Types and Judgment of Cable Faults
Whether it is high-voltage cable or low-voltage cable, it is often caused by short circuit, overload operation, insulation aging or external force during construction, installation and operation. Cable fault can be summarized as grounding, short circuit, broken three categories, the main types of failure in the following areas:
â‘ three-core cable one or two core grounding.
â‘¡ two-phase short circuit between the wires.
â‘¢ three-phase core completely short circuit.
â‘£ one-phase wire break or multi-phase break.
For direct short-circuit or disconnection fault can be measured directly with the multimeter to determine, for non-direct short-circuit and ground fault, shake with the megger to measure the core insulation resistance or core insulation resistance, according to its resistance can determine the type of fault.
After determining the type of fault, find the fault point is not an easy task, the following based on the author's experience, describes several ways to find the point of failure for reference.
2 cable fault point search method
(1) sound measurement method:
The so-called sound measurement method is based on the sound of the fault cable to find the discharge, the method of high voltage cable core insulation flashover discharge more effective. Equipment used in this method for the DC voltage testing machine. The wiring of the circuit is shown as in Fig. 1, among them SYB is the high voltage test transformer , C is the high voltage capacitor, ZL is the high-voltage commutated silicon reactor, 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 core of the faulty cable. At the fault, the core of the cable discharges to the insulation layer to produce a spark discharge sound of "Zi, Zi" If the buried cable, you must first identify and mark the cable to the noise and then the smallest noise time, with the help of deafness hearing aids or medical stethoscope and other audio amplifier equipment to find. Find, the pickup close to the ground, moving slowly along the cable, when you hear the "Zi, Zi," the largest discharge sound, the Department is the point of failure. Use this method must pay attention to safety, at the end of the test equipment and cable ends should be set to monitor.
(2) bridge method:
Bridge method is to use double-arm bridge to measure the DC resistance of the cable core, and then accurately measure the actual length of the cable, according to the length of the cable and the proportional relationship between the resistance to calculate the point of failure. The method for the direct short-circuit between the cable core or short circuit point contact resistance of less than 1Ω fault, the judgment error is generally not more than 3m, the fault point contact resistance greater than 1Ω fault, can be increased by voltage burn-in method to make the resistance down to 1Ω The following, then press this method to measure.
First, measure the resistance R1 between the cores a and b, then R1 = 2Rx + R, where Rx is the phase resistance of phase a or phase b to the fault point, and R is the contact resistance of the short contact. R2 = 2R (LX) + R, where R (LX) is the phase of the a'or the b'-phase to the fault, and then the DC resistance R2 between the wires a 'and b' Point of the phase resistance, measured after R1 and R2, then press the circuit shown in Figure 3 b 'and c' short, measured b, c two-phase DC resistance between the wires, the resistance of 1/2 for each phase of the core resistance, RL said, RL = Rx + R (LX), which can be derived from the point of contact resistance value of the fault: R = R1 + R2-2RL, The resistance value of the side core can be expressed by the following equation: Rx = (R1-R) / 2, R (LX) = (R2-R) / 2. Rx, R (LX), RL After determining the three values, calculate the distance X or (LX) of the fault point from the end of the cable according to a proportional formula: X = RX / RL L, LX = (LX) / RL) L, where L is the total length of the cable.
Bridge method should be used to ensure measurement accuracy, bridge cables to be as short as possible, the cable diameter should be large enough, and the cable core connection to be crimped or welded, the calculation of the number of decimal places to be retained.
(3) capacitance current measurement method:
Cable in operation, between the core, core to ground there are capacitors, the capacitor is evenly distributed, the capacitance and the cable length is linearly proportional to the capacitance current measurement method is based on this principle for the determination of the cable Wire breakage fault determination is very accurate. Measuring circuit shown in Figure 4, the use of equipment for the 1 ~ 2kVA single-phase voltage regulator, 0 ~ 30V, 0.5 AC voltmeter, 0 ~ 100mA, 0.5 AC mA table.
Measurement steps:
â‘ First of all, measure the value of Ia, Ib and Ic of the capacitor current of each phase wire (keep the applied voltage equal) at the head of the cable.
â‘¡ Measure the capacitance current Ia ', Ib', Ic 'of each phase wire at the end of the cable to check the ratio of the intact core wire to the capacitance of the broken core wire.
â‘¢ According to the capacitance calculation formula C = 1 / 2Ï€fU shows that in the voltage U, the same frequency f C and I. Because the power frequency voltage f (frequency) constant, as long as the measurement to ensure the same voltage, the capacitor current ratio is the capacitance ratio. Let the length of the cable be L, and the distance of the core wire break point is X, Ia / Ic = L / X and X = (Ic / Ia) L.
Measurement process, as long as the guarantee voltage does not change, ammeter readings accurate, accurate measurement of the total cable length, the measurement error is relatively small.
(4) zero potential method:
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