| With the increasingly severe global energy situation,the traditional single power system energy structure has been unable to meet the needs of development.In recent years,due to the increasing exhaustion of coal,oil and other traditional energy sources and people’s demand for the development of green economy,the exploration of clean and pollution-free energy and the improvement of comprehensive utilization efficiency of energy have become the focus of social research.Therefore,the concept of multi-energy microgrid system came into being.With the rapid development of electrolytic natural gas production technology and gas turbine technology,the connection between power system and natural gas system becomes closer.As the basis of the coupling analysis of power-gas system,energy flow calculation is particularly critical for its research;Multi-energy microgrid also includes a variety of energy combinations such as photovoltaic power generation,wind power generaion and traditional energy power generation.When the load in the microgrid changes suddenly,whether the system can operate normally in a given manner has an important impact on the stability of the system.This thesis mainly studies the energy flow calculation of the power-gas coupling to form a multi-energy microgrid,and considers the control problems of the grid-connected/grid-off operation mode of the multi-energy microgrid composed of photovoltaic power generation and wind power generation.The main research contents of this thesis are as follows:(1)The multi-energy microgrid formed by the coupling of the power system and the natural gas system is analyzed,and the principle of power-to-gas and gas-to-power conversion is explained.In view of the difficulty of Newton algorithm in selecting the initial value of node pressure in natural gas system,the particle swarm optimization algorithm is proposed to optimize the initial pressure value,and then Newton algorithm is used to sequentially solve the energy flow of the power-gas multi-energy system in two coupling modes.(2)The multi-energy microgrid composed of photovoltaic and wind power are analyzed,and the constant power control method applied in the grid-connected mode of the multi-energy microgrid can make the output power output according to the given power and keep it stable.The constant power control is analyzed in detail.The power control and current control included in the two links and their functions,the constant power control method given in this section can well realize the inverter according to the specified power output,and when the specified output power or load changes suddenly,the system can be kept in a stable range and the response speed is faster.The constant power control is simulated and verified on MATLAB/SIMULINK,and the simulation results show the correctness and effectiveness of the control method given in this thesis.(3)In this part,an improved droop control method is used to study the operation control of photovoltaic-wind power microgrid in off-grid mode,and the voltage control and current control in the droop control are analyzed.In view of the problem that the power of multiple inverters in parallel cannot be divided equally,the reason for the failure to be divided is analyzed,and the virtual impedance method is considered to solve it.Aiming at the voltage/frequency drop caused by traditional droop control and virtual impedance,voltage/frequency compensation is proposed to adjust.A simulation model is built on MATLAB/SIMULINK.The simulation results show that the virtual impedance method can solve the problem of unsatisfactory power sharing of multiple inverters in parallel in the droop control.The voltage/frequency drop after adjustment by the compensation link is obtained.Significant improvement has verified the correctness and effectiveness of the method proposed in this thesis. |
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