Why do electrical installations need to be earthed?

The correct specification of the earthing system for a high voltage electrical installation is critical for its safe functioning. Under specification of the earth grid (buried earth electrode) can lead to dangerously high touch and step voltages during system fault conditions. Over specification of the earth grid can lead to unnecessary expense. An optimum specification can both demonstrate compliance with national/ international safety standards, and minimise the cost of buried electrode.

Earth grid design - typical process• collect data: site layout, soil resistivity measurements, earth fault current levels/fault current scenarios and fault durations• size earth electrodes and conductors for adequate thermal and mechanical strength• determine tolerable step and touch voltages based on protection operating times

• carry out a preliminary design of the earth grid. The preliminary design usually encompasses the items to be earthed and provides sufficient cross conductors for all plant to be conveniently earthed• determine distribution of fault current, i.e. how much fault current flows through the earth grid and how much flows through cable sheaths/overhead line earth wires back to the source• calculate touch voltages on exposed equipment and step voltages in and around the vicinity of the installation. If safety criteria are not met, the earth grid must be extended by using horizontal conductors and/or buried earth rods• verify that the earth grid impedance is sufficient for lightning protection requirements• if the site is ‘hot’ from an earth potential rise perspective, consider extending the earth grid to make the site ‘cold’, or employ mitigation measures such as electrical isolation of incoming telecommunication circuits, separation of HV and LV earth systems etc• once the earth grid has been installed, its resistance needs to be confirmed using on-site measurements

HV Earthing Design ServicesSenergy Econnect’s consulting team provide specialist advice on all aspects of the connection of renewable generation to the electrical network, from initial feasibility to design, project management and advice on regulatory and financial challenges. Our team has facilitated the successful commissioning of projects worldwide, including many that would not have happened without our input.

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CDEGS from SES Technologies

CDEGS (Current Distribution, Electromagnetic Fields, Grounding and Soil Structure Analysis) is the industry leading software package for earthing system analysis and design. The MultiGroundZ+ package contains the following modules:• RESAP – determines equivalent soil structure models from on-site soil restivity measurements• MALT – low frequency earthing analysis• MALZ – frequency domain earthing analysis• TRALIN – computes conductor and cable parameters• FCDIST – calculates fault current distribution in distribution and transmission circuits• SPLITS – detailed fault current distribution and EMI analysis

Why use Senergy Econnect?

Senergy Econnect uses the CDEGS MultiGroundZ+ software package and has many years of experience designing earthing systems for HV installations and coupled with its use of the CDEGS Multiground+ software package can provide optimised earth grid designs for HV installations.

Examples of earthing design projects Senergy Econnect has completed are:• Cottonmount 11kV 2MW landfill gas generation site• Craig wind farm 11kV 10MW• Gunfleet Sands 132kV cable joints pits• Braich Ddu 11kV 3.9MW wind farm• Tangy wind farm 33kV reactive power compensation compound• Brookhurst Wood 33/11kV substation for energy from waste facility• New Holbeck Farm wind turbine and distribution substation (11kV) • Little Raith wind farm 25MW (33kV)• Record Hill wind farm (Maine, US) 51MW (34.5kV)

Senergy Econnect designs earth grids to minimise the amount of buried electrode required by:• considering alternative fault current paths• considering adjacent earth grids which can be tied to the earth grid being designed• consider structural rebar in concrete foundations as an integral part of the earth electrode

Standards

• BS 7430:1998, Code of practice for Earthing• BS 7354:1990, Code of practice for Design of high-voltage open-terminal stations• BS EN 62305:2006, Protection against lightning• Draft BS EN 50522:2008, Earthing of power installations exceeding 1kV a.c.• IEC 61936-1 Edition 2.0 2010-08 Power installations exceeding 1 kV a.c. – Part 1: Common rules• Electricity Association Technical Specification 41-24 Issue 1: 1992, Guidelines for the design, installation, testing and maintenance of main earthing systems in substations• The Electricity Council Engineering Recommendation S.34 1986, A guide for assessing the rise of earth potential at substation sites• IEEE Std 80-2000, IEEE Guide for Safety in AC Substation Grounding

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• choosing a combination of horizontal buried conductors and/or driven vertical earth rods depending on analysed soil models

Designs are clearly presented in AutoCAD DWG format and a detailed report will contain touch potential, step potential and hot zone contour plots.

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