Research On Topologies And Control Strategies Of MMC Based High-power High-voltage DC/DC Converter

Recently,much attention has been paid to offshore wind power,one of the latest development in global wind power technologies.It has been widely accepted that High Voltage Direct Current(HVDC)technology is likely to be the only feasible option for connecting large offshore wind farms over long distance.Compared to Multi-terminal HVDC(MTDC),DC grid is expected to provide higher power supply reliability,higher equipment redundancy,more adaptable power supply mode,and more flexible and secure power flow control.Although the prospect of DC grids offers many benefits,one of the principal challenges is the interconnection of different DC networks.Now,there are many HVDC schemes interconnecting different grids and they have used a wide variety of DC voltage levels.In the absence of a DC/DC converter,these schemes could only be integrated into the network by their AC connections,which cannot constitute a true sense of DC grid.Therefore,as one of the key equipments for DC interconnection,the high-power high-voltage DC/DC converter is an important part for constructing future multi-terminal multi-voltage levels DC grid.The topic of the thesis closely follows the latest global research and application on multi-terminal multi-voltage levels DC grid for large intermittent renewable energy integration.The thesis mainly focuses on the topologies,steady control and fault protection strategies of the high-power high-voltage DC/DC converters for future DC grid,which has theoretical innovation and practical application.The specific contents can be summarized as follows:The technical requirement of high-power high-voltage DC/DC converter for future DC grid is analyzed.The topologies of high-power high-voltage DC/DC converter proposed by domestic and foreign scholars have been investigated and classified,including isolated and non-isolated type.And the advantages and disadvantages of the corresponding topologies have been summarized.Then the topology of the Modular Multilevel Converter(MMC)is investigated and compared,and its working principle is analyzed,and the modulation and control methods are also studied.The detailed MMC model will be used for the modelling and simulation analysis of high-power high-voltage DC/DC converter.The thesis proposes an isolated two-terminal high-power high-voltage DC/DC converter based on hybrid MMC,and designs its modulation strategy and the capacitor voltage balancing strategy,and analyzes the DC fault blocking principle.First of all,through the investigation of existing MMC submodule(SM)topologies,a T-type full-bridge SM(T-FBSM)with DC fault current blocking capability is proposed.Based on this,a hybrid MMC based DC/DC converter is established,and the corresponding mathematical model is built.Then the modulation strategy and the capacitor voltage balancing strategy are designed,the strategy takes advantage of T-FBSM negative level output to achieve the stable operation of multi-voltage level DC system under reduced DC voltage.Then the DC fault blocking capability is achieved through the rational allocation of SMs.Finally,steady and transient simulation analyses are performed in the PSCAD/EMTDC simulation platform.The thesis proposes the topology of isolated multi-terminal high-power high-voltage DC/DC converter,and designs the steady and fault control strategies,and compares with the DC fault isolation scheme.First of all,the necessity of multi-terminal high-power high-voltage DC/DC converter is discussed.Then the multi-terminal DC/DC converter is applied to offshore wind farm integration with multi-voltage levels,and its master-slave control strategy is proposed.And then,a detailed analysis on the principle of the DC/DC converter operation under reduced DC voltage is presented.Based on this,a control strategy during DC fault is proposed,which can realise “DC fault ride through” under minimised arm FBSM ratio.Finally,the effects of blocking and unblocking of the faulty terminal during DC fault are compared through the time-domain simulation.The proposed SM configuration principle will help researchers rationally allocate arm SM according to the actual operation requirements of the DC grid,which can further reduce the converter cost and operating losses under the premise of DC fault isolation.The thesis proposes the SM configuration of HVDC-DC Auto Transformer(HVDC-AT)considering DC fault,and analyzes the cost of two-terminal and multi-terminal HVDC-AT,and carried out the full operation process simulation under DC fault conditions.First of all,the operation principle of the HVDC-AT is described.Considering the arm current differences,the total equivalent semiconductor number for the two-terminal and multi-terminal HVDC-AT are derived and compared with the corresponding isolated topologies.A full operation process for the multi-terminal HVDC-AT considering DC fault is then presented,including normal operation,fault isolation and continuous operation of healthy converters after fault.Finally,average MMC model with fault characteristics is established.Based on this,the validity of the above theoretical analysis is verified by time-domain simulation.The proposed analysis method will help researchers to select the topologies of high-power high-voltage DC/DC converter from the cost point of view according to the voltage level and capacity of the actual DC grid.The research results of the thesis will enrich the existing operation,control and fault protection theoretical framework of DC grid,and can provide important technical support for future large-scale offshore wind farm integration and the interconnection of multi-regional DC networks.