With the increase of cable lines, the morning noise of cable faults on power supply reliability is increasing day by day. Therefore, quickly and accurately detecting the location of the fault point is of great significance to ensure timely repair of the faulty cable. The detection method for cable faults depends on the nature of the fault, so the first step in the detection work is to determine the nature of the fault.
Cable faults can be roughly divided into two types: the first type is caused by insulation damage between cable cores or between cable cores and outer skin, resulting in short circuits, grounding, or flashover breakdown The second type is caused by the disruption of the continuity of the cable core, resulting in wire breakage and incomplete wire breakage, and sometimes a mixed type of robbery with both situations occurring. But usually the first type of fault is more common, among which short-circuit grounding can be divided into high resistance and low resistance.
The method of determining the nature of the fault can be used with a megohmmeter. Use a 2500 volt megohmmeter to measure the insulation resistance of each phase's buried wire. If the insulation resistance value of a certain phase to ground is below 30 kiloohms, it indicates that the phase wire has a grounding point When a broken core fault occurs in a local buried line, the three phase wires and neutral wire at the end of the buried line can be connected together, and a multimeter can be used to measure the resistance between each two phase wires and between the phase wire and neutral wire at the beginning of the line. If the resistance is not high, it indicates that the circuit is intact; If the resistance value reaches several thousand ohms, it indicates that there is a broken core in the circuit If the resistance between the three phases to the neutral wire is high and the resistance between the three phases is not high, it can be concluded that there is a broken core in the neutral wire.
After identifying the faulty cable, we can use the DC bridge method and pulse method to measure the distance of the fault point. Among them, the pulse method is the most widely used and can be divided into low-voltage pulse reflection method and high-voltage pulse reflection method. The low-voltage pulse reflection method can be used to detect wire breakage and low resistance short circuit faults, while the high-voltage pulse method is mainly used to detect high resistance short circuits, ground faults, and flashover faults.
Ranging can only estimate the fault section, and in practical engineering, more accurate determination of the fault location is required to reduce excavation volume. Therefore, before excavation, it is necessary to first locate the site, that is, to use instruments to search for suspicious areas and accurately determine the actual location of the fault. The absolute error of measurement should not exceed 1m. For short cables with a length of only tens of meters, it is not necessary to conduct initial measurement and can be directly fixed, and the faults are mostly at the terminal end. Even for cables that are several hundred meters long, if burning through distance measurement is required, it is advisable to use acoustic measurement to measure the fixed point before burning through to prevent the resistance from dropping too low and damaging the acoustic measurement conditions. There are many methods for fixed-point measurement, including acoustic measurement, induction method, probe method, and current direction method.
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