Research On Fault Risk Assessment And Recovery Strategy Of Multi-outfeed And Multi-infeed UHVDC System In Similar Path

At present,China’s power network in general presents the"west to east","north to south"power pattern..At the end of 2020,after the operation of Qingyu DC and Tianzhong DC,a typical multi-outfeed and multi-infeed UHVDC transmission system with similar path will be formed with the northwest grid as the multi-outfeed network and the central China power grid as the multi-infeed network.Its operation mechanism is more complex than the traditional multi-infeed transmission system,so it is of great significance to accurately grasp the DC system fault risk and develop an effective recovery strategy for the safe and stable operation of China’s DC transmission system network.Thesis begins with a detailed discussion of the main circuit and control method of the DC transmission structure,using the CIGRE standard test model as an example.Next,establish the mathematical model of the main circuit and control system of the multi-DC system,referring to the actual engineering parameters of the Qingyu-Tianzhong multi-outfeed and multi-infeed UHVDC transmission system with similar path of Northwest Grid-Henan Grid.Building Equivalent Circuit Topologies in PSCAD/EMTDC Simulation Platform.Finally,the simulation analysis verifies that the model can be used for the study of multi-outfeed and multi-infeed DC transmission systems.Second,by analyzing the mechanism of interaction effects of multi-outfeed and multi-infeed UHVDCV systems with similar path at the receiving end of the converter station,it is found that the results of direct application of multi-infeed analysis methods in multi-outfeed and multi-infeed UHVDCV systems with similar path tend to be optimistic.After considering the limitations of the multi-infeed index,the multi-outfeed and multi-infeed UHVDC transmission system with similar path electrical characteristics are combined to propose the multi-outfeed and multi-infeed interaction factor(MOMIIF_ji)to quantify the magnitude of interaction between multi-outfeed and multi-infeed UHVDC transmission system with similar path.Further,the critical condition of commutation failure and the calculation method of MOMIIF_ji are combined to construct the risk assessment method of simultaneous phase change failure in the multi-outfeed and multi-infeed UHVDCV systems with similar path.The simulation example of Qingyu-Tianzhong verifies that the multi-outfeed and multi-infeed interaction factor has high accuracy during the temporary steady state and the proposed commutation failure criterion considering the fault type,fault location,fault degree and fault moment.Finally,after considering the strength of each AC system,the transmitted DC power and the ability to support the remaining DC return,the multi-outfeed and multi-infeed recovery factor MOMIRF_ji is derived as the multi-outfeed and multi-infeed UHVDC transmission system recovery order indicator.And jointly MOMIRF and VDCOL change the inverter side DC current output command to make each system current lag voltage recovery and realize the orderly recovery after each line fault.Finally,it is verified in the PSCAD simulation platform that the MOMIRF_ji-based multi-level VDCOL coordinated recovery strategy can effectively reduce the reactive power consumption of the DC system during fault transients and improve the voltage recovery speed during faults in the multi-outfeed and multi-infeed UHVDC transmission system with similar path.