DC Distribution Network With Distributed Photovoltaic System

The distribution network is the terminal network of the power system,which undertakes the function of delivering electric energy to users.The current distribution networks are all AC distribution networks.In recent years,new energy and power electronic technology have made great progress.Distributed generation represented by photovoltaic,electric vehicles,etc.mostly use direct current,which requires an ACDC converters to be incorporated into AC power network.A large number of converters reduce the reliability and efficiency of the system.Moreover,the power quality of the existing AC distribution network is difficult to meet the needs of industries and life.DC distribution network with outstanding advantages in stability and controllability has great development potential.In this thesis,four typical DC distribution network topologies are analyzed by simulation.Open-loop simulation is carried out for the four topologies under normal state and fault state respectively,and the stability and reliability of the four topologies are analyzed.The maximum power tracking strategy of distributed generation represented by photovoltaic is studied,and a new photovoltaic maximum power point tracking algorithm is proposed.Compared with existing algorithms such as Particle Swarm Optimization and Cuckoo Search,the tracking speed is increased by 56% and 65% respectively.It provides the technical basis for the efficient access of distributed photovoltaic system to DC distribution network.Modular Multilevel Converter is studied,CPS-SPWM modulation mode with voltage sharing control is adopted,and improved DC voltage droop control suitable for DC distribution network is proposed.Meanwhile,DC transformer is studied.The topology of double active full bridge high voltage side connected in series and low voltage side connected in parallel is adopted.Aiming at the power backflow of high frequency chain,a backflow power suppression strategy based on single-side double phase shift is proposed,which greatly reduces the backflow power.Finally,combined with the improved droop control of MMC,the system-level control mode of DC distribution network was studied.Aiming at the unbalanced output of each converter station when the load was too large,the auxiliary power balance control was proposed,which can not only stabilize the medium-voltage DC voltage around the rated value,but also make the active output of each converter station equal,thus improving the balance of the system.Finally,based on the change of IEEE 9-node standard topology structure,a three-terminal encircling calculation example is set up.After analysis,the system is stable and balanced in operation.This thesis puts forward the new rapid photovoltaic maximum power point tracking algorithm,and the improved droop control for DC distribution network,and reflux power suppression control of DC transformer,and system-level power balance control method.Effectively improve the photovoltaic maximum power point tracking speed,reduced the reflux power of the DC transformer,ensure the multi-terminal DC voltage stability of distribution network and the output of the converter station is balanced,and lay a solid foundation for the construction of a new round of DC distribution network in the future.