How to use high-voltage cable fault tester and locator?

Author: Noyafa–CCTV Tester

Question: How to use the high-voltage cable fault tester and locator? Answer: The cable fault point test is to use a certain test instrument to find out the cable fault point in time, to eliminate the fault as soon as possible, and to restore the electrical equipment in the line to a normal working state: the use of high-voltage cable fault tester and locator skills ? Answer: The cable fault point test is a kind of test that uses a certain test instrument to find out the cable fault point in time, eliminate the fault as soon as possible, and restore the electrical equipment in the line to the normal working state. A cable is a rope-like wire composed of several or several groups of wires each stranded with at least two wires. Each group of wires is insulated from each other, and is often twisted around a center. The entire outer layer is covered with a high degree of insulation. Floor. There are two types of cables commonly used in power systems: power cables and control cables. Power cables are used to transmit and distribute high-power electrical energy.

This article mainly introduces several methods for testing the possible points of power cables that are prone to failure and how to test them. First, the basic test method of cable fault After the cable fails, generally use a shaker or high-resistance meter above 1500V to identify the type of fault, then use different instruments and methods to initially test the fault, and then use the fixed-point method to determine the fault point, and the precise measurement of the fault point There are two methods. One is the acoustic measurement method, the principle of which is to use a high-voltage pulse to promote the discharge of the fault point to generate a discharge sound, and use the sensor to receive the discharge sound on the ground to measure the location of the fault point.

There is also an induction method. The principle is that when the audio current passes through the cable core, there are electromagnetic waves around the cable. Because some carry electromagnetic induction receivers, when walking along the line, you can hear the sound of electromagnetic waves, and the audio current flows. When the fault point is reached, the current suddenly changes, and the audio frequency of the electromagnetic wave changes suddenly. This method is very convenient for finding low-resistance short-circuit faults between disconnected phases, but it is not suitable for finding high-resistance short-circuits and single-phase grounding faults. Second, the specific fault types need to be tested according to the following methods. 1. High-resistance grounding fault, the high-resistance grounding fault of the cable is that the insulation resistance value between the conductor and the aluminum sheath or the conductor and the conductor is much lower than the normal value, but greater than 100kΩ, while the core wire has good continuity.

High-resistance grounding fault test method: (1) High-voltage bridge method, the wiring principle is shown in Figure 3a. Due to the large resistance of the fault point, a high-voltage DC power supply must be used to ensure that the current passing through the fault point is not too small. The bridge arm resistance is 3.5 divided by 100ΩThe left and right sliding wire resistance, the voltage applied to the bridge is 10 ~ 200kV, and the microammeter indicates 100 ~ 20μA. The distance from the fault point to the measurement end can be calculated by the following formula, that is: when the positions of the faulty core wire and the intact core wire in Figure 3 are exchanged, there is the formula X——Distance from fault point to measurement, m; L——Cable line length, m; C——Sliding bridge readings. (2) One-time scanning oscilloscope (711 type) method, the so-called one-time scanning oscilloscope method is to use a high-voltage one-time scanning oscilloscope to record the discharge oscillation waveform of the fault point and determine the fault point. Calculated by the following formula: where V——Wave speed, m/μs; T——oscillation period,μs.

There are four things to pay attention to when measuring: the same cable must be reliably grounded even if the core wire is not measured to prevent induction of dangerous high voltage; the pressure should be gradually increased during the measurement. Stop the measurement to avoid burning the instrument; since the measurement is carried out under high voltage, it must be reliably insulated from the ground. The operator should wear insulating gloves, operate with an insulating rod, and keep a distance from the high-voltage lead; When replacing the wiring, the voltage must be reduced and the power supply must be cut off. Only after the residual charge in the loop can be discharged, the wiring can be exchanged for reverse connection measurement. 2. Low-resistance grounding fault, low-resistance grounding fault is divided into single-phase low-resistance grounding fault, two-phase short-circuit fault and three-phase short-circuit fault. (1) Test of three-phase short-circuit fault point: When a three-phase short-circuit fault occurs, other parallel lines or temporary circuits must be used as loops during measurement. When temporary circuits are installed, the resistance of the circuit must be measured. The wiring method is as follows. shown in Figure 2.

It can be calculated by the following formula, that is, where R is the single-wire resistance value of the temporary wire, and the meanings of other symbols are the same as those of formula (2). (2) Test of two-phase short-circuit fault point: When a two-phase short-circuit fault point occurs, the measurement wiring method is shown in Figure 2. When measuring, one faulty core wire can be used as the ground wire, and the other faulty core wire can be connected to the bridge. The calculation formula and measurement method are the same as the single-phase low-resistance grounding fault point.

(3) Test method for the fault point of single-phase low-resistance grounding fault: The single-phase low-resistance grounding fault of the cable means that the insulation resistance of one core wire of the cable to the ground is lower than 100kΩ, and the core wire has good continuity. Such faults have strong concealment, and we can use the principle of loop fixed-point method to test. The wiring diagram is shown in Figure 1a. The faulty core wire and another intact core wire form a measurement loop, and use a bridge to measure, one end is bridged with a jumper, and the other end is connected to a power supply, a bridge or a galvanometer, and the bridge resistance is adjusted. To balance the bridge, when the material and cross-section of the cable core wire are the same, it can be calculated according to the following formula: If the damaged core wire and the good core wire are exchanged with each other on the bridge, there is Z in the formula——The distance from the measuring end to the fault point, m; L——Total cable length, m; R1, R2——Resistor arm of the bridge.

Under normal circumstances, the measurement results of these two wirings should be the same, and the error is generally 0.1% to 0.2%. If it exceeds this range or X>L/2, the measuring instrument can be moved to the other end of the line to measure. In addition, we can also use the continuous sweep pulse oscilloscope method (MST—Type 1A or LGS—Type 1 digital tester) for testing. The reflected wave at the short-circuit or ground-fault point will be a negative reflection, as shown in Figure 1b on the oscilloscope screen.

At this time, the fault point distance can be calculated according to the following formula: X——reflection timeμs; V——Wave speed, m/μs. Matters needing attention during measurement are: the cross-section of the jumper wire should be close to the cross-section of the cable core wire, and the jumper wire should be as short as possible and kept in good condition. The measurement loop should bypass the branch box or the transformer and power distribution station as much as possible, and the shorter the better.

The DC power supply voltage should not be lower than 1500V. The negative pole of the DC power supply should be connected to the cable conductor through an electric bridge, and the positive pole should be connected to the inner sheath of the cable and grounded. And the operator should stand on the insulating pad and put the bridge arm resistance, galvanometer, shunt, etc. on the insulating pad.

3. Incomplete disconnection fault, incomplete disconnection points are high resistance disconnection (conductor resistance is greater than 1kΩ) and low resistance disconnection (conductor resistance less than 1kΩ) in two cases. It shows that the insulation of each phase is good, and the conductors of one or more phases are not completely continuous. (1) The high resistance disconnection can be measured by the AC bridge method, and the wiring schematic diagram is shown in Figure 5.

The ratio of the capacitance of the faulty phase to the standard capacitor is measured at both ends of the line, and the distance is calculated according to the following formula: where CE and CF are the capacitances measured at the E and F terminals of the faulty phase, respectively. (2) For low-resistance disconnection, first use low-voltage current to make it blown, and then test it according to the complete line failure. 4. Complete disconnection fault. The so-called complete disconnection fault refers to the good insulation of each phase and the discontinuity of one or more phase wires.

At this time, the same two methods can be used for testing. (1) Bridge method (capacitor bridge, QF1—A-type bridge), its wiring is shown in Figure 4a, measure the ratio of the fault capacitance to the standard capacitor at both ends of the line, and determine the distance of the fault point, which can be calculated according to the following formula: where CE and CF are the fault phase The capacitance measured at the E and F terminals. (2) Continuous scanning oscilloscope method (MST)—1A or LGS—Type 1), the oscilloscope method is used to transmit pulses, and at the fault point of disconnection, the reflected wave is a positive reflection.

The screen of the oscilloscope is shown in Figure 4b, and the distance of the fault point is calculated according to the following formula: where V——Wave speed, m/μs; T——reflection time,μs.