Research On Power Balance Control And Transient Power Support Method For Multi-voltage Level DC Distribution System

At present,the penetration rate of distributed new energy generation is gradually increasing,and the voltage levels of loads are gradually diversified.The application scenarios of multi-voltage DC distribution system are becoming more and more extensive.In-depth research on multi-voltage DC distribution system has become essential.Due to the large-scale operation,complicated structure and difficult energy management of the multi-voltage DC distribution system,it has brought severe challenges to the stable operation and economic operation of the system.In order to ensure the reliable and economic operation of the multivoltage DC distribution system and improve the utilization rate of distributed new energy,it is necessary to design an appropriate system steady-state power balance control strategy,and study the system transient operating characteristics and power support method.This paper takes distributed photovoltaic multi-port access to multi-voltage DC distribution system MW-class empirical platform as the main research object,and launches the research on multi-voltage DC distribution system power balance control and transient power support method.Firstly,according to the topological structure and design basis of the system demonstration platform,the mathematical models and control methods of the two-port converter and the multiport converter are introduced to provide design reference for the subsequent system-level power balance control strategy.For multi-port converters,multiple operating modes and control strategy of three-port optical-storage low-voltage DC converter and four-port optical-load medium and low-voltage DC converter are designed,which realizes the power balance among the port units of the multi-port converter.PSCAD/EMTDC simulation results verify the correctness of the coordinated control strategy of the three-port optical-storage low-voltage DC converter and the four-port optical-load medium and low-voltage DC converter.Secondly,a large-scale simulation verification model of the multi-voltage DC distribution system corresponding to the system demonstration platform is built as a verification platform for analyzing and verifying various control strategies.A power balance control strategy based on dual active bridge(DAB)droop phase-shift is proposed.On the basis of using droop control to realize the internal power sharing of the subsystem power,the secondary control by only adding the DC bus voltage per unit value does not increase the burden on the communication line.Under the premise,the global power balance of DC distribution systems with different voltage levels in grid-connected operation mode and island operation,ensuring stable and economic operation of the system.The RT-LAB experimental results verify the correctness of the power balance control strategy for different operating modes.Finally,for the DC faults of the multi-voltage DC distribution system,the transient operating characteristics of the system DC line short-circuit faults and single-pole grounding short-circuit faults under different fault stages are analyzed,and the fault stages with the most serious fault damage are summarized.The influencing factors of transient operation characteristics provide the basis for subsequent fault protection schemes.Aiming at the AC fault of the multi-voltage DC distribution system,a transient power support method based on the grid impedance ratio is proposed to raise the point of common coupling(PCC)voltage within an acceptable range,so that the DC distribution system can continue to be connected to the grid to operate.The RT-LAB experiment results verify the correctness of the transient power support method under AC faults.