In recent years, photovoltaic (PV) systems have become a significant sector within the energy market. They increase energy security, enhance sustainability, reduces pollution and keep fossil fuel prices lower than they would be otherwise. However, since these plants are frequently subject to climatic influence, measures to protect the sensitive electronic systems components from failure due to lightning flashes and surges are essential.
Lightning surges in the PV system can damage PV modules and inverters, leading to both high repair costs and lost revenue for the operator. For a complex PV installation, such as a solar power plant, the aim is to protect the operations buildings and the PV array against damage by fire (direct lightning strike), as well as protecting the electrical and electronic systems (inverter, remote diagnostics system, generator main line) against the effects of lightning electromagnetic impulses (LEMP).
Air-termination and down conductor system
For the protection of the PV array against direct strikes, it is necessary to arrange the solar modules within the protection zone of an isolated air-termination system. According to the class of lightning protection, the height and the quality of the air-termination rods required is determined by means of the rolling sphere method. Furthermore, it has to be ensured that the separation distance is kept between the PV supporting frames and the air-termination rods. Also, the operation building must be equipped with external lightning protection. Down conductors must be connected to the earth-termination system by using terminal lugs. Due to the corrosion risk at the point where the terminal lugs come out of the soil or concrete, they have to be made out of corrosion-resistant material or be protected by applying sealing tape or a heat-shrink sleeve.
Earth-termination system
The earth-termination of the PV system is designed as a surface earth electrode, while the earth-termination system of the operation building should be designed as a foundation earth electrode. The metal supporting frames, onto which the PV modules are fixed, must be connected to the earth-termination system approximately every 10 metres. The earth-termination of the PV system, and the one of the operation building, must be connected via at least one conductor. The interconnection of the individual earth-termination systems reduces the total earthing resistance, while the intermeshing creates an equipotential surface that reduces the induced lightning effects on the electric cables between the PV array and operation building.
Surge protection measures in the PV array
In order to reduce the load on the isolation inside the solar modules, thermally monitored surge protective devices are installed in a junction box connected to the PV generator. On the DC side, a surge protection device is installed in each generator junction box to provide local protection for the PV modules and ensure that no spark-over caused by conducted or field-related interferences occurs.
Surge protection measures for IT systems
The operation building house the remote diagnostics used to check the status of the PV systems, permitting operators to recognise and remedy any malfunctions as early as possible. Measurements of wind velocity and module and ambient temperature are performed via external sensors and can be read directly from the data acquisition unit. The unit provides an Ethernet interface to which a PC or modems are connected for remote enquiry and maintenance.
In order to provide reliable trouble-free and continuous transmission of data to the measuring unit, it is necessary to lead the sensor cables entering the building via surge protective devices. When choosing the protective devices, it must be ensured that measurements cannot be impaired. Safety in the forwarding of the measured data via the telecommunication network per ISDN modem must be given as well in order to provide for continuous monitoring and performance optimisation of the installation.
For more information contact Alexis Barwise, Dehn Protection South Africa, +27 (0)74 505 0312, [email protected], www.dehn-africa.com
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