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the Free Field PV System utilises dc currents which do not disconnect
under fault conditions. The cost of repair as well as the economic loss
caused by downtime must be taken into account. There are also vari-
ous other factors such as the premature ageing of the PV components
and the risk of not meeting the PV plant`s contractual requirements
on the amount of power being generated.
The risk of damage caused by lightning must be determined in
accordance with the SANS/ IEC 62305 part 2 [1] where the results of
the risk analysis must be considered at the design stage. In addition
in SANS/ IEC 62305 Part 3 [1] a minimum lightning protection level III
is specified for all PV systems greater than 10 kW.
Based upon the standards it must be considered as ‘Good Prac-
tice’ to employ the proper earthing, lightning and surge protection
systems.
The installation of non-compliant / sub-standard lightning protec-
tion systems to save on the initial PV farm build costs will result in
damage and downtime of the PV system and in the medium to long
term be far more costly.
Lightning protection measures
To ensure effective protection, the Lightning Protection Systemmust
have the following optimally coordinated elements:
• Air termination and down conductor system
• Earth termination system
• Lightning equipotential bonding
• Surge protection system to power supply and data systems
These elements form the complete lightning protection system, the
parameters of each element are derived from the selected lightning
protection level which is obtained from the lightning risk assessment
process.
Figure 4: Rolling Sphere versus Protective Angle of air terminals.
Air Termination and Down Conductor System
A properly designed air termination system will prevent lightning
striking the electrical systems of the PV plant directly. All electrical
systems including the PV panels must be located within the protection
zone of the air termination system. The air termination system must
be designed in accordance with the parameters of SANS 62305 Part
3 [1] and as a minimum the air termination design should be based
upon a lightning protection level III system.
ELECTRICAL PROTECTION + SAFETY
As shown in
Figure 1
, generally the rolling sphere and angle of protec-
tionmethods are employed for an air termination system that protects
a PV array. The air termination system forms a critical part of the
external lightning protection system. In the case of an uncontrolled
lightning strike to the PV system, lightning currents will be induced
into the electrical installation and cause severe damage to the system.
Figure 5: Earth Termination System as per IEC / SANS 62305-3.
Shadowing
When designing the air termination system, care should be taken
that no solar panels are shaded by the air termination masts or rods.
Diffuse shadows caused by distant masts or rods, do not negatively
affect the PV system or their yield. Core shadows, however, cause
stress to the PV cells and this leads to bypass diodes. The required
distances from the air terminals must be calculated, for example, the
calculated minimum distance from a PV module of a 10 mm diameter
air terminal is 1,08 m.
Umbra Effect
The region of the PVmodule which is completely obscured or shaded
is called the Umbra and should be prevented in all cases. The other
region called the Penumbra or diffuse shadow, is partly obscured
since the air terminal or conductor partly covers the sun. Depending
on the dimensions of the air terminal or conductor, the minimum
distance required to prevent an umbra can be calculated.
Figure 6: Umbra Effect.
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August ‘16
Electricity+Control