POWER CABLE TESTING
AN OVERVIEW OF
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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)
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(HVS
2011)
9/21/15
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
Dale Persad 810000263
(Direct Industry
2014)
9/21/15
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.
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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)
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Megger/Biddle 120 KV
DC High-pot Capable
of Testing 66KV
Cables (Mahabir
2007)
9/21/15
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
cables 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.
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Dielectric Response
INTRODUCTION - This type of test investigates the cables 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.
ADVANTAGES
DISADVANTAGES
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 Accuracy affected by factors e.g.
water treeing, and conductive type temperature/humidity and neutral
defects
corrosion
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Dielectric Response Dissipation Factor
INTRODUCTION - This investigates the loss factor of the cables 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:
Dale Persad 810000263
(IEEE 400
2012)
9/21/15
Dielectric Response Dissipation Factor
Cable
under
Test
VLF
VLF
Control
Measureme
nt Unit
Loss
Angle
Analyser
ADVANTAGES
DISADVANTAGES
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 .
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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.
ADVANTAGES
DISADVANTAGES
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
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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.
ADVANTAGES
DISADVANTAGES
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 Time required for cable discharge
fully automated
before & after test.
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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 cables
leakage currents are measured and analysed at frequencies between
0.001 Hz to 100 Hz.
The measured results are used to determine tan delta.
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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.
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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.
Dale Persad 810000263
(IEEE 400
2012)
9/21/15
14
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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