Choice of Surge Protection Devices for Photovoltaic Applications
To achieve the complete functionality of a photovoltaic (PV) power plant, whether small, installed on the roof of a family house or large, extending over vast areas, it is necessary to develop a complex project. The project includes correct selection of PV panels and other aspects such as mechanical structure, optimum wiring system (suitable location of components, correct oversizing of the cabling, protective interconnection or network protection) as well as external and internal protection against lightning and overvoltage. The LSP Company offers surge protection devices (SPD), which can protect your investment at a fraction of the total purchasing costs. Before projecting the surge protection devices, it is necessary to become familiar with the particular photovoltaic panels and their connection. This information provides basic data for selection of the SPD. It concerns the maximum open-circuit voltage of the PV panel or the string (a chain of panels connected in a series). The connection of PV panels in a series increases the total DC voltage, which is then converted into AC voltage in inverters. Larger applications can standardly reach 1000 V DC. Open-circuit voltage of the PV panel is determined by the intensity of sun rays falling on the panel cells and the temperature. It rises with growing radiation, but it drops with rising temperature.
Another important factor involves the application of an external lightning protection system – a lightning rod. The standard CSN EN 62305 ed.2 on Protection against lightning, Part 1 to 4 defines types of losses, hazards, lightning protection systems, lightning protection levels and the adequate arcing distance. These four lightning protection levels (I to IV) determine parameters of the lightning strikes and the determination is given by the level of hazard.
In principle, there are two situations. In the first case, protection of an object by an external lightning protection system is demanded, but the arcing distance (i.e. the distance between the air-termination network and the PV system) cannot be maintained. Under these conditions, it is necessary to ensure the galvanic connection between the air-termination network and the support structure of the PV panels or the PV panel frames. The lightning currents Iimp (impulse current with the parameter of 10/350 μs) are able to enter the DC circuits; thus it is necessary to install a type 1 surge protection device. LSP offers a more suitable solution in the form of combined 1+2 type surge protection devices FLP7-PV series, which are produced for the voltage of 600 V, 800 V and 1000 V with or without remote signalization. In the second case, there is no demand to equip the protected object by an external lightning protection system, or the arcing distance can be maintained. In this situation, the lightning currents cannot enter the DC circuit and only induced overvoltage is considered (impulse current with the parameter of 8/20 μs), where a type 2 surge protection device is sufficient, e.g. SLP40-PV series, which is produced for the voltage of 600 V, 800 V, and 1000 V, again with or without remote signalization.
When projecting the surge protection devices, we must consider the AC side as well as the data and communication lines, which are standardly used in a modern PV power station. A PV power station is also threatened from the side of the DC (distribution) network. On this side, the choice of a suitable SPD is much broader and depends on the given application. As a universal surge protector, we recommend a modern FLP25GR series device, which incorporates all three 1+2+3 types within five meters from the installation point. It features a combination of varistors and a lightning arrester. LSP offers several series of surge protection devices for measuring and regulation systems as well as data transfer lines. New types of inverters are usually equipped with interfaces that allow monitoring of the entire systems. The products include various types of interfaces and various voltages for various frequencies and a selectable amount of pairs. As an example, we can recommend the DIN Rail mounted SPDs FLD2 series or PoE surge protector ND CAT-6A/EA.
Consider the following examples of three basic applications: a small PV power station on the roof of a family house, a mid-size station on the roof of an administrative or industrial building and a large solar park extending over a large plot.