In this case study, we highlight Earthing System Design avoiding stray current corrosion and saving the client money in the process.
GreyMatters were engaged to perform earthing design for a utility-scale PV Solar Farm in the south west of England. As well as achieving compliance with BS EN 50522, it was necessary to comply with the requirements of G59 and the DNO. What makes this case study a worthy of note, is how the PV system build itself was utilised as a part of the end Earthing Design Solution, which not only saved on installation costs but also avoided anti-corrosion measures including Cathodic Protection.
Want to know what the Seven common mistakes in Earthing Design are for Solar projects in low resistivity soils?
Owing to the recent tightening in subsidies for utility-scale PV (photo-voltaic) in the UK, there is even more pressure to control costs on the larger scale PV projects. But it is vital to ensure that the focus on safety remains. At this site, GreyMatters produced a competitive cost design using our significant experience to balance cost against the risk to human safety.
High-end industry standard FEA (Finite Element Analysis) packages were used to develop an earthing network that integrated the sites extensive foundation system to reduce the number of Earth Electrode rods required. The routing of conductors was optimised to reduce installation costs as well as material costs. And finally, the installed system was verified against the original proposed Electrical Earthing Design model to ensure that the safety case remained valid.
Possible Stray Current Corrosion
The process began with GreyMatters attending site to collect soil resistivity data. Owing to the site location, the soil resistivity was very low. So using a high signal level tester was critical to obtaining accurate data. At the time, also determined that the soil structure was likely to cause galvanic corrosion between the support system for the panels and the earthing system. As a result, GreyMatters recommended the use of galvanised steel earthing instead. Galvanised Steel is reasonably popular in Europe, as it offers similar performance while reducing costs. Additionally, it is much less attractive to thieves, so the maintenance cost due to theft is reduced.
Once the soil resistivity testing had been performed using the Wenner method and processed using CDEGS RESAP, a ‘virtual’ model representing the entire site above and below ground created in CDEGS HIFREQ, e.g. the panel support structures (tables), inverter substation, fence and HV connection and tower, modelling, as well as the substation plinth and reinforcement.
From an economic perspective (value engineering) – as a direct result of the CDEGS modelling, a significant saving was achieved, given, a minimal amount of purpose-built earthing required. The table supports driven over 1m into the ground, so these were able to take the role of most of the rods (vertical electrodes) in the earthing system. Around the substation, numerous earth rod electrodes were installed to assist with surface voltage control around the substation plinth. The interconnections between the tables were installed in the same cable trenches as the cables back to the substation, so no additional trenching needed. This saving cut down further on the installation costs and accelerated the construction, allowing the client to start generating revenue without delay.
Less than three months later, GreyMatters was back on site performing post construction verification by Fall-of-Potential testing to enable the network grid connection works. While the testing showed there were some minor quality issues with the installation, these quickly rectified by the installation contractor, and within a week, GreyMatters had returned to the site to verify the installation was satisfactory. There was good agreement between the modelled resistance to earth and the measured resistance to the earth, and continuity tests confirmed system integrity. Without this level of information, the circle on the design process can’t be closed-out, ensuring that our model is accurate and calibrated, and the safety calculations confirmed as valid.
When compared to copper, galvanised steel is suitable for a wide range of applications in earthing. GreyMatters were able to design a technically compliant system using the only 250m of 105mm2 steel conductor and rod electrodes, significantly reducing the cost impact to the developer without compromising safety from the system.
Often, thinking-outside-the-box like this, considering the use of less apparent materials can future-proof the installations, protecting against corrosion due to stray current as well as enhanced corrosion due to inadvertent creation of galvanic cells.
If you believe your current Electrical Earthing System Design needs a rethink from some new GreyMatter, then get in touch, and we can discuss how to future-proof your project.