Summary of Cable Fault Detection Method and Development History

(1) Pulse reflection method: In the 1970s and 1980s, the cable fault test generally adopted the flash test method, and the principle was the pulse reflection method (also called the radar method). The instruments used are mainly electronic tubes and transistor circuits, which are bulky. The displays used successively include oscilloscope tube type flash tester, storage oscilloscope tube type flash tester and so on. After the 1990s, with the widespread application of computer technology, intelligent cable fault flashover testers (flashover testers) began to be put into use, and the testing principle used was still the pulse reflection method. The flash tester used is from the picture tube display to the liquid crystal display, and the single-chip microcomputer circuit is generally used for control, so that the rough test work of the cable fault enters a new stage.

(2) Bridge method: since the buried cable, the detection of cable faults has become a problem that must be solved. The initial rough testing of cable faults was carried out using the bridge balance test principle. At that time, resistance bridges, capacitor bridges, low-voltage bridges, and high-voltage bridges were used. Using the bridge principle to test the cable fault distance was a method commonly used in the 1960s and 1970s. After 2000, instruments using the bridge method test principle continued to be used and developed. After using computer technology, there are now more intelligent bridge testers (such as high-voltage digital bridges). The intelligent flash tester using the pulse reflection method (also called the flushing flash method) is currently a rough tester for cable faults with a wide range of applications and a large number of markets. For example, in Beijing power supply system, due to the long service time of buried cables and the large amount of cable laying, the history of using cable fault testers is also long. From 1993 to 10 years, the early products of DTC series detectors controlled by single-chip microcomputers were purchased. There are more than 50 sets of TC series large-screen LCD display cable fault testers, which are used by almost every power supply department. And in some power supply departments, the use of this type of cable fault tester is regarded as a skill that must be mastered in the cable test job examination. The author has conducted technical training on the pulse reflection method cable fault tester for the Beijing power supply system many times. Since this type of instrument has been used for a long time, there are many types of training materials and monographs on the use knowledge and experience of this type of flash tester, which is beneficial for users to master the use skills of the instrument in time.

There are several methods for cable fault location:

(1) Acoustic measurement method: Acoustic measurement method is used to determine the point, which is a method of cable fault determination that has been widely used from the past to the present. And it is an effective method. It's just that the instrument used has developed from the simple acoustic-electric amplifier in the past to the commonly used acoustic-magnetic synchronous fixed-point instrument. Acoustic measurement points are applicable to high-voltage cables, low-voltage cables, direct-buried cables, cable trench cables, etc.

Acoustic measuring point is to discharge the faulty cable by the high-voltage pulse generator, the fault point generates an arc, and produces a discharge sound. In the case of a cable buried directly, a seismic wave is generated, and the acoustic measuring probe (sound sensor) of the fixed-point instrument picks up the seismic wave signal And amplified and then output through earphone or meter. Through a large number of field tests, after the seismic wave is transmitted from the fault point of the cable to the ground, it quickly attenuates to a very small value outside the radius of 2 meters. once. When the seismic wave is heard, it means that the fault point is within 2m, and the fault point can be found as long as the louder sound is carefully found.

(2) Step voltage method: The step voltage method is used to fix the grounding fault of the outer sheath that requires the insulation of the outer sheath of the cable. Now it is used to ground the core wires of some directly buried low-voltage cables and wires without armor. Faults can also be fixed by the step voltage method.

(3) Electromagnetic method and audio method: It is feasible to use electromagnetic wave fixed point or audio frequency method to determine the fault point by using the change of electromagnetic wave signal or audio signal before and after the fault point of the cable. But judging from the current situation, there is no fixed-point instrument with reliable performance and practical application. In other words, the fixed-point instrument using electromagnetic waves is still under research and development by various scientific research institutions, and it needs to be further verified and improved in practice to reach the level of practical application.

(4) Acoustic-magnetic synchronization method: it is a comprehensive application of acoustic measurement method and electromagnetic wave method. For example, the DTC series acousto-magnetic synchronization fixed-point instrument adopts the combination of acoustic measurement method and acousto-magnetic synchronization fixed-point method. When the acoustic measurement method is used for pointing, the sound meter head of the pointing instrument indicates the seismic wave received by the acoustic measurement probe, and the earphone also reflects the seismic sound wave received by the acoustic measurement probe. Right above the fault point, the sound wave signal is strong, away from the fault point, the sound wave signal decreases, or there is no sound wave signal. When the acousto-magnetic synchronization method is used, the head of the sound meter reflects the seismic sound waves received by the acoustic probe, and the head of the magnetic meter and the earphone simultaneously indicate the electromagnetic wave received by the antenna when the fault point is discharged. When the sound measuring probe is placed above the fault point, the two meter indications of the fixed point instrument and the sound of the earphone are synchronized. When the acoustic signal is not received, the acoustic-magnetic synchronous electromagnetic wave receiving function can be used to grasp the discharge rhythm of the ball gap in time, which is beneficial to distinguish the weak acoustic signal of the fault point in a noisy environment. In addition, the acousto-magnetic synchronous locating instrument can simultaneously perform fault locating and cable path detection, greatly improving the efficiency of fault locating.

The fixed-point instrument adopting the acoustic-magnetic synchronous technology is the most widely used cable fault fixed-point instrument at present.

(5) Magnetic field predetermined point technology: The principle of cable fault magnetic field predetermined point technology is: through a high-voltage DC pulse generator, the fault point of the cable is generated with an arc, and during the existence of the arc, an audio signal is injected into the cable. At the fault point of the cable, this audio signal is short-circuited by the arc and does not continue to propagate to the end of the cable. The receiver used receives the audio electromagnetic wave signal radiated by the cable, and judges whether the receiver is located before or after the fault point by comparing the amplitude of the audio electromagnetic wave before and after the fault point, so as to achieve the purpose of fast predetermined point.