Evaluation of Cable Fault Test Methods

Author: Noyafa–CCTV Tester

Evaluation of cable fault test methods Evaluation of cable fault test methods 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. Evaluation of cable fault test methods Evaluation of cable fault test methods 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. Review and compare. 1. Fault location 1) Bridge method The bridge method is a classic test method. The connection of the test circuit by the bridge method is shown in Figure 1.5a. The faulty phase and the non-faulty phase of the cable under test are short-circuited, and the two arms of the bridge are respectively connected to the faulty phase and the non-faulty phase. Figure 1.5b shows the equivalent circuit diagram .

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 hours or even days, especially for hinged polyethylene plastic cables that are widely used at present. In most cases, the fault point cannot be burned through, which is very inconvenient. Sometimes the fault point will burn out, and the fault resistance will increase instead. High phenomenon, or the fault resistance burns too low, it is a permanent short circuit, so that the final point cannot be determined by the discharge acoustic measurement method.

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.

With the development of technology, the limitation of fault resistance has been greatly improved with the use of high voltage bridges. However, due to the particularity of the fault point, it not only shows resistance characteristics, but also cannot be used for flashover faults, and the reliability of the instrument is also tested. 2) Low-voltage pulse reflection method The low-voltage pulse reflection method, also known as the radar method, was invented by the radar of the Second World War. ).

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 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 Pulse current method is a test method developed in the early 1980s, which shows 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) Secondary pulse method Secondary pulse testing technology is a testing technology developed in the 1990s. Because the waveforms of the pulse current method and the pulse voltage method are not easy to analyze, it causes great inconvenience to the testers. Foreign countries began to study the secondary pulse testing technology very early.

Because of its simple operation and easy waveform analysis, it is welcomed by cable testers. However, due to the complex equipment, many technical difficulties, and high equipment prices, the domestic market has always been dominated by imported equipment. In recent years, with the efforts of domestic technicians, this technology has been mastered by several domestic companies. Zibo Xinyijie Electric Co., Ltd. is honored to be one of the leaders.

The first is the breakdown of the cable fault under the action of DC high voltage or pulsed high voltage signal. At the same time, arc delay technology and continuous arc technology are used to prolong the arc duration and maintain stability; secondly, when there is no breakdown fault point, the measurement signal is injected into the cable. , obtain the reference waveform; inject one or more measurement signals during arc delay to obtain one or more comparison waveforms; display the waveforms obtained twice at the same time, which can clearly show the difference, greatly reducing the difficulty of waveform analysis . The secondary pulse technology is well received by the majority of technicians for its simple operation and easy waveform analysis. With the gradual localization of equipment, the market share will inevitably increase gradually, and eventually become a mainstream product.

6) Suggestions on the selection of ranging methods and instruments At present, the traveling wave ranging method is generally used. The low-voltage pulse reflection method is used for low-resistance and open-circuit faults, which is simpler and more direct than the bridge method; the secondary pulse method and pulse current method are used to measure high-resistance and flashover faults. One round-trip time ranging, but the low-voltage pulse reflection method is to actively transmit detection voltage pulses to the cable, and the pulse current method is to passively record the instantaneous pulse current signal generated by the breakdown of the fault; When the breakdown and breakdown arc delay, the test signal is actively transmitted to the cable in two or more times; the recording and processing of the signal can be completed by the same circuit, so the instrument can easily realize three functions at the same time. 2. Fault location The precise location of cable faults is the key to fault detection.

At present, the more commonly used methods are the impact discharge acoustic measurement method, the audio frequency induction method mainly used for low-resistance fault location, and the fast magnetic field pre-positioning method innovated by our company. In practical applications, usually due to the complex environment of the cable fault point, such as excessive vibration and noise, and the cable burial depth is too deep, it is difficult to find the point of sound measurement, which becomes the main contradiction to quickly find the fault point. After using our company's innovative fast magnetic field pre-positioning method, the speed of positioning will be greatly improved.

Acoustic-magnetic synchronous detection method (see Chapter 6 for details) improves the ability to resist vibration and noise interference; by detecting the time difference of the received magneto-acoustic signals, the position of the fault point from the probe can be estimated; compare the pulses received on both sides of the cable The initial polarity of the magnetic field can also be used to locate the fault and find the cable path at the same time. The fast magnetic field pre-positioning method uses arc extension and arc continuation techniques to inject audio signals during arc pulling, and finds fault points by detecting audio signals. This method develops the traditional audio frequency method, overcomes the limitation that the audio frequency induction method can only test low-resistance faults, and promotes the convenience and speed of the audio frequency induction method.

It will greatly improve the speed of fixed point and provide convenience for technicians. 3. A new generation of intelligent cable fault detection instruments The development of modern microelectronic technology has promoted the progress of cable fault detection instruments. The instrument is developing in the direction of intelligence, which can perform complex mathematical processing on the collected signals, automatically calculate the fault point; memorize the measurement waveform; print out the waveform and measurement results; and has the advantages of small size, easy portability, and simple operation.

The CD series cable fault tester and cable fault locator provided by Shenzhen Noyafa Electronic Technology Co., Ltd. is a representative of this development, and is rapidly being promoted nationwide, and has been welcomed by the majority of users.