Selection and Application of Power Cable Fault Detection Technologies Under Multiple Operating Conditions--PartI

Target Readers: Technical engineers, equipment selection personnel, and professionals who need to understand method principles and standard basis.

Cable fault testing involves three tiers of standards: equipment manufacturing standards, on-site operation specifications, and acceptance test standards, forming a complete standard system.

Usage Tip: Standards are continuously updated. Please refer to the current valid version published on the official website of the Standardization Administration of China when citing.

Time Domain Reflectometry (TDR). A low-voltage narrow pulse is injected into the cable. The pulse propagates along the cable and reflects at positions with impedance mutation. The instrument calculates the fault distance by measuring the time difference.

Fault distance: S=V×Δt÷2

• V: Wave propagation velocity in cable (m/μs), varying with cable insulation materials
• Δt:ime difference between transmitted pulse and reflected pulse (μs)
• Divided by 2: The pulse travels a round trip, and the measured time is the round-trip duration

• Start point: Intersection of rising edge of transmitted pulse and baseline
• End point (Open circuit): Intersection of rising edge of reflected pulse and baseline
• End point (Short circuit): Intersection of initial rising edge of reflected pulse and baseline

Distance per sampling point: S=V÷(2×f)

• PILC cable: (160 ÷ (2 *25) = 3.2m/point
• XLPE cable: (170÷(2 *25) = 3.4) m/point

✅ Open circuit / broken cable faults

✅ Low-resistance short circuit ((R ≤ 100) Ω)

✅ Measure total cable length (for wave velocity calibration)

✅ Locate cable joints, T-joints and branch points

• Simple operation and low learning cost
• No high-voltage equipment required, high safety
• High ranging accuracy and clear, intuitive waveforms
• Capable of locating common joints, T-joints and branches simultaneously

• Only applicable to low-resistance faults (R ≤ 100 Ω)
• For high-resistance faults, the reflection coefficient approaches zero, and the reflected waveform cannot be identified
• Unable to directly detect high-resistance leakage faults, which account for about 80% of all cable faults

High-resistance faults maintain high impedance under low voltage, so low-voltage pulses cannot generate effective reflection. The impulse flash method applies high-voltage impulse to break down the fault point into an instantaneous electric arc (short-circuit state, lasting for tens of milliseconds). The traveling wave of pulse current is collected at this moment to obtain a clear reflected waveform similar to low-resistance short circuit.

220 V Mains → Voltage Regulator → High-Voltage Transformer → Energy Storage Capacitor → Discharge Gap → Fault Cable

Current Sampler → Test Host (Waveform Acquisition)

Key waveform features: Transmitted pulse, reflected pulse and negative reverse impulse.

• Start point: Intersection of rising edge of transmitted pulse and baseline
• End point: Intersection of falling front edge of negative reverse impulse and baseline (set at rising edge if no negative reverse impulse exists)

For individual high-resistance faults, the impulse voltage passes through the fault point to the cable terminal, and flashover is triggered after voltage superposition during the return trip. The waveform presents a two-section structure.

Reading rule: Take the distance of the second section as the actual fault distance.

✅ High-resistance leakage faults ((R > 100) Ω)

✅ High-resistance flashover faults

✅ Wet joint faults

✅ Faults causing tripping during operation with unobvious symptoms in low-voltage tests

• Applicable to about 80% of high-resistance leakage faults with strong on-site versatility
• No need for precise classification of fault types, high fault tolerance

• High-voltage equipment is required with strict safety operation requirements
• The amplitude of negative reverse impulse varies with fault properties, and waveform interpretation requires experience
• Lower accuracy for low-resistance faults than low-voltage pulse method (discharge delay leads to larger ranging error)
• Complex wiring and numerous operation steps

A combination of low-voltage pulse method (high accuracy but invalid for high-resistance faults) and impulse flash method (applicable to high-resistance faults but complex waveforms):

1. Transmit low-voltage pulses and record the reference waveform (no reflection at high-resistance fault point)
2. Apply high-voltage impulse to break down the fault point and form an electric arc
3. Transmit low-voltage pulses synchronously while the electric arc is burning. The fault point is equivalent to a low-resistance point, generating clear short-circuit reflection
4. Compare the two waveforms; the difference position is the fault point

This method uses clear waveforms of low-voltage pulse to realize positioning of high-resistance faults.

✅ High-resistance leakage faults (higher accuracy than impulse flash method)

✅ Operators with insufficient experience (easy waveform interpretation)

✅ Scenarios requiring reliable ranging results

Note: This is a positioning method, not a ranging method. It is used for precise ground positioning after rough ranging.

Apply high-voltage impulse to the fault cable. Discharge at the fault point generates two signals:

1. Acoustic signal: Discharge energy is converted into mechanical vibration and propagates to the ground through soil or concrete
2. Electromagnetic signal: Discharge current generates magnetic pulses and radiates to the ground

Ground sensors receive both signals simultaneously. When the sensor is directly above the fault point, the propagation path difference between acoustic and electromagnetic signals is the smallest, and the time difference approaches zero.

Positioning Rule: Move the sensor to find the position with the minimum time difference (close to 0).

• Move the sensor slowly near the rough ranging range along the confirmed cable path
• Do not judge only by the maximum sound volume; sound may be reflected by pipelines, manhole covers and joint structures to form false maximum values
• Repeat tests 2 to 3 times at the same position to confirm stable time difference
• Verify with cable path and rough ranging distance

The loop of single-core high-voltage cable sheath fault consists of metal sheath, fault point and earth. Pulse signals attenuate severely due to the high attenuation coefficient of earth, so conventional pulse methods have an extremely limited effective ranging range and cannot realize effective positioning.

Based on Wheatstone bridge principle, calculate fault distance by measuring the resistance ratio between the faulty phase and healthy phase (or auxiliary wire). This method does not rely on pulse reflection and is immune to earth attenuation.

Apply DC or pulsating signal between sheath and earth, and measure the potential difference (step voltage) between two adjacent points on the ground. Abnormal change of step voltage near the damaged sheath indicates the direction and position of the fault point.

DL/T 2530.2-2024 General Technical Conditions for Power Cable Testing Equipment Part 2: Cable Sheath Fault Locator, newly issued in 2024, specifies technical requirements, test methods and inspection rules for locators adopting step voltage method and pulsating rectangular wave current method.

Sheath faults are usually closed faults with extremely weak discharge sound, so acoustic-magnetic synchronous method has poor effect. Professional sheath fault locators are required, and high operational experience is needed.

Notes: All cited standards are valid versions at the time of compilation. Verify the current status of standards before use. Strictly comply with DL/T 474-2018 and relevant safety regulations during high-voltage tests.