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Four practical measurement methods for cable fault points

1. Types and judgment of cable faults

whether high-voltage cables or low-voltage cables, faults are often caused by short circuit, overload operation, insulation aging or external force damage during construction, installation and operation. Cable faults are divided into three categories: grounding, short circuit and disconnection. Three core cable fault types mainly include the following aspects: one core or two core contact; Short circuit between two-phase cores; Three phase core wire is completely short circuited; One phase core wire is broken or multi-phase wire is broken

for direct short-circuit or broken wire faults, use a multimeter to directly measure and judge. For indirect short-circuit and ground pool faults, use a megger to telemeter the insulation resistance between cores or the insulation resistance between cores to ground, and determine the fault type according to its resistance value

second, the search method of cable fault point

1. Sound measurement method

the so-called sound measurement method is to search according to the sound of the discharge of the faulty cable. This method is more effective for the flashover discharge of the high-voltage cable core to the insulation layer. The equipment used in this method is DC withstand voltage testing machine. The circuit wiring is shown in Figure 1, in which 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 resistance, q is the discharge ball gap, and l is the cable core

when capacitor C is charged to a certain voltage value, the ball gap discharges the faulty core of the cable, and the cable core discharges the insulating layer at the fault to produce "Zi, Zi" spark discharge sound. Then, when the noise is minimal, find it with the help of deaf hearing aids or medical stethoscope and other audio amplification equipment. When searching, put the pickup close to the ground and move slowly along the cable direction. When you hear the maximum sound of "Zi, Zi", this is the fault point. Pay attention to safety when using this method, and assign special personnel to monitor the end of test equipment and cable

2. Bridge method

bridge method is to measure the DC resistance value of cable core with double arm bridge, and then accurately measure the actual length of cable, and calculate the fault point according to the positive proportional relationship between cable length and resistance. In this method, for the fault of direct short circuit between cable cores or the contact resistance of short circuit point is less than 1 Ω, the judgment error is generally not more than 3m. For the fault of contact resistance of fault point is greater than 1 Ω, the method of increasing voltage burn through can be used to reduce the resistance to less than 1 Ω, and then measure according to this method. The measuring circuit is shown in Figure 2. First measure 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 phase B 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 on the other end of the cable, then R2 = 2R (l-x) + R, where R (l-x) is the phase resistance value from the A'phase and B' phase 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 B and C core wires, then 1/2 of the 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. Therefore, the resistance value of the core wires on both sides of the fault point can be expressed by the following formula: RX = (r1-r)/2, R (l-x) = (r2-r)/2. After the three values Rx, R (l-x) and RL are determined, the distance X or (l-x) between the fault point and the cable end can be calculated according to the 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 guaranteed. The connecting wire of the bridge should be as short as possible, the diameter should be large enough, the connection with the cable core should be crimped or welded, and the middle and small digits should be retained in the calculation process and in the key period of moving towards high truth in the industry

3. Capacitance current measurement method

during the operation of the cable, there is capacitance between the core wires and between the core wires and the ground. The capacitance is evenly distributed, and the capacitance is in linear proportion to the cable length. The capacitance current measurement method is based on this principle, which is very accurate for the measurement of cable core wire disconnection fault. The measuring circuit is shown in Figure 4, and the equipment used is 1 ~ 2K. "With the continuous accumulation and development of new material industry in Ningbo, there is a VA single-phase voltage regulator, a 0 ~ 30V, 0.5 AC voltmeter, and a 0 ~ 100mA, 0.5 AC milliammeter

measurement steps:

(1) first, measure the capacitance current of each core wire (keep the applied voltage equal) ia, IB, IC at the cable head end

(2) measure the capacitance current ia', ib', ic'of each phase core at the end of the cable to check the ratio of the specific capacity of the intact core to the broken core, and preliminarily judge the approximate point of the broken distance

(3) according to the capacitance calculation formula C = 1/2 π Fu, when the voltage U and frequency f are constant, C is proportional to I; Because f (frequency) of power frequency voltage remains unchanged, as long as the applied voltage remains unchanged during measurement, the ratio of capacitance to current is the ratio of capacitance. If the total length of the cable is l and the distance between the core wire breaking point is x, then ia/ic = l/x, x = (ic/ia) L

in the measurement process, as long as the voltage is constant, the ammeter reading is accurate, and the total length of the cable is measured accurately, the measurement error is relatively small

4. Zero potential method

zero potential method is also known as potential comparison method. It is suitable for short length cable core to ground fault. This method is simple to use. Mr. zobel expects to be accurate without precision instruments and complex calculations. Its wiring is shown in Figure 5. The measurement principle is as follows: connect the cable fault core in parallel with the comparison wire of the same length. When e is pressurized at both ends, it is equivalent to connecting the power supply at both ends of 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 must be zero. On the contrary, the two points with zero potential difference must be the corresponding points, because the negative pole of the microvolt meter is grounded and equipotential with the cable fault point. Therefore, when the positive pole of the microvolt meter moves to the point with zero indication in comparative conductivity, it is 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 four No. 1 dry batteries, and G is a DC microvolt meter. The measurement steps are as follows:

(1) first connect battery e to phase B and C core lines, and then lay a comparison wire s with the same length as the faulty cable on the ground. The wire should be bare copper wire or bare aluminum wire, and its cross section should be equal, without intermediate joints

(2) ground the negative pole of the microvolt meter, and connect the positive pole with a long soft wire. The other end of the wire is required to be fully contacted when sliding on the laid comparison wire

(3) close the knife switch K, slide the broken end of the flexible wire on the comparison wire, and the position when the microvolt meter indicates zero is the position of the cable fault point. Use graphene for artificial skin. (end)

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