1 GroundRod AC Substation Earthing Tutorial 1. Functions of an earthing system The two primary functions of a safe earthing system are: To ensure that a person who is in the vicinity of earthed facilities during a fault is not exposed to the possibility of a fatal electric shock. To provide a low impedance path to earth for currents occurring under normal and fault conditions. 2. Earthing standards There are a variety of national and international standards available, which provide empirical formulae for the calculation of earthing design parameters and shock potential safety limits. There is some variation in formulae between the different standards. Three standards, which are widely referred to, are: BS 7354-1990: Code of practice for Design of high-voltage open-terminal stations. IEEE Std 80-2000: IEEE Guide for Safety in AC Substation Grounding. Electricity Association Technical Specification 41-24: Guidelines for the Design, Installation, Testing and Maintenance of Main Earthing Systems in Substations.
2 3. Ground potential rise (GPR) The substation earth grid is used as an electrical connection to earth at zero potential reference. This connection, however, is not ideal due to the resistivity of the soil within which the earth grid is buried. During typical earth fault conditions, the flow of current via the grid to earth will therefore result in the grid rising in potential relative to remote earth to which other system neutrals are also connected. This produces potential gradients within and around the substation ground area as depicted in Figure 1. This is defined as ground potential rise or GPR. The GPR of a substation under earth fault conditions must be limited so that step and touch potential limits are not exceeded, and is controlled by keeping the earthing grid resistance as low as possible. 4. Step, touch, mesh and transferred potentials In order to ensure the safety of people at a substation, it is necessary to ensure that step and touch potentials in and around the yard during earth-fault conditions are kept below set limits. These maximum permitted step and touch potentials are addressed within various national and international standards. An illustration of step, touch, mesh and transferred potentials is provided in Figure 1. 4.1 Step potential The step potential is defined as the potential difference between a person s outstretched feet, normally 1 metre apart, without the person touching any earthed structure. 4.2 Touch potential The touch potential is defined as the potential difference between a person s outstretched hand, touching an earthed structure, and his foot. A person s maximum reach is normally assumed to be 1 metre. 4.3 Mesh potential The mesh potential is defined as the potential difference between the centre of an earthing grid mesh and a structure earthed to the buried grid conductors. This is effectively a worst-case touch potential. For a grid consisting of equal size meshes, it is the meshes at the corner of the grid that will have the highest mesh potential. 4.4 Transferred potential This is a special case of a touch potential in which a voltage is transferred into or out of a substation for some distance by means of an earth referenced metallic conductor. This can be a very high touch potential as, during fault conditions, the resulting potential to ground may equal the full GPR.
3 Figure 1: Basic shock situations 4.5 Maximum permitted step and touch potentials The maximum permitted values of step and touch potentials vary widely between the different standards. The value of maximum permitted touch potential has a dominant role in determining the design of the earthing grid. As a general rule, if an earthing grid design satisfies the requirements for safe touch potentials, it is very unlikely that the maximum permitted step potential will be exceeded. The IEEE 80 standard uses the maximum mesh voltage as the touch voltage, and this usually exists at the corner mesh. UK practice defines the touch voltage differently. In practice the voltage at the surface of the ground is a maximum adjacent to a corner of a grid. UK practice is to define touch voltage as the sum of the step voltage plus the voltage difference between the ground surface adjacent to a corner and the grid beneath. Although the mesh voltage is used as the defining touch voltage in American practice, the maximum permitted touch voltage used is less than that used in British Standards. In practice, compliance with American usage thus also ensures the arrangement will comply with UK requirements. CENELEC have issued a harmonisation document HD 637 S1 containing references to the maximum body impedance and permitted touch voltage. 5. Soil resistivity In order to calculate the GPR, the grid resistance firstly needs to be calculated. To do this, the resistivity of the soil at site needs to be determined. This should ideally be obtained via site measurements but if not possible can be determined from soil resistivity maps or tables available in the standards. The Wenner test method is commonly used for site measurements.
4 The soil resistivity can vary quite widely over a site and it is thus important that the measurements are taken at several points in the site area. The average site value is thus calculated. Due to the difference in resistivity with depth, two or more layer resistivity models are normally determined. 6. Design considerations 6.1 Conductors A substation earthing grid will consist of a system of bonded cross conductors. The earthing conductors, composing the grid and connections to all equipment and structures, must possess sufficient thermal capacity to pass the highest fault current for the required time. Also, the earthing conductors must have sufficient mechanical strength and corrosion resistance. It is normal practice to bury horizontal earthing conductors at a depth of between 0.5m and 1m. This ensures that the conductor has the following properties: Adequate mechanical protection. It is situated below the frost line. The surrounding earth will not dry out. 6.2 Vertically driven rods Where there are low resistivity strata beneath the surface layer then it would be advantageous to drive vertical rods down into this layer. To be effective the vertical rods should be on the periphery of the site. The length of rod is chosen so as to reach the more stable layers of ground below. The rods would stabilise the grid resistance over seasonal resistivity changes at the grid burial depth. 6.3 Substation fences The earthing of metallic fences around a substation is of vital importance because dangerous touch potentials can be involved and the fence is often accessible to the general public. Fence earthing can be accomplished in two different ways: Electrically connecting the fence to the earth grid, locating it within the grid area or alternatively just outside. Independently earthing the fence and locating it outside the grid area at a convenient place where the potential gradient from the grid edge is acceptably low. In America, the common practice is to extend the grid sides to 1 metre beyond the fence line. The common practice in the UK is to erect the fence away from the grid sides, typically 2 metres, and to earth the fence independently. This will, however, present a problem should the fence inadvertently be connected to substation equipment, and hence the earthing grid. 6.4 Other earthing The GPR at a substation is reduced by: Overhead line earth wires which are connected to the substation earthing grid. This diverts part of the earth fault current to the tower footing earthing.
5 Cables entering and leaving the site. The armouring of such cables is usually earthed to the substation earthing grid at both ends. Part of the earth fault current will thus be diverted to a remote earthing grid via the cable armouring. 7. Earthing design calculations Performing earthing design calculations, using one of the standards above, is an involved and time consuming process and there are various subtleties which need to be considered. The GroundRod spreadsheet provides an easy-to-use, fast and accurate means to perform these calculations. The program can perform the calculations in accordance with any of the three above standards. 8. Hot Zone In order to protect telecommunications staff, equipment and users, the International Telecommunication Union (ITU) has provided recommended limits for acceptable GPR in MV and HV networks. These limits have traditionally been used in the electrical industry as 430V for circuits with a fault clearance time of 200ms and 650V for fault clearance times normally less than 200ms (high reliability circuits). In addition, if the GPR exceeds these limits then the 650V or 430V surface potential contour extending into the ground surrounding the grid needs to be specified and is termed the Hot Zone (measured from the edge of the grid). The relevant telecommunications company needs to be notified when these limits are exceeded and what the extent of the Hot Zone is. The GroundRod program provides both these distances and advises that the latter is best practice. For further information on GroundRod please contact: Mr Richard Simmonds Cobham Technical Services ERA Technology Ltd. Cleeve Road Leatherhead Surrey KT22 7SA UK Tel + 44 (0) 1372 367073 richard.simmonds@cobham.com www.cobham.com/technicalservices ERACS Software Cobham Technical Services has also combined world-class electrical engineering experience with leading-edge computer technologies to produce ERACS, the new generation of power systems analysis software. The fully integrated suite features loadflow, fault, harmonics, protection coordination, arc flash and transient stability modules together with an intuitive graphical user interface, universal dynamic modeller and equipment data library. For further information on ERACS please visit www.era.co.uk/eracs