Electricity + Control August 2016

ELECTRICAL PROTECTION + SAFETY

Special Surge Protection measures for dc side The typical curve characteristics of PV current sources differ substan- tially from conventional dc current sources. They have a non-linear characteristic and a different dc arc behaviour. These differences does not only affect the size of the dc switches and fuses but it requires surge protection devices that are capable of coping with these unique dc follow currents. Supplement 5 of IEC 62305-3 [1] requires safe operation of surge protection devices on the dc side even in the case of overload. Lightning Current Distribution and Loading of SPDs The lightning current is mostly dissipated into the ground via a properly designed earth termination system (grid type) and only small partial lightning currents dissipate via the PV system`s cables which are equipotentially bonded to the earth termination system via SPDs.

Therefore, in PV ground-mounted systems with a central inverter, the partial lightning currents flow through the dc conductors between the generator junction boxes and the dc input of the central inverter. Type 1 PV-SPDs need to be installed both at the dc-sub-distribution boards and at the dc-input of the central inverter.

This happens for both low resistance and high resistance earth termination systems. It is therefore the shape and dimension of the earth termination that is important, not the earth ter- mination resistance.

Figure 9: Lightning current distribution and loading of SPDs.

Size and Placement of SPDs The most important factor in the selection of the type and placement of the surge protection meas- ures, is the type of PV system. Central Inverter A great portion of lightning current directly enters the earthing; and partial lightning currents also seek a path to the earth via the dc cabling. The as- sumption that the whole surface of the PV system can be considered as ‘equipotential surface’ is not realisable in practice. The energy system cables work as equipoten- tial bonding conductors between the ‘local’ earth termination at the module array where the direct lightning strike occurs and the ‘distant’ equipoten- tial surface of the central inverter.