Fundamental best practices have not changed as the principles are still sound: “Avoid lightning by diverting it away from the subjects to be protected”. It is the knowledge that adds value to best practices, and this has improved considerably with lightning empirical data.
The risk we run in over-simplifying the task of lightning protection is that we try and over-complicate it. By overcomplicating the task we resolve that the problem is both complex and fluctuating to such an extent that actually the entire exercise is pointless. The most common mistake is not carrying out a qualified risk analysis and thereafter not carrying out a sound fault analysis when the systems fail.
Prior to 2017, the industry did not have the benefit of a high resolution GFD map to indicate the level of long-term lightning threat for which mitigation is necessary. Prior to January 2006, the industry did not have the benefit of verifying the presence of lightning at any particular site in order to determine the extent of the lightning threat.
Now in 2018, South Africans can qualify the longterm threat down to four square kilometres (4km2) and can verify their risk models with actual lightning data at the specific location for a given period of time. Thus, clients can qualify whether the lightning protection and earthing they have on their sites is adequate. Ideally, the risk should not be discovered after suffering extensive lightning damage.
Thus the second most common mistake is judging the performance of the lightning protection and earthing without understanding the exposure. Lightning’s gradual impact over time is invisible until it injures or kills people, or causes assets and system breakdown. We do not see the underlying deterioration taking place until it is too late. In many cases in the power and energy industry, the final catastrophic failure is not even triggered by lightning but simply by increasing ambient temperatures under load conditions, or reclosing a breaker and introducing a switching surge.
The third most common mistake is implementing a lightning protection scheme without considering secondary implications such as induced currents and voltages, and material chemical stability. Induced currents and voltage give rise to secondary damage that is usually completely not anticipated. Chemical stability of materials involves both material corrosion and decomposition over time, as well as the impact of different materials bonded together or in a series electrical circuit.
In its strategic business plan, the International Electrotechnical Commission TC81 noted “a recent trend in many markets (notably North America and Europe) is looking to thunderstorm warning systems as an effective tool to reduce exposure to lightning risks”. It is important to note that in South Africa, Eskom has been developing this capacity since 2005 by guiding the South African Weather Service (SAWS) in the implementation and operations of the Southern African Lightning Detection Network (SALDN). The maps below show the change in lightning ground flash density (GFD) from low resolution flash counters results in 1986 to the high resolution remote sensing SALDN results of 2017.
Influence on voltage measurements and substation performance
In lightning transient conditions, a direct strike to a power line or substation structure will lead to an immediate rise in voltage as the lightning current passes through the power system. To explain this, think of how water flowing in pipes creates pressure on the water, and the smaller the pipes (higher the resistance) the higher the pressure (higher the voltage) for the same flow rate of water (transfer of electric charge).
The process of the lightning discharge is from (a) cloud to (b) ground and the power system in between has little impact on completing the process. Therefore the current flowing is the driving parameter. Good earthing (low resistance values) provides a ‘pipe’ with large dimensions allowing the lightning current to flow with very little resistance and managed increase in voltage. Two factors are critical in substation performance:
- Good earthing of the substation earth-mat for the reasons already described; and
- Bonding continuity between plant in the substation – it is vital that every component in the substation is at the same potential. This is similar to a boat on a wave, which will rise and fall with the wave. Any portion of the substation not at equipotential to the rest of the substation will experience voltage fluctuations and thereby compromise the applied insulation scheme.
Every competitor in the market providing some form of electricity supply service to a substation must take care of the above two factors specifically to ensure the safety of people on these sites at all times.
Common methods for earthing and lightning protection
It’s a matter of providing an alternative path for the lightning current to flow to ground and ensure that: 1. Lightning current flowing in the lightning protection system (LPS) does not induce dangerous currents in any parallel metallic systems near the LPS, and
- No dangerous voltages can arise between any of the conductive parts of the protected systems and the LPS.
In many instances, the system to be protected is itself incorporated into the LPS. That is, a portion of the structures is used to transfer the lightning charge to a point where it is then dissipated with a separate LPS component.
An example would be a building containing steel reinforced bars (rebar) in the foundations, columns and floors of a multi-storey building. By bonding air terminations at the top of the building to the rebar, bonding each rebar joint transition and finally the bonding to the rebar in the foundations and/or pilings where applicable, the lightning current can be transferred using existing materials. In this case, any electrical wiring in close proximity to the rebar throughout the building will experience voltage rise and must be managed with suitable surge arrester protection.
In addition, this technique requires suitable measurement of overall continuity of the path from top to bottom to ensure no increase in ‘pipe size’ such that the lightning current may seek an alternative path to ground via nearby electrical wiring circuits. Where the soil resistivity is suitably low, no additional earth electrode is required.
An alternative to using the rebar and foundations/pilings in the above example is a dedicated separate electrical conductor of suitable dimensions from the air terminations at the top to a separate earth electrode in the ground. The cost of the additional material may offset the need for rigorous testing and bonding throughout the construction phase of the project.
The final element dictating the most common method of earthing is the soil resistivity. This is a measure of how well the soil upon which the site is constructed will dissipate the lightning current being channelled into it. The higher the soil resistivity, the higher the resistance will be of the ‘earthing’.
In a foundation with rebar at a site with very low soil resistivity the desired target earth resistance values can usually be achieved with no additional earth electrodes other than the bonding already mentioned between the foundation’s rebar and the structure steelwork.
Where the soil resistivity will produce a higher resistance than required, additional vertical earth rods and trench electrodes must be applied. Several analytical computer based models can be used to determine how much material is required – in South Africa the basis for such calculations is given in SANS 10199, which addresses the design and installation of earth electrodes, published by the SABS TC067.
Where no foundation and rebar exists, such as planted wood poles typical in medium voltage networks, one or more vertical earth rods and trench electrodes must be applied where a controlled earth resistance is required. The design will be the same as specified above using SANS 10199 and helpful computer modelling applications.
Suitable methods for T&D networks and generation plants
In the power and energy sectors, substation-to-substation networks are usually shielded – that is, apply air terminations (overhead shield wire) and bond them to ground via the power line towers – either via the steel structures or with one or more dedicated earthing conductors on wood poles. The overhead shield wires intercept any downward cloud-to-ground lightning stroke.
This system requires diligent management of the tower bonding and the earthing electrodes of each and every pole or tower the shield wire is attached to. Measurements must be taken to verify the obtained earth resistance values, given the soil resistivity conditions at each tower. Maintenance is equally important through visual inspections and repeat measurements in the event of lightning back-flashovers at the specific location.
For MV networks, these usually have numerous customer connection locations where the MV is stepped down to LV. There is no permanent connection to another substation. These systems have a low basic insulation level (BIL) and as such the shielded network solutions at these voltages are impossible to manage cost-effectively.
At each of the MV/LV step down sites, a surge arrester on each phase of the step-down distribution class transformers serves as a high speed switch to dissipate lightning energy at the speed of the lightning transient. Thus the key here is healthy surge arresters and good earthing at each of the MV/LV customer service points.
That is, a part of the existing circuit, namely the phase conductors, is used as the dissipation circuit. In much the same way as the rebar bonding must be ensured, the surge arrester condition and bonding to the earth electrodes on the MV networks must be verified through measurements.
If the networks are widespread with very long distances between service points, additional localised surge arrester locations would serve to manage travelling wave surges. In a power utility such as Eskom, every pole has a downwire with a managed gap in order to:
- Discharge direct strikes to the phase conductors, and
- Increase the BIL to limit the impact of indirect strikes (lightning strokes terminating adjacent to the power lines).
R&D and trends influencing earthing and lightning protection
Current changes in the sector increases the number of players in the market needing lightning protection and therefore the need for additional support regarding the appropriate standards to apply. Where Eskom and the AMEU have set arrangements with NRS regulations, etc, the new players have different challenges.
- The PV requirements are being reviewed separately from that of wind farms, as an example. The PV industry specifically is recognising the need for training and proper certification of staff to install LPS.
- The blades of a wind turbine require very specific considerations given the material of the blades, the extended height of the blades, the rotating nature of the system and the lack of a dedicated path from ‘top to bottom’. Numerous academia and specialists have increased their involvement in this arena specifically to support the wind farm operators.
It may be premature but my opinion is that South Africans have largely been lax in implementing the correct lightning protection solutions as required. We have too long been satisfied with second best and have been conned into a sense of complacency.
Wits University is the recognised Centre of Excellence when it comes to lightning and lightning research. I would like to see Wits empowered to undertake a mentorship supporting role to all tertiary educational facilities with the objective of identifying those institutions with a desire to develop students’ knowledge in this field and to build a skills base of their own.
Such efforts would build synergy among academics either working together on common tasks or each researching niche areas with an applied outcome in South African and – even more importantly – the African economy, with particular emphasis on the socio-economic responsibilities in each country.
In the above collaboration, there is an important role for the SAWS if it is up for the challenge. We need to support SAWS by building its knowledge base and integrating its efforts into those of the academic institutions, with mutual responsibility to the country and skills development in this area.
Our responsibility is to assist government with the resource to self-regulate such that the ministry of each department can concentrate on what is critically needed. Loss of life due to lightning is alarmingly high and unacceptable. Through sharing of knowledge, ELPA will play a role in reducing this burden. ESI
About the author
Richard Evert is the national director of the Earthing and Lightning Protection Association (ELPA), which is supported by various institutions such as Wits University, the Electrical Contractors Association of South Africa (ECA) and the Department of Labour. www.elpasa.org.za