Power Cable Testing Solution

Power cables are designed to remain underground, inside tunnels, cable trays, substations or other restricted environments for many years. Once installed, most of the cable insulation system cannot be inspected directly. The condition of the cable therefore has to be evaluated through electrical testing.

A cable system is more than the cable itself. The complete insulation path includes the conductor, semiconductive layers, main insulation, metallic screen or sheath, joints and terminations. A defect introduced during manufacturing may remain hidden for years. Mechanical damage during transportation or installation can create another type of weakness. Poorly installed joints and terminations are also common sources of insulation problems.

For utilities, cable manufacturers, EPC contractors, commissioning companies and maintenance teams, the practical requirement is to answer several different questions:

• Can the cable withstand the specified voltage?
• Is the insulation condition acceptable?
• Has the cable sheath been damaged?
• Are there localized defects inside the insulation or accessories?
• If a cable has already failed, where is the fault?

UHV provides a range of power cable testing technologies covering VLF Testing, AC Hipot, DC Hipot, AC Resonance, Sheath Testing, Partial Discharge Testing and Cable Fault Location.

The equipment can be configured for factory testing, installation acceptance, commissioning, maintenance and fault investigation of medium- and high-voltage cable systems.

1. Testing Background

The increasing use of XLPE-insulated medium- and high-voltage cables has changed the way cable systems should be tested.

Traditional cable testing often focused primarily on insulation resistance and DC withstand. For modern extruded insulation systems, however, the test method must be selected carefully. XLPE and EPR cables are capacitive loads, and long cable sections can require substantial reactive power when tested at 50/60 Hz.

This creates two practical problems.

First, a conventional power-frequency AC test system can become very large and difficult to transport to the site.

Second, the selected test voltage and waveform must represent the actual electrical stress expected by the cable insulation.

VLF testing addresses the first problem by reducing the frequency to a very low level, commonly 0.1 Hz or lower. The capacitive current falls substantially, allowing a compact field test system to generate high voltage without the power requirements of a conventional 50/60 Hz system. UHV's VLF systems cover frequencies such as 0.1 Hz, 0.05 Hz, 0.02 Hz and 0.01 Hz depending on the model and cable capacitance.

For longer or higher-voltage cables, AC resonant testing provides another solution. The inductance of the reactor is tuned against the cable capacitance so that the circuit operates near resonance. Reactive energy circulates between the reactor and cable, reducing the active power requirement of the supply.

Cable testing therefore needs to be considered as a complete process:

Insulation Check → Withstand Test → Sheath Test → Diagnostic Test → Fault Location → Test Report

The appropriate combination depends on cable voltage class, insulation type, length, capacitance, installation condition and testing objective.

2. Cable Types & Testing Objects

UHV power cable testing solutions can be applied to different cable systems.

Main Testing Objects

The testing object is normally the complete cable circuit, including:

• Cable conductor
• Main insulation
• Metallic screen
• Metallic sheath
• Cable joints
• Terminations
• Link boxes
• Cross-bonding system
• Grounding connections
• Surge protection components where applicable

For a newly installed cable, testing should not focus exclusively on the cable manufacturer. Installation quality can have a direct influence on the final test result.

For a service-aged cable, the test strategy should also consider previous failures, operating history, cable length, joint count, moisture exposure and previous diagnostic results.

3. Testing Requirements

A cable test plan should be established before connecting the high-voltage test equipment.

New Cable Installation

Typical testing stages include:

After Cable Laying

Check cable insulation, screen and sheath condition.

After Jointing

Verify joint installation and perform the required electrical tests.

After Termination

Check termination insulation and connections.

Before Energization

Perform the specified acceptance or commissioning test.

After Repair

Repeat the relevant tests to verify that the repaired section has returned to an acceptable condition.

UHV's field-test guidance recommends disconnecting equipment connected to the cable, checking insulation parameters with a megohmmeter and confirming that the preliminary insulation test passes before conducting the VLF withstand test.

Service-Aged Cable

For an operating cable system, a diagnostic approach can be more useful than a simple pass/fail test.

Typical combination:

VLF + Tan Delta + PD

VLF evaluates insulation withstand, while tan delta and partial discharge provide additional information about insulation condition.

UHV recommends considering diagnostic options such as tan delta and PD when selecting VLF equipment for maintenance applications.

4. Testing Methods

4.1 VLF Testing

VLF stands for Very Low Frequency.

Instead of applying 50 Hz or 60 Hz AC, the tester generates a high-voltage AC waveform at a very low frequency, commonly 0.1 Hz or below.

The major advantage is the reduction in capacitive charging current.

For a long cable:

I = 2πfCV

Reducing the frequency substantially reduces the charging current required to test the cable.

This makes VLF particularly practical for field testing of medium-voltage and high-voltage cables.

UHV's VLF cable testers support different frequency/load combinations so that the operator can select a lower frequency when cable capacitance is high. For example, the published UHV VLF cable tester specifications provide different permissible capacitance values at 0.1 Hz, 0.05 Hz, 0.02 Hz and 0.01 Hz.

VLF Applications

• New cable commissioning
• Installation acceptance
• Maintenance testing
• XLPE cable testing
• EPR cable testing
• Long underground cable testing
• Cable insulation withstand testing

VLF can also be combined with diagnostic measurements such as tan delta or PD.

4.2 AC Hipot

AC Hipot testing applies an AC high voltage to the cable insulation and evaluates whether the cable can withstand the specified electrical stress.

Compared with DC testing, AC testing creates an alternating electric field similar to the cable's normal operating environment.

AC Hipot can be applied to:

• Shorter cable sections
• Factory cable samples
• MV equipment
• Cable accessories
• Test systems where the required capacitance is manageable

For long cables, however, the reactive power requirement at 50/60 Hz can become very high.

That is why VLF and AC resonant systems are often more practical for long field-installed cables.

4.3 DC Hipot

DC Hipot applies a high DC voltage to the cable insulation.

It has historically been widely used because DC test equipment is relatively compact and the power requirement is low.

However, the test method should be selected according to the cable insulation and applicable standard.

For modern extruded XLPE and EPR cables, routine DC withstand testing of service-aged cable systems is generally avoided in many modern testing practices because DC can produce space-charge effects in polymeric insulation. UHV specifically positions VLF as the preferred alternative for modern field testing of extruded cables in accordance with the applicable IEEE practice.

DC Hipot remains relevant for certain cable types, legacy systems and project-specific requirements. It should not simply be treated as interchangeable with VLF.

4.4 AC Resonance

For long high-voltage cables, AC resonant testing can be the more appropriate technology.

A typical system consists of:

Variable Frequency Power Supply → Excitation Transformer → HV Reactor → Cable Under Test → Voltage Divider / Measurement System

The cable behaves primarily as a capacitive load.

When:

XL = XC

the circuit reaches resonance.

The reactive energy circulates within the system, allowing the test voltage to become high while the active power supplied by the source remains comparatively low.

AC Resonant Testing Is Suitable For

• Long HV cables
• EHV cable systems
• Factory cable testing
• On-site commissioning
• Underground transmission cables
• Large cable capacitance
• High-voltage withstand testing

UHV publishes AC resonant systems capable of testing large cable loads. One published PD-free system, for example, is specified at up to 500 kV, with an adjustable 30–300 Hz output frequency and applications including 220 kV cable testing.

4.5 Sheath Testing

The metallic sheath provides mechanical protection and an electrical return/grounding path depending on the cable design.

A damaged sheath can allow moisture to enter the cable structure. Over time, moisture ingress can contribute to insulation deterioration.

Sheath testing is therefore particularly important after:

• Cable laying
• Backfilling
• Joint installation
• Cable repair
• Mechanical excavation
• Road or construction work near the cable

The test checks the integrity of the outer sheath by applying a specified DC voltage between the metallic sheath and ground.

A failed sheath test does not necessarily mean that the main insulation has failed. It indicates that the outer protective system requires investigation.

4.6 Partial Discharge Testing

Partial discharge testing is a diagnostic method rather than simply a withstand test.

A PD event is a localized discharge that does not completely bridge the insulation between conductors.

Possible sources include:

• Voids
• Contaminants
• Poor cable joints
• Defective terminations
• Electrical treeing
• Floating conductive particles
• Local field enhancement

PD testing becomes especially valuable when the objective is to determine where insulation defects exist and whether they are developing.

For cable systems, PD measurement can be performed during suitable AC or VLF-based diagnostic procedures depending on the test system and applicable standard.

The major challenge is background interference.

A useful PD system must distinguish cable discharge signals from:

• Radio interference
• Corona
• Switching noise
• External electromagnetic interference
• Grounding noise

UHV's PD-related systems incorporate data acquisition and analysis functions, while its VLF selection guidance identifies PD as an optional diagnostic function for cable condition assessment.

4.7 Cable Fault Location

When a cable has already failed, the testing objective changes.

The first question is no longer:

"Can the cable withstand the test voltage?"

It becomes:

"Where exactly is the fault?"

Cable fault location normally involves several stages:

Step 1 - Fault Identification

Determine whether the fault is:

• Conductor-to-ground
• Conductor-to-conductor
• High-resistance fault
• Low-resistance fault
• Intermittent fault
• Sheath fault

Step 2 - Pre-Location

Estimate the approximate distance from the test end.

Methods can include:

• Time Domain Reflectometry
• Arc Reflection Method
• Impulse Current Method
• Decay Method

Step 3 - Pinpointing

Once the approximate position has been identified, the operator uses acoustic/electromagnetic methods or other suitable techniques to identify the exact excavation point.

UHV's VLF cable testing guidance specifically describes combinations such as VLF + Arc Reflection Method, VLF + Impulse Current and VLF + Decay Method for fault pre-location and pinpointing.

5. Recommended Equipment

A complete UHV power cable testing configuration can include the following equipment.

The selection should be based on cable characteristics rather than simply choosing the highest-voltage instrument available.

6. Testing Procedure

A typical field testing workflow is:

Step 1 - Collect Cable Information

Record:

• Cable manufacturer
• Cable type
• Rated voltage
• Rated frequency
• Conductor size
• Insulation type
• Cable length
• Installation date
• Joint quantity
• Termination quantity
• Cable capacitance
• Previous test results

Step 2 - Isolate the Cable

Confirm:

• Cable is de-energized
• All connected equipment is disconnected
• Switching operations are completed
• Test area is secured
• Safety grounding is established

Step 3 - Visual Inspection

Inspect:

• Terminations
• Joints
• Grounding
• Sheath
• Link boxes
• Cable ends
• Mechanical damage

Step 4 - Insulation Resistance

Measure the insulation resistance before applying high voltage.

This is a preliminary check. If the insulation resistance result is abnormal, investigate the cable before proceeding to the withstand test.

Step 5 - Sheath Test

Perform the specified sheath integrity test before or after the main insulation test according to the project procedure.

Step 6 - Select the Main Test

Choose:

• VLF → MV/HV field cable
• AC Resonance → Long/high-voltage cable
• AC Hipot → Appropriate shorter-capacitance test object
• DC Hipot → Applicable legacy/specific cable requirements

Step 7 - Voltage Ramp

Increase voltage in a controlled manner.

The system should continuously monitor:

• Test voltage
• Current
• Frequency
• Leakage behavior
• Test time
• PD where applicable

Step 8 - Hold Test Voltage

Maintain the specified test voltage for the required duration.

Do not shorten or extend the test duration without reference to the approved test procedure.

Step 9 - Discharge

After the test:

Reduce Voltage → Stop Output → Discharge Cable → Ground Cable → Confirm Zero Voltage

A cable can retain substantial stored electrical energy after the test has ended. The discharge and grounding procedure is therefore an essential part of the test, not an optional final step.

Step 10 - Diagnostic / Fault Location

If abnormal results are obtained, move to:

Tan Delta → PD → Fault Pre-Location → Pinpointing

This avoids unnecessarily applying repeated high-voltage stress to a cable with a known abnormality.

7. Applicable Standards

The exact standard depends on cable type, voltage level, test purpose and project location.

IEEE 400 Series

The IEEE 400 family provides guidance for field testing and evaluation of shielded power cable systems.

Relevant documents can include:

• IEEE 400
• IEEE 400.2
• IEEE 400.3
• Other applicable IEEE cable-testing guidance

UHV's VLF guidance identifies IEEE 400.2 for VLF withstand testing and IEEE 400.3 for PD testing applications.

IEC 60502-2

Relevant to power cables with extruded insulation for specified voltage classes and includes requirements associated with testing.

IEC 60840

Applicable to extruded insulation power cables and accessories for higher-voltage systems within its defined scope.

IEC 62067

Relevant to extruded insulation power cable systems for higher voltage levels.

UHV identifies IEC 60502-2, IEC 60840 and IEC 62067 among the standards relevant to cable AC testing applications.

IEC 60060

High-voltage test techniques under IEC 60060 may also be relevant to test voltage generation and measurement.

The final test voltage, waveform, frequency and duration should always be taken from the applicable edition of the standard and project specification rather than using a generic value.

8. Test Parameters

The main parameters to define before equipment selection are:

For VLF equipment, load capacitance is especially important.

A VLF tester may support a much larger cable capacitance at 0.01 Hz than at 0.1 Hz. UHV's published VLF specifications demonstrate this relationship directly.

For AC resonant testing, the basic design relationship is:

f = 1 / 2π√LC

where:

• f = resonant frequency
• L = system inductance
• C = cable capacitance

The required reactor configuration must therefore be calculated from the actual cable capacitance and required test voltage.

For example, UHV describes a 5 km, 220 kV XLPE cable example in which the cable capacitance is estimated from the cable capacitance per kilometer and total cable length before calculating the resonant-system requirement.

9. Common Testing Problems

Problem 1 - VLF Tester Cannot Reach the Required Voltage

The first item to check is cable capacitance.

If the cable is too long or the total capacitance exceeds the tester's load capacity at the selected frequency, the system may be unable to reach the specified voltage.

Possible solutions:

• Reduce test frequency where permitted
• Test sections separately
• Select a higher-capacity VLF system
• Use an AC resonant system

Problem 2 - Excessive Leakage Current

Possible causes include:

• Wet cable termination
• Contaminated surface
• Damaged insulation
• Incorrect grounding
• Connected equipment not fully isolated
• Cable leakage associated with aging

Do not interpret high current immediately as an internal cable breakdown. First inspect the test configuration.

Problem 3 - High PD Background

Check:

• Grounding
• Shielding
• Test leads
• Nearby switching equipment
• Corona
• Radio interference
• Measurement bandwidth

PD diagnosis is only useful when the background is sufficiently controlled.

Problem 4 - Sheath Test Failure

Possible causes include:

• Cable sheath damage
• Mechanical penetration
• Poor joint bonding
• Incorrect grounding
• Water ingress
• Installation damage

The fault should be located before the cable is placed into long-term service.

Problem 5 - Repeated VLF Failure

If the cable fails repeatedly at approximately the same location or voltage level, the test should not simply be repeated indefinitely.

The next step should normally be:

Confirm Test Configuration → Locate Fault → Inspect Joint/Termination → Repair → Retest

Problem 6 - Fault Locator Gives Inconsistent Distance

Possible causes include:

• Incorrect cable length
• Incorrect propagation velocity
• High-resistance fault
• Branch connection
• Joint reflection
• Poor grounding
• Cable route not matching the assumed model

The cable structure and installation drawings should be reviewed before pinpointing.

10. Equipment Selection Guide

The most important selection factors are:

1. Cable Voltage

For example:

6/10 kV → 8.7/15 kV → 12/20 kV → 18/30 kV → 26/35 kV → 64/110 kV → 127/220 kV

The test system must cover the required voltage with adequate margin according to the applicable test procedure.

2. Cable Length

Longer cable means higher total capacitance.

This is one of the most important parameters when selecting VLF or AC resonant equipment.

3. Cable Capacitance

Do not select a VLF tester based solely on cable voltage.

The supplier should know:

Cable Voltage + Cable Length + Capacitance + Required Test Voltage + Test Duration

before recommending the system.

4. Testing Purpose

5. Field or Laboratory

For field testing, priority should be given to:

• Portability
• Fast setup
• Integrated control
• Automatic voltage ramp
• Safety interlock
• Discharge function
• Data recording
• For factory testing, the focus shifts toward:
• Output capacity
• Long-duration operation
• Automation
• Test repeatability
• Data management
• Integration with production testing

UHV's VLF equipment is designed with field applications in mind, while its AC resonant systems can be configured as modular or containerized systems for larger cable testing projects.

11. Case Studies

Case Study 1 - MV Underground Cable Commissioning

A utility has completed the installation of a new medium-voltage XLPE underground cable.

The test sequence is:

Visual Inspection → Insulation Resistance → Sheath Test → VLF Withstand → Test Report

A portable UHV VLF system allows the commissioning team to conduct the test at the installation site without transporting a large power-frequency AC test system.

The VLF tester applies the required low-frequency AC test voltage while recording test voltage, current, frequency and test duration.

Case Study 2 - Long HV Cable Testing

For a long high-voltage cable, the total capacitance makes conventional 50 Hz AC testing impractical.

UHV configures an AC resonant system based on:

Cable Length + Cable Capacitance + Required Test Voltage + Test Frequency

The system tunes the reactor against the cable capacitance and generates the required high-voltage AC test condition.

This approach is particularly suitable for long underground transmission cables where the reactive power requirement would otherwise become extremely large. UHV describes modular/containerized resonant configurations for large cable systems and field applications.

Case Study 3 - Service-Aged XLPE Cable

An operating XLPE cable has been in service for many years.

Instead of performing only a withstand test, the maintenance team uses:

VLF + Tan Delta + PD

The VLF test evaluates withstand capability. Tan delta provides information on overall dielectric losses, while PD testing can identify localized defects associated with joints, terminations or insulation.

This type of testing provides more information for maintenance planning than a simple pass/fail result.

Case Study 4 - Underground Cable Fault

An underground cable fails after a construction project near the cable route.

The maintenance team first identifies the faulty phase, then performs fault pre-location using an appropriate reflectometry/impulse method.

The approximate location is established before the team uses a pinpointing system to identify the excavation point.

The sequence is:

Fault Identification → Pre-Location → Cable Route Verification → Pinpointing → Repair → VLF/Required Retest

This reduces unnecessary excavation and shortens restoration time.

12. FAQ

13. Related Products

VLF Hipot Tester

Portable low-frequency AC high-voltage tester for medium- and high-voltage cable insulation withstand testing.

UHV's VLF range includes different voltage and load-capacity configurations for cable testing applications.

AC Hipot Tester

Used for AC withstand testing of cables and other electrical equipment where the required test capacity is suitable.

AC Resonant Test System

Designed for high-voltage AC testing of long cables and other high-capacitance electrical equipment.

UHV's resonant systems can be configured with variable-frequency power supplies, excitation transformers, high-voltage reactors, voltage measurement and protection systems.

DC Hipot Tester

Used for DC high-voltage testing where the cable type, project specification and applicable testing practice permit DC testing.

Cable Sheath Tester

Designed to verify metallic sheath integrity and identify sheath insulation faults.

Partial Discharge Tester

For localized insulation defect detection and diagnostic assessment.

Cable Fault Locator

Used for cable fault pre-location and pinpointing after an insulation or conductor fault occurs.

VLF + Tan Delta Test System

Combines withstand testing with dielectric-loss diagnostics for condition assessment of suitable cable systems.

Integrated Cable Testing System

For users requiring multiple cable test functions within one testing platform.

14. Request a Quote

Need a Power Cable Testing Solution?

UHV can configure the cable testing system according to the actual cable rather than selecting equipment from voltage rating alone.

Send UHV the following information:

Cable Type → Rated Voltage → Cable Length → Capacitance → Insulation Type → Test Voltage → Test Duration → Testing Standard → Field/Factory Application

UHV can then recommend the appropriate combination of:

• VLF Hipot Tester
• AC Hipot Tester
• AC Resonant Test System
• Cable Sheath Tester
• Partial Discharge Test System
• Cable Fault Locator

For long or high-voltage cable projects, UHV can also evaluate the required resonant reactor configuration based on cable capacitance and required test voltage.

Typical UHV Cable Testing Configuration

New MV Cable

Insulation Resistance
→ Sheath Test
→ VLF Hipot
→ Test Report

Service-Aged MV/HV Cable

Insulation Resistance
→ VLF
→ Tan Delta
→ PD
→ Condition Assessment

Long HV/EHV Cable

Insulation Check
→ Sheath Test
→ AC Resonant Test
→ PD Monitoring
→ Test Report

Failed Cable

Fault Identification
→ Pre-Location
→ Pinpointing
→ Repair
→ VLF / AC Retest

The UHV engineering team can recommend the test voltage, frequency, load capacity, reactor configuration and diagnostic functions according to the actual cable parameters.

Request a Quote for Your Power Cable Testing Solution.