Views:195 Author:Site Editor Publish Time: 2020-10-15 Origin:Site
Cables play a key role in power transportation. However, in the long-term use of cables, due to problems such as complex processes, failures will also occur. When repairing cable short circuit and earth faults, it needs to be detected by appropriate detection methods and fault indicators. So what methods are worth choosing when carrying out detection?
Connect the faulty phase and the non-faulty phase to the two arms of the bridge separately, adjust the adjustable resistor on the two arms of the bridge to balance the bridge, according to the proportional relationship and the known cable length, we can get the electrical short circuit fault distance. Use a low-voltage bridge to test the low-resistance breakdown of the cable, and use a capacitance bridge to test the cable open circuit and disconnection. The measurement result of the bridge method is accurate, but the core wire needs to be intact as the loop, and the power supply voltage cannot be increased too high.
Using the echo phenomenon when the special impedance of the transmission line changes, a certain voltage can be added to the cable core wire to prevent it from burning through and causing the discharge. The discharge pulse propagates and reflects on the cable. Use a digital oscilloscope to measure the position ratio of the reflected pulse to calculate the location of the dead short circuit fault point. This method is suitable for high-resistance breakdown, but the safety of operators will be threatened, and the waveform is difficult to distinguish.
For low resistance breakdown, cable short circuit, and open circuit faults, pulse signals can be applied to the cable core, the signal propagates and reflects on the cable, and the pulse waveform is measured with a digital oscilloscope or a laptop virtual oscilloscope, then calculate the location of the power short circuit fault point. The advantage of the low voltage pulse method is that it is simple and intuitive, and does not require detailed cable original data. It can also distinguish the fault type according to the polarity of the reflected pulse. The disadvantage is that it cannot be used to measure high resistance and flashover faults.
The second pulse method is one of the commonly used distance measurement methods in recent years. Its principle is to release a low voltage pulse to the faulty cable. As long as the ground resistance of the short circuit and overloading fault point is greater than 5 times the wave impedance of the cable, it can be considered that the electronic short circuit cable is relative to the low voltage pulse. If it is an open circuit, the reflected waveform received at the pulse release end is equivalent to the waveform of a cable with good core insulation. Release a high voltage pulse to the faulty cable that is sufficient to flashover the core insulation fault point, at the same time, release the second low voltage pulse, when the arc at the fault point is not extinguished, the fault point is completely short-circuited, then the low-voltage pulse reflection waveform received at the pulse release end is equivalent to the waveform of a core-to-ground complete short circuit. There will be an obvious divergence point when comparing the two waveforms. This divergence point is the reflected waveform point of the fault point. This method is characterized by easy operation, multi-function and simple echo pattern. The disadvantage is that it cannot be used to measure high resistance and flashover faults.
Now, as long as you understand the cable fault detection methods, you will find that there are several methods and short circuit fault indicators that can be used. Therefore, when the cable fails, you must consider these methods and earthing and short circuiting devices to complete the fault detection, and the fault point in the cable should be repaired as soon as possible.