Electricity + Control August 2016

ELECTRICAL PROTECTION + SAFETY

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.

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 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.

Figure 6: Umbra Effect.

August ‘16 Electricity+Control

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