Research On Line Protection Strategy Of Multi-terminal Flexible DC Distribution Network

The shortage of energy and global warming have gradually restricted human development,and energy transformation has become unstoppable.In the new power system,the installed capacity of clean new energy sources such as wind energy and solar energy has been rapidly increased,while stimulating a large increase in DC load.The power supply capacity and quality of the traditional AC distribution network can no longer adapt to the development of the new power system.The flexible DC distribution network has received extensive attention from practitioners.The line protection method of flexible DC distribution network is the core technology to support the development of flexible DC distribution network.At present,it still faces the problem of narrow threshold range,close to the set value and easy misjudgment.The research on the line protection method of flexible DC distribution network is one of the keys to the development of flexible DC distribution network.This thesis proposes a fault diagnosis method based on multi-terminal flexible DC distribution network.A bipolar short-circuit fault is selected from the identified faults,and a location methods is proposed.The above method provides technical support for the safe and stable operation of flexible DC distribution network when line fault occurs.Firstly,the simulation model was built and the fault characteristics were analyzed.The typical topology of flexible DC distribution network and the mechanism of its basic control mode were analyzed,and then a multi-terminal flexible DC distribution network model was built on PSCAD/EMTDC4.5.The fault characteristics of the flexible DC distribution network were analyzed for the model,and the accuracy of the theoretical analysis of the fault characteristics was verified by simulation analysis.Secondly,the thesis proposed a multi-terminal flexible DC distribution network line protection method based on current mode component characteristics.The protected areas of DC line was divided by the circuit breaker at two-terminal of the line.The fault occurrence interval was identified based on the fact that the positive current line mode component direction at two-terminal of the fault line is positive at the same time when the fault occurs,and the positive current line mode component direction at two-terminal of the non-fault line is one positive and one negative.The fault identification criterion was constructed to identify the fault type based on the difference in the mean value of the zero-mode component of the positive current at the fault point under different fault types.The applicability of this protection method was verified on the multi-terminal flexible DC distribution network model built in Chapter 2,and the ability of the method to withstand transition resistance and noise was analyzed.Finally,a bipolar short-circuit fault location method using effective information before fault was constructed.The equivalent circuit model after the fault was analyzed,and the equations were established based on the relationship between the two-terminal electrical quantities of the line and the line parameters.The fault distance equation expressed by the electrical quantity and parameters of the line was obtained.Wavelet transform was used to eliminate the influence of distributed capacitance on the electrical quantities at two-terminal of the line.The processed electrical quantities were substituted into the distance detection expression,and the average value was taken to obtain the distance detection result.The simulation results show that the fault location method can reliably determine the fault distance and meet the requirements of fault location.