Highly excessive voltages and currents can threaten
the operation of a PV plant. Such surges are mainly caused by lightning
strikes, but also by faults in the grid.
In principal, a PV plant does not generally
increase the risk of a building being struck by lightning. A separate lightning
protection system does not necessarily need to be constructed simply because a
PV plant has been installed. Nevertheless, VdS (the German institute for fire
protection and security) recommends installing a lightning and overvoltage
protection system for all plants with a capacity of ten kilowatts or more. In a
given case, the risks should be assessed in order to enable a decision in favor
of or against the construction of a lightning and overvoltage protection
system. If the building on which the PV plant is constructed is already
equipped with a lightning protection system (e.g. a public building), the PV
plant must be integrated into the protection concept.
The standard DIN EN 62305 (VDE
0185-305):2006-10 provides a comprehensive approach to internal and external
lightning protection for buildings and systems. In particular, the
supplementary sheets to this European standard offer practical support when
deciding whether or not to install a lightning protection system, as well as
details on how to install such systems properly. Photovoltaic installations are
primarily discussed in Supplement 5 "Lightning and surge protection for PV
power supply systems".
External lightning protection includes
all measures for arresting lightning and conducting it to ground, and consists
of a lightning current arrester, a down lead capable of carrying lightning and
a grounding system which distributes the lightning current in the earth.
Priority must be given to preventing
the lightning from directly hitting the modules. This is first and foremost
necessary when the PV generator has been installed in an exposed area (elevated
on a flat roof, for example). Rods or wires are used as lightning current
arresters, and the core shadow of these should not be cast on the modules as
far as this is possible. Somewhat smaller air terminal rods are, therefore,
placed in front of the solar modules and somewhat larger ones are placed behind
the modules. The exact number and spacing of the air terminal rods is given by
the class of protection desired and is calculated using methods such as the
"rolling sphere method".
Indirect effects
The probability of indirect lightning
effects occurring is significantly higher than that of a direct lightning
strike. This is because every lightning strike within a one kilometer radius
can generate current flow in the modules, module cables and in the main DC
cable by means of induction. Conductive and capacitive coupling are also
possible and can equally cause overvoltage.
An integrated lightning protection
system comprising measures and equipment within the PV plant and in the
building is, therefore, required. Its fundamental purpose is to prevent
inductive coupling and provide a path to earth for currents caused by
overvoltage.
In order to keep coupling in the
module cables to a minimum, the area of the open conductor loops in the
generator circuit must be as small as possible. The outgoing and return lines
of the strings are, therefore, laid as close as possible to each other. The use
of shielded single lines also reduces the risk of lightning effects.
Surge protection devices (SPD) not
only prevent inductive coupling but also the occurrence of grid-side
overvoltage, and are normally built into the generator junction box. Because
varistors used as voltage dependent resistors can age due to leakage currents,
the combination of two varistors and a spark discharger in Y connection is considered
the safest long term protection against overvoltage.
Reverse current and electric arcs
Increased currents can also occur if
there is a voltage drop in a string, caused for example by shading or a short
circuit. If this happens, the parallel-connected strings will function like an
external power source which drives a fault current in the direction of
consumption (reverse current) through the modules of the defective string. If
the reverse current resistance of the modules is exceeded they will start to
heat up, so string diodes are used to prevent such reverse currents. Many PV
plants today are, however, built without string diodes, as most modules now
have higher reverse current resistance and will easily withstand reverse
current of 10 to 20 A.
Since direct current and DC voltage
are generated in a PV plant, there is a danger that non-self-extinguishing arcs
could be created, which could cause fire. This danger is not present in an
alternating current circuit because the regular zero crossing of the
alternating current’s sine curve immediately extinguishes any electric arc
created. The electrical connections in the DC circuit of a PV plant must,
therefore, be extremely secure, because a loose connection can lead to sparking
and, consequently, trigger an electric arc. As a result, when laying the DC
cables of a PV plant it is standard to protect them from short circuit and
ground leakages. This is achieved by tidy cable routing (e.g. not running
unprotected over sharp edges) and the use of separate positive and negative
cables, as well as double cable insulation. The DC cables used should be tested
to "PV1-F" standards and marked accordingly.
String fuses in the GJB can also
generally prevent the cables from becoming overloaded in the event of faults.
These are intended to reduce the risk of electric arcs.
