Detection Method of Power Cable Fault
1. Rough measurement of power cable fault (1) bridge method bridge method is a classical method of power cable fault location, which has a long history. Including DC resistance bridge method, DC high voltage resistance bridge method and capacitance bridge method. The resistance bridge method can only test some cable faults with low insulation resistance between single phase and ground or between two phases; High voltage bridge method is mainly used to test single-phase grounding fault or phase to phase and ground fault of main insulation with resistance greater than 10k Ω but less than megohm; The capacitance bridge method is mainly used to test the open circuit and broken wire fault of cables. The bridge method is relatively simple and convenient to operate, but it needs to know the original data such as the accurate length of the cable in advance. At the same time, it is not suitable for detecting high resistance faults. Most of the actual power cable faults are high resistance faults. Because the bridge current is very small when the fault resistance is very high, it is difficult for general sensitive instruments to detect. (2) Traveling wave method 1) low voltage pulse method low voltage pulse method is mainly used to measure the fault distance of open circuit, short circuit and low resistance faults of cables; At the same time, it can also be used to measure the cable length, wave velocity and identify and locate the middle head, T-joint and terminal head of the cable. Test principle: input a low-voltage pulse signal from the test end to the cable, and the pulse signal propagates along the cable. When encountering the impedance mismatch point in the cable, such as open circuit point, short circuit point, low resistance fault point, etc., a reflected pulse will be generated. The position of the fault point is calculated according to the round-trip time difference DT between the reflected pulse and the transmitted pulse and the pulse propagation speed v. 2) High voltage pulse method high voltage pulse method uses high voltage signal to turn cable fault into short circuit or low resistance fault instantly, so that the reflection coefficient of fault point is close to - 1, and the fault point almost produces total reflection. There are usually two basic flashover methods, namely direct flashover method and impulse flashover method. When testing cable fault by flashover method, the reflected wave formed at the cable fault point is high-voltage pulse wave, which cannot be displayed directly through the instrument. Generally, a sampler is needed to convert the high-voltage pulse formed at the fault point under the action of high voltage into the low-voltage pulse signal required by the instrument. According to different sampling methods, it is divided into voltage method, current method and voltage induction method. Wherein, R1 is the voltage dividing resistance, R2 is the sampling resistance, LP is the current sampler, C is the energy storage capacitor and B is the transformer. DC high voltage flashover method (direct flashover method): apply DC voltage to the fault cable to make the fault click through the room for discharge and flashover. Then, the fault distance can be calculated according to the transmission speed V of the traveling wave in the cable. The direct flash method is mainly used to test the flashover high resistance fault of power cable, and can also be used to test the leakage high resistance fault with particularly high resistance but lower resistance compared with the intact phase. Figure 6 shows the principle circuit of direct flash test. Impulse high voltage flashover method (impulse flashover method): due to the large equivalent internal resistance of DC high voltage power supply used in direct flashover method, the power output power is limited to a certain extent. For most leakage high resistance faults, direct flashover method cannot be tested. The impulse flashover method uses the high-capacity charging capacitor as the DC high-voltage power supply, which is connected to the fault cable to cause the flashover and discharge at the fault point to form an instantaneous short circuit. It is mainly used to test the leakage high resistance fault of power cable, and can also be used to test the low resistance, open circuit and flashover high resistance fault of power cable. The test principle circuit is basically the same as that of the direct flash method, except that a spherical gap is inserted between the energy storage capacitor and the cable, as shown in Figure 7. 2. Precise measurement of power cable fault point (1) acoustic measurement method uses the sound wave generated when the fault point is discharged for fixed point. The sound sensor detects the sound signal above the power cable. The place with loud sound is the location of the fault point. The farther away from the fault point, the smaller the vibration sound. (2) When the fault point flashover discharges under the action of impulse voltage, the acoustic magnetic synchronization method receives the electromagnetic wave and vibration sound wave generated by the discharge of the fault point at the same time, and judges whether the measured signal is generated by the discharge of the fault point to accurately judge the location of the fault point. (3) The audio induction method adds a certain power low-voltage audio signal to the fault of the cable at one end of the tested circuit. When the tested signal is transmitted to the short-circuit or disconnection point, it cannot continue to transmit along the cable, so there will be obvious signal size changes on both sides of the cable fault point. If the signal changes are detected through the receiver above the cable path, The location of the fault point can be determined. At the same time, the accuracy and integrity of various basic data of cables are also very important to improve the efficiency of fault point finding, such as cable laying direction, total cable length, distribution of cable intermediate joints, etc.
