Effects of Lightning Strike on Solar PV system:
A solar PV installation is vulnerable
to the effects of lightning stroke. The potential impact of lightning stroke increases
with the PV system size i.e. the exposed surface area. PV plants in areas with
frequent lightning strikes can suffer repeated and significant damage to vital
components, resulting in substantial downtime, repair cost and loss of revenue
due non-generation. Although some reports say that the installation of Solar PV
modules does not increase the risk of a lightning strike. Therefore, the need
for lightning protection measures cannot be derived directly because of the
mere existence of Solar PV system.
A lightning protection system with
proper air terminals, down conductors, and equi-potential grounding protects
the Solar PV system against direct lightning strokes. A properly designed and
installed Surge Protection Device (SPD) further minimizes the potential impacts
of lightning stroke. It is suggested to hire a professional Electrical Engineer
to carry out the lightning risk assessment study at any Solar PV plant of
significant capacity.
Importance of Surge Protection Device:
A properly designed Lightning Arrestor
safely bye-pass a direct lightning stroke through the associated down
conductors to earth, thus protects the solar PV modules, structures, inverters
and other equipments connected to the circuit. Surge Protection Devices are
used in a Solar PV system or any other electrical system to provide a discharge
path to earth to save those components from high voltage transients caused by
direct or indirect lightning strokes or power system abnormalies. External lightning
protection system alone will not be sufficient.
Transients caused by direct or
indirect lightning strokes or utility switching operations expose the
electrical and electronic equipments to very high voltages of very short
duration (tens to hundreds of microseconds), causing their failure. Continued
exposure to transients of lower magnitude deteriorates the dielectric and
insulating materials of these important equipments and eventually leading to
breakdown.
Therefore, in conjunction with a appropriate
Lightning protection system, the use of Surge Protection Device at key
locations protects vital components such as PV modules, inverters, measuring,
controlling and communication equipments.
Technical characteristics of SPD:
The commonly used Surge Protection
Device in solar PV installations are Metal Oxide Varistors (MOV), which
functions as a voltage clamping device. A Surge Protection Device must be able
to quickly change its state (non-conducting to highly conducting) in presence
of transients and to discharge the dangerous voltage and current to earth
without fail. The voltage drop across the Surge
Protection Device circuit should be minimum to protect the equipment it is connected to i.e. the SPD should not interfere with the normal working of the PV plant.
Surge Protection Device key
operating characteristics include maximum continuous operating voltage, DC or
AC application, nominal discharge current, voltage protection level (terminal
voltage that is present when the Surge Protection Device is discharging a
specific current) and temporary overvoltage, a continuous overvoltage that can
be applied for a specific time without damaging the Surge Protection Device.
Fig.1: DC Distribution Box with DC SPD, type-2
Placing the DC Surge Protection Device at specified locations in the DC circuit mitigates the effects of these induced currents and voltages. The Surge Protection Device is placed parallel to the energised conductor and ground. It changes state from a high impedance to a low impedance when the over-voltage occurs. Surge Protection Devices are supposed to not to carry any load current.
The Surge Protection Device must have an
in-built self protecting device that disconnects it from the circuit should the
device fail. To make this disconnection apparent, Surge Protection Devices come
with a display flag that shows the disconnection status. The status can also be
sent to a remote location opting a remote signalling feature. Some Surge
Protection Devices come with a finger-safe, removable module that allows a
failed module to be easily replaced without tools or the need for switching off
the circuit.
AC Surge Protection Devices:
Just as DC SPDs, AC Surge
Protection Devices are also used to protect the system from surges at the AC
side. They should be placed as close to the AC terminals of the inverter to
protect it from dangerous transients. The transients encountered at this
location are of high magnitude and duration and therefore must be managed by
the Surge Protection Device with appropriate high discharge current rating. The
connections to the Surge Protection Device should be made through short
conductor pieces of sufficient cross sectional area; so as to reduce voltage
drop in the Surge Protection Device circuit during discharge and to avoid
exposure of protected equipments to higher transient voltages.
During lightning strokes to nearby
grounded structures, lightning protection system itself, and inter and intra
cloud flashes of higher magnitude can induce transient currents into the DC
cables of the PV system. These transient voltages appear at the terminals of
the key components such as inverter etc leading to insulation and dielectric
failures.
In large commercial and utility
scale Solar PV systems operating at a maximum Open Circuit voltage of 600 V or
1000 V usually use MOV Surge Protection Device in ‘Y’ configuration. Each leg
of the ‘Y’ contains an MOV module connected to each pole and the ground.
Failure of Surge Protection Device
can be due to ambient heating, discharging currents that are greater than the
normal rating, discharging too many times, or being exposed to continuous
over-voltage condition.
Types of Surge Protection Devices:
Surge Protection Devices can be
classified into three different classes –
1. Broad
Protection Type (Type-1): This class of Surge Protection Device have the
highest value of admissible surge current and are designed to handle a direct
lightning stroke. They are used where there is a possibility of lightning
currents being conducted through the external lightning protection system and
the electrical cables. For example, locations where the distance between the DC
cables and the external lightning protection is too small.
2. Medium
Protection Type (Type-2): This class of Surge Protection Device have a lower
value of admissible surge current and are used for protection against indirect
lightning effects. When a local lightning strike occurs on the external
lightning protection system it can induce dangerous voltages into the electric
circuit because of the associated electromagnetic fields. However, the value of
lightning current, its duration and inherent energy is low in this case. Type-2
class of Surge Protection Device are used to protect against these
over-voltages. The DC SPD shown in fig.1 is a type-2 class.
3. Fine
Protection Type (Type-3): These Surge Protection Devices have the lowest value
of admissible surge current and are used to protect sensitive electronic end
devices from the effects of lightning stroke far away from the location.
Surge Protection Device check list:
Effective Surge Protection Device
installation should consider –
1. Placement
in correct location,
2. Proper
rating and type,
3. Proper
grounding,
4. Local or
remote indication facility,
5. Easily
replaceable modules.
