Learnings

Outcomes

The main outcomes are detailed in deliverable reports and published conference papers.

(i)           Deliverables 1&2: Earthing of T-Pylons and safety voltages

The earth resistance of the T-Pylon base was calculated using a simplified analytical model as well as a more detailed model using numerical modelling performed win CDEGS software. The installed T-pylons required strong reinforced concrete bases with varying dimensions, 4 x 4 m up to 10 x 10 m, depending on location and type of tower: suspension, tension or angle type which impose different mechanical stress.  The concrete bases have several piles of up to 10 m long to enhance the mechanical strength. Such strong foundations have a beneficial effect on the Pylon’s earth resistance. The calculated resistance values for a soil with resistivity of 100 Ωm ranged from 2 to 3 Ω respectively. 

 
The measured earth resistance on completion of the installation of the pylons ranged from 0.01 to 9 Ω. Two T-pylons, LD91 and LD60, with high and low range resistances respectively were selected for full earthing 

measurements during this project. Similar orders of magnitude were measured for the earth resistance.

In addition to earth resistance measurement, which is a good indicator for Earth Potential Rise (EPR) following injection of current into the earth, the safety voltages, namely, the surface potential distribution, the step and touch voltages were computed. It was found that the step voltage is highest above the corner of the concrete base, i.e., up 7 m away from the body of the T-Pylon. Such findings are important to consider ensuring the safety of personnel and animals around the T-Pylon. They may also help improve NSI-5 recommendations. Special mitigation needs to be implemented to lower such elevated step voltages.

(ii)          Deliverables 3 & 4: Induced Effects on T-Pylon Overhead Lines

The new west country 57 km hybrid line made up of T-pylons, lattice steel tower and 8.5 km underground cable was simulated in an electromagnetic programme to compute the induced voltages and currents on the deenergized circuit when the other circuit is energised with system voltage and load current as well as under fault conditions. Various soil resistivities were used and the effect of the T-Pylons earth resistance were investigated. This allowed simulation NSI 5 scenarios including the use of DrESS earths, and determination of earth currents flowing through the towers/pylons were determined which allowed study of the safety voltages at selected towers. These computed results can now be applied to enhance safety around individual towers along the line.

(iii)         Deliverables 5 & 6: Evaluation of Software and Comparative studies with measurements

Currently, CDEGS is the leading earthing software used in industry and academia. In recent years, others have entered the market, but no independent comparative study is available. In this project, two established earthing software packages, CDEGS and XGSLAB were compared using generic and practical earth electrodes and grids using steady state, variable frequency and transient current injections. Furthermore, comparison with measurements at the Cardiff University tests site using generic electrodes (vertical rod, horizontal conductor and small grid) as well as measurements in the field on towers and substations were carried out. Additionally, earth potential rise, surface potential distribution, and safety voltages (step, touch, and transfer) were analysed using both tools. Overall, good agreement was observed, which will help competition. However, for high-frequency events, the segmentation of the earthing systems must be carefully defined by the user to ensure accurate results in both cases.

(iv)         Deliverables 7, 8, 9 & 12: Low resistivity material and their applications

To lower the earth impedance of earthing systems, more copper conductors are added to increase the effective contact area of the earthing system. However, such solution can be costly. If the soil resistivity surrounding the earthing system is low, then the impedance reduces accordingly.

In recent decades, several low resistivity materials, mainly concrete-based, have been introduced to encase earthing copper conductors. In this way, the contact area with the soil is larger and, hence, reduces the impedance/resistance of the earthing system. One such material is Conducrete which was introduced in recent years. In this project, the material has been characterised in the laboratory to determine its resistivity and permittivity under various conditions of temperature and moisture content. It was found that, despite the gradual drying process, the resistivity of the material falls rapidly with temperature. This was explained by the behaviour of the water contained with the test sample/material. It was noted that the measured resistivity is two orders of magnitude higher than that published by the manufacturer, and this needs to be further discussed with the manufacturer to explore how the lower value was measured.

To test the properties of Conductrete on practical earth electrodes, a comparative test was conducted at the Cardiff University test field where two rod electrodes were tested: one bare conductor and a similar one encased inside a 30 cm cylinder of Conducrete. It was found that the earth resistance of the bare conductor is more than three times that of the Conducrete-encased rod, which proves the benefits which come at the cost of the Conductrete material, digging a hole and preparing/ pouring the slurry. The two earth resistances were monitored over more than a year, and the lower resistance of the Conducrete was verified for all seasons showing less variation than the bare rod electrode.

The other claim for the Conductrete is its protective property against corrosion. Such practice is already adopted in electrical railways where the corrosion phenomenon is well known. As the duration of the tests during the project is not long enough to examine this property, numerical simulations have been carried out to quantify the impact of corrosion on earth electrode resistance, and these have proven that a significant increase in earth resistance would be expected due to corrosion layer. This is in addition to weakening the electrode metal.

Given the importance of earthing systems in relation to safety of people and animals at along tower lines and substations, combined with the risk of corrosion, high seasonal variation and increasing copper theft from substations, it is important to explore the feasibility of online continuous monitoring of earthing systems. New methods based on clamp on injection and measurements were shown to be suitable for such online monitoring. It is, therefore, proposed that a full system is developed as a follow up to this project.

(v)          Deliverables 10 & 11: Induced Capacitive Effects on Floating Plant

In large substations, it is common to find, at height, de-energised/floating circuit breaker terminals, conductors or even busbars adjacent to energised conductors/busbars. Significant voltages can be induced these non-energised terminals through capacitive coupling with adjacent energised circuits. If not adequately considered during maintenance work, these induced voltages, which are usually more than 10 kV in magnitude, can constitute a significant hazard. In this project, an analytical model was developed to estimate these induced voltages for simplified geometries. A numerical methodology was also developed to consider more complex geometries. As a result of these findings, it is suggested that a new measurement system is developed in a follow-up project and implemented in future before maintenance work is started.

Lessons Learnt

The largest contribution of this project was in quantifying the safety aspects of the new T-pylon line. For this, the induced voltage and current effects on the de-energised circuit were quantified under steady state and transient conditions. Moreover, the earth resistances of various pylon constructions were determined both though measurements and computations. These were combined with the current flowing into the ground to quantify the rise of earth potential and safety voltages for various operating regimes, including induced effects conditions. The key finding of elevated step voltage at corners of the T-Pylons concrete base is different from other types of towers, e.g., lattice steel towers. For such lattice towers, the touch voltage is highest, and the step voltage becomes much smaller away from the tower. In contrast, for the large T-Pylons bases, the highest step voltage can be between 7 and 10 m away from the pylon tube while the touch voltage is close to zero near it. This can form a safety risk for personnel, people and animals around the T-Pylons. Future work has been identified to implement a mitigation technique for this risk through the use of grading electrodes to control the distribution of surface potential and, hence, reduce the step voltage magnitude at the edge of the base. Implications for the revision of safety instructions, NSI 5, will be beneficial for future maintenance work on such lines.

 
One of the key practical difficulties encountered with the project involved access to site for earthing and induced effects measurements, and these tests can be challenging to arrange and conduct due to various 

parties involved and satisfying all safety requirements for all involved.

Other lessons learned include:

(a)          the suitability of alternative software to conduct earthing numerical studies, 

(b)          conditions of use of low resistivity materials to benefit earthing resistance/impedances of various earth electrodes,

(c)          quantifying the induced voltages on floating electrodes in substation environments indicated high magnitude voltages of up to 12 kV. It is, therefore, important to evaluate or measure these induced voltages before work commences on such floating terminals. A methodology for the measurement is identified as future work, and 

(d)          for continuous monitoring of earthing systems, the preliminary work carried out in this project indicated the feasibility of achieving monitoring without separate injection using clamp on techniques. A new system adopting such concepts could form the core of a future project to allow alerts for changing earthing resistance/impedance, which can be caused by theft, corrosion, damage and seasonal variations.

Dissemination 

2024

"Bulletin Board: Report on the 16th UHVnet Colloquium, May 9–10, 2024," in IEEE Electrical Insulation Magazine, vol. 40, no. 5, pp. 48-52, September/October 2024, doi: 10.1109/MEI.2024.10646176.

M. Mokhtari et al., "An Analytical Model for Low Frequency Earthing System of the T-Pylon Transmission Tower," 2024 59th International Universities Power Engineering Conference (UPEC), Cardiff, United Kingdom, 2024, pp. 1-5, doi: 10.1109/UPEC61344.2024.10892483.

A. R. Justo de Araújo et al., "Quantification of Induced Effects on T-pylon Transmission Lines," 2024 59th International Universities Power Engineering Conference (UPEC), Cardiff, United Kingdom, 2024, pp. 1-6, doi: 10.1109/UPEC61344.2024.10892566.

2025

·       1 paper and a keynote speech at the 17th UHVNet (Universities High Voltage Colloquium) in Liverpool from 2–3 June 2025. 

·       1 paper accepted to the 24th ISH, (International Symposium on High Voltage Engineering) from 24-29 August 2025 in Karuizawa, Japan. 

·       1 paper is being elaborated to the 60th UPEC (Universities Power Engineering Conference), Brunel University from 2–5 September 2025.