wire and cable engineering guide

Cable Testing
Excerpt
From
Prysmian’s
WIRE AND CABLE
ENGINEERING GUIDE
700 Industrial Drive
Lexington, South Carolina 29072
Phone: 1.800.845.8507
Fax: 1.803.951.1142
www.na.prysmian.com/energy
Cable Testing
Revision 1
June 2, 2011
Page 1 of 8
CABLE TESTING
Testing represents an integral part in the life
of a cable. A cable will be subjected to
multiple tests in its lifetime including a series
of tests beginning at the factory and
potentially continuing throughout the lifespan
of the cable. Cable testing is performed in
different phases including: materials testing,
qualification testing, production testing, and
field testing (including on-reel, installation,
acceptance, and/or maintenance testing).
The types of tests performed in the various
phases can depend on the environment of
the cable as well as the type of cable being
tested: low-voltage or medium-voltage,
shielded or non-shielded, etc. Various cable
testing practices are covered in the following
documentation.
It is important to recognize that many factors
must be considered to properly characterize
the test results obtained from any cable
testing program. Many of those factors are
controllable as part of material testing,
qualification testing, and production testing.
Unfortunately, field testing does not allow
control over many factors, two of which are
temperature and humidity. Keeping track of
the various factors that influence cable
testing results and accounting for them can
be the difference between passing or failing
results.
MATERIAL TESTING
In order to provide a quality cable, quality
materials must be utilized in the
manufacturing processes. To ensure quality
materials are used in the production of our
cables, Prysmian adheres not only to the
requirements of industry standards but also
to our own strict internal requirements.
Industry standards, such as those from
ICEA
(Insulated
Cable
Engineers
Association) and ASTM (American Society
for
Testing
and
Materials)
provide
requirements for anything from the
conductor to the metallic shields, armoring,
and all the extruded materials. The material
testing requirements apply to both the
physical and the electrical characteristics of
the cable, but NOT necessarily to the
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specific ingredients of the materials. Many
of the cable test results are commonly
summarized in a Certified Test Report
(CTR), which can be used as a means to
compare industry standard requirements
and/or customer specifications to as tested
values.
In recent years, ICEA has been transitioning
towards performance-based standards. After
all, while the quality of the materials is
indeed important to the cable, even more
important is the performance of the finished
product. For this reason, the end user
should give greater consideration to the
tests performed on the cable as a whole as
well as the overall performance of the cable
rather than to the individual ingredients of a
compound or the compounds themselves.
QUALIFICATION TESTING
Qualification testing, also known as type
testing, insures the credibility of the cable’s
overall design. Qualification testing is
performed on a particular cable design and
some tests encompass accelerated aging as
part of the testing protocol. ICEA S-94-649
and S-97-682 list five main types of
qualification tests including:
(1) Core Material Qualification (this
includes Conductor-Shield/Insulation
Qualification and Insulation/InsulationShield Qualification)
(2) Thermomechanical Qualification
(3) Jacket Material Qualification
(4) CV Extrusion Qualification
(5) Other
Qualification
Tests
(this
includes
Insulation
Resistance,
Accelerated Water Absorption Tests,
etc.).
These qualification tests, especially the Core
Material Qualification tests, provide the
consumer with a sound and relative means
for comparing the performance of cables
produced by various cable manufacturers.
This apples-to-apples comparison is based
on industry standard (ICEA) details, which
provides the exact method of testing and the
Cable Testing
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procedures to be used as well as the
specifics regarding the cable sample: 15 kV,
unfilled, 1/0 AWG Al Class B compressed
conductor, 100% insulation level, unjacketed
cable. Each manufacturer is required to
comply with the same set of tests according
to the same test parameters.
PRODUCTION TESTING
Production tests are performed on a routine
basis on various types of cables during and
immediately following the manufacturing
process. Production testing insures the
continuous quality of the products and the
products’
compliance
with
industry
standards while also providing a means for
evaluating
the
efficiency
of
the
manufacturing line and/or facility.
Virtually every mechanical and electrical
aspect of each element of the cable design
is governed by an applicable industry
standard as well as Prysmian’s internal
requirements. These physical and electrical
characteristics
are
then
tested
for
compliance with various industry and
internal requirements. While there are many
tests, in fact too many to discuss in this
forum, following are some of the main
production tests worth noting:
1. Elongation and Tensile Strength of the
extruded materials.
2. Hot Creep and Hot Set
3. Dimensional Analysis
4. High Voltage AC Withstand (MV
Cables)
5. Partial Discharge (MV Cables)
6. Spark Testing
Elongation and Tensile strength tests ensure
the materials have been extruded correctly
and the required physical properties are as
they should be. Hot Creep and Hot Set tests
indicate whether the applicable material has
been properly cross-linked or thermoset.
Dimensional analysis indicates whether the
cables comply with the limits set forth in
industry requirements for diameters and
thicknesses. The High Voltage AC
Withstand test ensures the electrical
integrity of the insulation system with
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regards to its dielectric strength while the
Partial Discharge test identifies significant
voids and possible contaminants with
surrounding voids that may be present
within the dielectric material. Spark testing is
an inline voltage test used for low-voltage
insulation
and
medium-voltage
nonconducting
jackets.
Spark
testing
continuously inspects for pinholes or other
breaches in the outer layer of the cable.
FIELD TESTING
Field testing of cable is commonly employed
to determine the as-received condition of the
cable, the as installed condition of the cable,
and/or the operating condition of the cable.
Field testing can be divided into two broad
categories: Type 1 - Destructive and Type 2
- Non-Destructive. Both of these categories
of tests can be conducted as part of on-reel
testing, installation testing, acceptance
testing and/or maintenance testing.
Today, there are more field test methods
available than ever before. The test method
chosen depends on multiple factors, such as
what is to be tested and what information is
to be obtained (i.e. instant analysis or
historical data). Six commonly referenced
field tests are listed below.
1)
2)
3)
4)
High Potential Testing (Hi-Pot)
Very Low Frequency Testing (VLF)
Partial Discharge Testing (PD)
Dissipation Factor/Tan Delta Testing
(Tan δ)
5) Megohmeter Testing of Insulation
Resistance (a.k.a. Megger Testing)
6) Time Domain Reflectometry Testing
(TDR)
Specific test methods may be more
applicable to one category, while some test
methods may be applied to both categories.
More importantly, the above list of tests is
not meant to be a comprehensive list of all
field tests available.
†
Type 1 – Destructive Field Tests
Destructive tests can be categorized as
“pass/fail” or “go/no-go” tests. By nature, a
withstand test that tries to breakdown a
Cable Testing
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cable defect during the time of testing is
considered a destructive test.
Destructive tests typically consist of applying
a high electric stress for a prescribed
duration. Three common voltage sources
used for withstand testing are DC, power
frequency AC, and VLF AC. It is important
to recognize that Type 1 field tests may
trigger failure mechanisms within a cable
that will not show up during the test but may
cause subsequent failures in service.
†
Type 2 – Non-Destructive Field Tests
Non-destructive tests can be categorized as
“diagnostic” tests which are used to provide
the relative condition of the insulation
system by comparison with figures of merit.
Diagnostic testing is typically performed by
means of moderately increased voltages
applied for relatively short duration or by
means of low voltages. Two common types
of diagnostic tests are PD Testing and
Dissipation Factor/Tan δ testing.
It is
important to recognize that, in an advanced
condition of insulation degradation, Type 2
field tests may aggravate the condition of
the cable and cause breakdown before the
results can be determined or before the test
can be terminated.
High Potential Testing
Hi-Pot testing can be conducted with a DC
potential or AC potential and can be applied
as a Type 1, withstand test or a Type 2,
diagnostic test.
The DC hi-pot withstand test is a Pass/Fail
test that has been applied to all types of
cable and accessories.
The DC hi-pot
leakage current technique, sometimes
referred to as a Megger Test, involves the
measurement of leakage current when a
high potential (above nominal) is applied to
the conductor while the metallic shield of the
cable is grounded. The behavioral
characteristics of the leakage current are
evaluated to determine the condition of the
cable, specifically the insulation.
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It is important to recognize that published
documentation provides details showing DC
hi-pot testing mostly finds conductive type
gross workmanship errors in extruded
†
dielectric cable systems . Consequently,
the practical use of the DC hi-pot testing is
recommended only for paper insulated cable
systems and for performing a safety check
before switching an extruded cable system
into service (to prove that the system is not
grounded).
The AC Hi-Pot withstand test is a Pass/Fail
test
routinely
applied
by
cable
manufacturers before the cable leaves the
plant. When used in conjunction with PD
Testing and/or Tan δ testing, the AC Hi-Pot
test can be considered a diagnostic test.
Additional details on DC high potential
testing can be found by referencing IEEE
400.1 IEEE Guide for Field-Testing of
Laminated Dielectric, Shielded Power Cable
Systems Rated 5kV and Above with High
Direct Current Voltage.
VLF Testing
Very Low Frequency (VLF) testing
incorporates the application of an AC
voltage at a low frequency in the range of
0.01 to 1.00 Hz. The typical frequency
applied is 0.1 Hz. The VLF AC withstand
test (Type 1) should not be confused with
the non-destructive (Type 2) diagnostic tests
which use a VLF voltage source as part of a
partial discharge test or a dissipation factor
(tan delta) test.
When VLF testing is
referenced as a stand alone test, it typically
refers to VLF AC hi-pot testing.
A very low frequency AC high potential test
is a destructive withstand test. It is generally
operated at 0.1 Hz or lower, which allows
the equipment to be smaller than power
frequency voltage sources. The withstand
test causes large electrical trees and
mechanical defects in the cable to grow
towards failure. The recommended test
voltage is 2.0 to 3.0 times the cables’ normal
line to ground voltage (2.0V0 – 3.0V0); while
the recommended test duration is 15 to 60
minutes. The test voltage and time are
Cable Testing
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dependant on the type of test being
performed (i.e. installation, acceptance, or
maintenance). A properly implemented VLF
test will not cause damage to good
†
insulation , but will reveal many cable
system defects during the test duration.
Additional information on VLF testing can be
found by referencing IEEE 400.2 IEEE
Guide for Field Testing of Shielded Power
Cable Systems Using Very Low Frequency
(VLF).
PD Testing
Partial Discharge (PD) testing is a diagnostic
(Type 2) test which analyzes cable systems
for voids or contaminants in the conductor
shield, insulation, and, insulation shield
caused by electrical trees, water trees,
cracks, delamination, and/or workmanship
error. A partial discharge is a localized
dielectric breakdown of a small portion of the
electrical insulation system under voltage
†
stress . PD testing can be implemented
online or offline and is the only test that can
detect, locate, and characterize defects in
cable insulation. Caution should be used
when PD testing is performed at elevated
voltages on discharge-resistant cables as
†
defined by ICEA S-94-649 due to the fact
that it may not be very useful.
Offline PD testing is performed through the
application of an elevated AC voltage
between the conductor and metallic shield.
An oscilloscope and/or proprietary digital
signal analysis platform is used to detect
transient microvolt or microampere level
signals that are generated at the discharge
site and travel through the cable to the
†
detection equipment .
PD testing is
performed on all medium voltage cables at
the factory as a production test and is
governed by ICEA T-24-380 which limits the
PD limit to 5 picocoulombs or less at a
stress of 200V/mil.
Online PD testing typically employs high
frequency current transformers (CTs) or
capacitively coupled voltage sensors to
detect transient signals from discharges.
Acoustic PD measurement techniques could
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potentially be applied to parts of the cable
†
system that allow direct contact .
No
external voltage source is needed as the
online technique provides testing under
normal operating conditions.
Partial discharge threshold levels have been
established
for
factory
testing
of
terminations, joints, connectors and cable.
Comparison against these values provides
excellent reference for the condition of the
cable system.
Additional information on PD testing can be
found by referencing IEEE 400.3 IEEE
Guide for Partial Discharge Testing of
Shielded Power Cable Systems in a Field
Environment.
Dissipation Factor/Tan Delta Testing
Tan Delta (Tan δ) testing is a diagnostic test
that provides a means of measuring the AC
dielectric losses of the insulation and then
making a determination of the condition of
the cable based on this information. In
theory, a medium voltage cable approaches
being a perfect capacitor; a dielectric
sandwiched between a center conductor
and a surrounding metallic conductor.
However, since the insulation is not a
perfect dielectric, the system is not a perfect
capacitor.
The Tan δ test essentially measures the
phase shift between the voltage and the
current. In an ideal or perfect capacitor, the
insulation is free of impurities as well as
dielectric losses and the angle between the
voltage and current is 90°. Based on the
degree of impurities in the insulation, the
angle will decrease from 90°. The results of
a dissipation factor test are generally
grouped into one of three categories: like
new, aged, highly aged.
Additional
information
on
dissipation
factor/tan δ testing can be found by
referencing the National Electric Energy
Testing Research and Applications Center
(NEETRAC) Project No. 04-211 Overview of
Cable System Diagnostic Technologies and
Application Overview.
Cable Testing
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Megohmeter Testing of Insulation
Resistance
Megger testing can be applied as a
destructive (Type 1) test or a diagnostic
(Type 2) test depending on factors such as
test duration and test voltage.
Typical
application of a megger test determines the
total insulation resistance of each cable.
The resistance measurement is used to
determine if the circuit will operate without
excessive leakage current through the
insulation when energized.
Measured
values can be impacted by certain external
factors (temperature, moisture, etc.), which
may result in questionable readings, even
when evaluated on a satisfactory length of
cable.
The megger test is performed by applying
an elevated DC voltage to the conductor and
measuring the current flow to a ground
reference. With the known voltage and
measured current, an insulation resistance
value can be calculated.
It is important to recognize that megger
testing non-shielded cables may produce
marginal results due to the inherent lack of a
completely encompassing and uniform
ground plane over the dielectric of the cable.
Additional information on megohmeter
testing of insulation resistance can be found
by referencing Megger’s “A Stitch in Time”
The Complete Guide to Electrical Insulation
Testing.
Time Domain Reflectometry Testing
Time Domain Reflectometry (TDR) testing,
or a RADAR test, by itself does not evaluate
the insulation of shielded power cables.
Therefore its ‘Type’ is not classified into
destructive or non-destructive. However,
when low voltage pulses are used (≤600V),
the test can be considered a non-destructive
diagnostic test.
The test uses pulse reflection to measure
the distance to changes of characteristic
impedance in the cable.
In theory, a
completely uniform cable that is properly
terminated will exhibit no characteristic
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impedance changes and will not reflect a
pulse. In reality, interruptions in the cable
shield,
joints,
open
circuit
faults,
terminations, and short circuit faults all
produce reflections.
A graphical interface is commonly used to
display the original pulse, any reflected
pulses, as well as cable data. When fault
locating, the TDR response of the cable
system will yield a positive polarity reflection
at an open circuit and a negative polarity
reflection at a short circuit.
Additional information on time domain
reflectometry testing can be found by
referencing
Megger’s
Fault
Finding
Solutions.
Field Tests
The tests detailed above are six commonly
referenced field tests in an industry that has
even more options.
These tests are
referenced to provide a general overview of
some of the field tests available today. It is
up to the end-user to determine which field
test method will provide the most accurate
and useful information. One test method
cannot completely assess the condition of
every cable. To properly determine the
condition of a cable, the best test method
may be a combination of tests.
On-Reel Testing
On-Reel (field) testing is a rather uncommon
practice that allows the end user or installer
to test the integrity of the cable on the reel at
the time of delivery and prior to installation.
On-reel (field) testing is uncommon due to
the fact that prior to shipping, the cable
undergoes a thorough testing program at the
factory. On-reel testing by the end-user
insures that the cable has arrived without
sustaining any damage while in transit.
During shipment the cable may be loaded
and unloaded several times after it leaves
the manufacturer and before it arrives at its
final destination. This extraneous, but
sometimes necessary, handling of the cable
provides added opportunity for the cable to
experience mechanical damage. If the cable
is damaged during transit and is not on-reel
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tested, the cable may then be installed,
prepared, and tested only to determine the
cable is failing installation test due to
damage incurred during shipping.
On-reel testing of cables incorporates
different test methods when testing nonshielded vs. shielded cables.
On-reel
testing of non-shielded cables is a difficult, if
not an impossible task. Unless there are
multiple cables wound on the same reel or
the cable and reel can be submerged in
water to afford a continuous ground plane,
on-reel testing cannot be successfully
performed for non-shielded cables.
A commonly used test method for nonshielded cables is a megger test. Megger
testing
non-shielded
cables
typically
involves applying a potential to the ‘test
cable’ and grounding the other cables on the
same reel, and then measuring the
insulation resistance. This test may produce
marginal results due to the inherent lack of a
completely encompassing ground plane.
The insulation resistance of a single nonshielded cable on a reel can be determined
if the end-user has the means of submersing
the cable and reel while performing a
megger test.
On-reel testing of shielded cables involves a
slightly different method of verifying the
cable integrity. Shielded cables inherently
provide a solid ground reference for the test
setup by means of the metallic shield.
Therefore, it is not necessary to have
multiple cables on a reel or have
submersion capabilities. DC hi-pot testing
has been commonly utilized as an on-reel
test of shielded cables.
Since partial
discharge testing is performed at the plant
on these reels, on-reel field testing using a
PD test set would provide results that are
the most readily comparable to the plants’
PD test results, as long as field ‘noise’ is
filtered out of the results.
It is important to note that any intention to
on-reel test cables should be pointed out to
the manufacturer. In order to facilitate onreel testing by the end-user, the
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manufacturer will have to ensure test ‘tails’
are present on the reel. Test tails will consist
of allowing access to appropriate lengths of
cable at the drum end of the cable. This will
enable the connection of both cable ends to
the necessary test equipment. Test tails
typically consist of a length of cable at the
drum end that is approximately 24 inches
long.
Installation Testing
Installation testing is conducted after cable
installation but before jointing (splicing) or
terminating. The test is intended to detect
†
shipping, storage, or installation damage .
Installation testing of cable offers the best
possible assurance that the cable has not
been damaged and will perform satisfactorily
when energized.
There are many ways cables can be
damaged during installation: pulling through
ducts that are in poor condition, improper
use of pulling equipment, exceeding
minimum bending radii or training radii, or
exceeding maximum pulling tensions or
maximum sidewall bearing pressures. If
damage has occurred during installation, it is
important to determine this prior to
energizing. Installation testing can prevent a
safety hazard or a potential cable or
accessory failure during inopportune times.
To test the integrity of only the cable, the
installation test should be performed. Once
the cable tests satisfactorily, the accessories
can then be applied and the system can be
‘acceptance
tested’
to
ensure
the
accessories were applied successfully and
are of good quality.
Installation testing non-shielded (<5 kV)
cables typically involves applying a potential
to the ‘test cable’ and grounding the other
cables in the same duct as well as possibly
grounding the duct itself, and then
measuring the insulation resistance. This
test may produce marginal results due to the
inherent lack of a completely encompassing
and uniform ground plane.
Hi-Pot testing, VLF testing, partial discharge
testing, etc. are all types of installation tests
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that may be used on shielded cables.
These tests involve applying a potential to
the conductor and grounding the inherent
ground plane (the metallic shield).
Acceptance Testing
Acceptance testing is conducted after the
cable system installation, including all
terminations and joints (splices), but before
the cable system is placed into normal
service. The test is intended to detect
installation damage and to show any gross
defects or errors in installation of other
†
system components/accessories .
Various test methods have been used to
acceptance test a cable system, from DC hipot testing to PD testing. The test data
obtained from these tests typically provides
information about the whole cable system
being tested. Some acceptance tests may
be able to differentiate the results between
the cable and any accessories.
An
acceptance test provides a good starting
point for verification of the cable system
installation; however, if marginal test results
are obtained, further testing should be
performed.
101245 “Effect of DC Testing on Extruded
Cross-Linked
Polyethylene
Insulated
Cables.”
Maintenance testing of cables typically
includes all the cable accessories as part of
the test results. Consequently, any test
result must be interpreted to properly
determine if there may be a problem with the
cable or an accessory. Maintenance testing
of cables can be accomplished through
multiple test techniques: AC Hi-Pot testing,
Tan Delta testing, PD testing, and/or VLF
testing. Each test offers unique advantages
as well as disadvantages.
Cable Testing
Multiple testing practices are covered within
this document; however, please note that
this document is only intended to provide
general information about cable testing. For
further information about cable testing,
please contact Prysmian’s Application
Engineering department or a reputable cable
testing company.
The interpretation of cable testing results
is the key to properly assess the
characteristics of a cable!
As part of an acceptance test, the accessory
manufacturer should be contacted for
appropriate testing practices. In no case
should the testing of the cable exceed the
limits of the accessories or cable.
Maintenance Testing
Maintenance testing is conducted during the
operating life of a cable system. It is
intended to detect deterioration of the
system (in cable or accessories) so that
suitable maintenance procedures can be
†
initiated .
While there are multiple maintenance test
methods available, DC Hi-Pot Testing is not
recommended as a maintenance test for any
solid dielectric insulated cable, especially for
Cross-Linked Polyethylene (XLPE/TRXLPE)
cables and definitely not after five years of
in-service life. More information concerning
this issue can be found in the Electric Power
Research Institute (EPRI) project report TR-
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†
Documentation references are available upon request.
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