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State of the Art Distribution Earthing Design

The Evolution of Distribution Earthing Design article (see T&D issue September 2023 or Safearth Website) gave an overview of how distribution earthing design has been changing. This article discusses typical situations where standard (common) earthing designs result in unacceptable safety risk, particularly from transferred voltages to the LV street neutral supplying home and business installations, and modern methods available to unravel and solve design complexities.

What makes for more difficult earthing designs?

  • Slower protection clearing times
  • Smaller MEN areas
  • Minimal network earthing interconnections
  • Higher soil resistivity of the lower layer of soil, decreases the gradient of the soil voltage profile and hence typically increases the needed separation distance to third party assets.
  • Lower soil resistivity of the lower layer of soil, increases the gradient of the soil voltage profile and hence may increase the touch voltage to affected assets.
  • Higher HV earth fault levels increase the EPR and associated voltages.
  • Higher supply voltages increase the maximum possible EPR.
  • Close third-party assets (especially water or gas valves) mean touch voltage are likely to exceed allowable limits.

Sometimes determining a complying design solution seems unachievable at reasonable expense. In such cases it is worth investigating if money may be better spent making earthing connections to existing upstream or downstream earthing areas.

Another complication for the distribution earthing designer is conductive poles. Often conductive poles carrying HV and LV are bonded to the LV neutral. If this is the case a HV fault to the pole will transfer the resulting EPR (earth potential rise) into the LV neutral. This means conductive HV poles with bonded LV neutrals should be checked for compliance with appropriate safety criteria too. Ideally this is done for the section of the network before conductive poles are employed.

CASE STUDY 1: Kiosk substation for new subdivision, 22kV supply from overhead line

The brief for this case is typically as follows. A new URD subdivision of 50 lots is proposed to be supplied from a neighbouring overhead 22kV distribution feeder. The earthing design brief is to extend supply from the nearest pole via an underground cable to a kiosk substation and provide a design that meets the requirements of the relevant Australian Standards and in line with best practice as per ENA Power System Earthing Guide (EG-0) Part 1 Management Principles. (Note: Safearth’s STREETS Distribution DesignerTM has been used for the analysis that follows.)

The designer follows the typical design process:

  1. Obtain the relevant design inputs from the DNSP:
  • Earth fault (E/F) level at the proposed cablehead pole (CHP) = 2.5kA
  • Fault clearing time = 0.35s
  • Upstream E/F protection settings = IEC SI relay curve, TMS = 0.12, PU = 120A
  • Zone Substation (ZS) grid resistance = 0.2Ω
  • Standard drawings for CHP and Kiosk earthing
  • The E/F capacity of the three core 22kV cable = 10kA
  • The HV and LV plans of the electrical network around and supplying the new project site.

  1. Determine other relevant information:
  • Soil resistivity via Wenner tests at the site
  • A suitable soil resistivity model based on the Wenner test data
  • Cable length between the proposed CHP and kiosk substation.
  • Any relevant network earthing connections that could or should be modelled as part of the earthing design.
  • Any existing third-party assets in the vicinity of the project such as metallic fences, accessible parts of metallic pipelines (e.g. exposed pipe section, valves, CP test points), houses, buildings, Telco pits or pillars with metallic communications cables, public or domestic swimming pools, schools, or other unusual areas where people may be exposed to earthing related hazards during a HV fault on the proposed assets for the URD subdivision. In greenfield sites like this, if a concept earthing design is completed very early in the development process, relevant third-parties can be advised of required separation distances from the earthing electrodes and a more cooperative solution determined.
  1. Develop suitable models for soil, standard earth grids, and cable screens

At this stage the designer needs to consider what range of soil resistivity models are probable at this location. Soil moisture levels affect soil resistivity, so it makes sense to check how the design varies with variation in soil resistivity. As an aside there is a strong case to develop an understanding of this variation for normal, drought and wet conditions.

The total cross-sectional area of the HV cable screens connecting the kiosk to the CHP is the key factor at this point rather than the voltage or CSA of the phase conductors.

  1. Calculate EPRs and check compliance with relevant standards.

The screenshot below shows the system model, the calculated impedances, as well as EPR at the new kiosk and CHP.

STREETS® allows the designer to select as many hazard voltage assessments as are relevant to the design under consideration and calculates the modelled voltage difference between the asset in question and the soil 1m from the asset (touch voltage). Typical assessments are shown in the screenshot below. The area on the right being the nominated Australian Standards and Safety Criteria selected, and the area on the left showing the allowable voltage and if compliance is achieved for the direct touch voltages (transfer out) for the kiosk (DU) and the LV neutral (TDMEN) and required in-ground separation distances from third-party assets (Telco pits and pillars with conductors, valves on metallic pipes etc).

The analysis below shows that the standard common earthing arrangement for the new kiosk substation:

  • will satisfy the AS2067:2016 DU (distribution urban) touch voltage criteria. This means the Quantitative Risk Assessment (QRA) assessment of the hazard is lower than 10-6 .
  • but will transfer an unacceptably high (>6,700V) into the local neutral.
  • Further, the separation distances to other third-party assets range from 6m to about 100m.

Clearly this design is not acceptable, and an alternative earthing arrangement is required. Alternatives are typically a separate HV-LV earthing design or if a large MEN area is nearby, a connection to it.

CASE STUDY 2.  Earthing connection(s) to a large MEN area nearby
The designer can use STREETS® to assess what input impedance would be required for a common HV-LV design to be compliant. This earthing input impedance could be in the form of cable screens and/or LV neutrals (e.g. bonding back to the source zone substation and/or a large MEN area).  In this example an input impedance of about 0.17Ω would be required and using the MEN modelling function this would equate to a subdivision size of about 900 lots for this soil model. Although not shown in this example, STREETS® allows further detailed study of inductive cable effects leading to higher screen or neutral fault current return and reduced EPR, voltage hazards and exclusion zones. This is of significant importance when balancing land use maximisation while designing to acceptably low safety risk.

CASE STUDY 3. Separate earthing design

While it may not be the preferred standard approach, often in isolated areas a separated earthing design is the only reasonable solution to manage earthing risk. In separate earthing arrangements the HV and LV earths are not bonded together, and the designer needs to determine how far to separate the LV grid from the HV grid so that the voltage transferred via the soil from the HV electrodes to the LV electrodes is acceptably low.

This distance is affected by several factors including the length of the electrodes and the soil resistivity model. This approach usually requires greater separation distances to third-party assets if they are to comply with the relevant Australian Standards.

Clearly the common earthing design requires far less separation from third-party assets (for example 8m vs 103m to a water valve) but to achieve this a 900 house MEN area is required. Designing for separated or common earthing has obvious implications on exclusion zones around a distribution substation which can impact choice of substation location and quarantining of development land.

Conclusion

We have briefly examined the complexities of distribution earthing design and shown how an electrical designer can use STREETS® to rapidly determine an earthing design that will provide the lowest risk and most cost-effective solution that satisfies the relevant Australian Standards.

By Gordon McMurray – Specialist Engineer, Safearth and Matthew Bale – Engineering Director, Safearth

For more information: enquiries@safearth.com or 1800 327 844