systematically analyzed.First,the fundamental principle of the method is introduced.Then,the details of two modeling methods,i.e.,the state space averaging method and the switching cycle averaging method,are deduced,which lay the basis of establishing the average value model of microgrid and VSC-HVDC transmission system.(2)The rapid simulation of the microgrid with typical power electronic devices is studied.First,based on the average value modeling method,the average value models of the typical power electronic devices,including DC/DC,DC/AC and AC/DC converters,are established.Second,the models of two typical distributed generators in the microgrid,photovoltaic and microturbine,are established.Third,the average value modeling method is applied to the rapid simulation of microgrid.The average value model of microgrid with typical power electronic devices is established.Last,both the detailed model and the average value model of the microgrid are built in MATLAB.The accuracy and speed of the average value modeling method are verified through the comparison of the simulation result.(3)The rapid simulation of the VSC-HVDC transmission system based on MMC is studied.First,the principle and modulation method of MMC are analyzed,as well as the topological structure and working mode of MMC sub-module.Second,the average value models of MMC and the VSC-HVDC transmission system are established according to the principle of average value modeling method.Finally,the detailed model and the average value model of 41 level MMC-based VSC-HVDC transmission system are built in PSCAD.The effectiveness of averaged MMC model in simulation accuracy and computational efficiency is verified by comparing the simulation results of the average value model and the detailed model under steady-state operation and transient fault.The results of this research reveal that the average value model can reflect the external characteristics of the modeling object with a reasonable precision and greatly improved simulation speed.It has great theoretical importance and practical value for further research on the operating characteristics and coordinated control strategy of microgrid and HVDC grid with a large number of power electronic devices through electromagnetic transient simulation.