一、 Types and judgments of cablefaults

Whether it is high-voltage cables or low-voltage cables, faults often occur during construction, installation, and operation due to short circuits, overload operation, insulation aging, or external damage. Cable faults are divided into three categories: grounding, short circuit, and disconnection. The main types of faults in three core cables are as follows: contact between one or two cores; Short circuit between two-phase core wires; The three-phase core wire is completely short circuited; One phase core wire breakage or multi-phase breakage. For direct short circuit or wire breakage faults, a multimeter can be used to directly measure and determine. For non direct short circuit and grounding faults, a megohmmeter can be used to remotely measure the insulation resistance between core wires or the insulation resistance between core wires and ground. The fault type can be determined based on their resistance values.

二、 Method for finding cable fault points

1. Acoustic measurement method; The so-called sound measurement method is to search for faults based on the sound of cable discharge. This method is more effective for high-voltage cable core wire flashover discharge on the insulation layer. The equipment used for this method is a DC withstand voltage testing machine. The circuit wiring is shown in Figure 1, 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 capacitor C is charged to a certain voltage value, the ball gap discharges the faulty core wire of the cable. At the fault location, the cable core wire discharges to the insulation layer, producing a "sizzling" spark discharge sound. Then, when the noise is minimal, audio amplification equipment such as hearing aids or medical stethoscopes can be used to search for it. When searching, place the microphone close to the ground and slowly move it along the cable route. When the maximum discharge sound of "zi, zi" is heard, this is the fault point. When using this method, safety must be taken into account, and dedicated personnel should be assigned to monitor the testing equipment and cable ends.

2. Bridge method; The bridge method is to use a double arm bridge to measure the DC resistance value of the cable core wire, accurately measure the actual length of the cable, and calculate the fault point according to the proportional relationship between the cable length and resistance. For faults where there is a direct short circuit between cable cores or the contact resistance at the short circuit point is less than 1 Ω, the judgment error is generally not greater than 3m. For faults where the contact resistance at the fault point is greater than 1 Ω, the method of applying high voltage to burn through can be used to reduce the resistance to below 1 Ω, and then measured according to this method.

The measurement circuit first measures the resistance R1 between core wires a and b, then R1=2RX+R, where R is the resistance value of one phase from phase a or b to the fault point, and R is the contact resistance of the short contact. Measure the DC resistance value R2 between the a 'and b' core wires at the other end of the cable, then R2=2R (L-X)+R, where R (L-X) is the resistance value of one phase from the a 'and b' core wires to the fault point. After measuring R1 and R2, short-circuit b 'and C' according to the circuit shown in Figure 3, and measure the DC resistance value between the core wires of phases b and c. Then, half of this resistance value is the resistance value of each phase core wire, represented by RL. RL=RX+R (L-X), from which the contact resistance value of the fault point can be obtained: R=R1+R2-2RL.

3. Capacitive current measurement method;During cable operation, there is capacitance between the core wires and between the core wires and the ground. This capacitance is uniformly distributed and has a linear proportional relationship with the length of the cable. The capacitance current measurement method is based on this principle and is very accurate for detecting cable core wire breakage faults. The measurement circuit is shown in Figure 4, using one 1-2kVA single-phase voltage regulator, one 0-30V, 0.5-level AC voltmeter, and one 0-100mA, 0.5-level AC milliampere meter

4. Zero potential method; The zero potential method, also known as the potential comparison method, is suitable for measuring ground faults in short cable cores. This method is simple and accurate, and does not require precise instruments or complex calculations. Its wiring is shown in Figure 5. The measurement principle is as follows: Connect the faulty core wire of the cable in parallel with a comparison wire of equal length. When applying pressure E at both ends, it is equivalent to connecting power to both ends of the two parallel uniform resistance wires. At this point, the potential difference between any point on one resistance wire and the corresponding point on another resistance wire must be zero. On the contrary, the two points where the potential difference is zero must be the corresponding points, because the negative pole of the microvolt meter is grounded and has the same potential as the cable fault point. Therefore, when the positive pole of the microvolt meter moves on the comparative conductivity to the point where the reading is zero and has the same potential as the fault point, that is, the corresponding point of the fault point.