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.