Large solar PV plants need lightning-protection systems
The fact that such PV plants are relatively new in South Africa means there is not a large body of local data available in this regard, points out Stephen Reynders, Function Manager, Power & Energy, SMEC South Africa.
“The design of the LPS requires an engineer with some specialist knowledge and experience in Electromagnetic Compatibility (EMC). Although the risk-evaluation matrix of SANS/IEC 62305 is a logical process, it can have dire results if a practitioner produces faulty output based on poor-quality input,” Reynders warns.
Solar PV plants are quite large compared to other outdoor electrical installations. In addition, they have complex AC and DC 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 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 SMEC South Africa comes 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 EPC 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.”
There are three key design elements to be taken into account for LPS design for large solar PV plants:
External Lightning Protection
The design must ensure that the lightning strike is intercepted with an air-termination system. The lightning current must be 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.
Earthing System
The 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.
Internal Lightning Protection
The design must ensure that the direct lightning current and the effects of the Lightning Electromagnetic Pulse (LEMP) do not penetrate sensitive electronic circuitry and cause damage and/or malfunction. To this end:
- Electrostatic shielding of all signal/control cabling with proper earthing has to be assessed;
- Magnetic shielding of internal electronic components has to be assessed;
- Zone boundaries, which are areas where there is an increase in the sensitivity of the internal components to damage/disruption, have to be defined. In addition, the need for a coordinated Surge Protection Methodology (SPM) has to be assessed.
All electronic components should be enclosed in metallic enclosures to ensure that all sensitive control circuits are shielded. Control cables should have continuous earth shields that are earthed accordingly. 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.
Ends
Notes to the editor
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About SMEC South Africa
SMEC is a global consulting engineering firm providing high-quality, professional services on major infrastructure projects. SMEC has joined forces with Surbana Jurong, Asia’s consultancy powerhouse in the urban, infrastructure and management advisory sectors. This partnership provides a talent pool of 10 000 dedicated people working across a network of 100 offices in 40 countries throughout Asia, Australasia, the Middle East, Africa and the Americas. SMEC’s strength in major infrastructure projects, coupled with Surbana Jurong’s expertise in urban planning, industrial development and management advisory services, enables us to provide critical value-chain services to clients around the world.
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