Photovoltaic (PV) systems are becoming ever more important and are now part of the standard repertoire of many electricians. However, PV installation requires some rethinking in order to ensure maximum system availability. CEE Norm’s Graham Fox explains
The requirements for the interfaces between the individual components of photovoltaic (PV) systems differ distinctly from the requirements for classical power distribution. A PV system consists of various components, notably PV modules and inverters, which must be electrically interconnected to the interfaces.
To start with, several PV modules are interconnected to form PV strings. Several such strings then form a DC power supply unit, a so-called PV generator. Each PV generator supplies DC voltages of up to 1,000V. The PV generators are connected via special junction boxes to the inverters, which convert the direct current into alternating current. Several inverters are then interconnected via inverter collectors and from there linked to the grid or the mains supply to the consumer.
Since the PV generator junction box installations are predominantly outdoors, the devices are exposed to various weather effects. In order to ensure long lasting, high operating safety, they must be able to handle climatic conditions such as rain, heat, frost and UV radiation. At the same time they must be resistant to mechanical effects. Impact strength, protection against accidental contact and foreign bodies, corrosion and fire resistance make high demands on material and design.
Similar demands are made of the distribution boards for power supply and measuring. Although they are usually located inside buildings and are thus protected, safety, EMC compliance and maintainability are key issues that must be met to very high standards.
PV generator junction boxes
PV components on the direct current side need to be seen as placed under voltage, even if the system is separated from the alternating current side. Protection to Class II or equivalent specification is usually considered the best option, particularly when the DC networks are not earthed.
In such a case, empty boxes which are tested in accordance with IEC 60 439 and fulfill the requirements of protection Class II must be used. Cable junction boxes in accordance with IEC 60 670-22 may not be used, since this standard does not meet Class II requirements.
With this in mind, Hensel offers PV generator junction boxes conforming to the standards for this area. The housings made from impact and UV resistant themoplastic are ready for connection and are already pre-wired and equipped with over voltage protection and are thus ready to use. Plug-in connections for the PV strings and the inverter also save installation time.
On the AC side, inverter collectors provide the connection between inverters and the power distribution. For this area, high flexibility is required in the installation, since the requirements differ from installation to installation. Therefore, Hensel offers the ENYSUN inverter collectors as modular housing sets, which can be individually customised for the respective environment.
Unlike the distributors in buildings, for which the simultaneity factor ranges from 0.3 to 0.6, it is 1 in photovoltaic systems. This must be taken into consideration when dimensioning the distributors. In case of wrong dimensioning, the dissipated power can lead to the equipment exceeding its maximum temperature, meaning that the protective equipment will trip at levels below the rated current.
ENYSUN PV inverter collectors from Hensel are characterised by devices set at a distance, additional ventilation slots and larger housings. The dissipation of heat energy is thereby improved and a safe operation is ensured. Standard assembly aids, for the optimum positioning on the DIN-rail, as well as for protection cover, facilitate the assembly.
Connecting to the network
An interface to the infrastructure must be present to be able to feed the generated power to the power supply network and to bill the same. For this purpose, distribution boards and switchgear assemblies are suitable in various different designs. The dimensioning depends on the one hand on the capacity of the PV system, and on the other hand on the requirements of the electric power supply. Thus, the systems must offer a place for the measuring instruments and the connections of the inverter collectors. In some cases, an isolator is also required, which can be effected manually or automatically by monitoring the network parameters.