Research Brief Impact of higher 25kV fault currents Background Increasing the maximum fault levels at traction feeder stations has the potential to reduce the costs of electrification schemes. The potential savings are related to performance gains achieved by better voltage regulation, and by extending feeding distances, which reduces the number of feeder stations needed. At present fault levels on the UK 25kV electrified system are limited to 6kA which imposes a limitation on the efficiency of the system. However, the consequences of raising the fault levels need to be understood, and any mitigation measures identified that are needed to maintain a safe system that complies with relevant Network Rail standards and BS EN 50122-1. This research has reviewed earthing standards and codes of practice related to touch potentials from a range of administrations and organisations, including BS EN 50122-1:2011, National Grid, Electricity Networks Association, and railway design specifications (HS1, DB, and SNCF). This research reviewed the requirements of BS EN50122-1, including a risk-based assessment for step and touch potentials, taking into consideration the requirements of both human protective provision and equipment functionality. Recommendations have been made for mitigation measures that would be necessary to enable the GB system to comply with the limits within these standards. These measures include mitigating the rise of earth potentials (ROEP) at grid sites, tower lines, feeder stations, and track sectioning cabins (TSC). Aims This research was commissioned by the Future Electrification Group (FEG), and its parent body, the Vehicle/Train Energy System Interface Committee (V/TE SIC). The aims of this research were: To understand if and, if so, how other railway authorities achieve compliance with step and touch potential limits where there are systems electrified at 25kV, with permissible fault levels higher than in the UK. RSSB R&D Programme Block 2 Angel Square 1 Torrens Street London EC1V 1NY enquirydesk@rssb.co.uk www.rssb.co.uk/research/pages/ default.aspx 1
Impact of higher 25kV fault currents To establish whether the use of additional mitigation measures (risk process) to achieve compliance (BS EN 50122-1 step and touch voltage limits) is common practice on other railway authorities. To understand the risks to maintainers, public, and equipment associated with adopting higher fault levels in the UK. To identify mitigation measures which would enable fault currents to be increased without introducing unacceptable risks to people and equipment. To establish whether the use of additional mitigation measures (risk process) would be an acceptable way to achieve compliance in the UK. Method The research reviewed a range of standards and codes of practice to gain an understanding of how safe levels of step and touch potential are maintained on different electrical systems. These included electrical distribution networks and railway systems including HS1, Channel Tunnel, DB, and SNCF. The investigation then reviewed the risk process, which allows higher fault levels to be utilised on other railways, to assess its suitability for Network Rail infrastructure. An industry workshop was held to identify the key hazards. Other issues outside the scope of the study were also identified; notably third parties likely to be affected by a rise in fault levels, ie LUL. The research then examined signalling and telecommunication equipment and its proximity to tower lines, grid sites, feeder stations and TSCs to determine whether the current requirement to maintain a 50/100m separation from HV switchgear could be relaxed. In order to do this, mathematical models were developed to assess the rise of earth potential associated with faults on tower lines and at grid sites. Recommendations have been made on the relaxation of these limits, based on the outcome from the modelling The research has been desk-based and has included input from RSSB, Network Rail, and SNCF. 2 RSSB
Findings Having adopted a fault level of 6kA, the GB rail network has had a conservative approach to rail potentials and the subsequent touch potential when compared to European railways that have safely implemented >12kA 1 electrification systems. These systems implemented an 'integrated earth' arrangement to ensure the functionality of the signalling and control systems. Risk mitigation processes can be used where the earths have been integrated, and step and touch voltages are based on the 'effective' limit rather than the limit to remote earth. Signalling systems utilising an 'integrated earth' will require additional integration at the design and installation stages in accordance with BSEN 61000-5-2, and manufacturer's design. To provide protection for humans, SNCF and DB both manage touch and step voltages with 'integrated earth' and risk mitigation processes. Site details can be extremely complex and include buried earths, multiple tower lines, and non-uniform grid earth mats. Consequentially, at specific locations where an accurate assessment is needed, it is recommended that detailed soil resistivity measurements are obtained and a rise of earth potential study is undertaken. In line with other railway companies, where the fault level is to be raised to a nominal limit of 12kA it is recommended that Network Rail adopts the EN 50122-1 Risk Assessment Process and an integrated earth. Additionally it will be necessary to ensure correct functional behaviour of the signalling and control system: Where the signalling earth can be integral with the electrification earth, an 'integrated earth' arrangement should be adopted for all track-based infrastructure. This will create an 'equipotential zone', and ensure that earth voltages do not disturb signalling equipment. On legacy signalling systems, where the earth cannot be integral with the electrification earth, adequate segregation must be maintained, including all limitations as specified in the signalling manufacturer's specification. 1 BS EN50388, 2005 Clause 11.2 specifies 15kA as the maximum fault current which rolling stock and new infrastructure will be built to withstand. However, the 12kA value has been commonly used and therefore defined as a nominal fault current. In future it is proposed, irrespective of the fault current value, that clause 9.2.2.4 of BS EN 50122-1 is applied, with appropriate risk assessment and mitigation measures. RSSB 3
Impact of higher 25kV fault currents For the segregation of Signalling and Telecommunication (S&T) assets near grid sites connected to the railway traction return system: Existing segregation limits for legacy signalling systems should not be relaxed, where an 'independent earth' is used close to grid sites (HV primary earth faults), without understanding the immunity of the specific signalling assets and the characteristics of the HV grid. Segregation limits for new signalling systems with an 'independent earth', compliant with European Standards (EN50121-4, BSEN61000-5-1 and BS EN61000-5-2) should be capable of being relaxed based on site assessment. (Typically >70m and 1000V cable insulation). New signalling systems designed to work on an 'integrated earth' should be immune to these ROEP disturbances near grid sites, providing the grid and railway earths are interconnected. For the segregation of S&T assets near grid sites NOT connected to the railway: Segregation limits should not be relaxed where grid sites are not interconnected to the railway traction earth. For the segregation of S&T assets near grid overhead towers: Existing segregation limits (50m) close to tower lines can be relaxed on legacy ('independent earth') equipment to typically >25m (based on 430V ITU-T limit). Segregation limits close to tower line sites on new signalling systems compliant with European Standards (EN50121-4, BS IEC 61000-5-1 and BS IEC 61000-5-2) should be capable of being relaxed to typically >10m (based on 1000V cable insulation). For segregation at feeder stations and TSCs: Segregation limits for legacy 'independent earth' equipment close to feeder stations and TSCs should be capable of being relaxed to typically >10m. Segregation limits for new signalling systems compliant with European EMC Standards (EN50121-4, BS IEC 61000-5-1 and BS IEC 61000-5-2), close to feeder stations and TSCs should be capable of being relaxed to typically >10m (where all 25kV extraneous earths are bonded to traction negative). Signalling systems designed to work on an 'integrated earth' should be immune to 25kV rail potentials. 4 RSSB
In relaxing the segregation distance, designers should give due consideration to other forms of disturbance at each specific location including, but not exclusively: Disturbances from neutral sections Disturbances from lightning conduction paths Disturbance from lightning induction Disturbance from railway electrification induction Deliverables Next Steps The research has delivered the summary report Impact of higher 25kV fault currents, which includes the research findings, conclusions and recommendations. This report is available to RSSB members on request. This research has yielded many benefits. However, the key benefits will be realised through the reduction in the cost of future electrification of the GB rail network and better improvement in the performance of trains. The research findings have been considered by the FEG and the V/TE SIC and will be used to: Assist the rail industry to take the necessary steps to ensure compliance with the step and touch potential limits as documented in BS EN 50122-1. Provide NR with the appropriate information when reviewing the existing installations of autotransformer feeder stations, to determine whether the fault level can be raised by removing fault limiting reactors. Design the future electrification of the GB network in compliance with BS EN50122-1. The FEG and V/TE SIC have agreed to implement the findings from this research by supporting the draft Railway Group Standard for Energy Subsystems (AC Electrification System) that is being produced by RSSB. Contact For more information please contact: Head of Engineering Research R&D Programme RSSB enquirydesk@rssb.co.uk RSSB 5