| In today’s society,the level of science and technology and living standard of human beings are constantly improving.In this context,the dependence on energy is becoming more and more serious.The development of flexible DC grid technology is of great significance to solve the energy problem.Among them,the flexible DC power grid based on Modular Multilevel Converter(MMC)is a general research direction.To some extent,the development of this technology has solved the problem of frequency modulation,which is difficult for the electric energy generated by wind power generation and solar power generation to be integrated into the traditional AC grid,and the power consumption problem of long-distance transmission.Due to the lack of high-power DC power electronic devices and immature technology,fault detection and system protection of DC power grid have been a major problem restricting the development of DC power transmission technology.Especially when the fault occurs in the DC grid line,the fault current rises quickly,which affects a wide area of the grid system and needs to be removed immediately.Therefore,the fault detection and protection methods of the flexible DC grid are proposed.First of all,according to the flexible system of DC power grid from the inverter type and control mode of difference choose symmetric bipolar half bridge module,the power grid of three common grid topology analysis comparison,to cut off the fault line in the high power DC power electronics device hybrid DC circuit breaker of the working mechanism and the system structure is analyzed.The simulation model is built in PSCAD/EMTDC,and the whole system can run stably.Secondly,the fault analysis of the DC side of the DC power grid using the symmetrical bipolar half-bridge sub-module converter is carried out,and the fault mechanism of bipolar short circuit and single-pole short circuit is analyzed.The mathematical model of inductance installed at both ends of the line is established,and the mathematical expression of inductance voltage is obtained.The fault category of the line is obtained by analyzing the information conveyed by inductance voltage when the fault occurs in the line,and a preliminary judgment is made on the fault area.Simulation is carried out in PSCAD/EMTDC to simulate the fault of the line.Finally,according to the voltage information conveyed on the inductance voltage,the main protection scheme is designed when the line fails.With the hybrid DC circuit breaker,the single-pole short-circuit and bipolar short-circuit of the line are detected and protective measures are taken.When the main protection fails,a backup protection scheme is designed to distinguish the faults inside and outside the zone and to isolate the fault lines with hybrid DC circuit breaker to achieve the purpose of protection.And consider the influence of different transition resistance and ac side failure on the protection scheme,simulation in PSCAD/EMTDC,verify the feasibility and reliability of the protection method,and effectively protect the fault line to deal with the fault current,improve the safety of the whole line.Figure [52] table [6] reference [80]... |
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