Power Cable Testing

April 18, 2023Dale Persad 810000263 1

POWER CABLE TESTINGAN OVERVIEW OF…

Dale Persad 810000263


Why Test Power Cables?• Cable testing is undertaken in order to determine the condition of a power system.

Therefore it is a means by which potential cable failures can be detected so as to improve overall system reliability. The pictures below shows power cable insulation breakdown.

(EEP 2013) (HVS 2011)


History of Power Cable Testing• The most common method of Cable Testing since the invent of

the Power Cable was the High Voltage DC (HVDC) withstand test.

• HVDC withstand test can only detect defects which are associated with conduction. Therefore defects were detected by the leakage currents.

• However 95% of faults in extruded dielectric cable are due to Partial Discharge (Lanz 2009)

• In the past equipment used to perform Partial Discharge tests were expensive, therefore conducting these tests were impractical.

• Today Partial Discharge tests are more accessible and easier to perform using acoustic equipment shown on the right (Direct Industry 2014)


Categorization of Power Cable Tests?• The flow diagram on the left,

details the different categories of power cable tests which were outlined in IEEE 400 standard.


Installation, Acceptance and Maintenance Testing • INSTALLATION TEST -This is a field test which is undertaken after cable installation, but

before the application of joints or terminations.

• ACCEPTANCE TEST – Also a field test, however it is undertaken after cable installation, that is inclusive of terminations and joints; before the cable system is commissioned.

• MAINTENANCE TEST - This test is undertaken during the operation life of the cable system.

Raytheon Explor IR, Thermal imaging camera with High Temperature Filter commonly used for Maintenance Testing (Mahabir 2007)

Megger/Biddle 120 KV DC High-pot Capable of Testing 66KV Cables (Mahabir 2007)


Withstand Tests• INTRODUCTION - This type of test involves the application of a voltage at a nominal

level or higher than for a prescribed period of time. Exceeding the voltages or times stipulated in the IEEE 400 standard may cause permanent damage to cable which can lead to failure. This test therefore determines the cable’s ability to withstand voltage without insulation breakdown.

• APPLICATION – The applied voltage source can be either AC, Very Low Frequency (VLF) AC, Damped AC (DAC) or DC. The magnitude of the applied AC voltage is normally 1.5 x Operational Voltage whilst for DC voltages the applied voltage is 3 x Operational Voltage.

ADVANTAGES DISADVANTAGES

Test is simple to perform, flexible (can be applied to different cable types) and easy to interpret results.

Can not detect all cable faults. Undetected faults may worsen, causing failures when in service.

Dielectric Response • INTRODUCTION - This type of test investigates the cable’s dielectric response to an

applied electric field. By measuring properties such as, the recovery voltage, DC leakage current, dissipation factor polarization/depolarization current and spectroscopy a fairly complete assessment of the cable is provided.

• APPLICATION – AC or DAC voltage is applied to the cable for a prescribed period at a frequency of 20 Hz to 300 Hz. VLF voltage applied requires a frequency of 0.1 Hz.



Provides an overall assessment of cable condition

Interpretation of results is difficult as cable may have a high dielectric response but may not be faulty.

Has the ability to detect degree of water treeing, and conductive type defects

Accuracy affected by factors e.g. temperature/humidity and neutral corrosion

Dielectric Response – Dissipation Factor • INTRODUCTION - This investigates the loss factor of the cable’s insulation. This loss

factor increases with age, therefore the dissipation factor or Tan Delta measurement can be used to determine cable condition.

• APPLICATION – The dissipation factor (DF) and Tan Delta is determined using the formulae below:


(IEEE 400 2012)


Dielectric Response – Dissipation Factor


Useful results can be obtained at low voltage inputs

Results are difficult to interpret due to nonlinear characteristics

Results obtained are display high immunity to external noise/ electric fields

Results are dependant on test voltage frequency .

VLF Measurement Unit

VLF Control Loss Angle

Analyser

Cable under Test

Dielectric Response – DC Leakage Current • INTRODUCTION – In this test a DC voltage lower than the withstand voltage is applied

between the conductor and the insulation shield whilst the leakage current through the cable insulation is measured.

• APPLICATION – HVDC must not be used especially if testing aged extruded cable systems. Measurements of Leakage Current are only undertaken when the applied voltage has reached its steady state value. After steady state, voltage is increased step wise, at each step, a time period allows for the steady state to be achieved.



Easy to conduct Pass or Fail criteria nor duration of voltage application is not firmly established

Test can be automated Time required for cable discharge before & after test - up to 4 x Test Time

Dielectric Response – Recovery Voltage• INTRODUCTION – In this test a DC voltage is used to charge the cable for a specified

time, after which the cable is discharged to ground through a resistor for a very short time. The open circuit voltage is then recorded and plotted against time.

• APPLICATION – This test is particularly sensitive to moisture ingress in cables, therefore it is commonly used to determine the level of water tree degradation in extruded insulation.



Sensitive to moisture ingress in cables Interpretation of results may be complicated due to frequency dependence of polarization effects.

Test equipment is small and can be fully automated

Time required for cable discharge before & after test.

Dielectric Response – Polarization/Depolarization I• This test involves the application of a DC voltage to the cable for a prescribed duration. The

current during this voltage application is measured and plotted. After charging the cable, it is then short circuited. The discharging current (depolarization current) is also measured and plotted.

• Through analysis of the plotted graphs cable characteristics can be determined.


Dielectric Response – Dielectric spectroscopy• In this final test, the real and imaginary components of the cable’s leakage currents are

measured and analysed at frequencies between 0.001 Hz to 100 Hz.

• The measured results are used to determine tan delta.

Partial discharge - Electric Measurements• This test one of the oldest tests used in power cable testing and is used to determine point

of failure of cable insulation i.e. discharge locations. This test is normal conducted by the cable manufacturer, before the cable leaves the factory .

• This test is undertaken by applying a test voltage, either DAC or VLF, cable is then examined for partial discharge.


Partial discharge - Acoustic measurements• In this test, one listens for partial discharge. When a partial discharge occurs, energy is

released which produces a mechanical wave. Therefore the site at which the Partial discharge occurs acts like an acoustic wave source.

Time Domain Reflectometry


• INTRODUCTION – Time Domain Reflectometry (TDR) uses changes in impedance of a cable to determine cable imperfections such as faults, open connections , poor neutrals , lossy connections and water ingress.

• APPLICATION – TDR can be compared to a radar system. This is because a voltage pulse is applied between the conductor and the insulation shield of the cable. As pulse travels through the cable imperfections reflect the pulse. The reflections are captured and analysed used the expression below.

(IEEE 400 2012)


Thermal Infrared Imaging • Cable imperfections such as insulation breakdown result in the formation of hot spots

along the cable length. This hot spots are caused by arcing within the cable

(IEEE 400 2012)

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Power Cable Testing