As the "underground lifeblood" of urban power grids, power cables bear the heavy responsibility of transmitting and distributing electrical energy. Its operational reliability is directly related to the stability and safety of social production and daily electricity consumption. However, due to long-term operation in complex underground environments, cables are inevitably prone to failure due to insulation aging, mechanical damage, external corrosion, and other reasons. Quickly and accurately locating and locating fault points is a crucial and challenging task in the operation and maintenance of power systems. This article will systematically explore the types of cable faults, modern detection methods, and localization techniques.

1、 Common types of cable faults

Understanding the type of fault is a prerequisite for selecting the correct detection method. Cable faults can be mainly classified into the following categories based on their nature:

1. Low resistance fault and high resistance faultThis is the most common classification method.

• low-resistance faultThe resistance at the fault point is relatively low (usually less than 10 times the characteristic impedance of the cable), such as a metallic short circuit (resistance is zero or close to zero). This type of malfunction is relatively easy to detect.

• High resistance faultThe fault point has a high resistance (up to megaohm level or above), usually caused by insulation aging, moisture, and other reasons. It exhibits high insulation resistance at low voltage, but insulation breakdown occurs during high voltage operation. This type of fault has strong concealment and is difficult to detect.

2. open circuit faultOne or more phases of the cable conductor are disconnected, and the resistance at the fault point is infinite. Although the cable cannot deliver power, the insulation may not have been damaged.

3. Flashover faultInsulation can break down instantly under certain conditions (such as high voltage impact), but the breakdown channel subsequently restores insulation. This type of malfunction occurs intermittently, is very unstable, and extremely difficult to locate.

2、 The "three-step" strategy for fault detection and localization

Modern cable fault location usually follows a systematic process, which can be summarized as three steps: "diagnosis", "distance measurement", and "positioning".

Step 1: Fault diagnosis and property judgment
Firstly, it is necessary to useInsulation resistance tester (megohmmeter)andmultimeterConduct preliminary testing on the faulty cable, measure the insulation resistance and continuity of each phase conductor to ground and between phases, in order to determine whether the fault belongs to low resistance, high resistance, or open circuit, laying the foundation for selecting the correct testing method for the next step.

Step 2: Fault location (rough measurement)
This step aims to determine the approximate distance between the fault point and the testing end. The mainstream technologies include:

1. Bridge methodTraditional methods are suitable for low resistance and open circuit faults. Calculate the distance by comparing the resistance difference between the faulty phase and the intact phase using the Wheatstone bridge principle. High precision, but powerless against high resistance faults.

2. Low voltage pulse methodInject a low-voltage pulse into the cable, observe the waveform of the pulse reflected back at the fault point (impedance mismatch point), calculate the time difference between the transmitted pulse and the reflected pulse, and calculate the distance based on the wave velocity. This method is effective for open circuit and low resistance faults.

3. Impulse high-voltage flashover method (impulse flashover method)This is for locating high resistance and flashover faultscore technologyBy applying a high voltage pulse generator to the faulty cable, it forces the high resistance fault point to instantly breakdown and discharge, forming an instantaneous short circuit. This method is usually used in conjunction with the following two ranging techniques:

• Arc Reflection Method (ARM)The short-circuit arc generated by the fault clicking through will simulate a low resistance point, triggering pulse reflection, and distance measurement can be carried out by analyzing the reflection waveform.

• Secondary Pulse Method (SDC): Todaycommonly usedThe method. It first uses a high-voltage pulse to penetrate the fault point, and then immediately injects a low-voltage measurement pulse. The low-voltage pulse will reflect at the ionized fault point (manifested as low resistance). The instrument compares the waveform after breakdown with the waveform before breakdown, and the points of divergence in the waveform correspond to the location of the fault point, which is very intuitive and accurate.

Step 3: Precise positioning (path confirmation and precise positioning)
Ranging provides an approximate distance, but cables are usually buried underground and precise vertical grounding points must be found. This process requires technologies such as acoustic magnetic synchronization.

1. Acoustic magnetic synchronous fixed-point methodWhile using the "flash method" to periodically discharge the fault point, useAcoustic magnetic synchronous fixed-point instrumentDischarge produces two acoustic signals (a loud "pop, pop" sound) and an electromagnetic signal. The fixed-point instrument captures surface vibration sound and electromagnetic signals through a probe, and ensures that only the sound of the point that receives both sound and magnetic signals simultaneously is the effective fault point sound. The operator listens through headphones and displays on the instrument screen, searching for the point with the highest sound and synchronized with the electromagnetic signal along the cable path on the ground. The point directly below it is the fault point.

2. Cable path recognitionIf the cable laying path is unclear, it is necessary to use it firstPathfinderTransmitting signals of a specific frequency and detecting them on the ground with a receiver to accurately map the buried direction of the cable, providing a basis for fixed-point work.

3、 Technological Development and Prospects

With the development of technology, cable fault location is evolving towards a more intelligent and efficient direction:

• Integration and AutomationModern testing instruments often integrate high-voltage generators, ranging systems, and fixed-point instruments into one system, making operation easier and achieving higher levels of automation.

• Traveling wave methodUsing transient traveling wave signals generated by faults for distance measurement is particularly suitable for long-distance cables and complex networks.

• Intelligent diagnosisCombining big data and artificial intelligence technology, analyze online monitoring data of cables, predict insulation aging trends, achieve fault warning, and shift from "post repair" to "pre prevention".

Conclusion

Power cable fault location is a highly integrated technology that combines theory and practice. From the initial preliminary diagnosis to modern secondary pulse method and acoustic magnetic synchronous positioning technology, a mature and scientific process has greatly improved the efficiency and accuracy of positioning, minimizing the time of power outages and excavation repair costs. Mastering these technologies proficiently is of irreplaceable importance in ensuring the sustained and strong beating of the "urban heart" of the power system.