Research On The Key Control Techniques Of Medium Voltage Flexible DC Distribution Network

In recent years,with the continuous expansion of urbanization,the rise of distributed energy,the high-density use of renewable energy and the widely use of DC loads,how to rationally develop and effectively use renewable energy,how to meet the power demand of large cities,how to further improve the power quality of the power grid and enhance the controllability of the power grid system are the great challenges we are facing today.Compared to traditional AC distribution network,flexible DC distribution network has shown great potential and great technical advantages in saving a large number of DC/AC commutation links,improving the conversion efficiency,increasing the power supply capacity and radius,reducing the occupied area of urban land,line electromagnetic radiation and interference.Moreover,it can also solve a series of power quality problems such as line loss,voltage fluctuation,grid harmonics,three-phase unbalance in the existing AC distribution network,and greatly improve the reliability and controllability of urban distribution.However,the current research on DC system mainly focuses on the HVDC transmission system and low-voltage micro-grid.Few research focuses on medium-voltage DC(MVDC)distribution network.Therefore,the research in this paper focuses on the MVDC system architecture and key control techniques.The main contents and innovative points are as follows:1)In the studies on overall structure of flexible MVDC distribution network,the system structure and key equipment suitable for flexible MVDC network are determined.The system control architecture on the secondary side is analyzed and a three-layer control system is proposed based on the design principle of "unified control of key equipment and autonomous control of power load".Moreover,the operation modes of MVDC distribution network and the control modes of key equipments are reserched.2)In the studies on the control system of key equipment in the flexible MVDC distribution network,the principle of MMC and DCSST are studied.Moreover,a maximum power transfer method under ac side faults is proposed which can reduce the impact of AC system faults on the power grid.3)In the studies on the operation control of MVDC distribution network,firstly,by studying the defects of the traditional master-slave voltage control strategy and the voltage droop control,the P-U-I controller with three-ring characteristics is proposed in this paper.This controller can provide a friendly power interface for the EMS system and realize the power regulation function in the steady state.In addition,in the transient state,the system transient voltage can be stabilized and seamless mode switching can be achieved.Moreover,based on the P-U-I controller,a voltage coordination control strategy suitable for multi-terminal flexible MVDC distribution network is proposed.The simulation results show that the control strategy improves the voltage and current control capability of the system under large disturbances,and achieves seamless operation mode switching.4)In the studies on the operation mode identification of the flexible MVDC distribution network,in order to solve the problem of listing operation mode for the system in master-slave control and EMS system,a control strategy based on breadth-first search algorithm is proposed.Fast identification of operation mode of complex MVDC distribution network is realized.5)In view of the diversity and complexity of equipment and control of flexible MVDC distribution network,system start/stop control strategy,"plug and play" control strategy and DC fault ride crossing control strategy are also proposed which enrich the control scheme of flexible MVDC distribution system under different operating conditions.6)A dynamic simulation test platform for flexible MVDC distribution network was built according to a certain simulation ratio.The practical problems involved in the construction of the platform,such as the choice of simulation ratio,system communication and interface,the selection of simulation equipment and the development of the main controller are introduced in detail.Finally,the correctness and effectiveness of the key control technology of MVDC distribution network system are verified by the dynamic simulation test platform.