Design Of Arc Faults Locator For Low Voltage Distribution Circuits Based On Time-domain Reflection

In the harsh environment conditions,physical friction,chemical erosion and many other factors may make low-voltage distribution line cables' insulation layer aging,wear,poor contact and etc.,which lead to fault arc.Fault arc is usually the culprit of the fire.After a fault occurs,the fault must be located in order to repair and replace the faulty cable.Different from the high-voltage transmission lines,low-voltage distribution lines are relatively short.The transmission voltage is low,and usually laid in the walls of buildings or closed pipes,difficult for visual distinguishing.Fault location technologies that suitable for high-voltage transmission lines are not suitable in low-voltage distribution lines.At present,the study of fault arc location at home and abroad has certain limitations in the application scope and precision.Therefore,the low-voltage distribution line arc fault location in-depth,comprehensive study,to find a fast and accurate location of the fault location method is important.Firstly,the transmission characteristics of the cables in low voltage distribution lines are analyzed in this thesis.By establishing the distributed parameter model of parallel double conducting wires,the wave equation and the characteristic impedance of transmission cable are deduced.Then,the basic principle of time domain reflection technique is discussed.In this process,the propagation velocity of the pulse signal in different dielectric cables and the reflection coefficient of the reflected signal are studied in order to improve the positioning accuracy of the time domain reflection technique and determine the type of fault arc.Secondly,the system design of the arc fault locator is carried out in this thesis.The design scheme of the hardware circuit of the arc fault locator based on the time domain reflection technique is introduced in detail from the core circuit and the external shaping circuit.In this process,this thesis improves the arrangement of the traditional delay line delay unit,and distributes the delay elements in the multi-column programmable logic module by using the method of equal interval and crossing arrangement to improve the delay line length and reduce the line error.In order to improve the recognition degree of time domain reflection signal and determine the type of arc fault point,this thesis also selected an operational amplifier chip with high slew rate and a high-speed comparator chip and designed off-chip shaping circuit.The experimental results show that the time interval measurement module can correctly generate the test clock signal with resolution of 625ps.Based on the time domain reflection method,the polarity of the return signal also successfully reflects the fault type of the fault point.The locator can accurately measure the length of the cable,which means,the exact location of the simulated arc fault point can be measured.The locating error is less than 0.1 meter,which meets the measurement needs.