"SPD" - 3 letters that help reduce downtime occurring from surges. At PV plants, SPDs have to fulfil specific requirements to ensure continuous operation and energy generation.
When designing a PV plant, it is important to consider the installation of surge protection devices (SPDs). Surges and network disturbances can lead to downtime, reducing the performance of the plant. Therefore, any conditions affecting energy generation and distribution should be taken into consideration when designing the electrical installation.
Why are SPDs a top priority in PV plants?
Let's start from the beginning - solar panels are installed outside to convert solar energy into electricity. This outdoor location makes them directly exposed to harsh conditions like rain, wind and dust. Among the weather conditions lightning strikes require specific attention as they can severely affect the safety and performance of a PV plant. They originate in a cumulonimbus cloud and terminate on the ground. When the lightning strike hits the ground, it discharges energy, affecting the electrical field on the ground. For the solar PV plant this poses two risks:
A direct impact which can physically destroy the solar equipment on a rooftop
Transitory over voltages passing through cables by magnetic coupling, which can lead to the damage of sensitive components such as printed circuit boards (PCB).
As far as the direct impact is concerned, ‘External Lightning Protects’ (ELP) provide the required protection according to IEC 62305, which describes how to evaluate if your location needs such protection, and what should be the preferred option (meshed cages, air terminal, etc.). The concept is simple: make sure the lightning will strike a metallic rod installed on the highest point of your plant and dispel the energy directly to the ground through a copper down conductor.
When it comes to transitory over voltages, however, SPDs are required. They are installed in parallel into the circuit protection boards to divert the energy to the ground and limit the overvoltage up to such value acceptable to the end equipment.
As soon as ELP is installed at a PV plant, it is mandatory that an SPD be installed, too. If the PV plant is not equipped with an ELP, the installation of an SPD is highly recommended to limit network perturbations (transient over voltages).
How does an SPD work to protect the DC side of solar plants?
To guarantee the energy will flow to the ground first to limit over voltages the most important component is the Metal Oxide Varistor (MOV, ‘Varistor’ stands for VARI-able resi-STOR). This component has such propriety that in normal conditions (no over voltages) the resistance is high enough to not make possible nominal currents passing through it. Starting at a certain overvoltage level, the resistance will quickly drop, opening the path to the ground and coming back to a normal condition once the energy has been dissipated. This process allows limitation of the overvoltage level reaching all equipment connected downstream.
Type1+2 SPD versus standard Type 2 SPD, which one is the right one?
There are different types of SPDs available which vary in terms of resistance: Type 1, Type 2, and Type 1+2. A Type 1 SPD can cope with a direct strike which bring an energetic surge, whereas a Type 2 limits over voltages from various sources. Both characteristics can be combined into a “Type 1+2” for complete protection.
In PV plants the challenge is to choose the appropriate surge protection to withstand pure energy 10/350µs waveform currents (almost 10 times more powerful than type 2 8/20 µs waveform) while at the same time taking space into consideration. In an inverter or junction box space always is top priority. To maximize the available space, ABB’s SPDs use the depth of the enclosure for stronger components with an increased depth of the device. With the new OVR PV QuickSafe series both AC and DC circuit protection boards in solar installations can be protected against over voltages due to lightning strikes or network disturbances.