How TDR locates faults in high-voltage cables

Time-domain reflectometry locates cable faults by injecting a fast-rise electrical pulse into the cable and measuring the round-trip travel time of echoes that return from impedance discontinuities. The fault distance is calculated directly from that time measurement — no frequency sweep, no spectral inversion, no post-processing pipeline. As described in a 2014 Vetco Gray Controls patent covering subsea umbilical cable applications, a TDR unit connected at the surface transmits a current pulse, detects the reflected pulse, and uses the measured time duration between transmission and reception to calculate fault distance. The method is inherently direct and requires minimal post-processing, making it well-suited for field deployment.

The practical strength of TDR lies in its interpretability. The time-domain reflectogram is a temporal map of impedance changes along the cable, with peaks and polarity shifts directly corresponding to specific fault types — open circuits, short circuits, and shield breaks. A 2025 Analog Devices International patent demonstrates that TDR echo responses can identify and map multiple reflections along a cable, with fault conditions determined by comparing peak reflection amplitudes against a normalised threshold. This direct visual readability makes TDR the dominant approach for field engineers who need a rapid, portable diagnostic.

However, TDR faces a fundamental resolution constraint. Shorter pulses improve spatial resolution but attenuate more rapidly over long cable lengths — a tension that is particularly acute in high-voltage cable systems spanning several kilometres. Research from WiN-MS (2020) explicitly notes that faults whose damaged zone remains small compared to the wavelength fall below standard TDR’s detection threshold; the paper demonstrates that combining frequency-domain numerical modelling with time-domain signature extraction can address this regime. Advanced correlation-based TDR implementations address the range-resolution trade-off by applying matched-filter or cross-correlation techniques to the reflected signal. A 2019 Korea Electrical Safety Corporation patent derives a correction signal by removing the applied signal from the reflected waveform, then calculates fault distance from the resulting time delay.

Spread-spectrum TDR (SSTDR) extends the live-wire capability of TDR further: by superimposing a low-power broadband probe signal on an energised cable, SSTDR enables online, continuous monitoring. A 2023 Rockwell Automation Technologies patent employs SSTDR to acquire reflections on power line systems while the power conversion components are operating — a capability that standard FDR cannot easily replicate due to interference from the cable’s operating voltage.

How FDR characterises faults through frequency analysis

Frequency-domain reflectometry injects swept sinusoidal or multi-tone signals across a band of frequencies and measures the complex reflection coefficient — amplitude and phase — as a function of frequency. That frequency-domain data is then transformed into spatial fault-distance information through an inverse Fourier transform. Unlike TDR, which produces a single temporal snapshot, FDR captures the full frequency-dependent response of the cable, enabling not just fault location but fault characterisation and severity assessment.

A 2005 BAE Systems patent describes the canonical FDR algorithm: it takes into account both attenuation per unit length and phase shift per unit length in a modified inverse Fourier transform that converts the frequency-domain complex reflection coefficient into a more accurate time-domain reflection coefficient, from which fault distance is determined. This modification — incorporating frequency-dependent propagation parameters into the IFT — is what allows FDR to de-embed multiple faults along the same cable, a capability that basic TDR struggles with because reflections from distal faults are masked by proximal ones.

FDR’s most significant advantage over TDR in high-voltage cable diagnostics is its sensitivity to soft or incipient faults — insulation degradation mechanisms such as water tree ingress or dielectric voids that produce frequency-dependent changes in propagation parameters rather than abrupt impedance steps. Line Impedance Resonance Analysis (LIRA), described in a 2022 paper as a powerful technique based on frequency-domain reflectometry, was originally developed for nuclear plants and has since been commercialised to determine the location of water trees and physical damage across cable voltage classes from LV to EHV. According to IEEE standards bodies, insulation degradation is the primary failure mode in aged high-voltage cable infrastructure, making this FDR capability commercially critical.

“FDR captures the frequency-dependent response of the cable, enabling not just fault location but fault characterisation and severity assessment — a capability not inherently available in basic TDR implementations.”

CEA’s 2016 patent on soft fault identification demonstrates a complete frequency-to-time domain processing pipeline: the method estimates propagation parameters — attenuation α(f), phase factor β(f), and reflection coefficient Γin(f) — as functions of frequency from a frequency-domain reflectogram, subtracts a fault-free reference, and then transforms the residual back into the time domain to identify amplitude peaks marking fault positions. This reference-subtraction architecture is a defining characteristic of advanced FDR implementations for high-voltage cable diagnostics, as it suppresses the dominant cable response and isolates the weak fault signature.

For long-distance high-voltage cable systems, FDR faces a specific challenge: signal attenuation at high frequencies degrades the accuracy of the inverse transform for distant faults. A 2024 Shenzhen Power Supply Co. patent addresses this directly by constructing a compensation curve derived from cable structural parameters, characteristic parameters of each material layer, and the frequency-domain incident signal parameters, then combining it with the raw FDR positioning curve to produce a corrected fault diagnosis result whose peak amplitude accurately indicates fault severity at long distances. The Polytechnic University of Bari’s 2020 stepped-frequency waveform reflectometry (SFWR) technique uses sinusoidal bursts to estimate the frequency response function of a cable with very low systematic error, then inverts this to locate and characterise faults with quantified error bounds.

Hybrid TFDR: resolving the time-frequency trade-off

Time-frequency domain reflectometry (TFDR) resolves the core trade-off between TDR’s time localisation and FDR’s frequency sensitivity by using a chirp signal multiplied by a Gaussian time envelope. The Gaussian envelope provides time localisation while the chirp simultaneously excites the system under test with a swept sinewave covering a frequency band of interest. High-resolution fault localisation is achieved via a time-frequency cross-correlation function, with fault distance derived from the frequency offset of the reflected signal rather than from round-trip time alone.

Park Jin-Bae’s foundational TFDR patents — filed across US, EP, AU, and WO jurisdictions from 2004 onwards — establish the chirp-Gaussian TFDR paradigm. Korea Electric Power Corporation has translated this into an operational underground cable diagnostic platform, as described in patents from 2020 and 2022: the system designs an optimal reference signal considering the propagation characteristics of the underground cable under test, acquires the reference and reflected signals, and computes their time-frequency energy distribution using the Wigner-Ville transform to decide on fault presence by similarity analysis. Korea Western Power Co.’s 2022 commercial implementation classifies both fault points and fault conditions through a TFDR algorithm applied to reference and reflected signals for power plant cable and junction box management.

CEA has independently developed Wigner-Ville-based time-frequency reflectometry for soft fault detection. A 2014 CEA patent decomposes the reflected signal into time components, constructs intermediate signals from them, and calculates the Wigner-Ville transform of each to produce a joint time-frequency representation whose maxima reveal fault positions with sub-pulse-width resolution — a precision improvement over both classical TDR and single-domain FDR. A separate 2021 CEA patent addresses topologically complex cable networks by measuring the reflection coefficient as a function of frequency at each network end and applying a simulation-based vector matching technique across all fault position hypotheses.

Machine learning is increasingly augmenting both TDR and TFDR architectures. Research from Wuhan University (2019) demonstrates unsupervised clustering for travelling-wave arrival-time identification in sheath current monitoring of cross-bonded HV cable systems. A French national research centre patent from 2021 describes distributed reflectometry with multiple sensor nodes monitored by principal component analysis. According to WIPO trend data on power cable diagnostics, machine learning integration in reflectometry patents has accelerated significantly since 2018, reflecting the industry push toward online, automated cable health monitoring.

Head-to-head: TDR vs. FDR across five critical dimensions

A direct comparison across five engineering dimensions — drawn from the patent and literature evidence — clarifies when each approach should be selected for high-voltage cable fault location.

1. Signal injection and hardware complexity

TDR injects a broadband pulse or step with a fast rise time and measures return time directly. The hardware is relatively simple and field-portable, as exemplified by the Vetco Gray / GE Oil & Gas underwater facility applications, where a TDR unit connected at the surface locates insulation breakdown in subsea umbilical cables. FDR requires a swept-frequency source — a vector network analyser or stepped-frequency synthesiser — and more sophisticated signal processing hardware. The BAE Systems FDR architecture employs a multi-port junction and a down-conversionless architecture to acquire the frequency-domain complex reflection coefficient, a design suited to laboratory or fixed-installation diagnostics rather than rapid field deployment.

2. Fault location precision and range

TDR resolution is bounded by pulse rise time and cable dispersion. FDR achieves finer effective resolution by virtue of its frequency diversity, but the inverse Fourier transform introduces artefacts when signal bandwidth is limited or attenuation is high. Research from Kazan Federal University (2017) confirms that TDR can function on both energised and de-energised lines and can resolve high-impedance faults — a class particularly relevant to ageing HV cable insulation. The 2024 Shenzhen Power Supply Co. patent demonstrates that raw FDR results must be compensated using cable structural and material parameters to achieve accurate severity assessments at long distances.

3. Soft fault detectability

FDR is inherently better suited to soft fault detection because frequency-domain parameters such as attenuation and phase velocity are directly sensitive to distributed dielectric degradation — demonstrated by the LIRA technique covering cable voltage classes from LV to EHV. TDR-based soft fault detection requires sophisticated extensions: CEA’s integral-based method (2019) and zero-crossing antisymmetry method (2018) extract soft fault signatures from the time-domain reflectogram at the cost of significant algorithmic complexity.

4. Multi-fault environments

Both methods face challenges when multiple faults exist along a cable, because reflections from distal faults are masked or distorted by proximal ones. The BAE Systems FDR algorithm explicitly addresses this by incorporating attenuation and phase shift per unit length into the IFT to de-embed multiple faults. TDR approaches this through differential or correlation methods: a 2022 CEA patent computes the difference between measured and fault-free time reflectograms to isolate individual fault signatures, then matches hypothesised reflectograms to characterise each fault independently.

5. Live-wire operation

TDR — particularly SSTDR — is more readily adapted to online, live-cable monitoring because the low-power broadband probe signal can be superimposed on the energised cable. FDR systems generally require a cleaner signal environment and are more susceptible to interference from the cable’s operating voltage. This asymmetry makes TDR the preferred architecture for continuous online monitoring, while FDR is more commonly deployed in scheduled offline diagnostic campaigns. Standards bodies including IEC and the IEEE have developed guidance on both offline and online cable testing methodologies that inform these deployment decisions.

Patent landscape and key innovation trends

The patent dataset of over 50 documents spanning ten jurisdictions reveals a concentrated innovation landscape with a small number of dominant assignees and several clearly emerging technology vectors.

Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA) is the single most prolific assignee in this dataset, with patents spanning soft fault TDR integrograms, Wigner-Ville time-frequency reflectometry, network-topology-independent FDR, data-fusion-based soft fault detection, and distributed multi-node reflectometry systems. A 2024 CEA patent on distributed reflectometry with multiple sensor nodes and a 2021 patent on data-fusion-based soft fault detection represent the frontier of this portfolio. CEA’s output represents the most comprehensive academic-to-patent pipeline in soft fault reflectometry globally.

Park Jin-Bae holds the foundational TFDR patent family across US, EP, AU, and WO jurisdictions, establishing the chirp-Gaussian TFDR paradigm from 2004 onwards. Korea Electric Power Corporation has operationalised this for underground HV cable diagnostics in active US and Korean patents. Korea Western Power Co. has extended it to commercial power plant cable and junction box management. Analog Devices International focuses on normalisation-based TDR with 2025 pending patents on cable-length-independent fault threshold determination and shield-fault identification. Vetco Gray Controls / GE Oil & Gas holds an extensive patent family for TDR application to subsea and underwater-facility cable fault location spanning US, EP, AU, SG, and GB. BAE Systems is the primary FDR algorithm innovator, with patents on the modified inverse Fourier transform for multi-fault FDR.

Three emerging trends are reshaping the reflectometry landscape. First, distributed reflectometry with multiple sensor nodes monitored by principal component analysis — as described in a 2021 French national research centre patent — enables continuous, network-wide cable health monitoring rather than point-in-time diagnostic campaigns. Second, machine learning integration is accelerating across both TDR and FDR architectures: from unsupervised clustering for travelling-wave arrival-time identification in cross-bonded HV cable sheath current monitoring (Wuhan University, 2019) to S-transform time-frequency analysis for VSC-HVDC cable fault location (Shandong University, 2017). Third, the convergence of reflectometry with digital twin and cable structural modelling — as demonstrated by the Shenzhen Power Supply Co. compensation curve approach — points toward a future where fault diagnosis integrates real-time measurement with physics-based cable models. Research published through bodies such as IEC on smart grid cable monitoring confirms this convergence is a priority for grid operators managing ageing HV cable infrastructure globally.