Cables commonly used in power systems are broadly classified into two categories: power cables and control cables. Power cables are used to transmit and distribute high-power electrical energy. Based on the insulation material, they can be divided into oil-impregnated paper-insulated power cables, rubber-insulated power cables, and polyvinyl chloride (PVC)-insulated cables. Oil-impregnated paper-insulated power cables are the most widely used in engineering. Since there are explicit national regulations regarding cable manufacturing, cable laying, ambient temperature, and construction principles, these will not be elaborated upon here. This article mainly introduces the potential fault points of power cables and several methods for testing them.
Types of cable faults and testing methods: After a cable fault occurs, the type of fault is generally determined first by using a megohmmeter or high resistance meter with a voltage of 1500V or higher. Then, different instruments and methods are used to initially test the fault. Finally, the fault point is accurately determined by the fixed-point method. There are two methods for precise measurement of the fault point: induction method and acoustic method.
The principle of the induction method is that when an audio current passes through the cable core, electromagnetic waves exist around the cable. Therefore, if you carry an electromagnetic induction receiver, you can hear the sound of electromagnetic waves when walking along the line. 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 very convenient for finding low-resistance short circuit faults between phases with open circuits, but it is not suitable for finding high-resistance short circuits and single-phase grounding faults.
The principle of acoustic detection is to use a high-voltage pulse to cause the fault point to discharge, generating a discharge sound. A sensor on the ground receives this discharge sound to determine the precise location of the fault point.
A single-phase low-resistance grounding fault in a cable refers to a fault where the insulation resistance of one core wire to ground is less than 100kΩ, while the core wire is continuous. This type of fault is highly concealed, and we can use the loop-based fixed-point method for testing. The wiring diagram is shown in Figure 1a. The faulty core wire is connected to another intact core wire to form a measurement loop. A bridge circuit is used for measurement, with one end connected to a jumper wire and the other end connected to a power supply, bridge, or galvanometer. The bridge resistance is adjusted to balance the bridge. When the cable core wire material and cross-section are the same, the following formula can be used to calculate the fault. If the positions of the damaged core wire and the good core wire are interchanged on the bridge, then the formula is: [Formula omitted for brevity]. Where Z is the distance from the measuring end to the fault point (m); L is the total length of the cable (m); and R1 and R2 are the resistance arms of the bridge.
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