THIS IS AN UNCONTROLLED DOCUMENT, THE READER SHALL CONFIRM ITS VALIDITY BEFORE USE Document Number: EDS 06-0013 ENGINEERING DESIGN STANDARD EDS 06-0013 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
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 06-0022 while the review of national standards ENA TS 41-24 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 41-24 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
Contents 1 Introduction... 7 2 Scope... 7 3 Abbreviations... 7 4 Overview... 8 4.1 Data Requirements... 8 4.2 Earth Potential Rise (EPR) Estimation... 8 4.3 Standard Design... 9 4.4 Detailed Design... 10 4.5 Review of Existing Substations... 10 5 Detailed Design Procedure... 11 5.1 Determination of Soil Resistivity... 11 5.2 Preliminary Site Assessment... 11 5.3 Initial Design of the Earth Grid - Standard Earthing Arrangement... 12 5.4 Initial Design of Earth Grid - Non-Standard Earthing Arrangement... 14 5.5 Calculation of the Earth Grid Resistance... 14 5.5.1 First Approximation... 14 5.5.2 Computer Modelling... 14 5.6 Calculation of the Grid or Overall Earth Impedance (taking into account parallel paths)... 14 5.6.1 The Earth Grid... 15 5.6.2 Bonded Foundation Structure Steel Reinforcement Bars... 15 5.6.3 Steel Tower Lines... 16 5.6.4 Cable Networks... 16 5.6.5 Fortuitous Earth Paths... 18 5.7 Data Required from the Power System Fault Study... 19 5.8 Earth Fault Current Returning via Earthwire/Cable Sheaths Due to Induction... 20 5.8.1 Introduction... 20 5.8.2 Overhead line with Earth Conductor... 21 5.8.3 Cables... 21 5.9 Calculation of the Earth Potential Rise (EPR)... 21 5.9.1 Primary Substations... 21 5.9.2 Grid Substations (including 132 kv and 33kV Connections)... 22 5.10 Calculation of External Potential Contours or Zones of Influence (e.g. HOT Zone).. 23 5.10.1 Accurate Representation... 23 5.10.2 Approximate Calculation... 23 UK Power Networks 2015 All rights reserved 3 of 59
5.11 Implications of a Substation being HOT... 24 5.11.1 Reducing the Area Covered by the HOT Zone... 24 5.11.2 Reduce the Earth Fault Current... 25 5.11.3 Reducing the Electrode Resistance... 25 5.11.4 Reduce the Impedance of Parallel Paths... 26 5.12 Calculation of Touch and Step Potentials... 26 5.12.1 Accurate Calculation... 26 5.12.2 Approximate Method... 26 5.13 Requirements of the Final Design... 27 5.14 Conductor Size and use of Metallic Structures... 28 5.15 Earthing Arrangements Applicable to 400/275/132kV Sites of Different Companies 30 5.16 Earthing Arrangements Applicable to Sites with Generation... 30 5.17 Earthing Arrangements at Railway Supply Substations... 31 5.18 Earthing Arrangements at GIS Substations... 32 5.19 Earthing Arrangements for Surge Arresters and Capacitor VTs... 34 5.20 Positioning of Metal Supports for Security Lighting etc Near Fences... 34 5.21 Communication Towers within or adjacent to Substations... 35 5.22 Communication Masts Fitted to Towers... 36 5.23 LV Supplies to Third Party Equipment at Substations... 36 5.24 Reactors and AC to DC Converters... 37 5.25 Use of Earth Plates (cast iron pipes etc)... 37 5.26 Earthing of Instrument Transformer Windings... 38 5.27 Earthing of Cables... 38 5.27.1 Power Cables... 38 5.27.2 Protection and Control Cables... 38 5.28 General Guidance for Achieving Electromagnetic Compatibility (EMC)... 38 5.29 Earthing of Fences... 39 5.29.1 Adjacent Metal Structures... 40 5.29.2 Palisade Gates and Removable Fence Panels... 41 5.29.3 Metal Anti-climbing Guards... 41 5.29.4 Temporary, Site-Perimeter and Adjacent Landowners' Fencing... 41 5.29.5 Safety Advantages of a Separately Earthed Fence... 42 5.30 Metalwork Bonding Requirements... 44 5.30.1 Ancillary Metalwork... 44 5.30.2 Metal Trench Covers... 44 5.30.3 Cable Tunnel Metalwork... 44 5.30.4 Basement Cable Support Systems... 44 UK Power Networks 2015 All rights reserved 4 of 59
6 References... 45 6.1 UK Power Networks Standards... 45 6.2 National and International Standards... 45 Appendix A Standard Substation Earthing Arrangements... 46 Appendix B Calculation of Fault Current returning via the Ground... 47 B.1 Calculation of Fault Current Returning via the Ground... 47 B.2 Example of Cable Information Necessary where Network Reduction is Required to Estimate Earth Contribution from Cable Network... 48 B.3 Examples of Fault Current Information Required for Fault Current Reduction... 49 Appendix C Special Conditions Applicable to Generating Stations within or Adjacent to a Substation... 51 Appendix D Maximum Resistance Values for Electrodes at Pole-mounted Plant... 51 Appendix E Standard Substation Earthing Arrangements Resistance Values, Surface, Touch and Step Potential Contours... 52 E.1 132/33kV Substation Arrangements (EDS 07-0105-1001/EDS 07-0105-1002)... 52 E.2 33/11kV Substation Arrangement Option 1 (EDS 07-0105-2001)... 54 E.3 33/11kV Substation Arrangement Option 2 (EDS 07-0105-2002)... 56 Appendix F Minimum Conductor Sizes... 58 Tables Table 5-1 Calculation of Ground Current for Cable Faults... 21 Table 5-2 Earthing and Bonding Electrode/Conductor Sizes... 28 Table 5-3 Tape and Stranded Conductor Specifications... 29 Table 5-4 Maximum Current Rating of Earthing Rods... 38 UK Power Networks 2015 All rights reserved 5 of 59
Figures Figure 4-1 Making a First Estimate of the Substation EPR... 9 Figure 5-1 Earthing System Components... 15 Figure 5-2 Estimating the Proportion of Fault Current Returning Through the Soil... 20 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... 40 Figure 5-5 Separately Earthed Fence 2m away from Earth Grid... 42 Figure 5-6 Separately Earthed Fence 500mm away from Earth Grid... 42 Figure 5-7 Earth Grid Bonded incorrectly to Fence, which is 2m away from Earth Grid... 43 Figure 5-8 Earth Grid Bonded incorrectly to Fence, which is 500mm away from Earth Grid... 43 Figure 5-9 Fence 2m away from Earth Grid, Fence and Earth Grid Bonded with Potential Grading 1m away... 43 Figure 6-1 Calculation of Fault Current Returning via the Ground in a 132kV Network... 47 Figure 6-2 Calculation of Fault Current Returning via the Ground in a 66kV or 33kV Network... 48 Figure 6-3 Example of Cable Network Information Required... 48 Figure 6-4 Example showing 132kV Phase Currents for Transformer-feeder Arrangement... 49 Figure 6-5 Example showing 132kV Phase Currents for a Parallel-feeder Arrangement.. 50 Figure 6-6 132/33kV Substation Electrode System Surface Potential Contours Expressed as a % of the EPR... 52 Figure 6-7 132/33kV Substation Electrode System Touch Potential Contours Expressed as a % of the EPR... 53 Figure 6-8 132/33kV Substation Electrode System Step Potential Contours Expressed as a % of the EPR... 53 Figure 6-9 33/11kV (Option 1) Substation Electrode System Surface Potential Contours Expressed as a % of the EPR... 54 Figure 6-10 33/11kV (Option 1) Substation Electrode System Touch Potential Contours Expressed as a % of the EPR... 55 Figure 6-11 33/11kV (Option 1) Substation Electrode System Step Potential Contours Expressed as a % of the EPR... 55 Figure 6-12 33/11kV (Option 2) Substation Electrode System Surface Potential Contours Expressed as a % of the EPR... 56 Figure 6-13 33/11kV (Option 2) Substation Electrode System Touch Potential Contours Expressed as a % of the EPR... 57 Figure 6-14 33/11kV (Option 2) Substation Electrode System Step Potential Contours Expressed as a % of the EPR... 57 UK Power Networks 2015 All rights reserved 6 of 59
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 06-0012. ECS 06-0022 provides construction guidance for grid and primary substations. There will be some situations where advice from an earthing specialist is required refer to EDS 06-0001 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 06-0019) 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
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
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 300-430V 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
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 5.11.1). 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 06-0025. 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
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 9. 5.2 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
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
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 5.29. 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 5.20. 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
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 5.5.1 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. 5.5.2 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
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 5.6.1 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. 5.6.2 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
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 2 34. 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 5.19. 5.6.4 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
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
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. 5.6.5 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
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
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 5.8.1 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
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. 5.8.3 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 14 132 630mm 2 PILC or PICAS All cable 13 33 Standard * PILC Via overhead line 16 132 630mm 2 3 x 1 PILC Via overhead line 17 132 630mm 2 3 x 1 PICAS Via overhead line 10 33 Standard* PILC Cable 15 132 630mm 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) 5.9.1 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