Discussion on Testing Methods for Power Cable Fault Points

Using certain testing instruments to promptly identify cable fault points and quickly eliminate faults, so as to restore the normal operation of electrical equipment in the line.

　　A cable is a core formed by twisting one or more wires, wrapped with an appropriate insulation layer, and then covered with a sealed sheath (aluminum, lead, or plastic, etc.). In power systems, commonly used cables include power cables and control cables. 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 PVC insulated cables. In engineering, oil-impregnated paper insulated power cables are widely used. Since the production of cables, as well as the laying of lines, ambient temperature, and construction principles, are all clearly stipulated by the state, this article will not elaborate on them. This article mainly introduces the possible fault points of power cables and several methods for testing them. Hubei Yitiancheng Electric Equipment Co., Ltd. is a high-quality supplier of CVT resonance, frequency conversion series resonance, and inductive series resonance, and we supply series resonance and related products to domestic customers all year round.

　　1　Types of cable faults and testing methods

　　After a cable fault occurs, generally use a megger or high-resistance meter above 1500V to determine the fault type, then use different instruments and methods for preliminary testing, and use the pinpointing method to accurately determine the fault point. The precise fault point testing 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, carrying an electromagnetic induction receiver and walking along the line, the sound of electromagnetic waves can be heard. When the audio current reaches the fault point, the current changes abruptly, and the audio frequency of the electromagnetic wave changes. 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 discharge sounds. Sensors are used on the ground to receive these discharge sounds 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 below 100kΩ, while the continuity of the core is good. This type of fault is highly concealed, and 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, connect one end with a jumper wire, and connect the other end 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, it can be calculated using the following formula:

　　If the positions of the damaged core and the good core on the bridge are swapped, 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, the measuring instrument can be moved 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 maintain good contact.

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

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