Power Cable Fault Detection Technology

Author: Xu Bingyin / et al.
Editor-in-Chief: Xu Tong
Publisher:
Publish Date: 1999-09-01
Features: This book systematically introduces the basic principles and applications of power cable fault detection technology. Starting from the classification and causes of basic power cable faults, it provides a detailed explanation of the voltage and current wave processes in cable lines, and conducts an appropriate analysis of the magnetic field distribution around the cables. Finally, it introduces the principles and usage methods of several practical testing instruments. The book strives for easy-to-understand language and includes certain graphical examples. The appendix provides some actual test waveforms for reference. The content of this book is technologically advanced among similar publications and highly practical, capable of promoting the development of power cable fault detection technology in China. It is widely welcomed by power cable fault maintenance personnel. The book is written for individuals with high school or above education and most cable testers with practical experience. Theoretical issues focus on explaining physical concepts, with language designed to be easy to understand for self-study.
Excerpt: The convenience of testing methods, based on currently popular fault ranging techniques, low-resistance faults can be detected using low-voltage pulse reflection methods, high-resistance faults require impulse flashover methods, and arcing faults can be tested using DC flashover methods. On-site personnel often refer to faults with Rf < 100kΩ as low-resistance faults, primarily because traditional bridge methods can measure such faults. Statistics show that high-resistance and arcing faults account for about 90% of all cable faults. On-site, fault methods are used to distinguish between high-resistance and arcing faults. Figure 1-3 shows the equivalent circuit for cable withstand testing, where Rs represents the internal resistance of the test equipment, E is the DC voltage the equipment can provide, and the resistor Rf in parallel with a gap representing the fault point is set to the critical breakdown voltage VB. For arcing faults, Rf is relatively large, allowing the voltage across the fault gap to increase significantly. When the test voltage reaches a certain level, the fault point breaks down and discharges, causing a sudden increase in current and a sharp drop in voltage. Faults that occur during preventive tests are often of the arcing type. High-resistance faults have a smaller fault point resistance (but greater than 10Ω), making it impossible to raise the voltage across the fault point above the breakdown voltage, thus preventing breakdown. Therefore, the presence or absence of breakdown phenomena during high-voltage insulation testing can help determine whether a cable has high-resistance or arcing faults. Clearly, the distinction between high-resistance and arcing faults is not absolute and depends on factors such as the capacity of high-voltage test equipment or the internal resistance of the test equipment. In reality, there is also a closed-type fault, which often occurs in cable joints or terminations, especially in oil-filled cable heads. When such faults occur, they may break down at a certain test voltage but recover, making the breakdown phenomenon completely disappear. These are called closed-type faults, and since they cannot be reproduced, locating them becomes more challenging.
1.4 Steps in Cable Fault Detection
Cable fault detection generally involves three steps: diagnosis, ranging, and pinpointing.
1. Cable Fault Nature Diagnosis
Cable fault nature diagnosis refers to determining the type and severity of the fault to help testers choose appropriate cable fault ranging and pinpointing methods.
2. Cable Fault Ranging
Cable fault ranging, also known as coarse measurement, involves using instruments at one end of the cable to determine the fault distance. Common on-site fault ranging methods include classical bridge methods and modern traveling wave methods (see Section 1.6).
3. Cable Fault Pinpointing
Cable fault pinpointing, also known as fine measurement, involves locating the general direction of the fault point based on the results of fault ranging and the cable's path. Within a small range, methods such as discharge sound detection or other techniques are used to accurately locate the fault point.
Generally, successful cable fault detection requires all three steps. Otherwise, haste can lead to poor results. For example, directly pinpointing a fault using discharge sound detection without ranging along a long cable path (which may be several kilometers long) makes it extremely difficult to detect the discharge sound. If the fault distance is known, determining a general direction allows for easier pinpointing within a small range (about 10 meters) by moving the detection instrument back and forth to locate the discharge sound of the cable fault.