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Abstract

Despite being a critical component, wiring for solar panels is rarely discussed. Khyati Vyas highlights that cable management is one of the most important aspects of the safety and longevity of nearly every photovoltaic (PV) system. This is primarily due to the extensive use of exposed cables used in the DC PV array. Since the equipment is installed outdoors on rooftops and in open fields, the electrical conductors must be rated for sunlight resistance and be supported and secured properly.
RE Feature
Managing Solar
Cables and Connectors
For Safety and Longevity of PV System
Despite being a critical component, wiring for solar panels is
rarely discussed. Khyati Vyas highlights that cable management is
one of the most important aspects of the safety and longevity of
nearly every photovoltaic (PV) system. This is primarily due to the
extensive use of exposed cables used in the DC PV array. Since the
equipment is installed outdoors on rooftops and in open fields, the
electrical conductors must be rated for sunlight resistance and be
supported and secured properly.
18 | Akshay Urja | February–April 2017
Managing Solar Cables and Connectors: For Safety and Longevity of PV System
A
significant amount of work
goes into the complex
process of designing and
planning a photovoltaic (PV)
power plant, whether on the rooftop
of a building or ground-mounted on
the field. This then translates into an
ecient, working PV power plant in
situ. With the rapid uptake of PV, it is
also common to see homeowners
who design and implement their own
small PV rooftop projects. With so
much already invested, it would be
vexing if careless cable management
after installation led to losses. And
dangling, untidy cables are simply
unaesthetic. Cables are subjected to
thermal, mechanical, and external
loads. Just like the rest of the system,
cables need to last the stipulated
25 years or more. Being exposed to
harsh environmental conditions like
temperature fluctuations and direct
ultraviolet (UV) rays can damage
unprotected cables and in turn the
wires in them that carry the
power generated.
Applications
To connect the components of a
solar energy system, you will need
to use correct wire sizes to ensure
low energy loss and to prevent
overheating and possible damage or
even fire.
There are four components to
connect together: the solar panels,
the charge controller, the batteries,
and the inverter. The charge controller
is used to prevent the batteries
from overloading; the wires that
connect the panel to the charge
controlled should be correctly sized
to minimize transmission power loss.
Correspondingly, the further away
the panels are, the larger the wire
gauge should be. The inverter is used
to convert the DC power collected
by the panels into AC power, which is
the most popular form of electricity
accepted by appliances. These
systems are typically outdoors, so any
cable used for this type of application
needs to be UV radiation resistant and
suitable for wet locations. For solar
tracking panels, the cables used need
to be flexible as the panels will be
moving along with the sun.
Depending upon the system capacity
cabling utility varies as follows:
Small-scale systems with string
inverters: A three-core AC cable is
used for connection to the grid if a
single-phase inverter is used, and
a five-core cable is used for three-
phase feed-in.
Large-scale system wiring with
central inverters.
Larger power collector cables are
used to interconnect from the
generator box referred to as the
Main DC and DC combiner to the
central inverter. These cables must
be shielded when over 50 m in
length (IEC62548).
Significance of DC and
AC Cables
DC cables are used predominantly
in solar projects and hence, issues
around their usage are still not
understood very well unlike AC cables,
which are used extensively across
the power sector. Moreover, intense
commercial pressure is forcing project
developers and contractors to reduce
capital cost resulting in the selection
of inferior products and/or
sub-optimal design.
DC Cable
DC cables connect modules to
inverters and are further segmented
into two types.
String DC cables
These cables are used to interconnect
solar modules and to connect
modules with string combiner boxes
or an array combiner boxes. Cables for
interconnecting modules come pre-
connected with modules, whereas the
cables required to interconnect strings
and to connect with combiner boxes
are procured separately. String DC
cables carry current of only around 10
Ampere (A) and a small cross section
(2.5 mm2 to 10 mm2) is sucient for
this purpose.
Main DC cables
These cables are used to connect
array combiner boxes with inverters.
These cables carry higher current
of around 200–600 A in utility
scale projects and require a larger
cross section (95 mm2 to 400 mm2).
DC cables, except for those pre-
connected with modules, account for
only around 2 per cent of solar project
Februar y–Ap ril 2017 | Akshay Urja | 19
cost, but can have a significant impact
on the power output. Improper design
and/or poor cable selection can lead
to safety hazards, reduced power
output, and other performance issues.
Experts believe that power output
loss in DC cables can be as high as
15 per cent but it is time consuming
and arduous to empirically isolate and
quantify the role of DC cables in poor
performance. Further, a higher voltage
drop typically leads to heating up of
cables and fire accidents. Power loss
in DC cables is measured in terms of
voltage drop from module to inverter.
As current in the cables remains
the same, voltage drop implies
proportionate loss of power.
LT and HT Cables
(AC Cables)
LT and HT cables are AC cables with
a higher voltage rated capacity. These
cables are used to connect inverters
to transformer and transformer to the
on-site substation. At present, cables
of 1,000 V rating are typically used
for this purpose but the trend is now
shifting towards the use of 1,500 V
cables. HT cables are used for power
transmission at high voltage from
on-site substation to transmission grid
substation. Depending on the project
capacity, voltage rating of these cables
can range from 11,000 V to 33,000
V. LT and HT cables are widely used
in the power sector including both
conventional and renewable energy
power generation plants. However,
DC cables are used primarily in
solar projects.
Aluminium is widely used in AC
cables, which have a life of over
35 years and have been in wide
operation throughout the world. In
AC cables, flow of current is usually
continuous, whereby the cable
reaches steady state with minimal
thermal stress. Operation in a solar
plant is discontinuous because of ever
changing irradiation. Figure 1 shows
the type of cables used in a solar
PV plant.
Cable Specifications
Economically generating electricity
from renewable sources requires
a cabling system engineered to
optimize eciency and minimize
line losses. This allows more of the
generated power to reach substations
where it is transmitted to the grid. To
optimize eciency, cables used at
the point of solar power generation
oer a higher voltage range of up to
2,000 V versus the standard 600 V
rating for conventional applications.
Medium-voltage cables used between
transformers and substations are
being re-engineered to provide better
eciency over the life of the cable
through cooler operation and lower
line loss.
Solar cables, which are UV and
weather resistant and can be used
within a large temperature range, are
laid outside. Single-core cables with a
maximum permissible DC voltage of
1.8 kV and a temperature range from
–40°C to +90°C are the norm here.
A metal mesh encasing the cables
improves shielding and overvoltage
protection, and their insulation must
not only be able to withstand thermal
but also mechanical loads.The
Figure 1: Type of cables used in Solar PV Plant
RE Feature
20 | Akshay Urja | February–April 2017
cross-section of the cables should
be proportioned such that losses
incurred in nominal operation do
not exceed 1 per cent. String
cables usually have a cross-section
of 4–6 mm2.
Cables used in solar generation
must be designed to withstand
long-term exposure to sunlight. To
maintain long-term performance
and reliability, solar cables have been
developed to resistant UV, ozone,
and water absorption, as well as
provide excellent flexibility for sub-
zero conditions and deformation
resistance during prolonged exposure
at high temperatures. Given the often-
extreme installation environments for
solar power systems, coupled with
the need to save time and ensure
reliability, pre-connectorized cable
solutions have been developed. Ideal
for utility-scale generation systems,
these solutions enable fast, easy
connections, simplifying installation
while removing the inconsistencies
associated with field termination.
Along those same lines, DC feeder
cables for connecting combiner
boxes to inverters are now oered
as all-in-one metal-clad cables that
increase reliability and eliminate the
need to install conduit. PV cables are
also being engineered in a full array
of colours to easily identify source,
output, and inverter circuits without
the need for time-consuming marking
tape or tagging cables.
Connecting Technology
There was a need to develop
connection technology rapidly over
the last few years, as inadequate
contacting can cause electric arcs.
Secure connections are required
that will conduct current fault-free
for as long as 20 years. The contacts
must also show permanently low
contact resistance. Since many plug
connectors are required in order
to cable a PV plant, every single
connection should cause as little
loss as possible, so that losses do not
accumulate. Given the precious nature
of the solar power acquired from the
PV plant, as little energy as possible
should be lost.
Screw terminals and spring clamp
connectors (e.g. in the module
junction boxes and for connection
to the inverter) are gradually being
replaced by special, shock-proof
plug connectors, which simplify
connection between modules and
string cables.
Crimp connection (crimping) has
proven itself to be a safe alternative for
attaching connectors and bushes to
the cables. It is used both in the work
carried out by fitters on the roof and
in the production of preassembled
cables in the factory.
An alternative plug connector
design has been developed to
allow the connection to be fixed in
place without the need for special
tools: in this instance, the stripped
conductor is fed through the cable
gland in the spring-loaded connector.
Subsequently, the spring leg is pushed
down by thumb until it locks into
place. The locked cable gland thus
secures the connection permanently.
Plug connectors and sockets with
welded cables are also available in the
market. Such connections cannot,
however, be used during installation
work on the roof, but only during
production in the factory.
Another development is preassembled
circular connection systems for
the AC range. These are intended
to reduce the high levels of
installation work required when
several inverters are used within one
plant. Owing to the sharp increase
in copper prices, aluminium has
recently gained significance as an
electrical conductor. It is possible
to save around 50 per cent by using
aluminium cables, particularly for
underground cables at low- and
medium-voltage levels. However,
their poor conductivity means that
they are thicker than copper cables.
Careful attention must be paid to the
default breakaway torque of their
screw connections, as, in comparison
to copper, aluminium tends to creep
under roofs which are very heavy. If
the screw connections are too tight,
the cable loosens over time, possibly
resulting in an electric arc, not to
mention the associated risk of fire and
all the consequential damage.
Standards for Plug
Connectors
Since PV modules generally come
equipped with preassembled plug
connectors, several modules can
Managing Solar Cables and Connectors: For Safety and Longevity of PV System
Februar y–Ap ril 2017 | Akshay Urja | 21
RE Feature
easily be connected to form a string.
Connecting these strings to the
inverter, on the other hand, is not
always straightforward. A variety
of dierent cable connectors are
available in the market, and as yet
no standards have been established
for these interconnection systems.
Plug connectors from dierent
manufacturers are usually either
completely incompatible or they fail to
provide a connection that will remain
permanently snug. If the connector
fits too tightly, this can cause the
insulating plastic parts to break. A
loose fit, on the other hand, poses
the risk of creating high-contact
resistance. This leads to yield losses
and the areas around the connection
heating up, even causing an electric
arc and the connector to melt. When
connecting a plug with a socket
from a dierent manufacturer, a
cross-over connection is created,
which can generally only be proved
to be reliable if complex, expensive
tests are performed. In addition to
measuring the contact resistance and
determining the connection strength,
accelerating aging tests and weather
exposure tests must also be carried
out. Such tests will make it clear
whether or not the dierent materials
are compatible. This concerns both
the metals used to manufacture the
contacts and the plastic materials
employed. There are currently no
cross-over connections which have
been tested in accordance with DIN
EN 50521 VDE 0126- 3:2009-10:
‘Connectors for photovoltaic systems;
safety requirements and tests’ and
approved by both manufacturers
(socket manufacturer A combined
with plug manufacturer B or socket
manufacturer B combined with plug
manufacturer A). A standard for
photovoltaic plug connectors, which
should be as international and uniform
as possible and is similar to that for
domestic Schuko plugs, is desirable
and necessary to ensure reliable
connections between products from
dierent manufacturers. If such a
standard were to be introduced,
manufacturers would be in a position
to oer reciprocal warranties for
specific cross-over connections.
Market Demand
Despite the promising growth
of solar power and related cable
developments aimed at ensuring its
economic viability, this emerging
market is not without challenge.
Codes and standards are struggling
to keep pace with new technologies
and applications, while a relatively
new contractor base is in need of
continuous on-going training to stay
one step ahead of evolving installation
practices. As such, the industry has
seen a variety of cable designs and
practices, many of which may not
necessarily support long-term solar
needs. Application-specific cables
and contractor certification are
paramount to ensuring the economic
viability of solar power systems.
Cable manufacturers are challenged
with balancing up-front costs with
long-term reliability while continually
meeting evolving requirements and
trends, from developing cables for
new micro inverter technology where
DC power is converted to AC at the
panel, to meeting more stringent fire
ratings, test methods, UL and CSA
standards, National Electric Code
requirements, and global standards
for halogen-free, fire-retardant,
and low-corrosive gas emissions.
To meet these advancing trends
and standards, develop application-
specific cables, and ensure the
performance and reliability to
support the long-term needs of solar
applications, manufacturers must
be committed to solar energy with
significant investment in R&D eorts
and a strong presence in the market
through continued participation with
standards bodies, utility regulators,
and renewable developers.Consumer
demand for distributed solar energy
systems is rapidly growing, and small-
to medium-scale solarphotovoltaic
(PV) systemsare turning up in any
location with available space and
abundance of sunlight—from rooftops
and parking lots, to brown fields and
highways. No matter what the size of
the system, all PV applications require
high-quality cabling that provides
excellent mechanical properties
and superior sunlight resistance for
outdoor installations, flame-resistance
for added safety, and flexibility for
easy handling.
Contributed by Er. Khyati V yas, BE , MTech,
Chemtrols Solar Pvt Ltd, Mumbai, India.
22 | Akshay Urja | February–April 2017
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