There are several ways to test the cable fault detector, and there must be something you don't

Author: Noyafa–Cable Tester Manufacturer

There are several ways to test the cable fault detector, and there must be a cable fault detector that you don't know. It is mainly to find out the fault point of the cable in time, to eliminate the fault as soon as possible, and to restore the electrical equipment in the line to a normal working state. instrument. There are several ways to test the general cable fault detector, and there must be other cable fault detectors that you don't know. It is mainly to find out the fault point of the cable in time, to eliminate the fault as soon as possible, and to restore the electrical equipment in the line to a normal working state. test instrument. Generally speaking, a cable is a rope-like wire composed of several or several groups of conductors, each of which is stranded with at least two wires. Each group of wires is insulated from each other and is often twisted around a center. Highly insulating cover.

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. Described below are several methods of how the cable fault detector performs the test. First, the basic test method of cable fault detector cable fault After the cable fails, generally use a shaker or high resistance meter above 1500V to identify the fault type, then use different instruments and methods to initially test the fault, and finally use the fixed-point method to accurately determine the fault point. There are two methods of precise measurement of the fault point.

One is the acoustic measurement method, the principle of which is to use a high-voltage pulse to induce the discharge of the fault point to generate a discharge sound, and use a sensor to receive the discharge sound on the ground to measure the precise 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. 2. 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 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, 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 measuring end can be calculated by the following formula.

(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. The distance of the fault point of the oscilloscope screen can be calculated according to 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 circuits must be used or temporary circuits must be installed as loops. When temporary circuits are installed, the resistance of the circuit must be accurately measured. Calculated by the following formula, that is, R is the single-wire resistance value of the temporary wire in the formula, and the meanings of other symbols are the same as those of formula (2). (2) Test of two-phase short-circuit fault point: When two-phase short-circuit fault point occurs. 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, while 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 faulty core wire and another intact core wire form a measurement loop, measure with an electric bridge, one end is connected with a jumper wire, and the other end is connected with a power supply, an electric bridge or a galvanometer, and the bridge resistance is adjusted to balance the electric bridge. When the wire material and section are the same, it can be calculated according to the following formula: If the damaged wire core 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. 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.