Whether it is high-voltage cable or low-voltage cable, it is often caused by short circuit, overload operation, insulation aging or external force damage during construction, installation and operation. Cable faults are classified into grounding, short-circuit, and disconnection. The three-core cable fault type mainly has the following aspects: one core or two core contacts; two-phase core wire short circuit; three-phase core wire is completely short-circuited; one-phase core wire is broken or multi-phase broken wire. For direct short circuit or disconnection fault, the multimeter can be directly measured and judged. For non-direct short circuit and battery fault, use megohmmeter to measure the insulation resistance between the core wires or the insulation resistance of the core to ground, and determine the fault type according to the resistance value.
Second, the cable fault point search method
1. Sound measurement method The so-called sound measurement method is based on the sound of the faulty cable discharge. This method is effective for the high-voltage cable core to the flashover discharge of the insulation layer. The equipment used in this method is a DC withstand voltage tester. The circuit wiring is shown in Figure 1, where SYB is a high voltage test transformer, C is a high voltage capacitor, ZL is a high voltage rectifier silicon stack, R is a current limiting resistor, Q is a discharge ball gap, and L is a cable core. When the capacitor C is charged to a certain voltage value, the ball gap discharges the faulty core wire of the cable, and at the fault, the cable core wire discharges the spark discharge sound to the insulation layer, and when the noise noise is minimum, the An audio amplification device such as a hearing aid or a medical stethoscope is used for searching. When looking for it, put the pickup close to the ground and move slowly along the cable. When you hear the "Zi, Zi" discharge is the loudest, it is the fault point. Be sure to pay attention to safety when using this method. Special monitoring should be provided at the test equipment end and cable end.
2. The bridge method of the bridge method is to measure the DC resistance value of the cable core wire by the double-arm bridge, and then accurately measure the actual length of the cable, and calculate the fault point according to the proportional relationship between the cable length and the resistance. The method is generally no more than 3m for faults with direct short circuit or short circuit contact resistance between cable cores, and the fault is generally not more than 3m. For faults with contact resistance greater than 1Ω, the method can be used to increase the resistance to 1Ω. Below, measure again according to this method.
The measuring circuit first measures the resistance R1 between the core wires a and b, then R1 = 2RX + R, where R is the phase resistance value of the a phase or b phase to the fault point, and R is the contact resistance of the short contact. Then 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 a' phase and the b' phase core. The value of one phase resistance from the line 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 two core wires of b and c, then 1/2 of the resistance is the core of each phase. The resistance value of the line, expressed as RL. RL=RX+R(L-X), from which the contact resistance value of the fault point can be obtained: R=R1+R2-2RL. Therefore, the resistance value of the core wires on both sides of the fault point can be expressed by the following equation: RX = (R1 - R) / 2, R (L - X) = (R2 - R) / 2. After the three values ​​of RX, R(L-X) and RL are determined, the distance X of the fault point from the cable end or (L-X) can be obtained by proportional formula: X=(RX/RL)L, (L -X) = (R(L - X) / RL) L, where L is the total length of the cable. When using the bridge method, the measurement accuracy should be ensured. The bridge connection should be as short as possible, the diameter should be large enough, and the connection with the cable core should be crimped or welded. The decimal places should be retained during the calculation.
3. Capacitance current measurement method In the operation of the cable, there is capacitance between the core wires and the core wire to the ground. The capacitance is evenly distributed, and the capacitance is linearly proportional to the cable length. The capacitance current measurement method is based on this principle. For the measurement, the measurement of the cable core breakage is very accurate. The measuring circuit is shown in Figure 4. The equipment is a single-phase voltage regulator of 1~2kVA, one 0~30V, 0.5-level AC voltmeter, 0~100mA, 0.5-level AC mA meter.
Measurement steps:
(1) First, measure the capacitance current of each core line (should keep the applied voltage equal) Ia, Ib, Ic at the head end of the cable.
(2) Re-measure the values ​​of the capacitance currents Ia', Ib', and Ic' of the core wires of each phase at the end of the cable to check the ratio of the specific volume of the intact core wire to the broken core wire, and initially determine the wire breakage distance. Approximate point.
(3) According to the capacitance calculation formula C=1/2Ï€fU, C is proportional to I when the voltage U and frequency f are constant; because the f (frequency) of the power frequency voltage is constant, the voltage is constant as long as the measurement is applied. The ratio of the capacitance current is the ratio of the capacitance. Set the cable length L, the core wire break point distance is x, then Ia / Ic = L / x, x = (Ic / Ia) L. During the measurement process, as long as the voltage is constant, the ammeter reading is accurate, the total cable length is measured accurately, and the measurement error is relatively small.
4. Zero-potential method Zero-potential method is the potential comparison method. It is suitable for cable core-to-ground faults with short length. This method is simple and accurate, and does not require precision instruments and complicated calculations. The wiring is shown in Figure 5. . The measurement principle is as follows: the cable fault core wire is connected in parallel with the equal length comparison wire, and when the pressure is applied to both ends, the power supply is connected at both ends of the two parallel uniform resistance wires. At this time, the potential difference between any point on one resistance wire and the corresponding point on the other resistance wire is necessarily zero. Conversely, the two points with zero potential difference must be the corresponding points, because the negative pole of the microvoltmeter is grounded and equipotential to the cable fault point, so when the positive pole of the microvoltmeter moves on the comparative conduction to the point where the value is zero The equipotential with the fault point, that is, the corresponding point of the fault point. In Figure 5, K is a single-phase knife switch, E is a 6V battery or a 4-cell dry battery, and G is a DC microvoltmeter. The measurement steps are as follows:
(1) First connect the battery E on the b and c phase core wires, and then lay a comparison wire S on the ground with the length of the faulty cable. The wire should be bare copper wire or bare aluminum wire, and the cross section should be equal. There must be no intermediate joints.
(2) Ground the negative pole of the microvoltmeter, connect the positive pole to a long flexible conductor, and the other end of the conductor should be fully contacted when sliding on the laying comparison conductor.
(3) Close the knife switch K and slide the broken end of the flexible wire on the comparison wire. When the microvoltmeter indicates zero, the position of the cable fault point is the position of the cable fault point.
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