Whether installed on machinery or buried underground, power cables are inevitably prone to failures over time, disrupting the lives of citizens and businesses. Severe failures can even cause serious fires and casualties. Buried power cables are extremely hidden, making fault detection and accurate location difficult, hindering cable maintenance. Given the significant role of power cables in cities and their unique characteristics, power cable diagnostic testing technology has garnered significant 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 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 revealed that over 90% of cable faults occur at cable joints. In operating power cables, overload and contact resistance can cause joint temperatures to rise, leading to rapid aging and failure. Using cable joint inspection technology to monitor joint temperature and analyze real-time joint temperature data allows operators to gain a more comprehensive understanding of the power cable's operating conditions and proactively implement protective measures to reduce the likelihood of failure.
② Ultra-high frequency inspection technology
If the power cable only experiences a high localized discharge pulse frequency, capturing the localized discharge signal requires increasing the inspection tool's sampling frequency to minimize external noise interference. Ultra-high frequency inspection technology utilizes a wideband partial discharge sensor and electromagnetic coupling to detect partial discharge within the 10 kHz to 28 MHz frequency range, achieving satisfactory results.
③ Electromagnetic coupling technology
This technology connects the partial discharge current signal of the ground wire of a cross-linked polyethylene power cable to the two aforementioned lines through the combined action of a measurement loop and an electromagnetic coupling line. This amplifies the localized signal and minimizes 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 to perform 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 monitoring of cable faults 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 distance measurement. Some studies have shown that expert systems based on relay protection can use C language integrated diagnostics to determine the fault type and current RMS value of power cables, ultimately accurately locating the fault point. ③ Causal network: Nodes including symptoms, initial causes, states, and hypotheses constitute a causal network. Symptom nodes represent symptoms of state nodes, such as a protective action being a symptom of a circuit breaker tripping; initial causes represent the initial cause of a cable fault; state nodes represent the state of a specific component within the domain, such as the challenge of a circuit breaker; and hypotheses represent diagnostic hypotheses for the research system. Some researchers have expanded on the causal network, utilizing the concept of temporal constraints on alarm information to construct a new temporal causal network and have proposed a power cable fault diagnosis technique 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 the test terminal. The instrument records the time difference (Δt) between the transmitted pulse and the reflected pulse received at the fault point, and the fault distance is then calculated. If the signal propagation speed in the power cable is v (m/μs), then the cable fault distance l = v × Δt/2.
② Pulse voltage method: This method receives the pulse signal generated by the discharge at the fault point. High-voltage equipment is used to discharge a faulted cable, generating a pulse signal. The instrument then receives the discharge signal from the fault at the test end, calculating the distance to the fault based on the time it takes to receive the signal. However, this method may pose safety risks because it lacks complete electrical isolation 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, essentially guaranteeing safety.
④ Secondary pulse method: This is a highly advanced fault location method. The technical principle is to apply high voltage to the faulty cable, creating a high-voltage arc. This transforms the fault into 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 can be determined. However, for more precise 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.
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