Power Flow Control And Power Flow Calculation Of AC/DC Microgrid

In order to solve the problems of traditional energy shortage and environmental pollution caused by it,many countries have begun to vigorously develop distributed power generation technology.Compared with the traditional centralized power generation mode,the distributed power generation technology with renewable energy as the main body has the advantages of short construction period,high energy utilization rate and little environmental pollution.At the same time,the large-scale application and access of distributed power supply also bring great challenges to the traditional power grid.The change of power grid structure and operation mode makes it difficult for the traditional research methods of AC power grid to fully adapt to AC-DC hybrid power grid.Mathematical modeling,power flow control and power flow calculation of power grid are the basis for studying new network structure,control scheme and operation mode.Therefore,this thesis focuses on the mathematical modeling,power flow control,power flow calculation and other key issues of AC-DC hybrid microgrid.The main research contents are as follows:Firstly,the research status of distributed power supply and AC/DC hybrid microgrid is sorted out,and the characteristics,working principles,mathematical models and node processing methods of four distributed power supplies,namely solar photovoltaic cells,wind turbines,fuel cells and micro gas turbines,are discussed and studied,and the processing schemes of PQ,PI,PV and PQ(V)nodes when they are connected to the grid are given,which provides a basis for subsequent power flow control and power flow calculation.Secondly,the classical power flow calculation methods,such as loop impedance method,implicit Zbus Gaussian algorithm,Newton-Raphson method and forward-backward method,are studied.Aiming at the problem that the forward-backward method can only deal with PQ nodes in traditional power flow calculation,a new algorithm using equivalent current injection method instead of injection power method is proposed.Compared with the traditional algorithm,the improved algorithm greatly simplifies the structure of the system,improves the convergence of the system and improves the convergence speed.Taking the IEEE33 improved node system as an example,this thesis analyzes and discusses the power flow distribution of different PQ distributed power sources with the same grid connection location,different PQ distributed power sources with different grid connection locations,different PI distributed power sources with the same grid connection location,and different types of distributed power sources with the same grid connection location.Finally,the power flow control scheme of AC/DC microgrid is studied.The mathematical model of the converter is derived,and the DC unit control scheme of the converter is analyzed in detail.By comparing the two power flow calculation methods of alternating iteration method and unified iteration method,the flow of solving the power flow of AC-DC hybrid microgrid network based on extended DC variables is given.Taking the improved IEEE33-node tree network system and three-machine nine-node ring network system as examples,the influence of control scheme on power flow calculation of microgrid is analyzed and discussed.The simulation results show that when the impedance angle of the system is large,it is appropriate to adopt constant current on the rectifier side and constant arc extinguishing angle on the inverter side.When the impedance angle of the system is small,it is appropriate to adopt constant voltage at inverter side,constant power at rectifier side or constant current control;It is not conducive to the voltage stability of AC/DC microgrid when using constant current at rectifier side and constant arc extinguishing angle at inverter side.