Research On Three-Port Coordinated Control System For Photovoltaic Microgrid

Power imbalance in microgrid will not only affect the normal operation of the microgrid itself,but also will have great harm to the utility power grid.Thus,it is the invariable subject to maintain the power balance of the microgrid by some means of control methods in the development of microgrid.Based on analyzing the stability of photovoltaic microgrid operation and the dynamic balance of photovoltaic power source,grid-connected inverter and energy storage device,a three-port coordinated control method is proposed in this thesis.The system constitution and the principle and control method of the three-port converters for photovoltaic microgrid are analyzed in this thesis deeply,and the work is focused on the following sevsral parts.(1)According to the system composition of the photovoltaic microgrid,the mathematical models of the subsystem are established,and the characteristics of each subsystem are analyzed.Then the simulation model of the subsystems is built in the Matlab/Simulink environment.(2)For photovoltaic power source,the MPPT control strategy of the perturbation and observation method(P&O)and constant voltage control strategy are adopted,to establish the PV system model.In the control of grid-connected inverter,the constant power(PQ)control is adopted,and the inverter can keep the power stable in grid-connected and islanded modes.In the charging and discharging control of battery,the method of constant voltage is used after constant current in charging mode while the method of constant current is used in discharging mode,the bidirectional charging and discharging control of the battery is realized.(3)There is no need to take the frequency and phase of the photovoltaic microgrid into consideration,and the voltage stability of the DC bus is the only indicator of the power balance.According to the relationship between actual value and reference value of DC bus voltage and the relationship between output power of photovoltaic power source and load power,the microgrid can be divided into five modes.And through setting the switching condition,the autonomous seamless switching of five modes will be realized,and the purpose of power balance control will be achieved.(4)The simulation model of the three-port coordinated control system is built in the Matlab/Simulink environment after the simulation parameters are designed.The steady state process and the transient process of mode switching in microgrid are simulated,and the correctness of the proposed method is verified by simulation results.(5)To further verify the feasibility of the three-port coordinated control method in practical application,the hardware circuit and software control method based on the TI TMS320F28335 DSP are designed,and the experimental platform is built.According to the actual operation data and experiment waveforms of the system,the three-port coordinated control system has achieved the expected results.