REI Mar-Apr 2012 - Renewable Energy Installer

Test case
Knowledge: Solar PV
Jim Wallace of Seaward Solar unravels the confusion surrounding solar PV
testing and the use of appropriate test equipment
he enormous expansion of the
T
solar PV market has presented
significant opportunities for
installers and suppliers of solar
PV equipment.
For the installer faced with the question
of what test instruments to use, the matter is
therefore one of being able to identify exactly
what is needed to fulfil their obligations to the
customer and also satisfy the various industry
standards and best working practices.
The requirement for all installers of
PV systems in the UK is to satisfy the
Microgeneration Installation Standard: MIS
3002 which requires that PV systems are
installed in accordance with the DTI’s “Guide
to the installation of PV systems, 2nd Edition
2006.”
In addition to the guidance given by the
DTI, a British Standard BS EN 62446:2009
Pass the test: Installers are faced with a range of
issues when testing
Grid connected PV systems – Minimum
requirements for system documentation,
commissioning tests and inspection has
been published, giving details of specific
electrical tests that should be performed all
on PV systems at the time of installation and
commissioning.
Fundamentally, a PV system is part
of the electrical installation and therefore
the electrical works should comply with
the requirements of BS 7671 IEE Wiring
Regulations 17th Edition, along with the
particular requirements for the inspection
and testing of DC circuits specified by BS EN
62446.
Earth continuity
Where protective earthing and/or
equipotential bonding conductors are fitted
on the DC side of the installation, for example
bonding of the array frame, the continuity
should be tested. The connection to the
main earthing terminal should be tested.
An instrument which complies with the
requirements of BS EN 61557-2 should be
used.
Open circuit voltage and polarity
Prior to installation, it is good practise to
measure the open circuit voltage of each PV
module and compare the measured value with
that on the rating plate to ensure that each
module is functioning. It is much easier to
correct problems or replace a faulty module
before it is installed. It should be remembered
that the data shown on the rating plate is
taken under standard test conditions and
values measured in the field may deviate from
those shown on the rating plate. The polarity
of the DC cables should also be checked to
ensure it is correct.
When the modules have been installed,
the open circuit voltage of each string should
be measured and compared with the expected
value. The polarity should also be confirmed.
For systems with multiple identical strings,
the open circuit string voltages should be
compared. For stable irradiance conditions the
values should typically be within 5 per cent.
The measured open circuit voltage for each
string should also be recorded on a PV array
test report.
Voltages less than the expected values
may indicate that one or more modules are
connected with incorrect polarity or insulation
faults. Higher voltages are usually the result of
wiring errors.
Short circuit current
The string short circuit current is measured by
applying a temporary short circuit across the
DC terminals of the PV string and measuring
the resultant DC current. The apparatus used
to apply the short circuit must have a rating
greater than the potential short circuit current
and open circuit voltage.
The temporary short circuit can be
applied by a short circuit cable connected
into a switching device already present in
the string circuit, a suitably rated test box or
a dedicated PV short circuit test instrument.
When the temporary short circuit has been
applied, the short circuit current can be
measured using a suitably rated DC current
clamp or in-line DC ammeter. In the case of
a dedicated PV test instrument, such as the
Seaward Solar PV100, the short circuit load
and in-line DC ammeter are housed within the
test instrument.
For systems with multiple identical
strings, the short circuit string currents should
be compared. For stable irradiance conditions
the values should typically be within 5 per
cent. The measured short circuit current for
each string should be recorded on a PV array
test report.
It should be remembered that applying or
removing a short circuit on a live PV system
is potentially hazardous. Care should be taken
to avoid risk of electric shock or damage
due to electrical arcing. The installer should
therefore give careful consideration as to how
this test is to be performed and ideally use an
instrument which is capable of automatically
applying a short circuit and measuring
accurately the current during the test.
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