Detection method of cable fault point

Author: Noyafa–Cable Locator Suppliers

Cable fault detection method of cable fault point detection Cable fault detection For the cable fault point detection, it generally goes through three main steps: fault type diagnosis, fault point distance measurement, and precise point determination. The fault type diagnosis is mainly to determine the fault phase of the fault point of the cable, which belongs to the high resistance grounding or the low resistance grounding, so that the tester can choose the appropriate detection method. The fault point distance measurement is also called pre-positioning. The test signal is applied to the faulty cable core wire or the fault information is measured and analyzed online, the distance of the fault is initially determined, and the fault scope is minimized to facilitate the accurate point determination.

Pre-positioning methods can generally be classified into two categories, namely classical methods, such as bridge method, etc.; modern methods, such as low-voltage pulse method, high-voltage flashover method, etc. Precise point determination is based on the pre-positioning distance to accurately determine the actual location of the fault point. The precise point determination methods mainly include the acoustic determination point method, the induction fixed point method, the time difference fixed point method and the synchronous fixed point method.

This paper mainly discusses the basic method of fault point pre-location. For low-resistance breakdown, short-circuit, and open-circuit faults, a pulse signal can be applied to the cable core. The signal is transmitted and reflected in the cable, and the pulse waveform is measured with a digital oscilloscope or a laptop virtual oscilloscope to calculate the location of the fault point.

The advantages of the low-voltage pulse reflection method are that it is simple and intuitive, does not require careful cable raw materials, and can distinguish the fault type according to the polarity of the reflected pulse. The disadvantage is that it cannot be used to detect high resistance and flashover faults. Using the echo phenomenon when the characteristic impedance of the transmission line changes, a certain voltage is added to the core wire of the cable to prevent it from burning through and causing discharge.

The discharge pulse is transmitted and reflected in the cable, the position ratio of the reflected pulse is measured with a digital oscilloscope, and the position of the fault point is calculated. This method is suitable for high resistance breakdown, but the operator's safety is threatened, and the waveform is difficult to distinguish. The three-pulse method is a new method for pre-positioning the fault point of the cable. Because the low-voltage pulse sent by the pulse reflection method will not reflect at the high-resistance fault point, the fault point will not appear on the waveform at this time. The low-voltage pulse forms a total reflection at the end of the test cable, and the reference waveform of the full length of the cable is obtained; the subsequent pulse impulse can form a stable arc at the fault point, and then a high-energy detection pulse is used to stop the impact on the fault point. , At this time, the pulse amplitude can reach 1500V, which can fully ensure that a negative reflection is formed at the fault point, and the fault waveform of the fault point can be obtained.

By comparing the two waveforms, it is easy to see the location of the fault point. This method is applicable to all fault types except the special condition of the middle head being damp or water inflow, including high-resistance connection low and low-resistance connection low. 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 grounding resistance of the fault point is greater than 5 times the cable wave impedance, it can be considered that the faulty cable is related to the low-voltage at this time. If the pulse is open circuit, then 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 enough to cause flashover at the fault point of the core insulation, and trigger the release of a second low voltage Pulse, when the arc at the fault point is not extinguished, the fault point is a complete short circuit with respect to the low-voltage pulse, then the reflected waveform of the low-voltage pulse received at the pulse release end is equivalent to the waveform of a complete short circuit between the core and the ground; the comparison of the two waveforms will have obvious The divergence point is the reflected waveform point of the fault point.

Its characteristics are easy operation, multi-function, and simple echo graph. The disadvantage is that it cannot be used to detect high resistance and flashover faults. Short-circuit the faulty phase and non-faulty phase of the cable under test, connect the two arms of the bridge to the faulty phase and the non-faulty phase respectively, adjust an adjustable resistor on the two arms of the bridge to balance the bridge, and apply the proportional relationship and the known The length of the cable gives the interval between failures.

Use a low-voltage bridge to measure the low-resistance breakdown of the cable, and use a capacitor bridge to measure the open-circuit and disconnection of the cable. The test 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 added too high. Detection method of cable fault point Cable fault detection Shenzhen Noyafa Electronic Technology Co., Ltd.