SPD for photovoltaic applications

Overvoltage may occur in electrical installations for various reasons. It may be caused by:

• The distribution network as a result of lightning or any work carried out.
• Lightning strikes (nearby or on buildings and PV installations, or on lightning conductors).
• Variations in the electrical field due to lightning.

Like all outdoor structures, PV installations are exposed to the risk of lightning which varies from region to region. Preventive and arrest systems and devices should be in place.

Protection by equipotential bonding

The first safeguard to put in place is a medium (conductor) that ensures equipotential bonding between all the conductive parts of a PV installation.

The aim is to bond all grounded conductors and metal parts and so create equal potential at all points in the installed system.

Protection by surge protection devices (SPDs)

SPDs are particularly important to protect sensitive electrical equipments like AC/DC Inverter, monitoring devices and PV modules, but also other sensitive equipments powered by the 230 VAC electrical distribution n twork. The following method of risk assessment is based on the evaluation of the critical length Lcrit and its comparison with L the cumulative length of the d.c. lines.

SPD protection is required if L ≥ L_crit .

L_crit depends on the type of PV installation and is calculated as the following table (Fig.J45) sets out:

Type of installation	Individual residential premises	Terrestrial production plant	Service/Industrial/Agricultural/Buildings
Lcrit (in m)	115/Ng	200/Ng	450/Ng
L ≥ Lcrit	Surge protective device(s) compulsory on DC side
L < Lcrit	Surge protective device(s) not compulsory on DC side

Fig. J45: SPD DC choice

L is the sum of:

• the sum of distances between the inverter(s) and the junction box(es), taking into account that the lengths of cable located in the same conduit are counted only once, and
• the sum of distances between the junction box and the connection points of the photovoltaic modules forming the string, taking into account that the lengths of cable located in the same conduit are counted only once.

Ng is arc lightning density (number of strikes/km²/year).

SPD Protection
Location	PV modules or Array boxes		Inverter DC side	Inverter AC side		Main board
	LDC			LAC		Lightning rod
Criteria	< 10 m	> 10 m		< 10 m	> 10 m	Yes	No
Type of SPD	No need	"SPD 1" Type 2*	"SPD 2" Type 2*	No need	"SPD 3" Type 2*	"SPD 4" Type 1*	"SPD 4" Type 2 if Ng > 2,5 & overhead line

• Type 1 separation distance according to EN 62305 is not observed.

Fig. J46: SPD selection

Installing an SPD

The number and location of SPDs on the DC side depend on the length of the cables between the solar panels and inverter. The SPD should be installed in the vicinity of the inverter if the length is less than 10 metres. If it is greater than 10 metres, a second SPD is necessary and should be located in the box close to the solar panel, the first one is located in the inverter area.

To be efficient, SPD connection cables to the L+ / L- network and between the SPD’s earth terminal block and ground busbar must be as short as possible – less than 2.5 metres (d1+d2<50 cm).

Safe and reliable photovoltaic energy generation

Depending on the distance between the "generator" part and the "conversion" part, it may be necessary to install two surge arresters or more, to ensure protection of each of the two parts.

Fig. J47 : SPD location