Overvoltage protection equipment
Surge protection devices (SPD) are divided into 3 classes:
Broad protection (SPD type I): SPD type I have the highest value for admissible surge current resistance since they have been designed to handle a direct lightning strike. They are deployed where lightning currents or lightning partial currents are not only conducted via the external lightning protection system but can also be conducted via electrical cables. This can be the case if the system to be protected is directly connected to the external lightning protection system or if, for example, the distance between the DC lines and the external lightning protection is too small. The level of the lightning partial currents is determined by the current distribution with the number of conductors in the lightning protection system and the number of cables. In accordance with this current value and the lightning protection class, you can select the surge protection device. Whereas the costs for SPD type I for alternating current are relatively low, the costs for DC surge protection devices which can carry lightning current can quickly reach dimensions which render a PV system uneconomic. In many cases, adapting the lightning protection system to increase the separation distance is the most economical solution.
Medium protection (SPD type II): These surge protection devices have a lower value for admissible surge current resistance and protect against the indirect effects of lightning. In the case of local lightning strikes, e.g. to the external lightning protection system, electromagnetic fields arise which can couple dangerous high voltages into electric circuits. However, the peak values from the currents resulting from the overvoltage are much lower than the lightning current in each case. Also the duration of the impulse and therefore the coupled energy is lower. SPD type II are used to provide protection against this type of overvoltage.
Fine protection (SPD type III): SPD type III have the lowest value for admissible surge current resistance. They protect sensitive electronic end devices from impact by lightning strikes that occur further away. SMA inverters are designed in such a way that SPD type III are not necessary.
If you wish to protect an SMA inverter against impacting overvoltages, an SPD type II is sufficient. If lightning partial currents are expected, an SPD type I with connected SPD type II should be used.
Information on the topic of lightning and overvoltage protection in general as well as specifics when combining overvoltage protection devices with SMA inverters can be found under the following link: Technical Information "Overvoltage Protection - Lightning Protection and Overvoltage Protection for Sunny Boy and Sunny Tripower" and at www.SMA-Solar.com.