Design And Research On Control And Protection System Of Medium And Low Voltage AC/DC Hybrid Distribution Network

With the increasing development and progress of society,low-voltage DC loads for industrial and household use are increasingly appearing in current production and life.At the same time,with the rapid development of distributed power sources,DC power sources such as photovoltaics and energy storage are used more and more widely.AC power distribution network is increasingly unable to meet the efficient access of DC power and load.At present,there are many research and demonstration applications of point-to-point transmission for DC power and load at home and abroad,but each demonstration project focuses more on the DC transmission scenario,while less on the power distribution scenario,and the application prospects of such projects are limited.Therefore,the research on the networked medium and low voltage AC-DC hybrid power supply grid,and the design of the control and protection system under the AC-DC mixed supply mode will promote the rapid development of the future medium and low voltage DC.This article will focus on the control and protection technology under the AC-DC hybrid distribution network.In terms of grid structure,the current common voltage levels of medium and low voltage DC systems in related industries,different countries and regions were discussed in the article,and the voltage levels of DC and AC systems suitable for this demonstration project were selected according to the characteristics of this project.The AC and DC power supply and the load situation near the site was analyzed.In order to absorb the surrounding photovoltaic power supply and solve the large DC load demand of the data center,a four-port flexible converter with medium and low voltage levels,AC and DC mixed power supply was proposed.The converter has multiple functions such as flexible interconnection and AC-DC power distribution,and a regional power supply network was builed based on this.On the basis of the grid,the wire cross section was selected according to the transmission capacity and voltage drop theory.This grid scheme provides a new idea for solving regional photovoltaic grid connection and power consumption of data center.In terms of control system,the requirements of the four-port flexible converter and the control system under the AC/DC distribution network frame were analyzed and studied in this article,while the control system structure was divided.The control functions were analyzed and designed from the entire system level,sub-equipment level,etc.System-level control was mainly to design the overall operation mode of the dual power supply system,and a master-slave control strategy based on voltage deviation was proposed.This strategy relies on communication to quickly realize DC voltage switching and takeover when there is communication,which can achieve smooth voltage takeover and accurate power control.This strategy relies on DC voltage deviation to take over when there is no communication,avoiding DC voltage instability,so as to achieve better control results no matter if there is communication or not.The control at the equipment level is mainly based on the system-level control strategy to select the constant voltage or constant power control.This control strategy of the project has a wide application range,which is simple and efficient.In terms of protection system,grid structure and power industry protection specifications were combined in this article,and the idea of dividing four types of protection areas: AC,DC,load,and device level according to the idea of hierarchical division was proposed.The AC-DC system protection suitable for this project was designed.And a DC protection mode based on converter blocking for fast current limiting,fast positioning of optical differential protection,and fast fault isolation of the monitoring system was proposed.This protection mode can isolate faults simply and quickly,and pass the capacity matching the capacity of the flexible converter station.The AC transformer and energy storage device ensure uninterrupted power supply for the lower load.In terms of simulation analysis,the boundary conditions of the power system of the project were analyzed,and the simulation model of the project in PSCAD software was builded according to the parameters of each voltage converter.Fault simulation tests and studies for different fault types and different fault points were conducted,analyzing the action behavior of the control system and the cooperation between each protection action behavior and the protection.At last the feasibility of the project control and protection scheme design were demonstrated.