Earthing Case Study
Objectives
Supplied from multiple HV Transmission lines. GreyMatters was given the task to examine the existing earthing system design for an established large scale Data Centre – to study how it would perform under electrical fault conditions and to provide the necessary electrical earthing design to EN50522 and IEC standards to cope with the increased capacity of the Data Halls.
Due to the ever increasing demand for cloud-based and remote hosting resources, there has been a significant increase in calls for more capacity on Data Centre providers worldwide.
Project features
| Soil resistivity (ρ): | high-low-high |
| Geology: | clay on mudstone |
| Primary Voltage (V1): | <200 kV |
| Number of line towers: | ~100 |
Results
For this site, multiple soil resistivity tests using the Wenner method with high signal energy soil resistivity testing instrument (400 V) were carried out in 6 locations across the large site. Wenner traverses varied in spacings from short surface samples, to long lead traverses (soundings) of up to 300m. Why so long? Every Earthing Electrode System has a sphere of influence in the ground – Not just across the surface but in all 3 dimensions. So, it’s just as important to understand what is going on at the deeper layer(s) of the electrodes influence as it is at the surface layers. This is because the deeper layers have a role to play during the discharge of electrons into the ground from a fault.
[NB: Take a minute to learn more about soil resistivity testing methods and mistakes to avoid by going to our section devoted to all things Soil Resistivity, or if you are pushed for time why not cut to the chase with our free download 7 Deadly Sins of Electrical Earthing Design]
The results from the soil resistivity tests were processed using CDEGS – RESAP engineering module, which indicated a multi-layer soil structure. The data modelled in CDEGS RESAP was contrasted with the previous data assumptions held, which allowed an improvement in soil model accuracy of 30%.
From the soil resistivity model for this site, it became clear a higher than average amount of earth-return-current would pass through the soil on its path back to source. As in all cases, it’s imperative to reach a clear understanding of the soil structure and electrical properties of the ground itself. The key point being an accurate Soil Model is the foundational cornerstone that ALL subsequent parts of a valid Electrical Earthing System Design are built upon. This means, eliminating error from Soil Resistivity Testing at this stage is of absolute capital importance.
Figure 1 – example of a high-low-high soil model
Once an appropriately accurate soil model had been derived from the soundings data, attention turned to characterising the HV source power from the overhead transmission lines, which supported a significant quantity of tower structures (tower chain impedance) and remote earthing arrangements feeding a local substation, including the build of the step-down transformers and associated earthing transformers in CDEGS HIFREQ virtual model space.
Using Finite Element Analysis software CDEGS HIFREQ, the model representing all the existing (and proposed) above and below earthing arrangements for the entire data centre and its data halls, was subsequently analysed including impedance contributions and parallel return paths from reinforcement, overhead earth wires, OHL towers, cable sheaths, pits, lightning protection and plant bonding arrangements.
Key Points:
- Performance & Safety: A comprehensive Earth Fault Study using CDEGS Hifreq of the Electrical Earthing Design, from source to the site’s local substation and data halls to establish a SAFE electrical earth system to EN 50522 capable of protecting life (touch and step voltages) against the risk of an imported fault.
- Performance & Safety: Electrical properties of the ground were comprehensively studied and an accurate model was delivered forming the basis of all present and future electrical earthing system designs.
- Economy: Utilising an accurate multiple layer soil model of the ground allows present and future electrical earthing designs to be optimised with a sense of certainty, which allows Electrical Earthing Designs to be right-sized, saving money by avoiding the risk of over-engineering on installed arrangements.
Further reading: The 10 Commandments of Electrical Earthing Design
If you are responsible for your Data Centre Earthing and believe your current Electrical Earthing System Design needs a rethink from some new GreyMatter then get in touch and we can discuss your challenge.