Research On Line Protection Scheme Of Medium Voltage DC Distribution Network

With the gradual improvement of the status of clean energy,the demand for power system integration of renewable energy is also increasing.The traditional distribution network has many problems,such as high power loss,poor power supply quality and many commutation links,when it accepts distributed renewable energy.Therefore,the feasibility of using DC distribution networks has become a research hotspot in the field of distribution networks.The DC distribution system of medium voltage grade has become a development direction of urban power distribution in the future due to its advantages of high power supply reliability and low output harmonics.However,the large-scale development of DC distribution networks still faces many challenges,and one of the main obstacles is the relay protection of DC lines.In this context,this thesis analyzes the fault characteristics of DC lines and carries out the following research on the protection scheme of medium voltage DC distribution network lines:First,the structure,wiring mode and grounding mode of the medium voltage DC distribution network are analyzed from the two aspects of economy and reliability.This thesis summarizes the basic principle of the key electrical equipment affecting the DC distribution network,and selects the voltage source converter as the AC/DC conversion equipment.At the same time,the simulation model of the medium voltage DC distribution network is established in PSCAD/EMTDC,and its stability is verified.Second,this thesis analyzes the fault principle of the DC side circuit of a medium voltage DC distribution network and selects bipolar short circuit faults,which will cause serious overcurrent of the DC system,as the research object.According to the existing DC line protection schemes at home and abroad,traveling wave protection based on single-end electric volume and differential protection based on double-end electric volume are selected for study in medium voltage DC distribution networks.The advantages and limitations of these two fault identification methods in a medium voltage DC distribution network are analyzed theoretically and verified by simulation.Finally,two kinds of DC line protection schemes based on single terminal traveling wave protection and differential protection are studied.First,the different establishment times of the traveling wave head are used to constitute the protection criterion.Faults inside and outside the DC line area can be identified reliably by detecting the polarity of the initial traveling wave head,and fault location can be carried out quickly according to the time constant calculated by the detection device after fault identification.Second,the frequency characteristic of the fault differential current is used as the protection criterion.On the basis of current differential protection,wavelet transform is used to decompose the differential current signal,and the decomposed signal is used to identify the fault area.The performance of these two protection methods is verified by using the distribution network model established in PSCAD/EMTDC.The verification shows that the two solutions and traditional traveling wave protection and DC differential protection in medium voltage distribution network adaptability are better;even in the case of high impedance fault and noise interference,they still have good motion performance.