Lightning protection - Pre-empting the strikes

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Many organisations seeking to lower their carbon footprint are tapping into solar energy using photovoltaic systems, which are highly vulnerable to lightning damage. Ian Langeveld, UK and Ireland sales manager with Wieland Electric, discusses the importance of suitable protection for these systems

As technology has become an integral part of everyday life, measures to protect our devices and the systems that serve them have also increased in importance. Indeed, in some cases, protecting these systems has become critical to the business’s ability to operate. Thus, for example, protection against the damage that can be caused by lightning strikes is now just as important for many businesses as securing their buildings against intruders.

Now, with the growing use of photovoltaic (PV) arrays to harness solar energy there is an additional area to be considered when it comes to protection strategies. And with feed-in tariffs encouraging electricity generation from renewable energy sources this is an area that will continue to grow.

So it seems sensible to assume organisations that invest in PV systems – which even with the support of feed-in tariffs is a sizeable investment – will also invest in systems to protect them from damage. This is the same logical train of thought that assumes those same organisations take measures to protect business-critical systems inside the building that are vulnerable to damage from lightning.

The volume of insurance claims, however, tells a different story and it’s clear many organisations are doing no such thing. This is probably because they are assessing the risk of suffering lightning damage against the cost of protection and deciding to take the risk. After all, we all know there’s very little chance of being struck by lightning! Or do we?

PV solar arrays, though, are particularly vulnerable to lightning damage because they tend to be mounted high up and also tend to occupy large areas to maximise their ability to capture solar energy. To add to the potential problem, climatologists are predicting climate change will lead to more extreme weather conditions, so we may well see the frequency of lightning flashes increasing above the 100 or so that are already happening every second across the world.

When these lightning flashes result in a lightning strike that affects a building, these may fall into one of two key categories - direct strikes and remote strikes.

Direct or close-up lightning strikes are lightning strikes into the lightning protection system of a building, in close proximity to it, or into the electrically conductive systems implemented in the building (e.g. low-voltage supply, telecommunications, control lines).

Remote lightning strikes are lightning strikes that occur far away from the object to be protected as well as lightning strikes into the medium voltage overhead system or in close proximity to it, or lightning discharge from cloud to cloud.

Statistics show a large PV system in an open space is likely to be struck by lightning (directly or remotely) within two years of installation – and overvoltage protection systems used inside buildings are not very practical or effective for protecting PV systems. It has also been shown damage to PV systems can be attributed to overvoltage errors in 45% of all cases.

Clearly any such damage is not only disruptive and can lead to operational downtimes, it also reduces the profitability of the system and can jeopardise financing or lead to an increase in insurance premiums. So building operators that are installing PV really should be thinking about lightning protection as well; ideally with guidance from in-house or consulting electrical engineers.

Practical experience in a wide range of situations shows the most cost-effective approach is to use a protection system that offers overvoltage protection, fire protection and personal protection in a single device.. Such a system will combine a separator and short-circuiting device with safe electrical separation in the event of a short circuit. These devices prevent fire damage resulting from DC switch arcs while the reliable, error-resistant Y circuit prevents damages to the overvoltage protection caused by faulty insulation in the generator circuit. The integrated DC fuse guarantees safe, arc-free replacement of the protection module.

Clearly, for ease of use, it makes sense to opt for a protection module that can be used in all photovoltaic systems and is absolutely safe in every application thanks to an integrated fuse. Suitability for a wide range of applications is also important so ingress protection to IP 20 is a requirement, as is the ability to operate in the wide range of temperatures permissible for use inside a distribution box outdoors - from -40° to 80°C.

Extending protection
As noted earlier, there are many buildings that also lack suitable or adequate protection for their internal systems so it is to be hoped the current high profile of PV will help to raise more general awareness of the need for lightning protection. In this way, a project that begins with protecting a PV array may be extended to other areas of the building within an integrated system that protects all of the building’s electrical infrastructure. Again, a cost-effective approach will be appreciated by the building owner or operator as this will help to shift the balance between capital outlay and risk described earlier.

A sensible compromise is to adopt a zone concept for lightning protection – as described in IEC 62305-4 (DIN EN 62305-4, DIN 0185-305-4). This enables planners, builders and owners to align the protective measures they adopt with the risk levels to the business of damage occurring. In this way, all relevant devices, plants and systems are afforded a level of protection commensurate with their importance to the business.

Over and above the lightning protection system in the building, there is a strong case for introducing additional measures for overvoltage protection of electrical and electronic systems. These will help to safeguard the integrity and availability of complex power engineering and IT systems even in the case of a direct lightning strike.

An external lightning protection system will typically be made up of an air termination system, down conductors and an earth termination system. All of these elements need to connected effectively so that if lightning strikes the building the current discharge is conveyed safely away and damage to the building is minimised. The most straightforward way to achieve this is to ensure all connection components comply with BS EN 50164.

An internal lightning protection system is designed to eliminate the risk of dangerous sparks inside the building or structure, following a lightning strike. This could be the result of current flowing in the external lightning protection scheme and sparking over to metallic elements inside the building. Or it could happen if current flows through any conductive elements on the outside of the building.

The risk of sparking is minimised by ensuring there is a sufficient distance between metallic parts or by implementing appropriate equipotential bonding measures so that no metallic parts are at different voltage potentials and there is no risk of sparking between them. Either bonding between conductive elements or the use of surge protection devices will address this. Surge protection devices are especially useful in situations where it would be inappropriate to create a direct connection, such as between power and communication lines.

Underlying the decision making process is the need to address the commercial realities that impinge on every project, balancing the level of risk against the importance of the systems at risk to the organisation’s operational performance. With the increased risk of damage that results from installing a large array of equipment at the highest point of the building, this risk:cost balance is changing for many organisations. So protecting the solar PV arrays falls into the high risk area of the zone concept for lightning protection described above.

In our experience, this zoned approach strikes the right balance between capital outlay and operational risk and is also one that is appreciated by the end client, so it is also a very effective way for electrical designers to add value for their customers.

Lightning facts and figures

At any one time there are more than 2,000 thunderstorms taking place somewhere in the world. The following are some key facts and figures about lightning:

Potential: Up to 1 billion volts
Current: Up to 200,000 amperes
Temperature: >30,000 deg C
Velocity: 304,800,000 m/s
Duration: Approx 1ms
Stroke diameter: 50mm
Stroke length: From 60m to 32km
Longest stroke recorded: 195km

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