Study On Protection Of DC Distribution System With High Degree Penetration Distributed Of PV

Flexible DC distribution system effciently and flexibly connects the growing DC load and power source in the distribution network,reduce the power loss resulting from multi-staged DC-AC conversion and harmonics accordingly generated,and improve the efficiency of power supply and power quality;it also avoids restrictions on traditional AC distribution network,namely,the limited short-circuit capacity and power supply radius.The flexible DC distribution system thus has emerged as one of the important studies at home and abroad.As China plans to build±10 kV multi-terminal flexible DC distribution system in Zhangbei County,Xiong'an New Area,and Southern Industrial Park in the name of many recent national key R&D programs,the protection based on the collaboration of DC circuit breaker and multi-type switch is one of the key technologies to ensure the safe operation of multi-terminal flexible DC distribution system.In light of the unclear fault characteristics of multi-terminal flexible DC distribution system and inaccurate fault area,this paper presents four major research innovations:(1)The characteristics of different converter faults at diffeerent stages in the DC distribution system are analyzed.A transient waveform analysis method based on the principle of curvature is proposed,which reveals the sudden change characteristics of DC fault current and the characterization of the waveform change.This paper proposes the method of the transient current waveform on the basis of polarity variation,and reveals the temporal variations of the transient waveform as the fault occurs,setting the stage for further research.(2)Single-ended protection based on the curvature fluctuation of the current waveform is proposed.This paper analyzes the characteristics of the rising process of the initial transient current waveform including the curving degree and the rising rate and the approximate relationship between the fault distance and the fault resistance.Premised on the curvature of the transient current waveform,a protection identification criterion is proposed to reliably identify the near-end transient resistance fault and a protection setting method is offered.Therefore,the DC fault can be identified fastly and reliably and the capability of-the single-ended protection to endure transition resistance also enhanced.(3)Double-ended protection based on polarity changes of the current waveform is proposed.This paper defines the different current waveform characteristics flowing thrOegh two ends of the line in the case of an internal and external fault and proposes the pilot protection according to polarity changes of the current waveform.The influence of various factors such as the transient current,load current,transition resistance and communication delay of distributed capacitance on the protection is further analyzed.The operation criteria,protection flow,and setting method are given and the operation performance of the proposed protection method is also tested through experiment and simulation.(4)A fault location method of a multi-terminal flexible DC distribution system based on control-protection coordination is proposed.Based on the analysis of the frequency and output characteristics of the converter,DC/DC converter of each terminal is provided with the capability of input characteristic signals by coordinating local protection and local converter of the end as well as multi-end converters.Moreover,the fault location method of a multi-terminal flexible DC distribution system is formed on the basis of the coordination of control and protection and the fault location process is given.Finally,the experiment and simulation are carried out to test the effectiveness and accuracy of the proposed protection location.