Electricity and Control December 2023

ENERGY MANAGEMENT + THE INDUSTRIAL ENVIRONMENT

Lightning protection level

Minimum value (kA)

Maximum value (kA)

Probability of current being < maximum

Probability of current being > minimum

I

3 5

200 150 100 100

99% 98% 95% 95%

99% 97% 91% 84%

II

III IV

10 16

Table 1: Lightning protection levels. meshed earthing system, it could be possible that the peak value of lightning current would overstress the SPDs and thus lead to damage of equipment such as inverters, string combiner boxes, tracker motor controllers, and suchlike. There are practical ways to achieve current distribution without having grids installed in the soil and these should be addressed early during the design stage. They could include using above-ground structures as natural compo nents, or adding other current distribution paths that safely carry lightning currents. Figure 3 shows the peak lightning current values through a string combiner box for different earthing mesh sizes. Loops in dc cables It is commonly seen that the installation of dc string cables can lead to creating large loops. In such configurations, cables are unintentionally routed apart from each other un der the PV modules (solar panels) where stringing includes the series and parallel connections of panels back to string combiner boxes or string inverters. The steepness of the lightning current impulse is responsible for possible induced voltages on string cables. In Figure 4 it can be seen that the induced voltage is a factor of the steepness of the lightning current impulse as well as the size of the loop where voltages could be induced. The induced voltage is calculated by: U = M x di / dt Where M is the mutual inductance of the loop. Figure 5 indicates the possible scenario on a free-field PV plant. If a lightning protection system is installed on a PV tracker structure, it is likely that lightning current will flow on the torque tube/ structure before reaching an earthing

Figure 3: Lightning current in SCB for different earthing mesh sizes.

Figure 4: Induced voltage in loop due to current carrying component.

connection. It is thus possible that large amounts of current flowing on the torque tube, with a high steepness of light ning current impulse , could create a large induced voltage in PV dc loops in the surrounding area, if care is not taken to minimise the loop size (a). Large induced voltages would result in high voltage stress on SPDs, PV modules, and oth er electronic equipment and possibly also cause flashover into the dc string cables. Incorrect dimensioning of lightning protection systems As indicated in Table 1, lightning protection is defined in

Figure 2: Meshed earthing system for current distribution in PV installation.

DECEMBER 2023 Electricity + Control

11