EDS GRID AND PRIMARY SUBSTATION EARTHING DESIGN

Transcription

1 THIS IS AN UNCONTROLLED DOCUMENT, THE READER SHALL CONFIRM ITS VALIDITY BEFORE USE Document Number: EDS ENGINEERING DESIGN STANDARD EDS GRID AND PRIMARY SUBSTATION EARTHING DESIGN Network(s): EPN, LPN, SPN Summary: This standard details the earthing design requirements for grid and primary substations and 132kV and 33kV connections. Owner: Allan Boardman Approved By: Steve Mockford Approved Date: 22/05/2015 This document forms part of the Company s Integrated Business System and its requirements are mandatory throughout UK Power Networks. Departure from these requirements may only be taken with the written approval of the Director of Asset Management. If you have any queries about this document please contact the author or owner of the current issue. Circulation UK Power Networks All UK Power Networks Asset Management Capital Programme Connections External G81 Website Contractors ICPs/IDNOs Meter Operators HSS&TT Network Operations UK Power Networks Services Other

2 Revision Record Version 3.0 Review Date 05/05/2017 Date 05/05/2015 Author Stephen Tucker Why has the document been updated: Periodic document review. Minor revision to include generation connections and ensure consistency with the earthing construction standard ECS while the review of national standards ENA TS and ENA ER S34 is being carried out. What has changed: Reference to generating station exclusion removed (Section 1 and Appendix C). Scope expanded to specifically include 132kV and 33kV connections including solar and wind farm generation (Section 2). Guidance on fault level for electrode sizing added and conductor sizes revised (Section 5.14 and Appendix F). Lightning protection reference updated (Section 5.16 and 6.2). Mobile phone base stations on towers reference added (Section 5.22). Bonding requirements for ancillary metalwork, metal trench covers, cable tunnel metalwork and basement cable support systems revised (Section 5.30). Version 2.0 Review Date 31/03/2015 Date 11/03/2013 Author Stephen Tucker Review date extended to tie in with the review of national standards ENA TS and ENA ER S34 Version 1.3 Review Date 31/03/2013 Date 22/08/2012 Author Stephen Tucker Reviewed for publishing on G81 website Version 1.2 Review Date 31/03/2013 Date 03/08/2011 Author Stephen Tucker Reclassification and reformatting of document from Earthing Design Manual Section 3. References to standard earthing arrangements updated Version 1.1 Review Date 31/03/2013 Date 17/03/2011 Author Peter Rix Document rebranded Version 1.0 Review Date 31/03/2013 Date 31/03/2008 Author Neil Fitzgerald Original UK Power Networks 2015 All rights reserved 2 of 59

3 Contents 1 Introduction Scope Abbreviations Overview Data Requirements Earth Potential Rise (EPR) Estimation Standard Design Detailed Design Review of Existing Substations Detailed Design Procedure Determination of Soil Resistivity Preliminary Site Assessment Initial Design of the Earth Grid - Standard Earthing Arrangement Initial Design of Earth Grid - Non-Standard Earthing Arrangement Calculation of the Earth Grid Resistance First Approximation Computer Modelling Calculation of the Grid or Overall Earth Impedance (taking into account parallel paths) The Earth Grid Bonded Foundation Structure Steel Reinforcement Bars Steel Tower Lines Cable Networks Fortuitous Earth Paths Data Required from the Power System Fault Study Earth Fault Current Returning via Earthwire/Cable Sheaths Due to Induction Introduction Overhead line with Earth Conductor Cables Calculation of the Earth Potential Rise (EPR) Primary Substations Grid Substations (including 132 kv and 33kV Connections) Calculation of External Potential Contours or Zones of Influence (e.g. HOT Zone) Accurate Representation Approximate Calculation UK Power Networks 2015 All rights reserved 3 of 59

4 5.11 Implications of a Substation being HOT Reducing the Area Covered by the HOT Zone Reduce the Earth Fault Current Reducing the Electrode Resistance Reduce the Impedance of Parallel Paths Calculation of Touch and Step Potentials Accurate Calculation Approximate Method Requirements of the Final Design Conductor Size and use of Metallic Structures Earthing Arrangements Applicable to 400/275/132kV Sites of Different Companies Earthing Arrangements Applicable to Sites with Generation Earthing Arrangements at Railway Supply Substations Earthing Arrangements at GIS Substations Earthing Arrangements for Surge Arresters and Capacitor VTs Positioning of Metal Supports for Security Lighting etc Near Fences Communication Towers within or adjacent to Substations Communication Masts Fitted to Towers LV Supplies to Third Party Equipment at Substations Reactors and AC to DC Converters Use of Earth Plates (cast iron pipes etc) Earthing of Instrument Transformer Windings Earthing of Cables Power Cables Protection and Control Cables General Guidance for Achieving Electromagnetic Compatibility (EMC) Earthing of Fences Adjacent Metal Structures Palisade Gates and Removable Fence Panels Metal Anti-climbing Guards Temporary, Site-Perimeter and Adjacent Landowners' Fencing Safety Advantages of a Separately Earthed Fence Metalwork Bonding Requirements Ancillary Metalwork Metal Trench Covers Cable Tunnel Metalwork Basement Cable Support Systems UK Power Networks 2015 All rights reserved 4 of 59

5 6 References UK Power Networks Standards National and International Standards Appendix A Standard Substation Earthing Arrangements Appendix B Calculation of Fault Current returning via the Ground B.1 Calculation of Fault Current Returning via the Ground B.2 Example of Cable Information Necessary where Network Reduction is Required to Estimate Earth Contribution from Cable Network B.3 Examples of Fault Current Information Required for Fault Current Reduction Appendix C Special Conditions Applicable to Generating Stations within or Adjacent to a Substation Appendix D Maximum Resistance Values for Electrodes at Pole-mounted Plant Appendix E Standard Substation Earthing Arrangements Resistance Values, Surface, Touch and Step Potential Contours E.1 132/33kV Substation Arrangements (EDS /EDS ) E.2 33/11kV Substation Arrangement Option 1 (EDS ) E.3 33/11kV Substation Arrangement Option 2 (EDS ) Appendix F Minimum Conductor Sizes Tables Table 5-1 Calculation of Ground Current for Cable Faults Table 5-2 Earthing and Bonding Electrode/Conductor Sizes Table 5-3 Tape and Stranded Conductor Specifications Table 5-4 Maximum Current Rating of Earthing Rods UK Power Networks 2015 All rights reserved 5 of 59

6 Figures Figure 4-1 Making a First Estimate of the Substation EPR... 9 Figure 5-1 Earthing System Components Figure 5-2 Estimating the Proportion of Fault Current Returning Through the Soil Figure 5-3 Scale Plan of Substation Showing Site Boundary Surface Potential Contours 23 Figure 5-4 Use of Separately Earthed and Bonded Fencing Arrangements at the Same Substation Figure 5-5 Separately Earthed Fence 2m away from Earth Grid Figure 5-6 Separately Earthed Fence 500mm away from Earth Grid Figure 5-7 Earth Grid Bonded incorrectly to Fence, which is 2m away from Earth Grid Figure 5-8 Earth Grid Bonded incorrectly to Fence, which is 500mm away from Earth Grid Figure 5-9 Fence 2m away from Earth Grid, Fence and Earth Grid Bonded with Potential Grading 1m away Figure 6-1 Calculation of Fault Current Returning via the Ground in a 132kV Network Figure 6-2 Calculation of Fault Current Returning via the Ground in a 66kV or 33kV Network Figure 6-3 Example of Cable Network Information Required Figure 6-4 Example showing 132kV Phase Currents for Transformer-feeder Arrangement Figure 6-5 Example showing 132kV Phase Currents for a Parallel-feeder Arrangement.. 50 Figure /33kV Substation Electrode System Surface Potential Contours Expressed as a % of the EPR Figure /33kV Substation Electrode System Touch Potential Contours Expressed as a % of the EPR Figure /33kV Substation Electrode System Step Potential Contours Expressed as a % of the EPR Figure /11kV (Option 1) Substation Electrode System Surface Potential Contours Expressed as a % of the EPR Figure /11kV (Option 1) Substation Electrode System Touch Potential Contours Expressed as a % of the EPR Figure /11kV (Option 1) Substation Electrode System Step Potential Contours Expressed as a % of the EPR Figure /11kV (Option 2) Substation Electrode System Surface Potential Contours Expressed as a % of the EPR Figure /11kV (Option 2) Substation Electrode System Touch Potential Contours Expressed as a % of the EPR Figure /11kV (Option 2) Substation Electrode System Step Potential Contours Expressed as a % of the EPR UK Power Networks 2015 All rights reserved 6 of 59

7 1 Introduction This standard (previously Section 3 of the Earthing Design Manual) details the earthing design requirements for grid and primary substations and associated connections at 132kV and 33kV. Definitions for the terms used and a catalogue of reference documents associated with earthing practice and the design criteria are detailed in EDS ECS provides construction guidance for grid and primary substations. There will be some situations where advice from an earthing specialist is required refer to EDS for further details. 2 Scope This standard applies to earthing design at: All new grid and primary substations. Existing grid and primary substations where a material alteration (including a significant increase in fault level) is to take place. All new connections at 132kV and 33kV (including solar and wind farm generation). Note: A new document (EDS ) has been prepared to provide additional guidance on all aspects of EHV and HV customer earthing. This document is intended for internal and external use. 3 Abbreviations Term DigSILENT PowerFactory EHV HV LV EPR ROEP Definition The power system analysis software used by UK Power Networks Extra High Voltage. Refers to voltages at 132 kv, 66kV and 33kV High Voltage. Refers to voltages at 20kV, 11kV and 6.6kV Low Voltage. Refers to voltages up to 1000V, typically 400V 3-phase and 240V single-phase Earth potential rise Rise of earth potential UK Power Networks 2015 All rights reserved 7 of 59

8 4 Overview 4.1 Data Requirements Before starting the following basic data is required: Substation layout drawing and an earthing drawing for existing substations. Plan of surrounding area (100m radius) with buildings and other utility services shown. Supply circuit type to source (cable or overhead line). Source earth fault current (see Section 5.7). Outgoing circuit type. Geographic plan showing existing bare metal sheathed or bare wire armoured cables and proposed cable routes within a 500m radius of substation. Outgoing earth fault current (max see Section 5.7). Thermal rating in ka required for earthing system. If in a rural location, estimate of soil resistivity. 4.2 Earth Potential Rise (EPR) Estimation This process is necessary as a first indication as to whether earthing is likely to be a significant issue at the site. If it is, then a more detailed design procedure can be followed from the outset. If not, then a standard arrangement with little or no further calculation should suffice. The main factors to consider are the incoming supply circuit type, whether the location is in ground that is detrimental to earthing (within infill material or on rocky ground), the outgoing circuit type and the fault level. The decision that can be reached by consideration of these factors is set out below. Using the flowchart in Figure 4-1 make a first estimate of the EPR. If the EPR is less than 430V follow the standard design procedure outlined in Section 4.3. If the EPR is greater than 430V follow the detailed design procedure outlined in Section 4.4. UK Power Networks 2015 All rights reserved 8 of 59

9 START Supply and outgoing circuits via cables? Yes No Supply circuits via towers and outgoing circuits cables? Yes Gross earth fault current less than 3kA? No No Carry out first estimates of fault current distribution (Section 5.2) From approx soil data and grid dimensions, estimate the approx grid resistance (see Section 5.5) and multiply by approx earth fault current No Gross earth fault current less than 500A? Yes EPR significantly less than 430V? Yes No Yes EPR V and sensitive installations nearby e.g. petrol storage (Section 5.12)? No Yes Use detailed design procedure (Section 4.4) Use standard design (Section 4.3) Figure 4-1 Making a First Estimate of the Substation EPR 4.3 Standard Design The standard design procedure can be used if the EPR is less than 430V and is outlined below. 1. Design earthing system as per standard design layout or approach (see Sections 5.3 and 5.4). 2. Lay earthwire with outgoing cable routes for 150m. UK Power Networks 2015 All rights reserved 9 of 59

10 4.4 Detailed Design A more detailed design procedure is required if the EPR is greater than 430V and is outlined below. 1. Obtain soil resistivity data or structure (see Section 5.1). 3. Design earthing system to optimise resistance in relation to soil structure or to avoid third party equipment (see Section 5.3 and 5.4). 4. Calculate earth grid resistance (see Section 5.5). 5. Calculate effect of parallel earth paths (Hessian cables, earthed tower lines, bare earthwire laid direct in ground with cables) (see Section 5.6). 6. Calculate overall earth impedance (see Section 5.6). 7. Obtain site specific earth fault current data and analyse how current splits to determine ground component (see Section 5.7 and 5.8). 8. Make more accurate calculation of EPR (see Section 5.9 and 5.9.2). If EPR <430V revert to standard design procedure (see Section 4.3). 9. Calculate touch and step potentials and compare to limits. Modify design to achieve compliance (see Section 5.12). 10. Estimate external voltage contours and effect on third parties. Liaise with third parties if necessary (see Section 5.10). 11. If significant mitigation costs likely, modify design to reduce impact if possible (see Section ). 12. If EPR exceeds threshold values, list additional design requirements and actions necessary. Produce HOT zone plot for third parties (see Section 5.11). 13. Augment design to cater for site specific equipment and arrangement (see Sections 5.13). For example, structure earths (5.14), surge arresters (5.19), security lighting (5.20) etc. 14. Select tape and joint sizes to match thermal and corrosion requirements (see Section 5.14). 15. Produce construction drawing. 4.5 Review of Existing Substations The assessment of existing substations for compliance with ENA ER S36 or this standard is covered in ECS Examples of the circumstances where this may be required are as follows: On receipt of a specific enquiry from a third party. Following a system incident. When modification work is being considered. Following changes to system fault levels. UK Power Networks 2015 All rights reserved 10 of 59

11 5 Detailed Design Procedure 5.1 Determination of Soil Resistivity An initial estimation of the soil resistivity can often be obtained from the EPR database or from published geological survey information. If the more pessimistic values of soil resistivity produce an acceptable EPR result then this may be used, otherwise soil resistivity measurements should be carried out according to Section Preliminary Site Assessment Before carrying out work at a green field site, the civil engineer will normally require a geotechnical survey. Wherever practicable, the same company should be required to carry out the soil resistivity and ph testing at the site. If boreholes are to be drilled, it may be possible for their positions to be selected such that they are suitable for earthing, whilst also providing the necessary data for the civil engineer (for example located just beyond the corners of the proposed building). On completion, 25mm x 4mm copper tape or stranded 70mm 2 conductor can be installed in each borehole prior to backfilling. The conductor should extend by about 500mm above the hole and this part is to be buried just below the surface of the soil to allow for later connection to the earth grid. The hole is to be backfilled with local soil or material that is non-corrosive to copper and electrically conductive. Concrete, soil, Bentonite or Marconite are all suitable for this purpose. The design engineer should avail himself of the Geo-technical Engineer s report plus any other published geological information relating to the site. The chemical analysis should include an assessment of the rate of corrosion to copper, lead and steel (normally the above average presence of chemicals such as chlorides, acids or sulphates increase the corrosion rate) and testing the ph value At an existing site, the buried electrode should be revealed at a number of locations and inspected to determine the conductor size, type and condition especially to see if there is any evidence of corrosion. If corrosion is evident, the new electrode size shall be increased and the copper tape surrounded by a minimum of 150mm radius of correct value ph soil. This may need to be imported if sufficient quantity is not available from other parts of the site. Stranded copper conductor should not be used, but if essential for small parts of the site the conductor shall have a small number of large cross sectional area strands. Monitoring checks (e.g. testing with clip on meters) shall also be included in the maintenance regime, together with a plan to alleviate any damage caused by corrosion to the existing conductor. Ash, cokebreeze or any other type of imported material which is corrosive to copper shall not be used as backfill. As an alternative to replacing the soil, the electrode may be surrounded by low sulphur content concrete, preferably with graded carbonaceous aggregate in place of the conventional sand or aggregate for a radius of 50mm. Marconite is suitable for this, but is more expensive. UK Power Networks 2015 All rights reserved 11 of 59

12 5.3 Initial Design of the Earth Grid - Standard Earthing Arrangement The standard is to make the substation safe and then to render the site COLD where practicable at reasonable cost. If it appears that extensive, costly modifications would be required to make the site COLD, an assessment shall be made of the costs involved in declaring the site HOT and this compared to the cost of extending the earthing. In most cases a compromise will provide the best solution, i.e. some additional earthing work will be needed to reduce the EPR, but to a level where the site is still HOT. It is preferable to use one of the standard designs given Appendix A but if these are not applicable, follow the general guidelines below to carry out the initial design of the earth grid. Ideally all services (such as water) should have standard insulated arrangements, to avoid possible transfer potentials. Metallic services should preferably be replaced with plastic type from 2m beyond the substation perimeter fence. If there is some uncertainty as to whether the site is HOT or not, it is sensible to introduce some of the less costly precautions at the construction stage. For example, insist on a plastic piped water supply and arrange for isolation units on any BT circuits. On the scale plan of the site, showing the plant arrangement, plot an earth grid to the following specification: 1. An outer (perimeter) loop of standard copper electrode should be installed, inside the fence line and ideally a minimum of 2m away (inside) from any metal fencing, at 600mm depth (installed in plastic duct where cable routes cross the earth tape). In addition, the electrode conductor should ideally be 1 metre outside any exposed metalwork of plant within the substation. This means in practice, that the gap between any item of plant and the metal fencing should ideally be 3m. The outer ring should encapsulate an area as large as possible. It is possible in many cases to allow the loop conductor to extend closer to the perimeter fence, so long as the resulting fence touch voltages are within safe limits. At least 2m separation between the fence and internal earthed equipment shall be maintained in all cases. 2. Convert the outer loop to a mesh by positioning standard conductor across the site, in two directions (at 90 to one-other), each conductor being parallel to one of the outer conductors, where practicable. The cross - members should form rectangles, should be spaced a nominal 10m maximum apart on the outer edges of the grid or 12m maximum apart in the central areas and installed to a depth of 600mm. They will be joined to the outer ring and at each crossing point. The conductor routes should be selected to coincide with planned excavations (such as adjacent to transformer bund walls) and run close to equipment/structures that require connection. 3. Care shall be taken when planning the grid layout to ensure that critical components such as transformer neutral connection points, switchgear earth bars etc., are provided with direct and duplicated routes through to the perimeter loop electrode. 4. At or near to the connection point of each cross member to the perimeter loop electrode, and at its corners, install one 3.6m x 16mm copper clad earth rod. Longer rods may be necessary in some soils or to reduce the grid resistance. Where rods of more than 5m length are used, at least two connections to permit testing are to be used. They should be on opposite edges of the site and will permit rapid testing and electrode location. 5. Provision may be necessary in the design of the grid layout to provide connection points for temporary neutral earth resistors or arc-suppression coils (ASC) to replace the normal unit during maintenance, particularly if the unit is shared between two or more transformers. UK Power Networks 2015 All rights reserved 12 of 59

13 6. Use shall be made of sheet piles and reinforcing bars in concrete piles wherever practicable. This will improve the resistance value and reduce installation costs. If vertical piles are to be plastic lined, then some copper tape should be installed on the outer edge of the piles to provide a low cost vertical electrode. An earth loop conductor shall be installed around the outside of the foundation any new switchroom and be incorporated into the design (this will have been installed as a standard feature at the civil stage). Methods of connecting rebar, piles and sheet steel are covered in Section 4. Horizontal foundation slabs of transformers and switch rooms are to have two rebar connectors fitted, one at either end of the foundation. Switch room rebar for new GIS equipment requires special attention and this will be addressed by the manufacturer or installer. Where the vertical piles have more than 5m of metal reinforcement in them, 20% of them are to be bonded direct to the earthing system. These will be selected at corner locations, on the outer edges of the structure or at locations that will assist with high frequency impulse attenuation. At Network Rail traction supply points the main reason for not bonding the rebar is the possible presence of DC return currents. 7. The metal fencing shall have its own electrode system. Details of fence earthing arrangements are covered in Section The clearance between the fencing and the earth grid is to provide electrical separation between the two systems. Fence earth rods should be close to the fence and not infringe the clearance zone. Any metallic pipes or cables which run under the fencing at a standard substation should be installed in plastic duct 2 either side of the fence. Metallic support columns for lighting or security cameras shall not be installed within the 2m clearance zone. Also see section Note: Electrode buried shallower than 600mm deep cannot be considered in calculating the resistance value, but can be included in calculations related to touch and step potentials. 8. Routing of electrode. Avoid laying earth electrode close and parallel to bare metal pipes, Hessian served power or multi-core cables. This reduces the risk of them being punctured due to high currents or voltage transients on the electrode. Ideally a separation of at least 300mm should be maintained. Where this is not practicable, for short lengths, then PVC tape or a split plastic duct should be applied around the Hessian sheathed cable or bare metal water pipe. This insulation should be applied for 500mm either side of a position were the cable or pipe crosses an earth electrode and the length over which the 300mm separation cannot be maintained. It is particularly important to ensure that this insulation is applied where a long earth electrode terminates. The end of the electrode shall be bent away from the cable or pipe, to increase the separation at this point, in addition to the insulation. For plastic sheathed cables, the separation can be reduced to 0.15m. When routing electrode off site, either to reduce the overall earth resistance or to provide a connection to external equipment such as terminal poles, routes that may be frequented by people with bare feet or animals are to be avoided. These include routes near caravan sites, animal drinking troughs or across access gates to stables or milking parlours. Where electrode crosses land that is to be ploughed, if it cannot be located near to hedgerows and so shall cross open areas, it is to be installed a minimum of 1m deep. UK Power Networks 2015 All rights reserved 13 of 59

14 5.4 Initial Design of Earth Grid - Non-Standard Earthing Arrangement This design is similar to the standard design with the following exceptions: Where the electrode is a loop outside the fence, this standard electrode shall be installed 1 metre deep and between 700mm to 2m outside any metallic fencing. Where the grid is being extended either to make the substation COLD or to limit the EPR, the outer conductor may be spaced more than 1 metre away from the fence-line at a distance indicated in the design study. Depending upon wayleaves and practical issues, the outer electrode spacing from the fence may not be symmetrical. For example, it may be 1 metre away on one side of the site and 15m away on the other. An additional electrode 1m outside the fence may also be needed in the latter case, if the EPR is high, to reduce fence touch voltages The internal electrode design will follow the same procedure as in Section 5.3, except that the cross members will pass under the fence and be bonded to it (see Section 4). Equipment bonded to the grid can be close to the fence, as there will be no touch voltage issue. When these guidelines have been translated into an actual design, the arrangement will be similar to those shown in Appendix A. 5.5 Calculation of the Earth Grid Resistance First Approximation The soil resistivity value is first required. The procedure for carrying out the resistance calculation is detailed in ENA ER S34. Table 1 of S34 contains a series of formulae to calculate the resistance of various forms of buried electrode. This method always has a margin of error, which can be significant if the soil is not uniform, especially when the soil has high resistivity layers underneath (e.g. rock). Where the soil is not uniform or the earth grid is complex, specialist advice shall be sought Computer Modelling The electrode arrangement can be modelled by computer if required to provide a more accurate value. Specialist advice should be sought. 5.6 Calculation of the Grid or Overall Earth Impedance (taking into account parallel paths) When a fault having an earth fault current occurs, the current that returns to the source(s) through the ground will pass through the earth grid together with any connected parallel paths. Before calculating the EPR it is necessary to estimate how much of the fault current flows in this way and how much returns via metallic routes (such as cable sheaths), to avoid over-estimating the EPR. In an urban located substation, the impedance of the parallel paths will often be an order of magnitude lower than the resistance of the substation grid, so it is vital that their contribution is accounted for. This will prevent unnecessarily declaring a substation as HOT, or overdesigning the grid earth. To be considered, the parallel paths shall be reliable and capable of carrying their proportion of anticipated maximum fault current, without duress. UK Power Networks 2015 All rights reserved 14 of 59

15 The overall impedance to earth is made up of the following components, shown also in Figure 5-1, connected in parallel: The earth grid. Bonded foundation sheet steel or reinforcement bars. Steel tower lines. Cable networks. Fortuitous earths, e.g. to/via other services and LV system. There are other parallel paths influencing the measured value. These include the electrode system of customers or adjacent substations, but these are not included in the design calculation unless they have been formally incorporated within an overall earthing design. EARTHING SYSTEM COMPONENTS Outgoing Earth Grid Incoming Electrical Sources Earth grid I Cable (XLPE) Cable (Pb) Steel reinforcing Cable (Pb) Cable (XLPE) KEY PVC serving on copper, aluminium or lead sheath. Hessian (conductive) serving on lead sheath. Figure 5-1 Earthing System Components The Earth Grid As described in Section 5.5, the earth grid resistance can be obtained by calculation, by computer modelling, or by graphical or interpolation methods Bonded Foundation Structure Steel Reinforcement Bars If these have been bonded to the earth grid, initially their effect can be ignored unless they increase the horizontal area encompassed by the earth grid or enter low resistivity soil underneath (such as long steel reinforced piles). If the total area is increased, the new total area should be used to recalculate the grid resistance. UK Power Networks 2015 All rights reserved 15 of 59

16 5.6.3 Steel Tower Lines Each tower has a natural resistance to earth of typically 5 to 20, due to the concrete-clad steel legs installed in the soil and the terminal tower at any substation shall be securely connected to the substation earth grid. The total combined resistance to earth of each tower, connected in series/parallel via the overhead earth conductor, is called the chain impedance. The calculated chain impedance value relies on the aerial earth conductor being bonded to the tower steelwork at each tower position and the actual value will be significantly higher if this has not been carried out. (Many older lines were originally constructed without bonds between the earthwire and tower) On site checks may thus be required to confirm satisfactory connection. Only towers continuously connected by an earthwire to the substation grid contribute to the chain impedance. For example, the contribution beyond a section of Trident or Portal type unearthed 132kV construction will not contribute. The tower line chain impedance has a reactive component due to the overhead earth conductor. On a new circuit, the individual tower footing resistance can be measured (before the earthwire is connected) and this is the preferred method. Alternatively the resistance can be calculated using the foundation depth, radius, spacing and soil resistivity. Computer modelling is the most accurate calculation method, for which specialist advice may be required. Once the tower footing resistance is known, a graph is provided (ENA ER S34, Figure 5) from which the chain impedance can be read. The graph assumes that there are at least 20 towers in a line, in similar soil conditions. In the absence of detailed information, a conservative estimate for a 132kV tower line with a minimum of 20 towers would be a chain impedance of 2 ohms at 34 degrees (lag), shown This assumes the tower line is not on rocky ground. If a tower line is shorter than 20 towers, or installed in rocky ground, then individual calculation is necessary and specialist advice may be necessary. The tower at a line cable interface should be fitted with potential grading and cable surge arresters. These are connected to earth directly via copper tape or stranded conductor and are also indirectly connected to earth via the tower. The rational behind this is that good earthing is necessary at the termination tower for insulation coordination and to prevent voltage doubling. The tower needs to be bonded to the substation earth at two points. This does not modify the substation earth resistance by very much. The difference the presence of the tower and earth wires make, is that a relatively low earth impedance will already exist. The high frequency impedance of a tower is much higher than at power frequency, so much so, that the impedance of the tower can have a second order effect. So a copper connection from the surge arresters is necessary with a few rods at the base. No measurements are necessary. Further details for high frequency earthing of surge arresters and capacitor voltage transformers (CVT) can be found in Section Cable Networks Where these exist, they are likely to be the most important factor governing the need, or otherwise, to extend the earthing system. Most existing 11kV cables have either a Hessian covered steel-wire (or tape) armouring, with a lead sheath; or are PVC served with an aluminium sheath. For Hessian covered cables, the outer lead sheath is in continuous contact with the soil throughout its length (except where installed in plastic ducts) and will provide a path for earth fault current to flow into the soil. UK Power Networks 2015 All rights reserved 16 of 59

17 Polymeric type cables are now increasingly being used and have a stranded copper sheath and a medium density polyethylene outer serving (MDPE). It is important that the sheath cross sectional area is sufficiently large to carry the anticipated fault current and not to have too high a longitudinal impedance. The sheath cross sectional area will also affect the proportion of fault current returning through the soil for an earth fault at the remote end of the cable and so will influence the EPR. This is often overlooked when cables are selected on a cost basis related to the load carrying capability only. A bare copper stranded earth electrode of 70mm 2 cross-sectional area is to be laid with each out-going group of insulated sheath cables for a typical distance of 150m from the substation (unless deemed not required by the Design Engineer) up to an overall maximum length as follows: In the absence of more specific design guidance, the total length of bare electrode installed in this way at a ground-mounted substation is to be 3.2 times the soil resistivity value, metres. The conductor required should ideally be shared equally amongst three or four routes running out in separate directions. If there are few routes and/or the distance to the first distribution substation is short (less than 200m), then it is advisable to run the electrode along it. If possible, the remote end of the electrode should be terminated at the electrode system of a distribution substation or to the sheath of a HV cable joint. This will provide a higher degree of reliability. Knowing the soil resistivity along the route, Figure 4 in ENA ER S34 provides a series of curves to enable the electrode resistance provided by the bare electrode or lead sheathed, Hessian served cables of different length to be estimated. As a first estimate, the resistance of up to three lengths of Hessian served cables, each following routes which are between 65 and 120 degrees apart, may be calculated using Figure 4 in ENA ER S34. The overall resistance can be obtained by assuming each of these and the earth grid to be in parallel. Short lengths of plastic sheathed cables or plastic ducts can normally be ignored. Also, for calculation purposes, any radial earth electrode installed along the route of a plastic sheathed underground cables can be treated the same as a Hessian served cable, provided it is not running in parallel with a Hessian served cable whose contribution is also to be included. The above approach will almost certainly provide a resistance value lower than the actual one. This is due to interaction between the outgoing cables and earth grid not being taken into account. Cables associated with different voltage systems may be included, provided their earth sheaths are connected at the substation concerned. The sheaths of many modern cables are plastic covered (PVC or MDPE) and hence insulated from earth. However, they are often connected to sections of older Hessian covered cable and to earthing systems at each 11kV substation. Due to this, a low overall impedance to earth may still be provided. The contribution from an entirely plastic served cable circuit may be calculated in a similar manner to that for a tower line, i.e. as a parallel set of chain impedances. The resistance of the HV electrode at the distribution substation (as specified in Section 4) is in series with the longitudinal impedance of the cable sheath (which may be found from ENA ER S34). As the distribution substations are generally several hundred metres apart, their individual electrode systems will not interact. This means that the contribution from nearly all the outgoing circuits may be taken as acting in parallel. It is important to note that the overall earth impedance will be significantly higher than for a comparative system of lead sheathed cables and will have a much larger inductive component. UK Power Networks 2015 All rights reserved 17 of 59

18 To enable an approximate calculation to be made for a typical network comprising of Hessian and plastic served cables, the following information is required: The area covered by the cable network, if less than 4km 2. The length of the buried cable. The proportion of Hessian served cable. Approximate distribution substation resistance if Hessian served cable accounts for less than 10% of the total. A value representing the proportion of the area used by the cable network (the active area). A factor which takes into account the effect of many Hessian served cables following similar routes. The approximate overall cable network impedance can then be found by use of an equation and/or graphs. Where a more precise estimate is required, for example when the cable network is relatively small or has a high amount of plastic sheathed cable, then specialist advice is required to calculate the overall earth return impedance. To facilitate calculations, the following information is required: Cable size and type. Sheath dimensions are particularly important. Typical installation depths. Distribution substation earthing value. Cable overlaying planned, especially where bare metal sheathed or bare wire armoured cable is to be replaced by PICAS/plastic served cable. Approximate cable routes, which show where bare metal sheathed or bare wire armoured cable is laid direct in the ground or in earthenware ducts. The above information is required for each circuit up to 1.5km from the site or up to the first section of overhead line. The areas of each circuit containing a high proportion of Hessian served cable are of particular interest. Where this cable is laid in earthenware ducts it has a measurable but much less effective earth resistance. A diagram showing the type and format of information necessary is provided in Appendix B. The overall earth return impedance may be calculated by considering the above paths to be in parallel. It is necessary to take proximity into account - for example if there is only one Hessian served cable and this runs in parallel with a tower line, the chain impedance of the tower line should be excluded as it will overlap that of the cable. Similarly, if the tower line passes through an area densely populated by Hessian served cable, its impedance should be neglected in the overall calculation Fortuitous Earth Paths These are provided by/via the LV system or other services at COLD substations. For example, at an old substation, there may be connection onto abandoned cable sheaths or pipes. Fortuitous earths are not included in design calculations, but may provide a further reduction in the overall resistance at the measurement stage. At a HOT substation, such fortuitous earths, especially if provided by LV cables, which leave the site boundary, could be a source of danger and are not permitted. If they exist, they shall be modified or disconnected. UK Power Networks 2015 All rights reserved 18 of 59

19 5.7 Data Required from the Power System Fault Study The objective is to determine the amount of earth fault current flowing into a substation s earth impedance (which includes the earth grid plus its parallel paths). This will be used to calculate the EPR and ensure that the earthing system can deal with the anticipated fault current. Note: The substation earth grid and main equipment bonds are sized to deal with the thermal requirements, i.e. the maximum earth fault current anticipated over a period of 3s (or 1 second at transmission sites). This means that any subsequent physical work on site should be kept to a minimum for moderate increases in fault current. Refer to Section 8 for the design issues it is necessary to consider when there is a significant increase in fault current. The network should be modelled using fault levels anticipated for five years into the future to obtain the maximum single phase to earth fault current available due to either an internal or external fault condition at the substation. The initial symmetrical short circuit current is to be calculated in accordance with IEC 909-HD 533. The internal fault is for example a highvoltage bushing or bus bar failure to earth or a fault thrower operation producing the highest earth fault current in the substation. (The highest voltage level may not necessarily produce the highest earth fault current). The worst case external fault condition normally involves an earth fault on a lower voltage outgoing circuit, particularly one supplied by unearthed overhead line, where the entire earth fault current will return through the ground. The switchgear earth fault rating should not be used for this calculation, as the rating is normally much higher than the actual earth fault current values. Switchgear with a high rating is often purchased due to high X/R ratios, not high earth fault current conditions. Use of these values would lead to un-necessary expenditure for internal earthing and external third party mitigation. Internal fault conditions within the substation at lower voltages do not normally require investigation, as most of the current should return via the installed electrode system back to the star point. This will not create a significant rise of earth potential, unless the earth grid has been incorrectly designed or is large (say at a power station). In this case, significant potential differences may occur across the substation. If the installed electrode system is not fully integrated, then specialist advice should be sought to determine whether a detailed analysis is required. A more detailed analysis is required for 132kV networks compared to those at 66kV and 33kV. These lower voltage networks are generally earthed at one point, whereas 132kV networks are multiple earthed i.e. at sending and receiving ends. This means that a portion of the fault current will return to source via phase conductors. To estimate the current returning in this way, the individual phase currents in each transformer winding during the fault are required and these need to be summated vectorially to find the residual current. This is subtracted from the gross earth fault current when calculating the EPR. Examples of the information required and how it is analysed for transformer feeder or parallel infeed arrangements are shown in Appendix B. For 66kV and 33kV overhead line networks supported on unearthed poles, because it is normal practice to provide a connection with earth at the source of supply only, all of the earth fault current returns to the source via the ground. UK Power Networks 2015 All rights reserved 19 of 59

20 It is important to include an estimate of the total earth return impedance (i.e. the series sum of the fault resistance, source resistance and circuit impedance) and fault position in the fault current analysis. Where the earth return impedance is significant, for example at rural locations supplied by 33kV lines, the prospective earth fault current will be significantly reduced. The above analysis, based on sequence modelling, will give approximate results where the earth fault current does not follow the same route back to source as the phase current. For example, when there are multiple cable sheath routes for the earth fault return current and only one route for the phase fault current. It is possible, to model the system more accurately by including independent phase and earth circuits. This can be carried out for specific cases, if required and specialist advice is necessary. 5.8 Earth Fault Current Returning via Earthwire/Cable Sheaths Due to Induction Introduction When a conductor lies parallel to another and an alternating current flows in one of them, then provided a complete circuit is available on the other, a current will be induced in it. This effect is experienced in tower lines and underground cables. The earthwire on a metal tower line forms a complete circuit as it is earthed at each end (and at intermediate towers), so current will be induced in it when earth fault current flows in a phase conductor. Similarly, if the metal sheath of a single core cable is earthed at each end, current will be induced in this when earth fault current flows in the cable core. The current induced into the cable sheath or aerial earth wire returns to source(s) without flowing through the ground locally. It will not pass through the earth grid and will therefore not contribute to any rise in potential. To arrive at an accurate estimate of the rise of earth potential under fault conditions, the fault current returning via metallic routes needs to be subtracted from the total fault current to give the current returning via the soil. Aerial earth I f I f Fault Substation A earth grid Substation B earth grid I gr (the current returned through the soil) for a cable fault is reduced by the value I f, which would be the current returning through the cable sheath. I f = Maximum earth fault current from fault study I f = Current induced into earth loop, opposite to I f I gr = I f I f Figure 5-2 Estimating the Proportion of Fault Current Returning Through the Soil UK Power Networks 2015 All rights reserved 20 of 59

21 5.8.2 Overhead line with Earth Conductor To determine the value of ground current, refer to Table 2 in ENA ER S34. However, note that the earthwire shall be continuous back to source. If a section of unearthed construction (e.g. Trident) is installed, then the table cannot be used and any induced return current will need to be individually calculated. Again, specialist advice should be sought to assist in this task Cables To determine the proportion of ground current, ENA ER S34 has a series of nomographs. These are summarised in Table 5-1. The nomographs are only suitable for relatively straight forward cases and are not available for polymeric cables. For these and more complex arrangements, for example where there are several sources or intermediate substations between the source and the substation of interest, specialist advice should be sought. Table 5-1 Calculation of Ground Current for Cable Faults ENA ER S34 Fault on Cable Section Figure No Voltage (kv) Size Type Supply Source 8 33 Standard PILC All cable mm 2 PILC or PICAS All cable Standard * PILC Via overhead line mm 2 3 x 1 PILC Via overhead line mm 2 3 x 1 PICAS Via overhead line Standard* PILC Cable mm 2 PILC or PICAS Cable * Standard includes 185mm 2, 240 mm 2 and 300mm 2. To use the nomographs, it is necessary to know the length of cable, the cable type, the earth impedance at each end, the fault position and method of earth connection at each cable end. Normally the sum of the two cable end resistances are divided by the cable length (in kilometres). The factor obtained is then used with the nomographs, to provide the percentage and phase angle of fault current returning through the ground, for each particular type of cable. 5.9 Calculation of the Earth Potential Rise (EPR) Primary Substations From Section 5.6, the overall earth impedance value will have been obtained. From Section 5.7 the maximum earth fault current will have been obtained. As a first estimate, the two should be multiplied together to obtain the EPR value. If this means a COLD substation, then no further calculation is generally necessary. If, the calculation indicates that the substation may be HOT, then the effects of current returning to the network by induction, as explained in Section 5.8 now have to be taken into account. See Appendix B. The EPR is then recalculated as the net (remaining) current through the overall earth impedance multiplied by that impedance. UK Power Networks 2015 All rights reserved 21 of 59

22 5.9.2 Grid Substations (including 132 kv and 33kV Connections) The overall earth impedance value will have been obtained as explained in Section 5.6. The current returning to the substation, and passing through the overall earth impedance will have been obtained as part of the earth fault study Section 5.7. Due to high fault current, there is no virtue in carrying out a rough estimate and it is necessary to first account for the effects of currents returning to the network via induction, as explained in Section 5.8. If this produces a COLD site, no further calculation is necessary. In most cases it will also be necessary to account for the current returning via phase conductors (see Section 5.7 and Appendix B) to more accurately calculate the EPR. See Appendix B. UK Power Networks 2015 All rights reserved 22 of 59

23 5.10 Calculation of External Potential Contours or Zones of Influence (e.g. HOT Zone) Accurate Representation When the substation earthing system has been analysed by computer modelling, the appropriate potential contours (including the 650V or 430V HOT zone and BT 1150 contour) may be readily obtained and figures should be provided in the appropriate report Approximate Calculation Appendix C of ENA ER S34 gives a formula for calculating the voltage profile from the edge of the substation grid under potential rise conditions. Nomographs are also provided to simplify the approach. Figures 24 and 25 in ENA ER S34 are the appropriate ones to use. Note: The formula and nomographs assume uniform soil. The appropriate potential contours shall be drawn on a suitably scaled plan (1:2500) of the substation and its surrounding area, in a similar way to that shown in Figure 5-3, for use by third parties such as BT Level 1-430V Contour Level 2-650V Contour Level V Contour Level V Contour Figure 5-3 Scale Plan of Substation Showing Site Boundary Surface Potential Contours UK Power Networks 2015 All rights reserved 23 of 59

24 5.11 Implications of a Substation being HOT The immediate practical requirements are: An isolation transformer is required on the termination of the telecommunication cable in the substation/control room. All metallic services to the site and building require attention to ensure they do not introduce a transfer potential risk. This can be prevented by introduction of insulated inserts (normally one inside the substation and another 2m beyond the perimeter fence). Alternatively, say the water supply could be provided by a plastic pipe from 2m outside the perimeter of the substation. Any exposed metal of services within the substation shall be bonded to the substation earth grid if there is any possibility of simultaneous contact. There are operational problems associated with work on pilot cables, telecommunication and power circuits. For example, when required to carry out jointing work on a cable between two substations, one of which is HOT. Appropriate operational procedures shall be used to reduce risk. The HV and LV electrode systems at the first distribution substation out on each cable fed circuit, or at any distribution substations situated within the HOT zone, shall be earthed separately from one another. Where the final arrangements mean that a substation will have a HOT zone (zone of influence) that extends outside the substation fence, there are a number of steps to be initiated. In general: BT or other telecommunication companies, that use metallic cables, need to be advised and will require the geographic map showing the surface potential contours, as shown in Figure 5-3. Telecommunication cables within the substation shall be terminated via an isolation transformer and mitigation work on cables passing through the HOT zone may be necessary. Reference should be made to ENA ER S36/1 to determine who is responsible for costs of telecommunication remedial work. Other bodies (gas, water, the petro-chemical industry, etc), having buried metallic pipework within the HOT zone or zone of influence, should be advised so that appropriate operational precautions can be taken by their staff whilst working on any metalwork within the zone and mitigation measures considered. There are operational implications when working on telecommunication circuits associated with the substation or within the HOT zone. It is necessary to ensure that touch and step potentials are below the appropriate limits. Where there is equipment belonging to other authorities within the zone of influence, then a number of methods may be adopted to reduce risk. This includes physical diversion, addition of further insulation, adoption of new protection schemes (e.g. to increase limit from 430V to 650V or install telecommunication protection devices) and operational procedures Reducing the Area Covered by the HOT Zone It is desirable that a substation should be COLD, but if this is not possible, then it should have a limited HOT zone area, preferably one that encompasses a minimum or no third party equipment. Since the EPR is a product of maximum earth fault current and earth return network impedance there are several methods to reduce the EPR and hence the HOT zone area. UK Power Networks 2015 All rights reserved 24 of 59

25 These are to: Reduce the overall earth fault current; Reduce the impedance of the earth electrode or its parallel paths; or Divert more of the fault current away from the earth electrode through parallel paths or metallic routes. It may also be possible to design the earth electrode to create a potential contour (or HOT zone) that avoids sensitive/costly third party equipment. This will almost always require a site specific computer aided design Reduce the Earth Fault Current In some cases, and in discussion with Asset Management, it may be practicable to alter the system running arrangement or the method of system neutral earthing in order to reduce the overall rise of earth potential. Caution shall be exercised to ensure that correct protection operation is maintained and customer supply quality is not compromised Reducing the Electrode Resistance The only effective methods of achieving this are to either significantly increase the length of the earth rods (where there is low resistivity soil at deeper levels) or increase the area enclosed by the grid and its electrodes. For example, it may be possible to extend the earth grid out from the fence on one or more sides of the site. This is most economically achieved by bare stranded electrode in each new route used by plastic served cables up to an appropriate distance as shown in S34 Figure 4. Alternatives, such as using greater cross section conductor or more earth rods of the same length, will only provide a marginal improvement and are rarely economically justified. Special back-fill materials can sometimes be useful. The most common are Bentonite and Marconite. Bentonite is a clay which, when mixed with water swells to many times its original volume. It absorbs moisture from the soil and can retain it for some time. Marconite is a conductive carbonaceous aggregate which, when mixed with conventional cement, has the effect of increasing the surface area of the earth electrode, thus helping to slightly lower its resistance. These back-fill materials normally only provide a marginal improvement but may be specified for other reasons; for instance to help to maintain the resistance value at a more constant level throughout the year, to provide protection against 3rd party damage, or to protect the electrode from corrosion. They are also useful for surrounding electrodes installed in rock. Where a decision is taken to use Bentonite, Marconite or any other special back-fill material, the design engineer should ensure that this information is passed to the construction staff. These materials can be quite costly, so the construction methods should attempt to limit the amount used. Examples are mixing bentonite with local clay, reducing the hole diameter drilled (for vertical electrodes) and minimising the width and volume of the horizontal trench section into which the electrode will be installed. Increasing the size of the grid may introduce practical problems (such as maintaining the integrity of long spurs against theft or damage) and difficulty in obtaining the necessary wayleaves. UK Power Networks 2015 All rights reserved 25 of 59

26 Reduce the Impedance of Parallel Paths There are a number of possible alternatives: Lay electrode in outgoing mains cable trenches (only useful where the cables have PVC outer sheaths). If calculations show that this will make the substation COLD, it the most economical solution. However, if the substation remains HOT, the electrode may extend the HOT zone some distance from the substation. Where the electrodes are critical to reduce the EPR and are long, steps shall be taken to maintain their security against damage or corrosion. An ideal arrangement is to route the electrode such that its end may be incorporated into a cable joint or the electrode system of a distribution substation. This means that there are two connections to the electrode, which also helps reduce the longitudinal impedance. If connection of each end is impractical, a test point shall be included in the substation so that the resistance of the spur electrode may be monitored by measurement. Make use of abandoned, Hessian served underground cable. Often reinforcement schemes involve replacement of cables. The phase conductors and sheaths may be joined together and connected to the electrode system. Because of the risk of damage, it is essential that multiple connections be provided to such cables. The start ends should ideally be connected via test points, to permit resistance measurements. Ensure that maximum benefit will be gained from the impedance of tower footings by ensuring that aerial earth conductors are bonded to the tower steelwork at each tower position. In some cases additional earth electrode (e.g. a loop 1m distance around the tower base or a counterpoise earthwire run along the tower route) can be beneficial. It might be possible to take advantage of any deep excavation or piling, to either install some additional earth electrode or incorporate the piles as part of the formal substation earth grid. Ensure that effective earthing systems are installed at adjacent substations, where these are directly connected to the site by short cable sections Calculation of Touch and Step Potentials Accurate Calculation Where the substation earthing system has been analysed using computer modelling, the report will provide figures that show the touch and step potentials across the site, expressed as a percentage of the EPR value. By applying these percentages to the calculated EPR, then the maximum touch and step values can be identified Approximate Method ENA TS 41-24, Section 9, provides formulae for calculating touch and step potentials both within the grid area and at fencing: For substations with separately earthed fence, i.e. Standard design; and For substations with an integrally earthed fence, i.e. non-standard design. Approximate values can be obtained by using the earth grid dimensions, soil resistivity and grid current in the ENA TS formulae. The values obtained shall be compared with the maximum acceptable values as given in Section 2. Note: If the control/switchroom is remote from the formal earth grid and has not had potential grading electrodes specified, the approximate touch potential there can be calculated in the following way. The potential on the surface of the soil around the control /switchroom is calculated and then subtracted from the EPR value to give the touch voltage. UK Power Networks 2015 All rights reserved 26 of 59

27 If the touch potential of any exposed metalwork within the grid area exceeds the acceptable value, the solution is normally to reduce the spacing between cross-members of the grid in that area. The value of 10m chosen for the initial design guidance should ensure that there is seldom a problem of excessive touch potential, especially if the site is covered with crushed rock/gravel as specified. If the touch potential on any metallic fencing exceeds the acceptable value, there are a number of options: Provide potential grading protection by laying an electrode 1 metre beyond and parallel to the fence, buried 0.5 to 1 metre deep, and connected to it. If the fence is independently earthed, this electrode shall be kept segregated by at least 2m, from the earth grid, otherwise it will be bonded to the earth grid and fence or Arrange for the affected short section of fence to be insulated and 'earth free', by insetting an insulated fence section with insulated bushes at support positions and at any point where the fence is connected to an earthed section of fence (refer to Section 4) or Provide a non-metallic barrier at this point, such as a brick wall. It is essential that precautions are in place to ensure that third party fences are not connected to the metal substation fence, especially if the substation fence is bonded to the earth grid. Connection of a third party fence will introduce a transfer potential. Any method used needs to be obvious to avoid future maintenance compromising the separation. Therefore physical barriers like brickwork are ideal Requirements of the Final Design The final design will provide the following: The earth grid resistance and overall earth return impedance values. The EPR, for a maximum value of earth fault current. Specify whether the substation is HOT or COLD. Show the limits of the HOT zone and appropriate zones of influence - if applicable. Confirm that the internal maximum touch and step potentials at all points are below the safe acceptable value. Confirm that, the maximum touch potentials at the fence are below the safe, acceptable value. Confirm that the maximum external step potentials are below the safe acceptable value. Ensure that there is an earth electrode reasonably close to each item of plant, which requires connection to it. Guidance is provided in Section 4 on how to install the electrode and make the necessary connections/joints. UK Power Networks 2015 All rights reserved 27 of 59

28 5.14 Conductor Size and use of Metallic Structures The earth electrode and bonding conductor standard sizes are given in Table 5-2. These are based on the minimum conductor sizes in Appendix F. The thermal loading i.e. the rating of the earthing electrode, conductors and equipment connections shall be based on the worst case steady state symmetrical RMS earth fault level (or the three-phase fault level if it is not available). This shall be the corresponding maximum earth fault level at the source grid or primary substation. The EPR and safety voltage calculations shall be based on the calculated foreseeable worst case earth fault level. This shall be the corresponding maximum earth fault level at the substation or point of connection (including any generation contribution) plus 10%. An example of the PowerFactory fault level format is shown below. The RMS break value (Ib) should be used for earthing calculations. PowerFactory Studies Name Ik" A (ka) Ik' A (ka) Ib A (ka) ip A (ka) ib (ka) Sub-transient Transient RMS Break Peak Make Peak Break Busbar Poc At sites shared with National Grid the electrode embedded within their area shall comply with the National Grid requirements (typically 400kV - 63kA for 1 second, 275kV - 40kA for 1 second, 132kV kA for 3s - in some special cases 40kA). Parts of the site peripheral to National Gird may not need to be sized for this rating. Table 5-2 Earthing and Bonding Electrode/Conductor Sizes Function Connection Conductor (Table 5-3) Minimum Standard Size (mm or mm 2 ) 12kA/3s 26kA/3s 31.5kA/3s 40kA/3s Earth grid Primary equipment connections Duplicate or loop brazed or welded Single (spur) brazed or welded Single (spur) double bolted Duplicate or loop brazed or welded Duplicate or loop double bolted Copper tape 25 x 4 40 x 4 50 x 6 50 x 8 Copper tape 25 x 4 40 x 6 50 x 6 50 x 8 Copper stranded 120mm 2 300mm 2 300mm 2 400mm 2 Copper tape 40 x 3 50 x 6 50 x 6 50 x 8 Copper stranded 120mm 2 300mm 2 400mm 2 400mm 2 Copper tape 25 x 3 40 x 4 40 x 4 40 x 5 Copper stranded 70mm 2 185mm 2 185mm 2 240mm 2 Copper tape 25 x 3 40 x 4 38 x 5 40 x 6 Copper stranded 70mm 2 185mm 2 240mm 2 240mm 2 UK Power Networks 2015 All rights reserved 28 of 59

29 Function Connection Conductor (Table 5-3) Minimum Standard Size (mm or mm 2 ) 12kA/3s 26kA/3s 31.5kA/3s 40kA/3s Secondary equipment connections Above ground equipment connections or internal earth bars Equipment connections via structure legs Fence bond Gate post bond Gate bond Lighting and security equipment connections Other bonding e.g. staircases, cable supports etc. Single (spur) bolted Single (spur) bolted Double or loop bolted Copper tape 25mm x 4mm Copper stranded 70mm 2 Aluminium tape 40 x 6 n/a n/a n/a 40 x 4 40 x 6 50 x 6 n/a Single leg Galvanised steel 380mm 2 870mm 2 970mm mm 2 Duplicate legs Single (spur) bolted Single (spur) bolted Single (spur) bolted Single (spur) bolted Single (spur) bolted Copper tape or stranded Copper tape or stranded Copper stranded or braid Copper tape or stranded 225mm 2 522mm 2 582mm 2 738mm 2 25 x 3 or 70mm 2 25 x 3 or 70mm 2 16mm 2 Copper stranded 16mm 2 25 x 3 or 70mm 2 Table 5-3 Tape and Stranded Conductor Specifications Material Specification Copper Tape* High conductivity copper tape to BS EN Copper Conductor Hard drawn stranded copper to BS 7884 with a minimum strand radius of 3mm Copper Braid High conductivity copper wire to BS 4109-C101 Aluminium Tape* Hard drawn to BS or aluminium alloy to BS 3242 *Tape should be embossed with Property of UK Power Networks UK Power Networks 2015 All rights reserved 29 of 59

30 5.15 Earthing Arrangements Applicable to 400/275/132kV Sites of Different Companies The entire earth conductor at connected sites does not necessarily need to be sized to cope with the highest fault current. The fault studies carried out should account for the fault current flows through the earthing system and will enable the high fault current routes to be identified. Conductors there will need to be fully rated, but elsewhere the conductor can be sized to account for through current and also for faults that occur there at different voltage levels. This means that smaller electrode should be acceptable elsewhere, for example in the 33kV area of a site that shares a 132kV zone Earthing Arrangements Applicable to Sites with Generation Conceptual and practical guidance on earthing of generating plant is covered in ENA ER G59 and ENA ER G75, which should be referred to. They do not cover design of the electrode systems, whilst ENA TS does not presently apply to generating stations. The guidance below is provided to deal with design of the electrode systems until such time that the subject is addressed by updated versions of the ENA Standards. Where generation plant is to be situated adjacent to or within a substation, it is essential that the earthing system is designed to a similar specification to that of the substation and incorporates any requirements of the external connected network. The same general design rules, as set out previously in this document will apply, especially the touch and step voltage limits. Where appropriate, declared LV earth loop impedance values should be maintained. However, additional studies will be required at the design stage to ensure that the impedance of the earthing system interconnecting the generator earthing to that of the rest of the site is sufficiently low as to prevent undesirable potential differences. The routing and longitudinal impedances of these inter-connectors can have a detrimental effect on the overall earth impedance, if not designed correctly. Appendix C of this document includes relevant guidance from American and other standards, which may assist designers. Earthing of wind farm collection substations should be in accordance with the appropriate part of this earthing document and comply with ENA TS Where wind company substations or plant is situated adjacent or within a substation, this shall be to a similar specification. Earthing of wind farm generators themselves and the overall wind farm is a specialist subject and beyond the scope of this document. Normally an earth loop and rod array is required at each generator to provide lightning protection in accordance with BS EN and potential grading is normally also necessary. Each turbine group cluster is normally interconnected via bare electrode and the sheaths of the power cables. Because of the size of the electrode system, calculations taking into account longitudinal impedance are normally required and the as installed earth impedance should be measured on commissioning. UK Power Networks 2015 All rights reserved 30 of 59

31 5.17 Earthing Arrangements at Railway Supply Substations This is a highly specialised topic, for any installations, reference shall be made to the relevant standards. The subsequent information is provided as an introduction to the topic. Generally, railway substations are supplied via 132/25kV single-phase transformers. The arrangements should comply with ENA ER P24. It should be noted that, at these locations, there are very large earth return currents. This is exacerbated by use of single-phase cables where the earthed sheath, in parallel with the soil, acts as the return route. Ideally, the transformers should be situated close to the supply point and share the same electrode system. This will enable earth return currents to flow via metallic routes, rather than through the soil. This, in turn will reduce the EPR on the electrode system which occurs when the railway system is drawing current. The main issues are therefore negative phase sequence voltages and transferred voltages. Where the supply point is some distance away, the standing voltage on the transformer earthing system can be significant. Touch and step voltages which are lower than the ENA TS limits apply on railway systems, mainly due to the regular exposure of the travelling public to the structures and facilities on which an EPR may occur. The design shall therefore ensure that the BS EN limits are complied with in areas to which railway staff or the public have access. Irrespective of the fault clearance protection time, it is preferred to limit the EPR to less than 430V, to avoid damage to signalling cables, etc. Difficulties also concern LV supplies to line side equipment and stations on 25kV AC electrified lines. It is unusual for PME to be applied, because the possibility of excessive neutral voltages on exposed metalwork is undesirable. A PME supply is not permitted if a voltage exceeding 25V, due to faults, starting or normal operation, can arise on the traction return rail or similar, if this is bonded to the main earth terminal and hence the supply cable neutral/earth. The LV supply would thus normally be via a TNS supply, an isolation or interposing transformer (with an earth supplied on the railway side) or via a dedicated transformer with supply via a circuit with separate neutral and earth (TNS) conductors. For supplies to Signalling Supply Points (SSP), if PME, the neutral cannot be used outside the SSP building. Supplies beyond this are normally treated as TT type, requiring their own electrode system and RCD. If the electrode systems are segregated (i.e. a TT supply), consideration of the possible voltage rise on each electrode is necessary to ensure that a sufficient segregation distance is maintained. Some special arrangements for the earthing of the SSP and outlying Trackside Location Cabinets have been discussed between Network Rail contractors and ENA. A TT type supply is generally a lower risk arrangement which requires little technical attention. If PME is to be provided, a detailed consideration of the signalling and traction supply system arrangements will be necessary. Further guidance may be found in ENA ER G12. Another cause of concern is earthing of DC traction systems, which use earth return, as this can cause accelerated corrosion to cable sheaths and earth electrodes. In most cases, supplies need to be of an isolated type, unless transferred voltages are of an acceptable level. Further guidance can be obtained from ENA ER P24 and EDS As mentioned, the main reference document is ENA ER P24 and this will apply to the railway supply substation. There are other standards to which reference is necessary and the main ones are: ENA ER 41-15, Standard Circuit Diagrams for Equipment in 132kV Substations. Part 9 AC traction supplies to British Rail. UK Power Networks 2015 All rights reserved 31 of 59

32 BS EN , Railway Applications-Fixed Installations. Part 1. Protective provisions relating to electrical safety and earthing, June This is the main reference and contains the touch and accessible voltage limits. Tables of touch voltage limits for short duration faults (less than 0.5s) are provided. There are also tables of accessible voltage limits; these being for temporary and steady state conditions (i.e. not fault situations). No tables for step voltage are included. The limit tables are based on the traditional one hand to both feet touch voltage scenario. When a hand to hand situation can arise, new limits based on the same criteria as used in the tables will be necessary. EN contains guidance on limiting stray currents by correct earthing and bonding on electric tram type systems. Network Rail Standards RT/E/S/21085, Design of Earthing and Bonding for 25kV AC Electrified Lines, October This is quite a high level standard that was originally produced to accompany procurement specifications. As such, some fine detail is missing. It describes the railway high voltage network, service conditions and performance requirements. Sections 8.9 to 8.20 cover treatment of metalwork, fences and services close to the railway. The philosophy is similar to that used in substations i.e. bond metalwork to the traction return rail or earthwire, but avoid third party structures being used to pass traction return current. Section 8.17 is particularly relevant as it concerns non-traction electricity supplies. The earths of buildings on the railway system shall be bonded to the traction earth, but it is recognised that separation of this and the incoming supply earth may be necessary in some circumstances. Auxiliary and pilot cables between Electricity Company and Network Rail facilities shall be single point earthed. Network Rail Southern Territory Power Upgrade Project, Procurement Specification Number 15, Earthing and Bonding Systems for DC traction substations and similar. This has been extensively updated to over-ride the RT standards and ensure that earthing systems comply with the European standard and modern practice. Has only been issued in draft form and does not have national application. RT/E/S /21032, Earthing Systems for DC traction substations, track paralleling huts and similar equipment locations (Issue 1, December 1996). The earthing designs described are to old standards and would not comply with modern practice or European Standards. Awaits updating, but the new procurement specification (15) ensures that new sites in the southern area have modern earthing designs. GM/RT 1010, Electrified Lines Traction and Bonding, This creates a formal link to the EN limits for touch and accessible voltages and requirements for connecting parts of the system to the general mass of earth Earthing Arrangements at GIS Substations Earthing of gas insulated switchgear (GIS) and associated plant and equipment is complex and the designer should consult with the manufacturer at an early stage. Typically the issues to be considered are: High fault current. Residual AC current. Occasionally GIS equipment uses earthed metal screens around individual phase conductors. If single phase or when unbalanced currents flow in a three phase enclosure, then current is induced in these screens and a residual AC current is likely to flow continuously via the earthing system. There is presently concern that these AC currents may cause accelerated corrosion, particularly in steel electrodes. UK Power Networks 2015 All rights reserved 32 of 59

33 High frequency currents. The nature of the equipment means that switching transients can occur whilst electrical current is being interrupted. These transients include components at very high frequencies. Some flow within the confines of the local earth grid, whilst others flow into the ground. The electrode system to deal with high frequency current flow into the ground is different to that for 50Hz operation. The most often quoted solution is to increase the density of the earth electrodes in the immediate vicinity and to use vertical rods. However this needs to be accompanied by specific screen terminating arrangements and secondary control wiring needs to be routed to minimise inductive interference. The design seeks to ensure that high frequencies are confined to the inside of screened enclosures, but the presence of interfaces (such as at air terminations, insulated CT flanges and transformer bushings) allow some opportunity for these to escape. It is also important to ensure that the earthing design does not permit circulating currents to flow between plant and connections, which would cause interference. It is normal to provide a significant number of vertical earth rods close to the GIS enclosure, indeed some rods may pass through the floor into underlying soil such that an earth is provided as close as possible to the equipment. It is also common to have a copper or steel mesh electrode embedded in the concrete floor of the building and earth bars either within or buried immediately outside the building walls. All equipment is connected to this via short spur connections. Connections between plant items are run close to and parallel with earth mesh conductors. GIS equipment is generally earthed via vertical connections, which are connected to the internal mesh near the following equipment locations, to disperse externally referenced currents: Close to circuit breakers. Close to cable sealing end. Close to the SF6/air bushing. Near to instrument transformers. At each end of the busbars, and at intermediate points (for long busbars). The three enclosures of a single-phase type GIS shall be bonded together before earthing using bonding conductors rated to carry the nominal current of the bays or busbars. Flange joints would not normally require a bonding strap if the contact pressure is high, but these may become a source of interference at high frequency and tests may be required at the factory acceptance stage. The plant earth connections to an internal grid which has conductors of relatively small cross sectional area should be distributed by additional connections forming a cross or star type arrangement until sufficient grid conductors are bonded to carry the required current. The connection shall not be to one or a few small conductors. Metallic sheaths of cables (nominal voltage greater that 1kV) should be connected directly to the GIS enclosure. If the connection needs to be separated from equipment under metal enclosures, then voltage surge protection devices are recommended. Where the soil conditions are suitable for quite long vertical rods, these can be so positioned to cater for high frequency (lightning protection and GIS) and low frequency applications. As they are critical elements of the design, test facilities are to be provided for such rods. UK Power Networks 2015 All rights reserved 33 of 59

34 5.19 Earthing Arrangements for Surge Arresters and Capacitor VTs Switching transients from the high voltage system contain high order harmonics, which will see a CVT as virtually a short circuit to earth and will be dispersed through them with little attenuation. The connection to the grid and the electrode near its point of connection shall be designed to cater for this. The current flowing through a surge arrester under fault conditions needs similar consideration. The connection from the CVT or surge arrester to earth and the electrode system itself has an impedance which has an inductive component. The inductive part is small at power frequency compared to the resistive part, but will increase with length of the down lead and any bends. It only becomes relevant at increasing frequencies and can dominate the resistive part at high frequencies. This effect can reduce the efficiency of the surge arrester operation and the system of insulation co-ordination. In CVTs, where a high frequency electrode has not been installed, companies have experienced flashover across secondary wiring glands and between the support frame and other earthed conductors. To counteract the adverse effects, special earthing arrangements are necessary. Two connections are required. The first is a standard bond from the support metalwork to the main earth grid. The second is a high frequency earth connection, which should be as straight as possible, through to an earth rod, which is as close as possible to the equipment being protected. A cross connection is made from the down lead or earth rod to the adjacent grid. At higher voltages, there is a high frequency earth rod for each phase arrester or CVT. At 11kV, 20kV, 33kV and sometimes 66kV pole-type surge arresters, the ideal arrangement is to have two down leads and earth rods connected down the H pole legs. The three phase devices are then bonded together on the top of the pole or structure. Only at 11kV is it generally acceptable to have one shared earth electrode. For these cases the resistance values required are given in the Section 5. Within substations, the surge arresters are generally bonded together and at least two connections taken down to the earth grid. When seeking to achieve particular impedance value, the electrode system for high frequency currents will need to be designed differently to that for normal frequency currents and will require more vertical electrodes close to the structure and sometimes longer rods. Where the soil conditions are difficult (say underlying rock), then three or more radial spur electrodes should be taken out from the downlead/grid connection point for about 2m radius and an approximately equal angle apart Positioning of Metal Supports for Security Lighting etc Near Fences Metal supports for security lighting and/or cameras can require special attention to protect against touch potentials. Ideally these items should be situated within the confines of the earth grid and their electricity supply referenced to the substation earth. This generally means positioning the column about 1 metre inside the perimeter electrode of the earth grid, or at least 3m from a separately earthed fence. Where this distance cannot be achieved, then a non-metallic support column should be used. Any metal support within 2m of the fence shall be bonded to it. This may require a different fence earthing arrangement or a modification to the support supply arrangement. In the latter case, the LV cable earth shall be terminated in an insulated connector and only the neutral and phase (or switch) conductor taken up the column. The column and other exposed metalwork are then earthed via the fence and its independent electrodes. UK Power Networks 2015 All rights reserved 34 of 59

35 Where the substation earth grid extends outside the substation perimeter fence there is no problem locating metal supports close to the fence inside the substation. Any metal support shall be bonded to the grid, and the low voltage supply shall come from the substation supply. Barrier equipment is required in cables or wiring to remote locations to prevent any potential on the substation earthing system being transferred there, if the substation is HOT Communication Towers within or adjacent to Substations Because of the increased lightning risk associated with communication masts and the high frequencies involved from this and the equipment itself, special earthing arrangements are necessary. These include earth rods and/or an increased density of electrode in the immediate vicinity of the structure, where it is necessary to minimise the impedance of the earthing system. At a microwave dish or large aerial, it is normal to have a number of parallel earth down leads terminating near the base of the structure, onto earth rods. This arrangement reduces the inductance of the down leads and the earth impedance seen at the mast. Electrodes that run out radially, are relatively close together and arranged symmetrically have traditionally been used in place of rods, where there is underlying rock, to offer a low earth impedance value. Where the communication facility shares the same site as a substation, then the two earthing systems should be well interconnected wherever possible. There shall be rods, radial electrodes or other means of reducing the earth impedance at the interface of the two systems. This is to minimise the transfer of high voltage, low energy disturbances from one system to another. The substation earthing system will be especially important in the event of a lightning strike to the communication tower, as it will help disperse the energy associated with this. Good interconnection (at least two standard electrodes) is necessary to restrict any potential difference across the earthing system whilst the lightning energy is being dispersed. Attention is also required to the bonding/termination of pilot and communication cables and the earthing arrangement for the LV supply. In dealing with a request for supplies, the following strategy is to be followed: If the communication tower is to be situated within the substation earth grid, wherever practicable it should be located away from areas which may be susceptible to high transient voltages (such as SCADA rooms) or locations of expensive equipment (such as power transformers). The tower should be reliably connected to the earth grid at three or four points. Earth rods shall be installed on each of these connections where the mast is high enough to significantly increase the risk of a lightning strike. If the communication tower is situated close (within 10m) to, but outside the substation fence, wherever practicable, the site earthing and fence arrangement should be extended to include this area, using the same earthing philosophy as within the substation. This means the same fence earthing arrangements of both the substation and the cellular facility, in particular at the interface fence sections. Where it is not possible to maintain this, it is usual to introduce insulated fence panels either side of the communication tower fencing. The tower fencing would then be of the bonded type with potential grading electrode outside. For both of the above arrangements, wherever possible, the LV supply to the communication tower should be taken from the substation, either from the LV supply busbar or a dedicated 11kV transformer. Caution is necessary where LV supplies are derived from a combined auxiliary/earthing transformer. High secondary voltages occur when remote earth faults occur and have resulted in damage to IT cards and communication equipment. UK Power Networks 2015 All rights reserved 35 of 59

36 If the existing LV supply does not have sufficient capacity, this should be augmented if possible. If the LV supply shall be provided from outside the site, this can only be accomplished using standard arrangements if the substation is COLD. If it is a HOT site, then an isolation transformer (minimum 4kV insulation voltage) or similar facility is required and specialist advice is necessary. If it is necessary to extend the site area to accommodate the communication tower and the site is HOT, then the associated earthing should be modified if possible such that it can provide a COLD site. If this is not possible, specialist advice is necessary as the extent of the HOT zone and any increased impact on third party equipment will need to be considered. Sites a significant distance (typically more than 10m away) away from the substation and outside the HOT zone, should be supplied on a standalone basis and not connected in any way to the substation. The LV supply should be provided from the network, not the substation. If the tower is of a height and/or location such as to substantially increase the risk of a lightning strike, additional earth rods are to be installed at the base of the communication tower, in particular on the sides which interface with the substation equipment Communication Masts Fitted to Towers These masts can often be fitted to towers in an effective manner and avoid the need for planning permission. However, there may be problems in providing the base station with an electricity supply. The three main issues are: The high voltage, which could occur across the distribution transformer when an earth fault occurs on a tower, associated with 132kV and higher voltages. Possible high touch and step voltages around the tower and associated equipment. Possible extension of the HOT zone. Because of the complexity of this work, some special arrangements have been developed at national level and are set out in ENA ER G78. Specialist advice should be sought for guidance on introducing such installations. In ground earthing designs have been successfully developed for use on 132kV towers where the earth fault current magnitudes are moderately low (i.e. below 10kA) and the soil has relatively low resistivity (below 100m). In other cases, insulated base arrangements are available where the LV supply and base station equipment is located on a steel platform that is insulated from earth. Refer to EDS for further details on providing supplies to mobile phone base stations on towers LV Supplies to Third Party Equipment at Substations To make optimal use of sites, there are more cases of third parties locating their equipment within or adjacent to substations. Wherever possible, these installations should be installed within the area enclosed by the main earthing system and be provided with an electricity supply derived from within the substation, such as the house/auxiliary supply. If the equipment is located just outside the main earthing system (say within 2m to 5m), if possible the fence and earthing should be extended using the same earthing philosophy as in the main substation, i.e. the earth grid extended and the method of fence earthing continued in the new part, wherever practicable. UK Power Networks 2015 All rights reserved 36 of 59

37 There may be a specific type of earth electrode design for the installation and the customer is responsible for designing and installing this part of the earthing system. This shall be bonded to the main substation earth grid, in a manner, which provides the required potential grading, or physical separation against adverse touch voltages. If the site is HOT, then the electrode and fencing arrangement of the extended area should be designed to minimise any detrimental effect on the HOT zone. Particular care is required not to extend the HOT zone into areas where third party mitigation will become an issue Reactors and AC to DC Converters Normally there are high electric and magnetic fields associated with such devices. These can, in turn, induce high currents in any nearby metal structures or earth conductors. Additional precautions are required to prevent induced circulating currents. One method is to ensure that such equipment is only earthed at one point. Another solution is to use nonmetallic fencing or supports where these are in close proximity to these devices. Where thyristors are used, again high frequency harmonic currents may be present and the earth electrode may need to be positioned close to their source to prevent significant potential differences arising. Any individual spur parts of the main earth grid (except the reactor earth connection) shall be at least 0.6 x the reactor diameter away and any earth grid loops at least 1.2 x the reactor diameter away. Care shall be taken that a metal tool of 300mm length cannot cause these distances to be infringed to create a closed loop. Interconnecting leads to other equipment should be run close to earth grid conductors Use of Earth Plates (cast iron pipes etc) Many of the older earthing installations made use of earth plates or pipes, mainly because of the much higher current they are capable of dissipating, compared to a rod (in the order of thirty times). The reality is that they are not required to carry large currents because these are diverted through other earth components that have a lower resistance. An example of this is shown in ENA TS (example 14.1). Where an existing substation is being refurbished, the plates/pipes, if in good condition, would normally be retained and connected to the new earthing system, provided they do not introduce a touch voltage hazard. If they are corroded or in an inconvenient location then they can usually be designed out by the new electrode system. If they are situated close to transformers, surge arresters or CVTs, they should be replaced by several rods (two or three) placed close together, say 500mm apart. These are cheaper to install and help equalise potentials on the soil surface as they are used over a greater area. The ideal locations for a plate are at the corner position of the perimeter electrode and near the neutral point of transformers. They would only be required if a detailed design study showed that the current there exceeded the rating of a vertical rod. Maximum current ratings for rods are shown in Table 5-4. UK Power Networks 2015 All rights reserved 37 of 59

38 Table 5-4 Maximum Current Rating of Earthing Rods Resistivity Protection Clearance Time Copper Clad Steel (30%) 16mm Diameter Solid Copper 16mm Diameter 1s 2s 3s 1s 2s 3s 30m 40.8kA 28.8kA 23.4kA 69.7kA 49.2kA 40.2kA 50m 31.6kA 22.3kA 18.2kA 54.0kA 38.1kA 31.2kA 100m 22.3kA 15.7kA 12.9kA 38.2kA 27.0kA 22.0kA 150m 18.2kA 12.8kA 10.6kA 31.2kA 22.0kA 18.0kA 200m 15.8kA 11.1kA 9.1kA 27.0kA 19.0kA 15.6kA 5.26 Earthing of Instrument Transformer Windings ENA TS and ENA ER S15 require that instrument transformer windings be wired out to a terminal board in an accessible place, outside the metal enclosure. The appropriate connections are to be bonded to earth at this terminal board and the link identified so that it cannot be removed by mistake Earthing of Cables Power Cables The earthing of power cables is outside of the scope of this standard. Further guidance can be found in the Jointing Manual and ENA ER C55/4 (1989), Insulated sheath power cable systems Protection and Control Cables Provided there is continuous earth bonding between plant and equipment located within the same substation site, protection and control cables shall be earthed at both ends. The only exception is at the RTU where only the end remote from the RTU shall be earthed. Where protection and control cables are run out to remote sites or third party sites then single end earthing shall be adopted. Any necessary precautions against transferred potential shall also be observed General Guidance for Achieving Electromagnetic Compatibility (EMC) The sources of electromagnetic radiation are: Low Frequency Short circuits or earth faults. Fields generated by equipment. High Frequency Switching on the power system. Lightning. Gapped surge arrester operation. High frequency radio transmitters. Electrostatic discharges. UK Power Networks 2015 All rights reserved 38 of 59

39 The guidance provided elsewhere in this document helps ensure practices, which should minimise electromagnetic radiation. Further information is provided here as background. Practices which reduce low frequency interference are: Separating control cable routes from those of power cables. Installing cables in trefoil rather than flat. Avoiding cable runs in parallel with busbars or power cables. Control cables to avoid single-phase transformers and inductances. Avoid cable earth loops. All wires of the same circuit in one cable or one route. Auxiliary cable routes to have radial rather than ring configuration. Use of twisted pair cables. Practices which reduce high frequency interference are: Suitable instrument transformers with adequate inter-winding shielding. Suitable shielding of secondary circuit cables. Group circuits associated with the same function, wherever possible. Equipment should be selected and grouped according to its working environment and filters and voltage limiting devices used where necessary Earthing of Fences The ideal and preferred arrangement is for all external fences to be separately earthed, and for internal fences (i.e. those crossing or subdividing the site) to be bonded to the earth grid. (Section and Figure 5-5 to Figure 5-9 below illustrate in terms of touch-voltage that a separately-earthed fence is the safer option.) However at some sites it may be necessary to treat separate sections of external fencing differently; or if more practicable, to apply a common earthing method to all compound fencing. When fencing is separately earthed, adequate separation (minimum 2m) shall be maintained throughout between the fencing and any bonded plant (although at sites with low EPR it may be permissible to reduce the distance from the fence to the buried earth-electrode, only, down to 500mm). Any bare metal, armoured or sheathed, cable bonded to the substation main earth and running under the separately earthed fence shall be in an insulated duct for 2m either side and perpendicular to the separately earthed fence. This also applies to conductive pipes and any other conductive materials buried below the separately earthed fence. When fencing is bonded, a detailed calculation is necessary to ensure touch-voltages are safe - unless it is possible to install around the outside either a potential grading electrode or the perimeter electrode itself - typically running 1m outside the fence and buried 1m deep. Wherever fence-lines with different earthing methods meet, an insulating section of minimum 2m length is required to separate them. This may comprise a brick building, a short section of brick wall, or an insulated fence panel. UK Power Networks 2015 All rights reserved 39 of 59

40 An example illustrating these principles and the use of an insulated panel is shown in Figure 5-4. The panel may either be non-conductive (e.g. fibreglass), or a conventional steel panel supported on small stand-off insulators. For the latter it is important that suitable insulators are specified, having a voltage withstand of 3kV for 3s and adequate mechanical durability. If the EPR of the substation is likely to exceed 3kV, then more robust insulators will be required. 2m 33kV Switch House 2m 11kV Switch House Control Room 11kV Switch House Auxiliary Plant Building Key Buried Earth Electrode Fence Building Wall Insulated Fence Panel Bond Figure 5-4 Use of Separately Earthed and Bonded Fencing Arrangements at the Same Substation Adjacent Metal Structures Where there are earthed metal structures within 2m of the fence (such as a terminal tower), then the preferred fence earthing arrangement may not be achievable. It may be necessary to bond the fence run adjacent to the structure, install a perimeter grading electrode outside, and fit insulated fence panels to separate this part of the fence from the rest. Whether this option is followed or the whole fencing arrangement bonded depends upon the site layout and dimensions, and a decision needs to be taken at the design stage. UK Power Networks 2015 All rights reserved 40 of 59

41 Palisade Gates and Removable Fence Panels Gate openings in a fence-line shall be bonded across between posts, to prevent potential differences arising, using 70mm 2 stranded conductor (minimum). Posts supporting removable metal fence panels shall also be bonded across. Gate hinges should also be bonded across, using 35mm 2 flexible braided conductor. Where gates associated with a separately earthed fence open inwards, it is important that they cannot inadvertently bond this to the grid, or allow personnel to touch the gate and bonded metalwork at the same time. For example, the gate retaining fittings shall not be bonded to the grid, and shall be at least 2m away from other earthed metalwork. In cases where the EPR is high (above 1kV) it may also be necessary to design the earth mat such that the open gate does not pass over or close to it. A small inset may be formed in the nearby electrode, such that the 2m separation is maintained whilst the gate is open, or else the infringing part of the electrode may be installed in PVC ducts. At existing sites where the earth mat has not been modified, it may be necessary to show by calculation that touch voltages are within the safe limit Metal Anti-climbing Guards Where anti-climbing razor wire or similar is fitted to the top of palisade fencing, it shall be earthed by connection to the metallic fence upon which it is situated. Where there is a change in the fence earthing method, there shall be electrical breaks in the anti-climbing wire (e.g. 100mm gaps at either side) co-incident with those of the fencing, or, the anti-climbing wire shall be in a situation where it is not realistically possible for someone to touch it and the panel below (the wire shall be supported on insulated mountings as it passes over this section. Where wire or guard is fitted along a short insulating section of brick wall, this may be left isolated (as for a steel panel on insulators) provided that a 100mm gap is maintained at both ends of the wall. In other situations, wire or guard fitted on a brick type wall shall be earthed either to the adjacent fence or to the earth grid - whichever is the most appropriate, and does not introduce a touch voltage risk Temporary, Site-Perimeter and Adjacent Landowners' Fencing Temporary fences inside the substation installed for construction and other purposes are to be earthed in the same manner as permanent fences, i.e. bonded to the main grid within the site, with insulated panels used if necessary to abut them to the external fence when this is separately earthed. Where galvanised or plastic coated mesh fencing is used, a separate 70mm 2 earth conductor shall be installed along the fence (or buried). This should be connected to the fence at 10m intervals, and to independent earth rods or the earth grid (as appropriate) at a minimum of 50m intervals. Site outer perimeter fencing, and any other metal fencing belonging to adjacent landowners, presents a transferred potential hazard if connected to the substation fence. Such fences shall be kept electrically isolated from the substation palisade fencing by means of 2m gaps, insulating 'spur' panels, or brick wall sections as appropriate. UK Power Networks 2015 All rights reserved 41 of 59

42 Safety Advantages of a Separately Earthed Fence Figure 5-5 to Figure 5-9 below illustrate why a separately earthed fence is the safer option to use. They show the difference that connecting the fence to the electrode system and then adding a potential grading conductor, makes to the touch potential on the fence. Figure 5-5 shows that placing the separately earthed fence 2m from the main electrode produces a fence touch voltage of only 3.4% of the EPR. If the fence separation from the grid is reduced to 500mm, the touch voltage only increases to 7.6% (Figure 5-6). Bonding the fence to the grid increases the touch voltages to 44.6% and 37.3% of the EPR, respectively (see Figure 5-7 and Figure 5-8), which would normally be too high. Adding an external potential grading electrode reduces this back to a maximum of 15.4% of the EPR when the fence is bonded (Figure 5-9). A detailed calculation to ensure touch voltages are safe is necessary if it is not possible to install either a potential grading electrode or the perimeter electrode outside a bonded fence. Earth grid dimension 50m x 40m, with 10m mesh spacing, 600mm deep METRES Uniform soil resistivity 100m EPR = 1000V Maximum touch voltage on fence = 34V METRES Figure 5-5 Separately Earthed Fence 2m away from Earth Grid Earth grid dimension 50m x 40m, with 10m mesh spacing, 600mm deep METRES Uniform soil resistivity 100m EPR = 1000V Maximum touch voltage on fence = 76V METRES Figure 5-6 Separately Earthed Fence 500mm away from Earth Grid UK Power Networks 2015 All rights reserved 42 of 59

43 Earth grid dimension 50m x 40m, with 10m mesh spacing, 600mm deep METRES Uniform soil resistivity 100m EPR = 1000V Maximum touch voltage on fence = 446V METRES Figure 5-7 Earth Grid Bonded incorrectly to Fence, which is 2m away from Earth Grid Earth grid dimension 50m x 40m, with 10m mesh spacing, 600mm deep METRES Uniform soil resistivity 100m EPR = 1000V Maximum touch voltage on fence = 373V METRES Figure 5-8 Earth Grid Bonded incorrectly to Fence, which is 500mm away from Earth Grid Earth grid dimension 50m x 40m, with 10m mesh spacing, 600mm deep METRES Uniform soil resistivity 100m EPR = 1000V Maximum touch voltage on fence = 154V METRES Figure 5-9 Fence 2m away from Earth Grid, Fence and Earth Grid Bonded with Potential Grading 1m away UK Power Networks 2015 All rights reserved 43 of 59

44 5.30 Metalwork Bonding Requirements Ancillary Metalwork All exposed and normally un-energised metalwork inside the substation perimeter, including doors, staircases, ventilation ducts, cable supports etc., shall be bonded to the main earth grid to avoid any potential differences between different items of metalwork. The appropriate bonding conductor shall be selected from Table Metal Trench Covers Metal trench covers within substation buildings which are not sitting on an earthed metal frame shall be indirectly earthed as follows: Install a copper tape strip (25mm x 3mm) along one edge of the trench top edge so that trench covers are in contact with it when in position. Connect the copper tape to the switchgear earth bar or internal building earthing system Cable Tunnel Metalwork Metal trays are used within cable tunnels to support power, pilot and communication cables. Where the power cables are of the single core type, the risk is that during normal or fault conditions voltages or currents may be induced into the tray metalwork causing damage, cable de-rating or a risk of shock. To prevent excessive induced or transfer voltages on the tunnel metalwork: Cable supports and cable trays in tunnels shall not be connected to the substation earthing system. Cable trays in tunnels shall be broken into sections with a 50mm gap approximately every 50 metres. Cable trays in tunnels shall be separated by 2 metres from any metalwork connected to the substation earthing system Basement Cable Support Systems Cable support structures and cable trays in basements shall normally be bonded to the substation earthing system using a suitable bonding conductor from Table 5-2. Individual dispersed cable supports do not need to be bonded to the substation earthing system provided they are separated from other earthed metalwork by a minimum of 2 metres. UK Power Networks 2015 All rights reserved 44 of 59

45 6 References 6.1 UK Power Networks Standards EDS EDS EDS EDS EDS EDS EDS EDS EDS ECS ECS EDS Earthing Standard HOT Site Requirements (internal document only) Design Criteria Secondary Substation Earthing Design Pole-mounted Equipment Earthing Design LV Network Earthing Design Customer LV Installation Earthing Design NetMap Earthing Information System (internal document only) Customer EHV and HV Connections (including Generation) Earthing Design and Construction Grid and Primary Substation Earthing Construction Earthing Testing and Measurements Grid and Primary Civil Design Standards EDS LV supplies to Mobile Phone Base Stations Mounted on 132, 275 and 400kV Towers (internal document only) 6.2 National and International Standards ENA TS ENA ER S34 ENA ER S36 BS EN BS 7430:2012 IEEE Standard 80 (2000) Guidelines for the Design, Installation, Testing and Maintenance of Main Earthing Systems in Substations A Guide for Assessing the Rise of Earth Potential at Substation Sites Procedure to Identify and Record HOT Substations Protection Against Lightning Code of Practice for Protective Earthing of Electrical Installations Guide for Safety in AC Substation Grounding IEEE Standard 655 (1987) Guide for Generating Station Grounding UK Power Networks 2015 All rights reserved 45 of 59

46 Appendix A Standard Substation Earthing Arrangements The following substation earthing drawings are available: EDS /33kV Substation General Earthing Arrangement Option 1 EDS /33kV Substation General Earthing Arrangement Option 2 EDS /11kV Substation General Earthing Arrangement Option 1 EDS /11kV Substation General Earthing Arrangement Option 2 EDS /11kV Substation General Earthing Arrangement Option 3 UK Power Networks 2015 All rights reserved 46 of 59

47 Appendix B Calculation of Fault Current returning via the Ground B.1 Calculation of Fault Current Returning via the Ground Total Fault Current Current returned to the source via earth wires and cable sheaths by induction Current returning to site transformer neutral(s) Ground Current Earth Grid Grid Current Parallel Paths Current Sum of parallel paths Tower footings, Cables Transformer Neutral At source Figure 6-1 Calculation of Fault Current Returning via the Ground in a 132kV Network UK Power Networks 2015 All rights reserved 47 of 59

48 Current returned to the source via earth wires and cable sheaths by induction Total Fault Current Ground Current Earth Grid Grid Current Parallel Paths Current Sum of parallel paths Tower footings, Cables Transformer At source Figure 6-2 Calculation of Fault Current Returning via the Ground in a 66kV or 33kV Network B.2 Example of Cable Information Necessary where Network Reduction is Required to Estimate Earth Contribution from Cable Network 185MM 2 PICAS 185MM 2 PICAS L o c a l Substation 0.15 //2 PILCSWA 0.15 //2 PILCSWA L o c a l Substation 300mm 2 PICAS 300 mm 2 PICAS 300 mm 2 PICAS 300 mm 2 PICAS L o c a l Substation 95mm 2 XLPE Figure 6-3 Example of Cable Network Information Required UK Power Networks 2015 All rights reserved 48 of 59

49 B.3 Examples of Fault Current Information Required for Fault Current Reduction a R a n b n T 0 OHM c n b c 0 Y R B Faulted substation b c a n b n c n Y T B 0 OHM Figure 6-4 Example showing 132kV Phase Currents for Transformer-feeder Arrangement Individual transformer and line phase currents are required in phasor format, indicating convention of current direction and the fault current at the point of fault. Calculations are to be in accordance with IEC 909 and Engineering Recommendation G kV Transformer and Line Currents Transformer Neutral Currents a a n b b n c c n a a n b b n c c n Vector sum Vector sum I f = I r + I n I f = UK Power Networks 2015 All rights reserved 49 of 59

50 Other information required includes the tower type (e.g. L4), tower footing resistance or foundation dimensions (average depth and separation). If underground cable, the cable size, type, length and sheath earthing arrangement are necessary. In some cases, the earth impedance of the source(s) and intermediate substations are also required. a b c A B C a n F a b c a n c n I f = b n c n I r = I n = Figure 6-5 Example showing 132kV Phase Currents for a Parallel-feeder Arrangement Line Currents Transformer Neutral Currents a a n b b n c c n a a n b b n c c n Vector sum Vector sum I f = I r + I n I f = UK Power Networks 2015 All rights reserved 50 of 59

51 Appendix C Special Conditions Applicable to Generating Stations within or Adjacent to a Substation Refer to EDS for additional guidance on all aspects of 132kV and 33kV customer earthing. Appendix D Maximum Resistance Values for Electrodes at Pole-mounted Plant Refer to EDS UK Power Networks 2015 All rights reserved 51 of 59

52 Y AXIS (METERS) Grid and Primary Substation Earthing Design Document Number: EDS Appendix E Standard Substation Earthing Arrangements Resistance Values, Surface, Touch and Step Potential Contours E.1 132/33kV Substation Arrangements (EDS /EDS ) Soil Type Typical Soil Resistivity (m) Resistance of Substation Earthing () Loam 25 or less 0.27 Chalk 50 or less 0.55 Clay 100 or less 1.10 Sand/Gravel/Clay Mix 150 or less or less or less 3.29 Slate/Shale/Rock 500 or less 5.49 SINGLE-ELECTRODE/SCALAR POTENTIALS [ID:EDF_132_100ohm_soil] LEGEND MAXIMUM VALUE : MINIMUM VALUE : Level 8 ( ) Level 7 ( ) Level 6 ( ) Level 5 ( ) Level 4 ( ) -50 Level 3 ( ) Level 2 ( ) Level 1 ( ) X AXIS (METERS) Potential Profile (% reference GPR) Figure /33kV Substation Electrode System Surface Potential Contours Expressed as a % of the EPR UK Power Networks 2015 All rights reserved 52 of 59

53 Y AXIS (METERS) Y AXIS (METERS) Grid and Primary Substation Earthing Design Document Number: EDS SINGLE-ELECTRODE/TOUCH VOLTAGES/WORST SPHERICAL [ID:EDF_132_100ohm_soil] LEGEND Maximum User Limit: Minimum Value : X AXIS (METERS) Touch Voltage (% Ref. GPR) [Wors] Figure /33kV Substation Electrode System Touch Potential Contours Expressed as a % of the EPR SINGLE-ELECTRODE/STEP VOLTAGES (SPHERICAL)/WORST SPHERICAL [ID:EDF_132_100ohm_soil] LEGEND Maximum Value : Minimum Value : X AXIS (METERS) Step Voltage-Worst (% Ref. GPR) Figure /33kV Substation Electrode System Step Potential Contours Expressed as a % of the EPR UK Power Networks 2015 All rights reserved 53 of 59

54 Y AXIS (METERS) Grid and Primary Substation Earthing Design Document Number: EDS E.2 33/11kV Substation Arrangement Option 1 (EDS ) Standard Design Soil Type Typical Soil Resistivity (m) Resistance of Substation Earthing () Rural/Overhead Line Fed Design Resistance of Substation Earthing () Loam 25 or less Chalk 50 or less Clay 100 or less Sand/Gravel/Clay Mix 150 or less or less or less Slate/Shale/Rock 500 or less SINGLE-ELECTRODE/SCALAR POTENTIALS [ID:EDF_33_Option_1] LEGEND MAXIMUM VALUE : MINIMUM VALUE : Level 8 ( ) Level 7 ( ) Level 6 ( ) Level 5 ( ) 0 Level 4 ( ) Level 3 ( ) -50 Level 2 ( ) Level 1 ( ) X AXIS (METERS) Potential Profile (% reference GPR) Figure /11kV (Option 1) Substation Electrode System Surface Potential Contours Expressed as a % of the EPR UK Power Networks 2015 All rights reserved 54 of 59

55 Y AXIS (METERS) Y AXIS (METERS) Grid and Primary Substation Earthing Design Document Number: EDS SINGLE-ELECTRODE/TOUCH VOLTAGES/WORST SPHERICAL [ID:EDF_33_Option_1] LEGEND Maximum User Limit: Minimum Value : X AXIS (METERS) Touch Voltage (% Ref. GPR) [Wors] Figure /11kV (Option 1) Substation Electrode System Touch Potential Contours Expressed as a % of the EPR SINGLE-ELECTRODE/STEP VOLTAGES (SPHERICAL)/WORST SPHERICAL [ID:EDF_33_Option_1] LEGEND Maximum Value : Minimum Value : X AXIS (METERS) Step Voltage-Worst (% Ref. GPR) Figure /11kV (Option 1) Substation Electrode System Step Potential Contours Expressed as a % of the EPR UK Power Networks 2015 All rights reserved 55 of 59

56 Y AXIS (METERS) Grid and Primary Substation Earthing Design Document Number: EDS E.3 33/11kV Substation Arrangement Option 2 (EDS ) Standard Design Soil Type Typical Soil Resistivity (m) Resistance of Substation Earthing () Rural/Overhead Line Fed Design Resistance of Substation Earthing () Loam 25 or less Chalk 50 or less Clay 100 or less Sand/Gravel/Clay Mix 150 or less or less or less Slate/Shale/Rock 500 or less SINGLE-ELECTRODE/SCALAR POTENTIALS [ID:EDF_33_Option_2] LEGEND MAXIMUM VALUE : MINIMUM VALUE : Level 8 ( ) Level 7 ( ) Level 6 ( ) Level 5 ( ) 8 0 Level 4 ( ) Level 3 ( ) -50 Level 2 ( ) Level 1 ( ) X AXIS (METERS) Potential Profile (% reference GPR) Figure /11kV (Option 2) Substation Electrode System Surface Potential Contours Expressed as a % of the EPR UK Power Networks 2015 All rights reserved 56 of 59

57 Y AXIS (METERS) Y AXIS (METERS) Grid and Primary Substation Earthing Design Document Number: EDS SINGLE-ELECTRODE/TOUCH VOLTAGES/WORST SPHERICAL [ID:EDF_33_Option_2] LEGEND Maximum User Limit: Minimum Value : X AXIS (METERS) Touch Voltage (% Ref. GPR) [Wors] Figure /11kV (Option 2) Substation Electrode System Touch Potential Contours Expressed as a % of the EPR SINGLE-ELECTRODE/STEP VOLTAGES (SPHERICAL)/WORST SPHERICAL [ID:EDF_33_Option_2] LEGEND Maximum Value : Minimum Value : X AXIS (METERS) Step Voltage-Worst (% Ref. GPR) Figure /11kV (Option 2) Substation Electrode System Step Potential Contours Expressed as a % of the EPR UK Power Networks 2015 All rights reserved 57 of 59

58 Appendix F Minimum Conductor Sizes Fault Level Material Connection Type Temperature ( o C) Calculated Conductor Size (mm 2 ) Minimum Tape Size (mm) Minimum Stranded Conductor Size (mm 2 ) 12kA/3s Copper Single (spur) bolted Aluminium Single (spur) brazed or welded Duplicate or loop bolted Duplicate or loop brazed or welded Single (spur) bolted Single (spur) brazed or welded Duplicate or loop bolted Duplicate or loop brazed or welded 26kA/3s Copper Single (spur) bolted Aluminium Single (spur) brazed or welded Duplicate or loop bolted Duplicate or loop brazed or welded Single (spur) bolted Single (spur) brazed or welded Duplicate or loop bolted Duplicate or loop brazed or welded x x x x x x x x x x x x n/a n/a x x UK Power Networks 2015 All rights reserved 58 of 59

59 Fault Level Material Connection Type Temperature ( o C) Calculated Conductor Size (mm 2 ) Minimum Tape Size (mm) Minimum Stranded Conductor Size (mm 2 ) 31.5kA/3s Copper Aluminium Single (spur) bolted Single (spur) brazed or welded Duplicate or loop bolted Duplicate or loop brazed or welded Single (spur) bolted Single (spur) brazed or welded Duplicate or loop bolted Duplicate or loop brazed or welded 40kA/3s Copper Single (spur) bolted Aluminium Single (spur) brazed or welded Duplicate or loop bolted Duplicate or loop brazed or welded Single (spur) bolted Single (spur) brazed or welded Duplicate or loop bolted Duplicate or loop brazed or welded x x x x n/a n/a n/a n/a x x x x x x 5 or 50 x n/a n/a n/a n/a n/a n/a 323 UK Power Networks 2015 All rights reserved 59 of 59