Teach you a practical measurement method for several types of cable faults. I. Types and judgments of cable faults Regardless of whether they are high-voltage cables or low-voltage cables, they are often caused by short circuit, overload operation, insulation aging, or external damage during construction, installation, and operation. malfunction. There are three types of cable faults: grounding, short circuit, and broken wire. Three-core cable fault types mainly include the following aspects: one-core or two-core contact; two-phase core-line short-circuit; three-phase core wire completely short-circuit; one-phase core wire breakage or polyphase breakage. For direct short-circuit or disconnection fault with a multimeter can be directly measured and judged, for non-direct short-circuit and tank fault, with megohmmeter telemetering insulation resistance between core wire or core wire insulation resistance, according to its resistance value can be determined type of failure.
Second, the cable fault point search method 1, the sound measurement method called sound measurement is based on the sound of fault cable discharge to find, the method for high voltage cable core insulation layer flashover discharge is more effective. The equipment used in this method is a DC voltage tester. The circuit connection is shown in Fig. 1, where SYB is a high voltage test transformer, C is a high voltage capacitor, ZL is a high voltage rectified silicon stack, R is a current limiting resistor, Q is the discharge ball gap, and 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. At the fault, the cable core wire discharges the insulating layer to generate a sparking sound of “nose and noxiousnessâ€, and when the noise noise is minimal, the noise is reduced. An audio amplification device such as an earwax hearing aid or a medical stethoscope is used for searching. When searching, pick up the pickup close to the ground and slowly move along the direction of the cable. When you hear the sound of “chicken and urine†discharge, it is the fault point. Use this method must pay attention to safety, in the end of the test equipment and the end of the cable should be set up to monitor.
2. The bridge method bridge method is to measure the DC resistance value of the cable core by two-arm bridge, then accurately measure the actual length of the cable, and calculate the fault point according to the proportional relationship between the length of the cable and the resistance. In this method, the error for the direct short-circuit or short-circuit contact resistance between cable cores is less than 1Ω, and the error is generally not more than 3m. For faults with contact resistance greater than 1Ω at the fault point, the method of increasing the voltage to burn-through can be used to reduce the resistance to 1Ω. Below, measure again according to this method.
The measuring circuit first detects the resistance R1 between the cores a and b, then R1=2RX+R, where R is the resistance value of phase a or phase b to the point of failure and R is the contact resistance of the short contact. Then measure the DC resistance R2 between the a' and b' cores 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 A phase resistance from the line to the point of failure. After measuring R1 and R2, and then short-circuit the b' and C' according to the circuit shown in Figure 3, and measure the DC resistance between the two phases of the b and c cores, then 1/2 of the resistance is the core of each phase. The resistance value of the line is represented by RL. RL=RX+R(L-X), from which the contact resistance at the point of failure can be found: R=R1+R2-2RL. Therefore, the resistance value of the core wire 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 from the fault point to the end of the cable or (L-X) can be obtained by a proportional formula: X=(RX/RL)L, (L -X)=(R(L-X)/RL)L, where L is the total length of the cable. The use of bridge method should ensure the measurement accuracy, the bridge connection line should be as short as possible, the diameter should be large enough, and the cable core connection to use crimping or welding æ‚, the decimal point in the calculation process to be retained.
3. Capacitance amperometry cable In operation, there is a capacitance between the cores and the core to the ground. The capacitance is evenly distributed. The capacitance is linearly proportional to the length of the cable. Capacitance amperometry is based on this principle. For the determination, the measurement of the breakage of the cable core is very accurate. The measuring circuit is shown in Fig. 4. The device is a single-phase voltage regulator with 1 to 2 kVA, 0 to 30V, 0.5 AC voltmeter, 0 to 100 mA, and 0.5 AC milliammeter.
Measurement steps:
(1) Measure the value of the capacitance current of each core at the leading end of the cable (shall keep the applied voltage equal) Ia, Ib, Ic.
(2) Measure the values ​​of the capacitance currents Ia', Ib', and Ic' of the core of each phase at the end of the cable to check the ratio of the specific capacitances of the perfect core to the broken core, and to determine the breakage distance Approximate point.
(3) According to the capacitance calculation formula C=1/2Ï€fU, when the voltage U and the frequency f are constant, C and I are proportional; because the frequency f of the power frequency voltage is constant, only the applied voltage is guaranteed to be measured during measurement. The ratio of capacitance current is the ratio of capacitance. With the full length L of the cable, the distance of the broken point of the core is x, then Ia/Ic=L/x, x=(Ic/Ia)L. During the measurement process, as long as the voltage is kept 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 the potential comparison method, it is adapted to the short length of the cable core ground fault, this method is simple and accurate measurement, does not require precision instruments and complex calculations, the wiring shown in Figure 5 . The principle of the measurement is as follows: When the cable fault core is connected in parallel with the equal length of the comparison conductor, when both ends are pressurized E, it is equivalent to connecting the two ends of the two uniform resistance wires. At this point, the potential difference between any point on one resistance wire and the corresponding point on the other resistance wire is necessarily zero. On the contrary, the two points where the potential difference is zero must be the corresponding point, because the negative terminal of the microvoltmeter is grounded and the potential point of the cable is equipotential. Therefore, when the positive terminal of the microvoltmeter moves on comparative conduction to the point where the indication value is zero. Equipotential with the fault point, which is the corresponding point of the fault point. In Figure 5, K is a single-phase switch, E is a 6V battery or 4 AA batteries, G is a DC microvoltmeter, and the measurement steps are as follows:
(1) Connect the batteries E to the cores of phases b and c first, and then lay a comparative wire S equal to the length of the faulty cable on the ground. The bare wire or bare aluminum wire should be used, and the cross-section should be equal. There can be no intermediate connector.
(2) Connect the negative pole of the microvoltmeter to ground and the positive pole to a long soft wire. The other end of the wire must be fully accessible when sliding on the comparative wire.
(3) Close the knife switch K and slide the broken end of the soft wire on the comparative wire. When the microvoltmeter indicates zero, the position is the position of the cable fault point.
Second, the cable fault point search method 1, the sound measurement method called sound measurement is based on the sound of fault cable discharge to find, the method for high voltage cable core insulation layer flashover discharge is more effective. The equipment used in this method is a DC voltage tester. The circuit connection is shown in Fig. 1, where SYB is a high voltage test transformer, C is a high voltage capacitor, ZL is a high voltage rectified silicon stack, R is a current limiting resistor, Q is the discharge ball gap, and 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. At the fault, the cable core wire discharges the insulating layer to generate a sparking sound of “nose and noxiousnessâ€, and when the noise noise is minimal, the noise is reduced. An audio amplification device such as an earwax hearing aid or a medical stethoscope is used for searching. When searching, pick up the pickup close to the ground and slowly move along the direction of the cable. When you hear the sound of “chicken and urine†discharge, it is the fault point. Use this method must pay attention to safety, in the end of the test equipment and the end of the cable should be set up to monitor.
2. The bridge method bridge method is to measure the DC resistance value of the cable core by two-arm bridge, then accurately measure the actual length of the cable, and calculate the fault point according to the proportional relationship between the length of the cable and the resistance. In this method, the error for the direct short-circuit or short-circuit contact resistance between cable cores is less than 1Ω, and the error is generally not more than 3m. For faults with contact resistance greater than 1Ω at the fault point, the method of increasing the voltage to burn-through can be used to reduce the resistance to 1Ω. Below, measure again according to this method.
The measuring circuit first detects the resistance R1 between the cores a and b, then R1=2RX+R, where R is the resistance value of phase a or phase b to the point of failure and R is the contact resistance of the short contact. Then measure the DC resistance R2 between the a' and b' cores 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 A phase resistance from the line to the point of failure. After measuring R1 and R2, and then short-circuit the b' and C' according to the circuit shown in Figure 3, and measure the DC resistance between the two phases of the b and c cores, then 1/2 of the resistance is the core of each phase. The resistance value of the line is represented by RL. RL=RX+R(L-X), from which the contact resistance at the point of failure can be found: R=R1+R2-2RL. Therefore, the resistance value of the core wire 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 from the fault point to the end of the cable or (L-X) can be obtained by a proportional formula: X=(RX/RL)L, (L -X)=(R(L-X)/RL)L, where L is the total length of the cable. The use of bridge method should ensure the measurement accuracy, the bridge connection line should be as short as possible, the diameter should be large enough, and the cable core connection to use crimping or welding æ‚, the decimal point in the calculation process to be retained.
3. Capacitance amperometry cable In operation, there is a capacitance between the cores and the core to the ground. The capacitance is evenly distributed. The capacitance is linearly proportional to the length of the cable. Capacitance amperometry is based on this principle. For the determination, the measurement of the breakage of the cable core is very accurate. The measuring circuit is shown in Fig. 4. The device is a single-phase voltage regulator with 1 to 2 kVA, 0 to 30V, 0.5 AC voltmeter, 0 to 100 mA, and 0.5 AC milliammeter.
Measurement steps:
(1) Measure the value of the capacitance current of each core at the leading end of the cable (shall keep the applied voltage equal) Ia, Ib, Ic.
(2) Measure the values ​​of the capacitance currents Ia', Ib', and Ic' of the core of each phase at the end of the cable to check the ratio of the specific capacitances of the perfect core to the broken core, and to determine the breakage distance Approximate point.
(3) According to the capacitance calculation formula C=1/2Ï€fU, when the voltage U and the frequency f are constant, C and I are proportional; because the frequency f of the power frequency voltage is constant, only the applied voltage is guaranteed to be measured during measurement. The ratio of capacitance current is the ratio of capacitance. With the full length L of the cable, the distance of the broken point of the core is x, then Ia/Ic=L/x, x=(Ic/Ia)L. During the measurement process, as long as the voltage is kept 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 the potential comparison method, it is adapted to the short length of the cable core ground fault, this method is simple and accurate measurement, does not require precision instruments and complex calculations, the wiring shown in Figure 5 . The principle of the measurement is as follows: When the cable fault core is connected in parallel with the equal length of the comparison conductor, when both ends are pressurized E, it is equivalent to connecting the two ends of the two uniform resistance wires. At this point, the potential difference between any point on one resistance wire and the corresponding point on the other resistance wire is necessarily zero. On the contrary, the two points where the potential difference is zero must be the corresponding point, because the negative terminal of the microvoltmeter is grounded and the potential point of the cable is equipotential. Therefore, when the positive terminal of the microvoltmeter moves on comparative conduction to the point where the indication value is zero. Equipotential with the fault point, which is the corresponding point of the fault point. In Figure 5, K is a single-phase switch, E is a 6V battery or 4 AA batteries, G is a DC microvoltmeter, and the measurement steps are as follows:
(1) Connect the batteries E to the cores of phases b and c first, and then lay a comparative wire S equal to the length of the faulty cable on the ground. The bare wire or bare aluminum wire should be used, and the cross-section should be equal. There can be no intermediate connector.
(2) Connect the negative pole of the microvoltmeter to ground and the positive pole to a long soft wire. The other end of the wire must be fully accessible when sliding on the comparative wire.
(3) Close the knife switch K and slide the broken end of the soft wire on the comparative wire. When the microvoltmeter indicates zero, the position is the position of the cable fault point.
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