Lightening Arrestor (LA) or Surge Protection Device (SPD): What is of utmost necessity for a solar plant?
2nd July 2018
A solar PV plant has lot of metallic components i.e. the frames of module (generally made of aluminum), the understructure (made of aluminum or hot dipped galvanized iron) which are exposed to the open atmosphere. Nowadays with the solar power plants being scaled from few MW's to around 3~5 GW (Figure 1) and with requirement of adequate substations and transmission lines in/around the plant, the actual amount of metal exposed to the atmosphere is immense. Areas in or around such high metal density are highly susceptible to lightning strikes. While few EPC's/ installers take appropriate considerations and give necessaryprotection required in the power plant, we have seen many plants (usually small in size) where there are no protection for lightening at all. Any lightning strike on such plants would result in (almost) complete destruction of solar modules (Figure 2 left). This lightning strike may also (in few cases) cause fire outrage leading to total devastation of solar power plant (Figure 2 right). With such effects, adequate protection from lightening is of utmost importance. This blog hence aims to educate its readers on two such important protection devices i.e. Lightening Arrestor (LA) and Surge Protection Device (SPD). It would also inform you on what or which of the protection device is important for your solar power plant.
Figure 1: A typical solar power plant (Source: Waaree Energies)
Figure 2: Solar module (left) and power plant (right) destroyed due to lightening (Source: Google images)
A lightning strike can be defined as an electrical discharge from atmosphere to an object on earth. As explained above, due to heavy metallic parts around solar plant, it becomes a critical spot for lightning strike. While such lightening is not inevitable, its effects can be averted by using lightening arrestor (LA). LA as the name suggests arrests or captures the lightening and transfers the discharge to the earth via a conductor cable (Figure 3). In order to make the earth more conducting, a low resistant ground enhancement material (GEM) may be added while digging the hole (2 to 3 meter) for grounding. While the other parts of the system are more or less standard, LA comes in variety of types and techniques.
Figure 3: A typical arrangement of LA on house (source: Google images)
The standard LA used in a solar power plant is more or less dependent on size of solar power plant. For a ground mounted and/or a utility scale power plant an early streamer emission LA is used. During a stormy weather, the ambient electric field of the atmosphere rises from 600V (when normal) to 10-20 kV/m. Under such conditions the internal circuit of the LA activates and starts generating early pulses (known as upward leader). This upward leader of LA is earlier (in time) compared to other (natural) upward leaders arising in its vicinity. From the fundamental laws we know that electricity tries finding least resistance path. Thus the tip of lightning strike (also known as downward leader) meets the upward leader and the entire lightening is grounded via this LA. It is due to this (early) upward leader that the structure in the vicinity of this LA survives the lightning strike (Figure 4). The advantage of using such system is that given a certain height and required level of protection, it can protect items in radius from 19 meters to as much as 120 meters. For a solar PV plant, this means that the lightening rod can be quiet far away from the power plant but still protect the plant. Additionally the shadow effect of such LA can be easily avoided. For a rooftop scale power plant a simple rod like structure may be installed which may a tapered/ spherical tip. They may not be able to form the upward leader before the strike, but their upward leader is usually the first and nearest that the lightning would come in contact in case of a strike. Usually both rooftop and utility scale power plant two grounding pits associated with a single LA. This is to ensure that the entire strike is grounded from either of/both the pit(s).
Figure 4: Working principal of early streamer emission LA (source: Earlystreameremission)
The area of protection of LA from lightening can be determined by a technique known as rolling sphere method. In this method, based on the class of protection and height of the LA a sphere with an imaginary radius r is drawn on/around the LA (Figure 5). This sphere is imaginarily rolled over the entire vicinity. The area's where the sphere cannot touch upon is deemed to safe. The area where the sphere touches is unsafe and requires LA around it or either the height/class of the existing LA should be enhanced.
Figure 5: Rolling sphere method of LA (Source: Google images)
With a background on lightning, let us now come to second part of the blog i.e. surge protection device (SPD). A surge may be defined as an exposure of an instrumentto overvoltage/ current/ power which may damage an instrument. With reference to PV plant, a surge may be a result of (mostly) an indirect lightning strike around the vicinity of the plant. This strike may cause surges in the areas around the spot where it was grounded. This sudden surges may be in thousands ofvoltsand/or hundreds of amperes which can significantly damage to the solar module. Additionally the electromagnetic radiation associated with such grounded strike may affect the current carrying cables, communication cables, inverters, etc.This is where SPD comes into picture.A SPD (as shown below in Figure 6) is used to limit the over voltage and surge currents or both. They are generally connected in parallel to the system they are intended to protect. In case of surge, the SPD acts as a switch that turns off the normal operation of system. This forces the surge to flow through the SPD which is then grounded (via the green grounding wire as seen in figure below). The SPD's generally available in market may be classified based on its classes and types i.e. type 1, type 2, type 3 and so on. The SPD used particularly in solar power plant is of type 2. This type of SPD is specifically to ground any surge arising from indirect lightning. These SPD's are installed on both DC and AC sides in the solar power plant.
Figure 6: A typical SPD (encircled) installed in a junction box (Source: Google images)
As evident from above, we could conclude that both LA and SPD's have a separate role to play in the power plant. Thus none of the above protective equipment should be ignored while installing the power plant. Additionally in order to ensure the plant runs smoothly for 25 years, it becomes necessary that it is fitted with all the necessary protective equipment's. Waaree has an experience of executing more than 500+ MW EPC projects. We ensure to follow all the safety standards and necessary guidelines in order to erect a power plant. An expertise like such enables end customer to be assured that their plant would be up and running for 25 years.
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HAMARA NATION