The following elaborates from several aspects, focusing on how to reduce the time for locating line faults, providing the fastest solutions for rapid restoration of the power system, and minimizing production losses caused by line faults. The so-called acoustic method involves locating faults based on the discharge sound of the faulty cable, suitable for cable fault diagnosis in cable tunnels and trenches. This method is particularly effective for flashover discharge from the core wire to the insulation layer of high-voltage cables. The equipment used in this method is a DC withstand voltage tester. The circuit connection is shown in the figure, where SYB is the high-voltage test transformer, C is the high-voltage capacitor, ZL is the high-voltage rectifier silicon stack, R is the current-limiting resistor, Q is the discharge ball gap, and L is the cable core wire. When the capacitor C is charged to a certain voltage, the ball gap discharges to the faulty cable core, causing a spark discharge sound of "crackling" between the core and the insulation at the fault point. Then, when ambient noise is minimal, audio amplification devices such as hearing aids or medical stethoscopes are used for detection. During detection, the pickup is placed close to the ground and moved slowly along the cable route. When the "crackling" discharge sound is loudest, that location is the fault point. Safety must be emphasized when using this method, with dedicated personnel monitoring both the test equipment end and the cable end. This method has been used multiple times in the external line projects of Xingang Steel Vanadium Steelmaking Plant, achieving excellent results. The bridge method involves using a double-arm bridge to measure the DC resistance of the cable core, accurately measuring the actual cable length, and calculating the fault point based on the proportional relationship between cable length and resistance. It is suitable for directly buried or conduit cables that cannot be directly observed. For faults where the core wires are directly short-circuited or the contact resistance at the short-circuit point is less than 1Ω, the judgment error is generally not greater than 3m. For faults with contact resistance greater than 1Ω, the method of applying high voltage to burn through can be used to reduce the resistance to below 1Ω, then measure using this method. The measurement circuit is shown in Figure 2. First, measure the core wire RX+R, where R is the resistance of one phase (a or b) to the fault point. Then, at the other end of the cable, measure the resistance R(L-X)+R for phases a and b, where R(L-X) is the resistance of one phase (a or b) to the fault point. After measuring R1 and R2, short-circuit b and C as shown in Figure 3, and measure the DC resistance between phases b and c. Half of this resistance is the resistance per phase core, denoted as RL. RL = RX + R(L-X). From this, the contact resistance at the fault point can be derived: R = R1 + R2 - 2RL. Therefore, the resistances of the core wires on both sides of the fault point can be expressed as: RX = (R1 - R)/2, R(L-X) = (R2 - R)/2. Once RX, R(L-X), and RL are determined, the distance X or (L-X) from the cable end to the fault point can be calculated using the proportional formula: X = (RX/RL)L, (L-X) = (R(L-X)/RL)L, where L is the total cable length. When using the bridge method, measurement accuracy must be ensured: bridge connection wires should be as short as possible, with sufficient diameter, and connections to the cable core should be made by crimping or soldering. All decimal places should be retained during calculations. 3. Capacitance current measurement method: During cable operation, capacitance exists between core wires and between core and ground, uniformly distributed, and the capacitance is linearly proportional to cable length. The capacitance current measurement method is based on this principle and is very accurate for detecting open-circuit faults in cable cores. The measurement circuit is shown in Figure 4, using a 1-2kVA single-phase voltage regulator, a 0-30V, 0.5-class AC voltmeter, and a 0-100mA, 0.5-class AC milliammeter. The resistances of the core wires on both sides of the fault point can be expressed as: RX = (R1 - R)/2, R(L-X) = (R2 - R)/2. Once RX, R(L-X), and RL are determined, the distance X or (L-X) from the cable end to the fault point can be calculated using the proportional formula: X = (RX/RL)L, (L-X) = (R(L-X)/RL)L. When using the bridge method, measurement accuracy must be ensured: bridge connection wires should be as short as possible, with sufficient diameter, and connections to the cable core should be made by crimping or soldering. All decimal places should be retained during calculations. 3. Capacitance current measurement method: During cable operation, capacitance exists between core wires and between core and ground, uniformly distributed, and the capacitance is linearly proportional to cable length. The capacitance current measurement method is based on this principle and is very accurate for detecting open-circuit faults in cable cores. The measurement circuit is shown in Figure 4, using a 1-2kVA single-phase voltage regulator, a 0-30V, 0.5-class AC voltmeter, and a 0-100mA, 0.5-class AC milliammeter. (1) First, measure the capacitance current of each core at the cable head (keeping the applied voltage equal) to obtain Ia, Ib, Ic values. (2) At the cable end, measure the capacitance current of each phase core Ia, Ib, Ic again to compare the ratio of capacitance of intact cores to broken cores, preliminarily determining the broken core. (3) According to the capacitance calculation formula C = 1/2πfU, when voltage U and frequency f are constant, C is proportional to I; since the frequency f of power frequency voltage is constant, as long as the applied voltage is kept constant during measurement, the ratio of capacitance currents equals the ratio of capacitances. Let the total cable length be L, and the distance to the core break point be x, then Ia/Ic = L/x, so x = (Ic/Ia)L. 1. Determine the cable fault type - main insulation fault / outer sheath fault to avoid bringing wrong equipment; the detection equipment and methods for the two fault types differ, with over 95% being main insulation faults. The positioning power supply output voltage differs: for voltages below 10KV, a 15KV output cable fault locating power supply can be used, which is small and lightweight; for 10KV and above, a 32KV output cable fault locating power supply is used, which has high energy but is heavier. 1. Use a megohmmeter (insulation resistance tester) or multimeter to measure the insulation resistance of each phase of the faulty cable to diagnose the fault type, such as open circuit, high resistance, or low resistance faults. Determine whether the cable fault is phase-to-phase or phase-to-ground. 4. Use a low-voltage pulse reflection instrument to measure the waveform at the cable end to determine whether it is the fault waveform or the full cable length waveform. Use a cable path locator to determine the cable position and direction. The accuracy of the path greatly affects the precise location of the cable fault. 1. At the cable head, use the cable fault locating power supply to discharge pulses to the faulty phase, with a reasonable output voltage to ensure breakdown discharge at the fault point. 2. Use a precise locator, preferably with acoustic-magnetic synchronization, to listen within the pre-located range of about 30 meters, comparing digital values; the point with the loudest sound and smallest digital value is the fault point, and the precise range can be narrowed to within one meter, allowing excavation to reveal the fault point. Underground power cables are increasingly used for safety and convenience, but once a cable fault occurs, it is difficult to quickly locate the exact fault point. Failure to promptly eliminate faults and restore power often leads to major losses due to power outages and production halts. Therefore, how to restore power in the fastest time and at the lowest maintenance cost is a challenge faced by power supply departments when encountering cable faults.
