Discuss the test method of cable fault point distance
The detection of cable faults generally requires three steps: diagnosis, ranging and fixed point. The test of cable fault is generally divided into two processes: that is, the test of the distance between the fault points of the fault cable; Test of fault point fixed point. There are three methods for measuring the distance between fault points of fault cables: Loop Bridge balance method; Low voltage pulse reflection method; Flashover method. Loop Bridge balance method is a method of using DC Bridge to locate cable faults, which is called Bridge method for short. Field personnel have the habit of calling Rf <100kΩ faults as low resistance faults, this is mainly because the traditional bridge method can measure such faults. Bridge method for short distance cable fault location, accuracy is quite high, therefore, is still in use. Based on the uniformity along the cable, the cable length is proportional to the cable core resistance, and according to the principle of Wheatstone bridge, the short-circuit grounding of the cable and the loop resistance on both sides of the fault point are introduced into the DC Bridge to measure its ratio. From the measured ratio and the total length of the cable, the distance from the measuring end to the fault point can be obtained. The principle of using bridge method to locate the single-phase grounding fault of the cable is to jumper the cable conductor of the fault phase of the cable and the normal phase with a wire not smaller than the cable section at the other end of the cable. Then connect the cable conductor of the faulty phase to the other end of the bridge at one end. Using Bridge method to cable two-phase short circuit or two-phase short circuit and ground, fault location, need to have a non-fault conductor and fault conductor together to form a ring, when the bridge is balanced, the distance of the fault point can be obtained. Low voltage pulse reflection method. The low-voltage pulse reflection method is used to detect cable faults. The pulse generator of the instrument sends a pulse wave to the cable fault phase through the lead wire, and the pulse wave propagates along the cable core, when it is transmitted to the fault point, because the wave resistance of the cable at the fault point changes, a pulse signal is reflected back, use the oscilloscope to record the time interval between the transmitted pulse and the reflected pulse at the test end to calculate the distance between the test end and the fault point. The low-voltage pulse reflection method can be used for open circuit and low resistance faults. The advanced point of the low-voltage pulse reflection method is to greatly simplify the fault waveforms measured on site, the complex high-voltage impact flashover waveform is changed into a short-circuit fault waveform similar to the low-voltage pulse method, which is very easy to interpret. The technical requirements and experience requirements for operators are reduced, the judgment accuracy of on-site faults is greatly improved, and the purpose of testing cable faults quickly and accurately is achieved. Flashover method. The basic principle of the flashover method is similar to that of the low-voltage pulse method, which uses the principle of reflection at the fault point when the electric wave propagates in the cable, write down the time when the radio wave goes back and forth between the fault points of the test end of the faulty cable, and then calculate the location of the cable fault point according to the wave velocity. According to statistics, high resistance and flashover faults account for about 90% of the total number of cable faults. High resistance fault should use impact flashover method, and flashover fault can be tested by DC flashover method. In fact, high resistance and flashover faults are distinguished through test methods on site.