Cable Fault Detection and Search Method

For direct short circuit or broken cable fault, the multimeter can be used for direct measurement and judgment; For indirect short-circuit cable fault and grounding cable fault, use a megger to telemeter the insulation resistance between core wires or the insulation resistance between core wires and ground, and judge the cable fault type according to its resistance value. The following describes the method of cable fault finding: zero potential method. Zero potential method is also known as potential comparison method. It is suitable for short cable core to ground fault. This method is simple and accurate, and does not need precision instruments and complex calculation. The measurement principle is as follows: connect the cable fault core wire in parallel with the comparison wire of the same length. When applying voltage ve 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. On the contrary, the two points with zero potential difference must be the corresponding points. Because the negative pole of the micro voltmeter is grounded and equipotential with the cable fault point, when the positive pole of the micro voltmeter moves to the point when the indicated value is zero on the comparison wire, it is equipotential with the cable fault point, that is, the corresponding point of the cable fault point. S refers to single-phase knife switch, e refers to 6e battery or 4 No. 1 dry batteries, and G refers to DC microvolt meter. The measurement steps are as follows: 1) connect battery E on phase B and C core lines, and then lay a comparison conductor s with the same length as the fault cable on the ground. The conductor shall be bare copper wire or bare aluminum wire, with equal cross section and no intermediate joint. 2) The negative pole of the micro voltmeter shall be grounded, and the positive pole shall be connected with a long flexible conductor. The other end of the conductor shall be fully contacted when sliding on the laid comparison conductor. 3) Close the knife switch s, slide the end of the flexible conductor on the comparison conductor, and the position when the microvolt meter indicates zero is the position of the cable fault point. High voltage bridge method high voltage bridge method is to measure the DC resistance of cable core with double arm bridge, then accurately measure the actual length of cable, and calculate the cable fault point according to the positive proportional relationship between cable length and resistance. For direct short circuit between cable cores or short circuit point, the contact resistance is less than 1 & omega; The judgment error is generally not greater than 3m, and the contact resistance at the fault point is greater than 1 & omega; The resistance can be reduced to 1 & Omega by increasing the voltage and burning through; Next, measure according to this method. When measuring the circuit, first measure the resistance R1 between core wires a and B, R1 = 2RX R, where Rx is the phase resistance value from phase a or phase B to the cable fault point, which is only the contact resistance of the short contact. Then move the bridge to the other end of the cable and measure the DC resistance R2 between A1 and B1 core wires, then R2 = 2R (l-x) r, and R (l-x) is the phase resistance 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 Figure 3, and measure the DC resistance value between B and C 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 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 of 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-2) At the end of the cable, measure the capacitance currents IA1, IB2 and IC3 of the core wire of each phase to check the capacitance ratio between the intact core wire and the broken core wire, and preliminarily judge the approximate point of the broken wire distance. According to the capacitance calculation formula C = I / (2 & PI; Fu), when the positive voltage U and frequency f do not change, C is directly proportional to I. because the frequency of power frequency voltage is f (frequency) Constant, as long as the applied voltage is kept constant 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 of the core wire breaking point is x, then IA / ic = L / x, x = (IC / IA) 50. 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 measured accurately, the measurement error is relatively small. The so-called sound measurement method is to find out according to the discharge sound of faulty cable. This method is more effective for the flashover discharge of high-voltage cable core to insulating layer. The equipment used in this method is DC withstand voltage tester. TB is high-voltage test Transformer, C refers to high-voltage capacitor, ve refers to high-voltage rectifier silicon stack, R refers to current limiting resistance, Q refers to discharge ball gap, and l refers to cable core. When capacitor C is charged to a certain voltage value, the ball gap discharges to the cable fault core, and the cable core discharges to the insulating layer at the cable fault, producing a loud and loud spark discharge sound. For open laying cables, it can be found directly by hearing. If it is buried electricity For cable, first determine and mark the cable direction. When searching, put the pickup close to the ground and move slowly along the cable direction. When the sound of Zi and Zi discharge is the largest, this is the cable fault point. Pay attention to safety when using this method, and assign special personnel to monitor at the end of the test equipment and the end of the cable. The cable fault locator produced by our company can not only measure the sound, but also To detect the magnetic signal, the signal strength is displayed visually and synchronously, which can locate the cable fault point faster and more accurately.

Cable Fault Detection and Search Method 1

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What Are the Methods of Cable Fault Detection
The fault types of power cable mainly include parallel fault and series fault. Series fault refers to the disconnection of multiple or one conductor in the cable. Usually, it is difficult to detect the series fault before disconnecting a conductor in series. It is easy to find the series fault only when there is a real short circuit. Parallel fault is caused by external insulation aging caused by cable overload operation for a long time, and then partial discharge, resulting in parallel fault. The cable fault different from the length difference and resistance value is broken down, and the cable fault can be divided into high resistance type, low resistance type and open circuit fault. So what are the methods of cable fault detection? Bridge method: bridge method is a traditional method of cable fault detection, which can achieve very ideal results. This method is convenient and fast with high detection accuracy. It belongs to a common cable fault detection method. However, due to the low bridge voltage difference and galvanometer sensitivity, it is only suitable for fault detection with small cable resistance. Moreover, it is difficult to detect the open circuit fault of equipment and cable with high resistance. High voltage bridge method: high voltage bridge method is a common fault detection method in cable fault detection. Its detection principle is that in the high-voltage bridge, when the constant current power supply pierces the cable, it will ensure a large bridge current to a certain extent, so as to form a certain potential difference on both sides of the overall bridge, so as to count the fault points on the basis of coordinating the bridge. This method is suitable for high voltage constant current power supply, and can effectively expand the detection range of high resistance of bridge. Relatively speaking, the result is more convenient and accurate. In addition, for the research theory of bridge method, the characteristics of cable centerline resistance and proportional distribution of the whole line can promote the formation of bridge detection system. Impulse high voltage flashover method: impulse high voltage flashover method is widely used in some methods of cable fault detection. The detection principle of this method is to apply impulse high voltage at the beginning of the faulty cable, so that it can quickly puncture the place where the fault occurs, and record the data information of voltage sudden change at the moment of the fault place. Through the analysis of wired fault point and wired initial data information, the time distance is tested, so as to obtain the fault location and uncertain countermeasures. Low voltage pulse reflection method: when low voltage pulse emission method is used to detect cable fault, low voltage pulse shall be injected into the damaged line. When the pulse is transmitted along the cable to the fault point, when the inappropriate impedance is encountered in the process of transmitting current, the reflected pulse is displayed on the detection device and reflected through the data record of the device, and then the round-trip time difference of the output pulse and the cable wave speed are calculated, so as to obtain the distance between the fault point and the test point. This method is very simple and can make the test results particularly significant. When it is difficult to determine the fault data, it can be detected directly. However, it also has disadvantages, that is, it is not suitable for high resistance fault and flashover fault. Secondary pulse method: for the secondary pulse method, it is effectively applied to form the impulse high-voltage pulse at the moment of the integrated high-voltage generator and introduce it to the cable fault location. On the premise of effectively piercing the fault location, it can prolong the uninterrupted time of arc formation at the fault location after breakdown. Obviously, it must be clear that at the same time, a trigger pulse can trigger the automatic trigger device of the secondary pulse and the operation of the cable detection instrument. On the basis of starting the automatic trigger device of the secondary pulse, two low-voltage pulses are transmitted. Through the device of forming the secondary pulse, the cable is effectively transmitted on the fault detection cable, so as to break down the cable. Using the cable fault detector, observe the floating characteristics of voltage waveform and the reflection wavelength in the whole process of forming arc, comprehensively and systematically record the screen of detection equipment, and distinguish various current fluctuations, one of which reflects the actual length of cable and the other reflects the actual distance of short-circuit fault.
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