Evaluation of Cable Fault Detection Methods

Author: Noyafa–CCTV Tester

Evaluation of cable fault detection methods For a long time, many measurement methods and instruments have emerged. These methods and instruments are suitable for different fault conditions and have their own advantages and disadvantages. Here, we will briefly evaluate and compare the fault location and fixed-point instruments. 1. Evaluation of fault detection methods for faulty cables For a long time, many measurement methods and instruments have emerged. These methods and instruments are suitable for different fault situations and have their own advantages and disadvantages. Here is a brief evaluation and evaluation of fault location and fixed-point instruments. Compare. 1. The method of fault location ①. Bridge Method The bridge method is a classic test method.

ab Figure 1.5 The principle of bridge distance measurement The connection of the bridge method test circuit is shown in Figure 1.5a. The fault phase and the non-fault phase of the cable terminal under test are short-circuited, and the two arms of the bridge are respectively connected to the fault phase and the non-fault phase. The equivalent circuit diagram is given in 1.5b. By carefully adjusting the value of R2, the bridge can always be balanced, that is, the potential difference between CDs is 0, and no current flows through the galvanometer. At this time, according to the bridge balance principle, it can be obtained: R3/R4=R1/R2 (1.1) R1 , R2 is a known resistance, set: R1/R2=K, then R3/R4=K Since the DC resistance of the cable is proportional to the length, the resistivity of the cable conductor is set to R0, the full length of L represents the full length of the cable, LX, , L0 are the distances from the fault point of the cable to the measuring end and the end respectively, then R2 can be replaced by (L full length + L0) R0, according to formula (1.1), it can be deduced: L full length + L0 = KLX and L0 = L full length - LX, Therefore, LX=2L full length/(K+1) The open-circuit fault of the cable can be measured by a capacitive bridge, and the principle is similar to the above-mentioned resistance bridge. The advantages of the bridge method are that it is simple, convenient and accurate, but its important disadvantage is that it is not suitable for high resistance and flashover faults, because when the fault resistance is very high, the current in the bridge is very small, and the general sensitivity of the instrument , it is difficult to detect, in fact, most of the cable faults are high resistance and flashover faults.

Before using the bridge method to measure the fault distance, it is necessary to use high-voltage equipment to burn through the fault point to reduce the fault resistance value to the range that can be measured by the bridge method. However, burning through the fault point is a very difficult task, often It takes several hours or even days, which is very inconvenient. Sometimes the fault point will burn out, and the fault resistance will increase instead, or the fault resistance will burn too low and become a permanent short circuit, so that it cannot be measured by the discharge sound. method to make the final point. Another disadvantage of the bridge method is that it is necessary to know the original technical data such as the exact length of the cable. When a cable line is composed of cables with different conductor materials or different cross-sections, it needs to be converted. The bridge method cannot measure three-phase short circuits. or open circuit fault. The field bridge method is less and less used now, but some testers, especially older testers, are still used to using this method.

Especially for some special faults, there is no obvious low-voltage pulse reflection, but it is not easy to use high-voltage breakdown. If the fault resistance is not too high, the bridge method can often solve the problem. ②. Low-voltage pulse reflection method - suitable for disconnection, low resistance and short-circuit faults. The low-voltage pulse reflection method, also known as the radar method, was invented by the inspiration of the World War II radar. It observes the time difference between the reflected pulse and the transmitted pulse at the fault point. Ranging (see Chapter 3 for details). The advantage of the low-voltage pulse reflection method is that it is simple, intuitive, and does not require original technical information such as the exact length of the cable.

The locations of cable joints and branch points can also be easily identified from the pulse reflection waveform. The disadvantage of the low-voltage pulse reflection method is that it is still not suitable for measuring high resistance and flashover faults. ③. Pulse voltage method - suitable for high resistance and flashover faults (obsolete) Pulse voltage method, also known as flash test method, is a high resistance and flashover fault test method developed in the 1960s.

There are several domestic companies that produce and sell cable fault flash testers based on this principle. First, the cable fault is broken down under the action of DC high voltage or pulsed high voltage signal, and then, by observing the time distance of the discharge voltage pulse going back and forth between the observation point and the fault point. An important advantage of the pulse voltage method is that there is no need to burn through high resistance and flashover faults, and the instantaneous pulse signal generated by the fault breakdown can be directly used. The test speed is fast and the measurement process is simplified. It is a major progress in cable fault testing technology.

The disadvantages of the pulse voltage method are as follows: A. Poor safety, the instrument measures the voltage pulse signal through a capacitor-resistance voltage divider, the instrument is electrically coupled with the high-voltage circuit, and the high-voltage signal is easily connected in series, causing damage to the instrument. B. When the flash measurement method is used for distance measurement, the high-voltage capacitor is in a short-circuit state to the pulse signal, and a resistor or inductor needs to be connected to generate a voltage signal, which increases the complexity of wiring and reduces the voltage added to the faulty cable when the capacitor is discharged. voltage, so that the fault point is not easy to break down. C. During the fault discharge, especially during the flashover test, the voltage waveform coupled by the voltage divider is not sharp and difficult to distinguish.

④. Pulse current method - suitable for high resistance and flashover faults Pulse current method is a test method developed in the early 1980s, which has shown strong vitality with the advantages of safety, reliability and simple wiring. The difference between the pulse current method (see Chapter 4 for details) and the pulse voltage method is that the former uses a linear current coupler to measure the current pulse signal generated by the breakdown of the cable fault, and successfully realizes the electrical coupling between the instrument and the high-voltage circuit. The series resistance and inductance between the capacitor and the cable are eliminated, the wiring is simplified, and the pulse current waveform coupled from the sensor is easier to distinguish. ⑤. Arc reflection method (second pulse method) - suitable for high resistance and flashover faults This is the most advanced fault location method at present, and it should be used first in the test.

It is a new test method developed based on the easy analysis of low-voltage pulse waveform and high test accuracy. The basic principle is: before the high-voltage pulse generator applies a high-voltage pulse to the cable, a low-voltage pulse signal is injected into the cable, and the low-voltage pulse waveform at this time (called arc-free waveform) is recorded. At this time, because the fault point is high resistance, the low-voltage pulse has no reflection or very little reflection at the fault point.

Then, a high-voltage pulse is applied to the cable through the high-voltage pulse generator, so that the fault is broken through and arc discharge occurs. Due to the small arc resistance, the originally high resistance or flashover fault becomes a low resistance short circuit fault during arcing. At this time, a low-voltage pulse signal is injected into the faulty cable through the coupling device, and the reflected waveform of the low-voltage pulse at this time (called the arc waveform) is recorded, and the low-resistance reflected pulse at the fault point can be clearly seen.

Comparing the waveform without arc and the waveform with arc, the two waveforms will be significantly different in the position of the fault point, and the distance between the obvious divergence point of the waveform and the test end is the fault distance. ⑥. Recommendations on the selection of ranging methods and instruments Currently, traveling wave ranging methods are generally used. The short-circuit, low-resistance and open-circuit faults adopt the low-voltage pulse reflection method, which is simpler and more direct than the bridge method; the arc reflection method or the pulse current method are used to measure the high-resistance and flashover faults; both are measured at the fault point through the pulse signal. One round-trip time ranging between points, but the former is to actively send detection voltage pulses to the cable, and the latter is to passively record the instantaneous pulse current signal generated by the breakdown of the fault; the recording and processing of the signal can be completed by the same circuit, so it is convenient So that the instrument can perform two functions at the same time.