Development Trend of Power Cable Diagnosis and Testing Technology

[Summary] Power cables, whether installed on machinery or buried underground, are subject to inevitable failures after prolonged use, disrupting the lives of citizens and businesses. Serious failures can even cause serious fires and casualties.

Power cables, whether installed on machinery or buried underground, are subject to inevitable failures after prolonged use, disrupting the lives of citizens and businesses. Serious failures can even cause serious fires and casualties. Buried power cables are highly concealed, making fault detection and accurate location difficult, hindering cable maintenance. Given the significant role of power cables in urban areas and their unique characteristics, power cable diagnostic testing technology has attracted considerable attention from industry insiders. 1. Overview of Power Cable Diagnostic Testing Technologies
1.1 Traditional Testing Technologies
The DC superposition method, DC component method, and TGδ dielectric loss method are all commonly used traditional power cable testing methods. While their application value cannot be completely denied and they do provide reference for diagnosing power faults, these traditional technologies are ultimately unsuitable for the testing and diagnosis of ultra-high voltage power cables, significantly limiting their scope of application.
1.2 New Testing Technologies
① Cable Joint Testing Technology
A statistical survey of power cable failures in operation found that over 90% of cable failures occur at cable joints. Overload and contact resistance in operating power cables can cause joint temperatures to rise, leading to rapid aging and failure. Using cable joint testing technology to measure joint temperature and analyze it based on real-time joint temperature data provides a more comprehensive understanding of the power cable's operating status, enabling proactive preventative measures to reduce the likelihood of failures.
② Ultra-High Frequency Testing Technology
If a power cable experiences a high localized discharge pulse frequency, capturing that localized discharge signal requires increasing the testing tool's sampling frequency to minimize external noise contamination. Ultra-high frequency detection technology utilizes wideband partial discharge sensors and electromagnetic coupling methods to detect partial discharge phenomena in the 10 kHz to 28 MHz frequency range with satisfactory detection results.
③ Electromagnetic coupling technology
This technology connects the partial discharge current signal of the grounding wire of a cross-linked polyethylene power cable with the two lines mentioned above through the interaction of a measurement loop and an electromagnetic coupling line. This amplifies the local signal and controls noise interference.
2. Development and Application of Power Cable Diagnostic Testing Technology
2.1 Online Detection Technology
① Wavelet Transform: This technology requires the use of filters. Some studies have proposed two methods for measuring fault distances—single-ended detection and dual-ended synchronous detection. Other studies have used wavelet transforms for single-ended traveling wave ranging, resolving the issue of choosing between traveling wave propagation velocity and arrival time. Extensive practical experience has confirmed that the accuracy of this single-ended traveling wave ranging technology fully meets the standards for accurate fault location at the fault site. Other studies have explored online cable fault monitoring and precise cable distance measurement methods, and have delved into cable fault distance measurement using wavelet transform technology. ② Real-time expert system: This technology, developed based on network remote services, addresses cable fault location. Research indicates that expert systems based on relay protection can, through C language integrated diagnostics, identify the fault type and current RMS of power cables, ultimately pinpointing the fault location.
③ Causal network: A causal network consists of nodes: symptoms, initial causes, states, and hypotheses. Symptom nodes represent the symptoms of state nodes, such as a protective action indicating a circuit breaker trip; initial causes represent the initial cause of a cable fault; state nodes represent the state of a specific domain, such as a circuit breaker fault; and hypotheses represent diagnostic hypotheses for the research system. Some researchers have expanded on the causal network, leveraging the concept of temporal constraints on alarm information to construct a new temporal causal network and have developed a power cable fault diagnosis technology based on this network.
2.2 Offline Detection Techniques
① Low-voltage pulse method: A low-voltage pulse signal is input into the cable through a test terminal. An instrument records the time difference (Δt (μs)) between the transmitted pulse and the reflected pulse received at the fault point, and then calculates the fault distance. If the signal propagation speed in a power cable is v (m/μs), then the cable fault distance l = v × Δt/2.
② Pulse voltage method: This method receives a pulse signal generated by a discharge at the fault point. High-voltage equipment is used to cause a discharge at the fault point in the cable, generating a pulse signal. The instrument then receives the discharge signal from the fault point at the test end, and the distance to the fault point is calculated based on the time it takes to receive the signal. However, this method may pose safety risks because it does not completely isolate the electrical connection between the high-voltage section and the tester.
③ Pulse current method: This method works similarly to the pulse voltage method, but uses a current coupler, completely isolating the high-voltage section, ensuring safety.
④ Secondary pulse method: This is a highly advanced fault distance measurement method. The technical principle is to apply high voltage to the faulty cable, creating a high-voltage arc. This creates a low-resistance short circuit, which can then be detected using a low-voltage pulse method.
2.3 Power Cable Fault Location Technology
Once the path and distance of the faulty cable are measured, the approximate location of the fault point can be determined. However, for more accurate fault location, fault location technology is required. ① Acoustic detection technology: A discharge device is used to generate vibrations at the fault point. Once the vibrations reach the ground, a vibration pickup is used to receive the acoustic signal from the fault point, allowing the specific location of the fault to be determined. Acoustic detection technology can be used for any cable fault detection where a high-voltage pulse signal generates a discharge sound at the fault point.
② Acoustic-magnetic synchronization technology: Discharge at the fault point simultaneously generates both acoustic and electromagnetic waves, allowing for precise fault location. A high-voltage pulse signal is applied to the faulty cable. During discharge, both an acoustic signal and a pulsed magnetic field signal are generated at the fault point, but these signals propagate at different speeds. The minimum propagation time difference is used to locate the fault point.
③ Audio sensing technology: Technicians use their ears to identify the strength of the acoustic signal and ultimately determine the location of the cable fault. An audio current signal of 1kHz or other frequency is applied between two phases of the cable, or between the metal sheath and a phase. This generates an audio electromagnetic signal, which creates a strong magnetic field directly above a nearby open-circuit fault or a metallic short-circuit fault, thereby locating the fault point.