Photovoltaic installations: the risk of lightning striking is not inevitable
Multiplication of photovoltaic installations in stormy weather regions, will provoke more and more damages from lightning. Damages can be estimated at several hundred euros; it is therefore crucial to apply security rules and study the risks before the development of a project.
The development of photovoltaic energy
For a few years, given the strong decrease of regulated pricing of photovoltaic energy, stakeholders of the branch oriented themselves towards projects of big dimensions installations in order to save considerable money and to meet the criteria of public procurements. Great ground Photovoltaic Installations, whose nominal power reaches several hundreds of MWc (Méga Watts peak), were created and continue to be produced and installed, in particular in the sunny southern areas.
In the South, storms are frequent and often violent. Average lightning strike density in South of France, in Corsica and in Italy, for example, exceeds 4 lightning strikes per km² per year. This phenomenon was stronger during the summer.
By the way, photovoltaic installations are often built in peaks, to take advantage of better sunning and everybody knows that lightning strikes culminating points in priority! Finally, in uneven and rocky terrain of most of these regions, the ground is particularly resistive, so the risk of overvoltage rises and potential differences increase between two close points.
What are the effects of lightning?
A lightning strike, or cloud-to-ground lightning, is the electrical shock that happens between the cumulonimbus and the ground. This shock generates a very strong electric power, of several tens of thousands ampere. When a lightning reaches the ground, it can not only directly provoke damages by heating effect but can be at the origin of phenomena that may be devastating. Around the impact point, an electrical field develops in the ground. A few tens of meters from the point of impact, this field can create a potential difference of several thousand volts between two points at one meter distance. The lightning is also accompanied by an electromagnetic radiation, that can provoke strong power in the conductors.
The lightning is therefore at the origin of several electrical and electromagnetic phenomena, that can create different kinds of damages.
How to hedge against a risk?
First of all, we need to remind that there is no system allowing to totally master the risk related to lightning. However, there are solutions to considerably reduce this risk. The best action consists in conducting a specific study in each installation, based on the CEI EN 62305 standard.
- Grant a good equipotential bonding
The terrestrial network must be designed to ensure a perfect equipotential between metallic masses (supporting structures, photovoltaic modules framework, …), in order to avoid the presence of potential difference between several pieces of equipment of the installation.
If it is discontinued, it will have a reverse effect than the expected: it will allow a potential rise since the ground until the equipment, that will discharge in the networks of the plant.
It is therefore crucial to endure a good equipotential bonding, through suitable connecting and wiring systems, in accordance to the standard C15-100 and the specific application guide to photovoltaic UTE C15-712-1.
- Avoid communication networks of type bus (ethernet, profibus, …) in the field.
Most plants are equipped with a security system (alarm, peripheral protection, surveillance cameras). Some have secondary boxes (direct current junction boxes) equipped with a monitoring system allowing to check the production of each string (series) of photovoltaic modules. Tracker plants (optimization systems of the production through follow up of the course of the sun) are equipped with an additional network for the command and the fueling of electrical engines.
Bus type networks (low tension communication) are very frequently stricken by lightning, generating damages on an equipment that has the feature of being sensitive to overloads and expensive (electronic cards, measurement systems, cameras, automatons…).
We need then to avoid to use this protocol of communication and prefer the optical fiber or Wi-Fi networks.
- Install lightning arresters DC, AC, com
Lightning arresters, or overvoltage arresters, is an equipment allowing to deviate energy of an overload to the ground. They allow to efficiently protect most equipment, be they connected to the power network, on the command network or on the communication networks.
The choice of lightning arresters and their installation must however strictly observe the rules, clarified by the guide UTE C15-443.
- Avoid induction loops
The tension originated by the electromagnetic field of the lightning depends mostly from three parameters: surface comprised between conductors, variation and intensity of the magnetic field.
For example, at 10 meters from the point of impact of a lightning, the tension can rise to several tens of thousands of volts in the cables creating a loop of 20 to 30 m².
Modules, terrestrial and communication networks wiring must therefore be designed to reduce at most the presence of loops: cables must advance together (see the guide UTE C15-712-1).
This prevention measure can have a high cost (considerable length of cables), and that can push entrepreneurs to turn a blind eye on this precaution, that is vital to ensure a good protection of photovoltaic modules.
However, some constructors were able to prove their inventiveness and find satisfactory technic-economic agreements, by just placing modules in a way to respect this rule while reducing the length of cables.
To sum up…
In the summer, photovoltaic plants on the ground are frequently striken by lightning, and most of all their effects. Damages provoked by a single lightning can amount to hundreds of thousands of euros when overloads propagate in the modules, UPS and security equipment.
Basic rules must be observed from the design of the plant in order to reduce the risks at most. We can only advise developers to realize a specific survey that will allow to adapt the means of prevention while reducing their implementation costs.