Earthing Solar Panels: Grounding of photovoltaic modules for safety and efficiency
The earthing of solar panels is an important aspect for the safety, functionality, and reliability of the photovoltaic system
Photovoltaic systems are usually installed on rooftops or in open fields and, due to this exposed position, are particularly subject to the risk of lightning and overvoltages. The failure of a system due to damage from overvoltage results in a loss of income during the repair period and, on the other hand, the need to incur additional costs, for example, due to the replacement of the inverter or a defective panel.
In the design phase of a photovoltaic system, it is therefore necessary to foresee, where necessary, the protection of the various sections of the system against overcurrents and overvoltages of atmospheric origin.
Earthing, in particular, is an essential component for the safety of photovoltaic systems, both to protect people and electrical equipment, considering the exposure of these systems to variable weather conditions and the presence of significant electric currents. This is a fundamental aspect to consider to ensure the safety and longevity of the system.
To ensure that you include all necessary protection devices, also considering surge protectors in the various sections of the system, use a photovoltaic software that allows you to automatically obtain the single-line diagram of the designed photovoltaic system, on which you will only need to choose the protections to include and where to position them.
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What is an earthing system?
The earthing system, also known as earthing, is an essential safety system for any electrical installation, designed to ensure the protection of the system itself and, above all, the safety of the people who use it.
Its purpose is to protect users from the risk of electric shock by establishing a safe electrical connection between the ground and the parts of the system that, although normally inactive, could become electrically charged in the event of faults or malfunctions.
Essentially, earthing keeps the electric potential of metallic parts at ground level, preventing voltage differences between them or with respect to the ground.
What is meant by earthing solar panels?
In the context of a photovoltaic system, earthing can relate to both the masses (such as the metal structure of the modules) and the electrical production system (for example, the photovoltaic cells). A photovoltaic system can only be earthed if there is galvanic separation from the electrical grid, usually achieved through a transformer.
Even though an isolated photovoltaic system might seem safer for people who could come into contact with live parts, in reality, the insulation resistance to ground of the active components is not infinite.
As a result, a person can be traversed by a current that closes through the insulation resistance. This current increases with the increase in the ground voltage of the system and its size, as a larger extent reduces the insulation resistance to ground.
Moreover, the aging of insulators and the presence of moisture over time further lower this resistance. Therefore, in large systems, a person in contact with a live part could suffer an electric shock. Consequently, the advantage of isolated systems compared to earthed ones is significant only for small systems.
Earthing solar panels: systems with transformer
In systems with a transformer, in addition to analyzing the isolated or earthed photovoltaic system, it is necessary to differentiate the masses upstream and downstream according to the direction of the electric energy produced by the photovoltaic system for protection against indirect contacts.
Upstream masses of the transformer (IT system)
In a system with an IT system, the active parts are isolated from the ground, while the masses are connected to the ground.
The earth resistance (Ra) must satisfy the following relationship (CEI 64-8):
Ra ≤ 120/Id
where Id is the current of the first fault to ground, not known a priori but usually very small in modest-sized systems.
In the case of a double fault to ground, since the photovoltaic generator is a current generator, the voltage assumed by the interconnected masses must be less than:
Icc · Reqp ≤ 120V
where:
- Icc is the short-circuit current of the affected cells;
- Reqp is the resistance of the conductor connecting the affected masses.
Upstream masses of the transformer (TN system)
In this type of system, the active parts and the masses are connected to the same earthing system (earthing system of the user installation). In this way, there is a TN system on the DC side. In the event of a ground fault, a short circuit occurs as in usual TN systems, but this current may not be detected by maximum current devices because the characteristic of photovoltaic systems is to generate fault currents with values not much higher than the nominal current.
Downstream masses of the transformer
Let’s consider a connection system between the grid and the user of the TT type. In this context, the masses of the user installation are protected by a differential switch, located at the beginning of the system, which ensures safety both with respect to the electrical grid and the photovoltaic generator.
It is not possible to place masses between the grid and the photovoltaic generator, as this would violate the regulatory requirement that all masses of a user installation in a TT system must be protected by a differential switch.
For the upstream masses, such as those of the transformer or the inverter integrated into the transformer, it is necessary to insert a differential that can detect any leakage currents from both the grid and the photovoltaic generator.
In the event of a ground fault, the intervention of the differential causes the inverter to go into standby due to the interruption of power from the grid. If, on the other hand, the connection system between the grid and the user is of the TN type, both the power from the grid and that from the photovoltaic generator do not require the use of differential switches, provided that the fault currents on the AC side promptly activate the protection devices against overcurrents, complying with the times set by the regulations.
Earthing solar panels: systems without transformer
In the absence of a separation transformer between the photovoltaic system and the grid, the photovoltaic system itself must be isolated from the ground in its active parts and becomes an extension of the power supply network, which generally has a grounding point (TT or TN system).
On the DC side, a ground fault on the masses causes the intervention of the differential switch placed downstream of the inverter. After the intervention of the differential, the inverter goes into standby due to the lack of grid voltage, and the fault remains powered by the photovoltaic generator.
