REI Mar-Apr 2012 - Renewable Energy Installer

Knowledge: Solar PV
Big is beautiful
Cumbria’s largest commercial solar PV system at Lakeland Livestock Mart,
Cockermouth, benefits from new technology
S
undog Energy says its latest
commercial installation at
Mitchell’s Lakeland Livestock
Mart, Cockermouth, is not only
the largest commercial system in Cumbria but
is also the first solar PV installation in Europe
to benefit from an innovative mounting
system called Trinamount. The system, from
solar panel manufacturer Trina Solar, has
been designed to significantly speed up the
installation time of Trina’s solar PV panels by
reducing the number of parts and eliminating
the need for traditional mounting rails.
The 50kWp retro-fit installation comprises
212 Trina Solar high efficiency PV modules
mounted on a pitched steel roof. The system
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Operational current
With the system switched on and in normal
operating mode, the current from each PV
string should be measured and compared
with the expected values. The operating
circuit current can be measured using a
suitably rated DC current clamp. For systems
with multiple identical strings, the string
operating currents should be compared. For
stable irradiance conditions the values should
typically be within 5 per cent.
Insulation resistance
One of the most important tests required
by BS EN 62446 and the DTI Guide is
the insulation resistance measurement.
Inadequate insulation not only increases the
risk of electric shock and/or fire but reduces
the performance of the system, as a portion of
the DC power generated by the PV modules is
lost through leakage currents to earth.
The insulation resistance measurement is
made between the live DC conductors of a PV
array and earth. The test should be performed
either between the array positive and earth
followed by the array negative and earth
or alternatively, the test can be performed
between the array positive and negative
shorted together and earth. Where the array
66 | www.renewableenergyinstaller.co.uk
will provide enough renewable energy to light
and power Mitchell’s business during the day
with surplus power being exported and sold
to the national grid.
According to Martin Cotterell, founder
and technical director of Sundog, and who
led the installation team on site from start to
finish: “We had a race against time in very
testing weather conditions to complete this
landmark installation before the December
12, deadline for the Feed-in Tariff, so we were
really able to put the new mounting system
through its paces and appreciate the very
significant benefits it brings.”
Bill Roberts, managing director of Sundog
commented: “The recently announced
frame is bonded to earth, the earth connection
can be to any suitable earth connection or
the array frame. Where the array frame is
not bonded to earth, two tests are required
between the PV DC conductors and earth and
between the PV DC conductors and the frame.
If the array has no accessible conductive
parts, the test is performed between the
DC conductors and the building earth. The
measured insulation resistance should be
recorded on a PV array test report.
A short circuit can be applied in the
same manner as that used to perform the
short circuit current measurement, taking
the same precautions when applying or
removing the short circuit. The ideal solution
is a test instrument with a dedicated PV
insulation resistance test. Such instruments
are connected directly to the PV string cables
using dedicated test leads and apply an
internal short circuit between the positive and
negative DC conductors whilst at the same
time applying the appropriate test voltage
to measure the insulation resistance. This
reduces the risk of exposure to hazardous live
parts or damage to the PV modules.
The test instrument used should
have a range of test voltages and should
automatically discharge any voltage when
the test is terminated. In addition, the test
instrument should meet the requirements of
reduction in solar subsidies means that we
are adopting innovative ways to reduce
installation times and therefore costs for our
customers and the installers we supply. We
believe that this new system will be quickly
taken up by installers around the UK.”
Hot news: Sundog Energy says this PV
installation is the largest in Cumbria
BS EN 61557-2. The open circuit voltage, and
to a greater extent the short circuit current,
vary with the intensity of sunlight striking
a PV module. If the irradiance is not stable
during open circuit voltage or short circuit
current measurements a measurement of the
irradiance can be used to adjust the measured
values.
The intensity of sunlight, known as solar
irradiance, is measured in watts per square
metre (wm-2) and should be recorded on the
PV array test report.
The most accurate and representative
readings are produced from irradiance meters
which utilise sensors which are similar to
the technology utilised in the PV modules
themselves. The ideal solution is an irradiance
meter where the sensor is a photovoltaic
cell rather than a photo-diode which will
not necessarily have the same characteristic
response to sunlight as a PV module itself. For
accurate measurements of in-plane irradiance,
the irradiance meter must be at the same
angle to the sun as the PV modules.
With such a wide range of test
instruments available, the best advice for the
solar PV installer is to ensure the appropriate
instruments are selected to satisfy the
mandatory requirements and then extend the
range as necessary with those test products
which provide more diagnostic information.