[Repost] A brief analysis of the three major detection methods for faulty cables

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[Repost] A brief analysis of the three major detection methods for faulty cables [Copy link]

The initial detection method of the faulty cable is divided into the loop bridge balance method, the low-voltage pulse reflection method, and the flash tester method. In the cable fault initial detection method, first select the appropriate cable fault tester according to the nature of the fault, and use the cable line technical data to measure and calculate the distance between the fault point and the test point and the path position of the fault point. The initial detection method mainly includes the loop bridge balance method, the low-voltage pulse reflection method, and the flash tester method.

The loop bridge balance method, referred to as the bridge method, is a method that uses the linear relationship between the length and resistance of the faulty cable to change the DC bridge to a method that is convenient for measuring the low impedance grounding of one or two phases of the cable. The principle diagram of the bridge method is shown in Figure 1. The range of the grounding resistance value depends on the operating voltage of the bridge and the sensitivity of the galvanometer, generally <100kΩ. When the grounding resistance value is large, a large part of the voltage of the bridge power supply will fall on the grounding resistance, making the galvanometer very insensitive, resulting in increased measurement errors. In order to expand the application scope of the bridge method, the sensitivity of the galvanometer can be improved and the bridge power supply voltage can be increased, but both measures are limited. The way to improve the sensitivity of the galvanometer is to install a DC amplifier in front of the galvanometer, but the amplifier gain is too high, which will produce serious zero drift and cause the bridge to be unbalanced. Increasing the voltage of the bridge power supply requires a corresponding increase in the insulation strength of the bridge body to the ground and taking safety measures to protect the operator. When the power supply voltage rises to a certain value, the fault resistance usually appears to be unstable. If the grounding resistance suddenly drops to zero, the high voltage will be added to the bridge and the galvanometer will burn out. Figure 1 Schematic diagram of the bridge method 25)]The characteristics and requirements of the bridge method are: ① It is only applicable to measuring single-point faults in cable lines. If there are several fault points in the line at the same time, it cannot be used; ② The jumper wire at the other end of the cable line should be as short as possible, and its cross-sectional area should be close to the cross-sectional area of the cable conductor, and it should be tightly connected so that its contact resistance is close to zero; ③ When increasing the test voltage for high-resistance grounding fault testing, special attention must be paid to safety issues.

After using the fault burn-through device to convert the flashover fault into a grounding fault, it is necessary to immediately use the bridge method to measure, so as to avoid the insulation recovery and re-closing the fault point. At the same time, do not burn the grounding resistance of the fault too low, so as to avoid the difficulty of too small volume when using the precise measurement point method.

The low-voltage pulse reflection method, referred to as the pulse method, uses the principle that when a pulse signal propagates in a cable line, it encounters a wave impedance mismatch point to generate electromagnetic wave reflection. The pulse wave reflection time and the cable wave velocity are measured on the oscilloscope to determine the distance of the cable fault point. The wiring diagram of the pulse method is shown in Figure 2. Usually, in addition to conductor breakage (open circuit), short circuit and grounding fault, the impedance mismatch points in the cable line are also impedance uneven points at the cable joints and the cable passing through the metal pipe, which will also produce wave reflections and must be carefully identified during testing. Especially when the grounding resistance value is more than 2 to 3 times the cable wave impedance, the reflected wave amplitude is very small, making it more difficult to identify the fault. The pulse method is most suitable for detecting broken wire faults, and is also suitable for detecting cable faults with a grounding resistance of less than 100Ω. The digital cable fault tester can directly read the distance to the fault point using a five-digit digital tube display. The principle is to use the transmission pulse of the input cable and the reflected pulse formed by the reflection at the fault point to control the electronic timing switch, and use the operator to calculate the measured time and the speed of signal propagation in the cable according to a certain functional relationship. Finally, the distance value of the fault point in meters can be directly read from the digital tube.

[color=rgb(25, Figure 2 Pulse method wiring diagram

The cable fault flash tester method applies DC high voltage or impulse high voltage to the fault phase of the cable to make the insulation of the fault point instantaneously break down. The distance to the fault point can be obtained by using the time of the electric wave reflection back and forth during the breakdown discharge and the propagation speed of the electric wave in the cable. Cable fault flash testers can be divided into two types: digital direct reading type and oscilloscope type.

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