Two different methods for locating fault points in power cables

There are many methods for fixed-point measurement, including acoustic measurement, induction method, etc.
Acoustic measurement method
The acoustic measurement method is sensitive and reliable, and is commonly used. Except for grounding faults with particularly low grounding resistance (less than 50 Ω), the principle wiring of the acoustic measurement method is similar to that of the high-voltage pulse reflection method. When the high-voltage capacitor C is charged to a certain voltage, the ball gap breaks down, and the capacitor voltage is applied to the faulty cable, causing breakdown between the fault point and the gap, resulting in spark discharge, electromagnetic wave radiation, and mechanical audio vibration.
The principle of acoustic measurement method is to utilize the mechanical effect of discharge, that is, the phenomenon of energy stored in capacitors dissipating in the form of sound energy at the fault point. A sound wave receiver probe is used to pick up the seismic waves on the ground surface, and the fault point is determined based on the strength of the seismic waves. When using acoustic measurement method, the following points should be noted:
(1) The tested cable should be able to withstand the selected test voltage without generating new faults, and the testing equipment should have sufficient capacity and margin.
(2) Due to the surge current flowing through the protective layer from the fault point at the moment of discharge, the potential of the protective layer is instantly raised. Therefore, in addition to the discharge at the fault point, there may also be stray and parasitic discharges at other grounding points, which should be carefully distinguished.
(3) Due to the high current generated by impulse discharge, the voltage rise caused by flowing through the main grounding grid may endanger other equipment connected to the grid. Therefore, transformers and capacitors should not only be reliably grounded, but also directly connected to the protective layer inside the cable;
(4) Wire breakage and flashover faults often occur in intermediate joints, therefore, after determining the approximate section using pulse method, acoustic measurement method can be used to check the intermediate joints at fixed points.
Audio current induction method
This method is suitable for phase to phase faults with low resistance, including two-phase short circuits and grounding, three-phase short circuits, and three-phase short circuits and grounding. But it is usually not suitable for single-phase grounding faults because the metal sheath of the cable head is generally grounded at both ends, so the audio current from the signal generator is divided into two sides at the fault point and flows back, and the signal on either side of the grounding point does not change.
The principle of induction method is mainly based on the magnetic effect of current, and the fault point is determined by detecting the fluctuation of the magnetic field along the cable. During the experiment, an audio current is passed between the two cores of the faulty cable. The current enters from one conductor and returns from the other conductor through the fault point. The magnetic effect of round-trip current tends to cancel each other out. But due to the distance between the wire cores, the synthesized magnetic field of the two currents can exist and twist with the twisting of the wire cores.
A synthetic magnetic field can be detected on the ground using a detection coil and receiver, and the received signal will fluctuate with the position of the wire core arrangement as it moves along the line. If the audio signal suddenly interrupts after the fault point, it can be confirmed that the fault point is at the location where the audio signal is interrupted.