For direct short circuit or broken cable fault, the multimeter can be used to measure and judge directly; For non direct short circuit cable fault and grounding AAC Cable fault, the insulation resistance between cores or insulation resistance between cores to ground can be measured by megohmmeter, and the type of cable fault can be judged according to the resistance value. The following describes the method of cable fault finding:
Zero potential method
The zero potential method is also called potential comparison method. It is suitable for short cable core to ground fault. The application of this method is simple and accurate, and does not need precise instruments and complicated calculation. The measurement principle is as follows: parallel the cable fault core with the same length of comparison wire, when the voltage VE is applied at both ends of B and C, 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, otherwise, 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, the point when the positive pole of the microvolt meter moves to zero on the comparison wire is equipotential with the cable fault point, that is, the corresponding point of the cable fault point. S is a single-phase knife switch, e is a 6e battery or four No. 1 dry batteries, and G is a DC microvolt meter
1) First, connect battery e to B and C phase core wires, and then lay a comparison wire s with the same length as the fault cable on the ground. The wire shall be bare copper wire or bare aluminum wire with equal cross section and no intermediate joint.
2) The negative pole of the microvolt meter shall be grounded, and the positive pole shall be connected with a long flexible wire. The other end of the wire shall be fully contacted when sliding on the laid comparison wire.
3) Close the knife switch s, slide the end of the flexible wire on the comparison wire, and the position when the microvolt meter indicates zero is the position of the ACSR Cable fault point.
High voltage bridge method
High voltage bridge method is to measure the DC resistance value of the cable core with double arm bridge, and then accurately measure the actual length of the cable, and calculate the cable fault point according to the positive proportional relationship between the cable length and resistance. This method is suitable for direct short circuit between cable cores or short circuit point contact resistance less than 1 Ω Generally, the judgment error is not more than 3m, and the contact resistance is greater than 1 Ω The method of high voltage burn through can be used to reduce the resistance to 1 Ω Next, measure according to this method. When measuring the circuit, first measure the resistance R1 between core a and B, R1 = 2RX + R, where Rx is the resistance value of a phase or B phase to the cable fault point, only the contact resistance of short contact. Then move the bridge to the other end of the cable and measure the DC resistance value R2 between A1 and B1 core wire, then R2 = 2R (l-x) r, R (l-x) is the resistance value of one phase from A1 phase or B1 phase core wire to the cable fault point. After measuring R1 and R2, short circuit B1 and C1 according to the circuit shown in Fig. 3, and measure the DC resistance value between B and C two-phase core wires, then 1 / 2 of the organization is the resistance value of each phase core wire, expressed by RL, RL = RX R (l-x), from which the contact resistance value of the fault point can be obtained: r = R1 r2-2rl table. Therefore, the resistance value of 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 Rx, R (l-x) and RL are determined, the distance X or (l-x): x = (Rx / RL) l, (l-x) = (R (L-2) at the end of the cable, the capacitance current IA1, IB2 and IC3 of each phase core are measured to check the capacitance ratio between the core and the broken core, and the approximate point of the broken distance can be judged preliminarily.
3) According to the capacitance calculation formula C = I / (2) Π When the positive voltage U and frequency f are constant, C is proportional to I. Because f (frequency) of power frequency voltage is constant, the ratio of capacitance current is the ratio of capacitance as long as the applied voltage is constant. If the total length of the cable is l and the distance between the broken point of the core is x, then IA / ic = L / X and x = (IC / IA) L. In the process of measurement, as long as the voltage remains unchanged, the reading of ammeter is accurate and the total length of cable is accurate, the measurement error is relatively small.
The so-called sound measurement method is based on the sound of fault cable discharge, which is more effective for the flashover discharge of high voltage cable core to insulation layer. The equipment used in this method is DC withstand voltage testing machine. TB is the high voltage test transformer, C is the high voltage capacitor, VE is the high voltage rectifier silicon stack, R is the current limiting resistance, q is the discharge ball gap, l is the cable core. When the capacitor C is charged to a certain voltage value, the ball gap discharges the cable fault core wire, and the cable core wire discharges the insulation layer at the cable fault location, producing “Zi, Zi” spark discharge sound. For the open laying cable, it can be found directly by hearing. If it is buried cable, it is necessary to determine and mark the cable direction first. When searching, put the pickup close to the ground and move slowly along the direction of the cable. When the “Zi, Zi” discharge sound is the largest, this is the cable fault point. It is necessary to pay attention to safety when using this method, and special personnel should be set up at the end of test equipment and cable. The cable fault locator produced by our company can not only measure the sound, but also detect the magnetic signal. The signal strength is visually displayed synchronously, which can locate the cable fault point faster and more accurately.