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EuroWire – November 2011
56
technical article
Cables for
Photovoltaic Applications
By Jorg Bor of Freidrich Lütze GmbH & Co KG, Stefan Grunwald of Lutze Inc, and Ilona Hirtz of Kabelwerke Villingen GmbH
Abstract
The recent growth of the renewable
energy market applies to both wind
energy as well as to solar energy. In many
countries this growth is supported by
governmental subsidies.
Outdoor
applications
in
renewable
energy plants demand high thermal and
mechanical requirements from cables
and other components. Therefore, some
national standardisation bodies defined
general requirements for cables used in
solar plants.
In 2005 UL 4703 was published in the
USA. In Germany a joint working group
“Cables for Photovoltaic Application”
was established in the same year, which
resulted in a specification published in
February 2008.
The performance requirements in high
temperatures have been set very high
for cables. Various German institutes
accepted this specification and now offer
certifications for it. There has been a huge
demand in the European as well as Asian
markets for such certified cable.
Due to the differences the UL 4703
requirements
cannot
be
combined
together with the German specification.
The different solar-cable requirements
have advantages as well as disadvantages
in respect to the specific regional
conditions.
1 Introduction
The renewable energy market is growing
rapidly. A tenfold increase for the next five
years is anticipated by Merill Lynch
[1]
.
This growth applies to wind energy as well
as to solar energy. Due to public interest, a
number of governments have decided to
support the renewable energy economics
with large amounts of subsidies. Thus,
even a further increase in this market can
be expected.
2 External Conditions
The photovoltaic market is a very specific
market. Various national regulations are
to be taken into account. The system size
can vary from a small home power supply
with a nominal power of few kilowatts
up to large centralised solar plants in the
Gigawatt range.
The components used in these systems
must be suitable for these specific
applications.
One common determining factor for all
photovoltaic power systems is the out-
door use, which brings along high
temperatures and, of course, high UV
radiation. Weathering and humidity
need to be taken into account, as well.
Furthermore, safety and reliability aspects
are very important.
The improvements of the solar panel
technology define the electrical perfor-
mance of components. Usually solar
systems are working with low voltage
direct current, and the panels are switched
in parallel.
Thus, the current is the determining
electrical parameter. In terms of cables,
this results in different conductor cross
sections.
3 Common
Specification of
German Technical
Institutes
In 2005 the German national electro-
technical standardisation body, DKE,
established a working group called
“Cables for Photovoltaic Application”
which resulted in a public specification.
This
document
was
published
in
February 2008
[2]
. Due to the close
cooperation between different institutes,
this specification is identical to the
TÜV-Specification 2 PfG 1169/08.2007 and
the VDE-Specification E PV 01:2008-02.
DKE-Subcommittee UK 411.2 released
the document for publication and
application, but due to CENELEC rules it
was not adopted as a national standard in
Germany.
4 Technical Details
of the German
Specification
In this German specification
[2]
the
requirements have been raised to a very
high level regarding the high temperature
performance of cables. Additionally,
high mechanical stability is required;
the cable has to be flame retardant and
free of halogens.
To meet this specification the manu-
facturers generally use halogen free
flame retardant cross linked polyolefin
copolymers as material for insulation and
jacketing.
The solar cable according to DKE is a two-
layer insulated single core cable with
a minimum wall thickness of 0.5mm in
each layer.
The most important tests required in
this specification
[2]
are discussed in the
following clauses:
4.1 Temperature range
Solar cables according to the German
specification
[2]
are defined for an environ-
mental temperature range from -40°C
up to +90°C. The maximum conductor
temperature is specified at +120°C.