Exploring Cable Fault Point Testing Methods



A cable is a core formed by twisting one or more wires, wrapped with a corresponding insulation layer, and then covered with a sealed sheath (aluminum, lead, or plastic, etc.). In power systems, the commonly used cables are power cables and control cables, among which power cables are used to transmit and distribute high-power electrical energy. According to different insulation materials, they can be divided into oil-impregnated paper insulated power cables, rubber insulated power cables, and polyvinyl chloride insulated cables. In engineering, the most widely used is the oil-impregnated paper insulated power cable. Since there are national regulations for cable manufacturing, laying routes, ambient temperature, and construction principles, they will not be repeated here. This article mainly introduces the possible points where power cables are prone to faults and several methods for testing them.



　　1　Types of Cable Faults and Testing Methods

　　After a cable fault occurs, first use a megger or high-resistance meter above 1500V to determine the fault type, then use different instruments and methods for preliminary testing, and finally use the pinpointing method to accurately determine the fault point. The precise fault point measurement methods include the induction method and the acoustic method.

　　Induction method: Its principle is that when audio current passes through the cable core, electromagnetic waves exist around the cable. Therefore, when carrying an electromagnetic induction receiver and walking along the line, the audio of the electromagnetic wave can be heard. When the audio current flows to the fault point, the current changes abruptly, and the audio of the electromagnetic wave changes abruptly. This method is convenient for finding low-resistance short-circuit faults between phases, but it is not suitable for finding high-resistance short-circuit and single-phase grounding faults.

　　Acoustic method: Its principle is to use high-voltage pulses to cause discharge at the fault point, producing a discharge sound. A sensor is used to receive this discharge sound on the ground to determine the precise location of the fault point.

　　Specific fault types are tested according to the following methods.

　　1.1　Low-Resistance Grounding Fault

　　1.1.1　Single-Phase Low-Resistance Grounding Fault

　　(1) Testing the fault point.

　　A single-phase low-resistance grounding fault in a cable refers to a situation where the insulation resistance of one core to ground is lower than 100kΩ, while the continuity of the core is good. This type of fault has strong concealment, so we can use the loop pinpointing method principle for testing. The wiring diagram is shown in Figure 1a. Connect the faulty core and another intact core to form a measurement loop. Use a bridge for measurement, with one end connected by a jumper wire and the other end connected to a power supply, bridge, or galvanometer. Adjust the bridge resistance to balance the bridge. When the cable core material and cross-section are the same, the following formula can be used for calculation:

　　If the positions of the damaged core and the good core on the bridge are interchanged, then:

　　Where　Z——distance from the measuring end to the fault point, m;

　　L——total length of the cable, m;

　　R1, R2——resistance arms of the bridge.

　　Under normal circumstances, the measurement results of these two wiring methods should be the same, with an error generally of 0.1% to 0.2%. If it exceeds this range or X>L/2, move the measuring instrument to the other end of the line for measurement.

　　In addition, we can also use the continuous scanning pulse oscilloscope method (MST—1A type or LGS—1 type digital tester) for testing. The reflected wave at the short-circuit or grounding fault point will be a negative reflection, as shown on the oscilloscope screen in Figure 1b. At this time, the distance to the fault point can be calculated using the following formula:

　　Where　X——reflection time, μs;

　　V——wave speed, m/μs.

　　(2) Precautions during measurement.

　　a. The cross-section of the jumper wire should be close to that of the cable core, and the jumper wire should be as short as possible and kept in good condition.

　　b. The measurement loop should avoid branch boxes or substations/distribution rooms as much as possible, and the shorter the better.

　　c. The DC power supply voltage should not be lower than 1500V.

　　d. The negative pole of the DC power supply should be connected to the cable conductor through the bridge, and the positive pole should be connected to the inner sheath of the cable and grounded.

　　e. Operators should stand on insulating pads and place the bridge arm resistance, galvanometer, shunt, etc., on insulating pads.

　　1.1.2　Testing Two-Phase Short-Circuit Fault Points

　　When a two-phase short-circuit fault point occurs, the measurement wiring method is shown in Figure 2. During measurement, any faulty core can be used as the grounding wire, and the other faulty core is connected to the bridge. The calculation formula and measurement method are the same as for single-phase low-resistance grounding fault points.

　　1.1.3　Testing Three-Phase Short-Circuit Fault Points

　　When a three-phase short-circuit fault occurs, the measurement method is similar to that of two-phase short-circuit. During measurement, one faulty core can be used as the grounding wire, and the other two faulty cores are connected to the bridge. The calculation formula and measurement method are the same as for single-phase low-resistance grounding fault points.

　　1.2　High-Resistance Fault Testing

　　For high-resistance faults, the bridge method is not suitable. We can use the pulse reflection method or the high-voltage bridge method for testing. The pulse reflection method uses a pulse generator to send a pulse signal into the cable. When the pulse encounters a fault point, a reflected wave is generated. By measuring the time difference between the transmitted and reflected pulses, the distance to the fault point can be calculated. The high-voltage bridge method uses a high-voltage DC power supply and a bridge to measure the resistance of the fault point, thereby determining the fault location.

　　1.3　Open-Circuit Fault Testing

　　For open-circuit faults, we can use the capacitance method or the pulse reflection method. The capacitance method measures the capacitance of the cable to determine the fault location. The pulse reflection method is similar to that used for high-resistance faults, but the reflected wave characteristics are different.

　　1.4　Flashover Fault Testing

　　For flashover faults, the acoustic method is commonly used. By applying a high-voltage pulse to cause discharge at the fault point, the discharge sound is received by a sensor on the ground to determine the precise location.

　　2　Conclusion

　　In summary, the testing of cable fault points is a complex and meticulous task. Different fault types require different testing methods. In practical work, we should choose appropriate testing methods based on the specific fault situation, and combine multiple methods for verification to accurately determine the fault point. At the same time, attention should be paid to safety during testing to prevent electric shock accidents.