Although the global solar industry is growing rapidly, photovoltaic (PV) plants are still relatively new in South Africa and require professional design expertise to ensure best practice engineering, says consulting engineering firm SMEC South Africa power and energy function manager Stephen Reynders.
He highlights that POWER-GEN & DistribuTECH Africa provides an ideal platform to inform the electricity industry about the latest trends and developments in the power sector, with the standards affecting lightning-protection systems (LPS) for utility-scale PV plants one of the issues under discussion.
“The design of the LPS requires an engineer with specialist knowledge and experience in electromagnetic compatibility. Although the risk-evaluation matrix defined by the South African National Standards and the International Electrotechnical Commission (SANS/IEC 62305) is a logical process, it can have dire results if a practitioner produces faulty output based on poor-quality input.”
Reynders points out that solar PV plants are large, compared with other outdoor electrical installations and, in addition, have complex alternating current and direct current electronic systems and cabling for power, control and monitoring, especially where the panels are required to track the sun.
“Designing a system to ensure minimal damage during a lightning strike, while balancing cost-benefit considerations, is, thus, a significant challenge,” Reynders highlights. SANS/IEC 62305, in particular, allows for cost-benefit trade-offs between the costs associated with loss of equipment, production losses and repairs as a result of lightning strikes, versus the costs of further improvements to the LPS.
However, such a cost-benefit approach needs to be calculated thoroughly, which is where the expertise of professional firms, such as SMEC South Africa, come into play. “The fact that lost production costs have a major impact on such trade-offs for large solar PV plants means that all relevant factors have to be analysed.”
Reynders argues that the end result is to embody the decisions taken and trade-offs approved in the eventual energy performance certificate contract and final detailed design. “Thus, there must be no doubt about the scope of work and the reasons driving the cost decisions taken, versus best-practice design and the resultant risks undertaken by the employer and the contractor respectively.”
Key Design Considerations
Reynders explains that there are three design elements to be taken into account in LPS design for large solar PV plants: external lightning protection, the earthing system and internal lightning protection.
The external design of the plant must ensure that the lightning strike is intercepted with an air-termination system, conducted safely towards the earth by means of a down conductor system and dispersed by means of an earth termination system. Air termination masts can be incorporated into the support structure of the panels, provided there are significant clearances between the lightning current path and sensitive components. In the absence of such clearances, damage to panels and electronic components in the event of a lightning strike is a real possibility.
Where the PV panel support structure includes a steel pipe, the down conductor should never be run inside such a pipe, since the magnetically induced opposing currents from the magnetic field in the pipe during a high-current strike will, in fact, render this down conductor totally ineffective.
The earthing system design must ensure a low impedance path for conducting lightning current into the earth, provide equipotential bonding between the down conductors, and ensure that lightning and surge currents are dissipated effectively, without causing excessive potential differences.
Most solar plants do have extensive cable trench routes that can be used to bury earth mat conductors. However, it is critical that the earth conductors are interconnected into a grid structure, since the cable-route network normally follows a tree-type structure. This is because every structure requires multiple paths for current or surge dissipation.
The internal design of the plant must further ensure that the direct lightning current and the effects of the lightning electromagnetic pulse do not penetrate sensitive electronic circuitry and cause damage or malfunction. To this end, electrostatic shielding of all signal and control cabling with proper earthing and magnetic shielding of internal electronic components has to be assessed.
In addition, zone boundaries, which are areas where there is an increase in the sensitivity of the internal components to damage or disruption, have to be defined and the need for a coordinated surge protection methodology must be assessed.
All electronic components should also be enclosed in metallic enclosures to shield sensitive control circuits. Control cables should have continuous earth shields that are earthed accordingly and appropriate surge suppression devices should be installed where cables enter sensitive zones.
SMEC South Africa provides a comprehensive suite of consulting services, including detailed specialist designs, assessment of claims, dispute resolution, expert determination and technical audit services.