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EuroWire – March 2012
184
Technical article
Quality materials can
improve reliability of
distribution cables
By Peter Pang, Shawn Miao, Simon Leung and Simon Sutton, of Dow Electrical & Telecommunications,
a business unit of the Dow Chemical Company
Introduction
Not all materials are created equal. It’s a
simple but true concept that needs to be
considered and embraced throughout
the value chain. Performance is essential
to the overall functionality of the grid
and can only be delivered by materials
that undergo thorough testing. Specifiers
who require distribution cables that
will deliver decades of reliable electrical
performance must stay abreast of material
developments.
Choosing materials for any reason
other than performance, such as price,
availability or regional origin, could result
in early and frequent failures. This means
additional time and expense required for
repair and/or replacement as well as a
decrease in consumer confidence.
Medium voltage cables using TR-XLPE
materials have been shown to maintain
excellent electrical properties, such as
ACBD strength, after many years of service,
thus guaranteeing long life and improving
reliability for distribution grids.
Besides
insulation
materials
which
can inherently retard water trees,
semi-conductive shield materials with
high cleanliness have also been shown
to improve electrical properties of cables
aged under laboratory conditions.
To achieve long-term success, it is
recommended that cable makers and
end-users (utilities) specify cables that
are
constructed
with
high-quality
materials
that
meet
specific
test
criteria. Installation should be done by
experienced professionals who also
run post-installation tests to ensure
system-wide performance. Pricing can and
should be evaluated by a life-cycle cost
analysis.
Failures and prevention
Early power cable failures are usually
caused by defects that are not detected
during factory testing and by poor
workmanship associated with preparation
and installation of cable joints and
terminations.
During the majority of the life of the cable
system, failures tend to be caused by
external factors such as accidental dig-ins.
In the later stages, cables fail usually by
aging of insulation materials.
Typical late-stage failure mechanisms
include initial formation of water trees
that subsequently generate electrical
trees, causing partial discharge, and cable
failures.
The presence of water trees of various
lengths in field-aged cables have been
found in recent studies by Shanghai
Municipal Electric Power Company of
SGCC.
The formation of water trees can be
prevented by the inclusion of water
barriers in the cable, like metal shields
and filled conductor strands, or by the
use of insulation and semi-conductive
shield materials that have been shown to
significantly slow water tree growth.
This is seen in cables aged in actual field
conditions or under laboratory accelerated
aging conditions.
Accelerated Water Treeing Test (AWTT)
protocols such as those found in industrial
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Figure 1
:
Comparing retention of dielectric strength of TR-XLPE and XLPE cables
Service Years
AC Breakdown Strength
(KV/min)