swr rh11 introduction to overhead line electrification.pdf

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RAILWAY ELECTRIFICATION & POWER ENGINEERING

REPE Handbook: Introduction to Overhead LineElectrification

RH11

August 2008

Version 4

This is an uncontrolled copy once printed or copied from its controlled location at

\\calcium-sw\non project\REPE_Shared\REPE Handbook\Current Version PDFs


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REPE HandbookRH11: Introduction to Overhead Line Electrif ication

For

Scott Wilson RailwaysTricentre 3Newbridge SquareSwindon SN1 1BY

and

Scott Wilson RailwaysBuchanan House58 Port Dundas RoadGlasgow G4 0HG

Report Verification

Name Posit ion Signature Date

Prepared by: Garry Keenor Technical Manager (OLE)Checked by: Ian Moore Section Manager (OLE)

Approved by: Rob Tidbury Head of Railway Electrif ication &Power Engineering

Revision Schedule

Version Date Details of Revision Issued by

3 25 October 2004 Minor Revisions GPK

4 15 August 2008 See details fol lowing page GPK


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Garry KeenorTechnical Manager (OLE)

Scott Wilson Railways

Tricentre 3Newbridge SquareSwindon SN1 1BY

Tel: +44 (0) 1793 508870Fax: +44 (0) 1793 508891

Email: [email protected]

Significant Changes in this Revision

Section Date

all Converted to REPE handbook.

5.2 AC supply principles section expanded.

5.6 DC supply principles section expanded.

5.8 Protection section expanded.

6.2 Materials section added.

6.8 Turnout wiring section added.

6.11.7 Spanwire portal added.


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This document has been prepared for use within Scott Wilson's Railway Electrification and PowerEngineering (REPE) Unit. It is addressed to and for the sole use and reliance of Scott Wilson'sREPE staff. Scott Wilson accepts no liability for any use of this document other than by REPE staffand only for the purposes, stated in the document, for which it was prepared and provided. Noperson other than REPE staff may copy (in whole or in part) use or rely on the contents of thisdocument, without the prior written permission of the Company Secretary of Scott Wilson Ltd. Anyadvice, opinions, or recommendations within this document should be read and relied upon only inthe context of the document as a whole. The contents of this document are not to be construed as

providing legal, business or tax advice or opinion.

Scott Wilson Group PLC 2008


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Railway Electrification & Power EngineeringREPE Handbook: Introduction to Overhead Line Electrification

TABLE OF CONTENTS

1. PURPOSE.........................................................................................................................16

2. SCOPE ..............................................................................................................................16

3. DEFINITION OF TERMS...................................................................................................16

4. BASICS OF OLE...............................................................................................................16

4.1 What is OLE? ...................................................................................................................16

4.2 Unique Features of OLE..................................................................................................17

4.3 Advantages and Disadvantages of the System ............................................................18

4.4 RAMS ................................................................................................................................18

4.4.1 Reliability ...........................................................................................................................19

4.4.2 Availability..........................................................................................................................19

4.4.3 Maintainability....................................................................................................................19

4.4.4 Safety.................................................................................................................................19

4.5 Development of OLE systems ........................................................................................19

4.5.1 Electric Beginnings............................................................................................................20

4.5.2 Mainline DC Growth...........................................................................................................21

4.5.3 AC Developments..............................................................................................................22

4.5.4 High Speed Lines ..............................................................................................................23

4.5.5 UK Developments..............................................................................................................26

4.6 Categor ies of OLE System..............................................................................................29

4.6.1 Tram Systems....................................................................................................................29

4.6.2 Light Rail Systems.............................................................................................................29

4.6.3 Mainline Systems...............................................................................................................29

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4.6.4 High Speed Systems .........................................................................................................29

5. ELECTRICAL PRINCIPLES .............................................................................................31

5.1 Supply voltages and currents ........................................................................................31

5.1.1 Transmission and Supply Voltages ...................................................................................31

5.1.2 Supply Current...................................................................................................................32

5.2 AC Supply Princ ipals ......................................................................................................33

5.3 AC Supply Equipment .....................................................................................................36

5.3.1 AC Transformers ...............................................................................................................36

5.3.2 Auxiliary Transformers.......................................................................................................36

5.3.3 AC Circuit Breakers ...........................................................................................................36

5.3.4 AC Cables..........................................................................................................................37

5.4 AC Sectioning Pr inciples ................................................................................................37

5.5 AC Feeding and Immunisation Methods .......................................................................39

5.5.1 Classic feeding arrangement .............................................................................................39

5.5.2 Auto Transformer Feeding Arrangement...........................................................................42

5.6 DC Supply Principals ......................................................................................................44

5.7 DC Sectioning Principles ................................................................................................44

5.8 Protection, Monitoring and Control ...............................................................................46

5.8.1 Fault Protection..................................................................................................................46

5.8.2 Control and Monitoring ......................................................................................................47

5.9 Electrical Clearances.......................................................................................................48

5.10 Earthing and Bonding .....................................................................................................50

5.10.1 AC Systems.......................................................................................................................53

5.10.2 DC Systems and Stray Currents........................................................................................53

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5.10.3 Temporary Earthing Arrangements ...................................................................................53

5.10.4 Buffer Sections and Permanent Earths .............................................................................53

6. MECHANICAL PRINCIPLES............................................................................................55

6.1 Interface with the Pantograph ........................................................................................55

6.2 Materials ...........................................................................................................................59

6.3 Wire Types........................................................................................................................59

6.3.1 Contact Wire......................................................................................................................59

6.3.2 Contact Bar........................................................................................................................61

6.3.3 Catenary Wire and Auxiliary Catenary...............................................................................61

6.3.4 Droppers............................................................................................................................61

6.4 Insulators..........................................................................................................................62

6.4.1 Materials ............................................................................................................................62

6.4.2 Mechanical Requirements .................................................................................................63

6.5 Suspension Arrangements .............................................................................................66

6.6 Tensioning Arrangements ..............................................................................................69

6.7 Transferring the Pan between Tension Lengths ..........................................................70

6.7.1 Zero Span Overlap ............................................................................................................70

6.7.2 Single Span Overlap..........................................................................................................72

6.7.3 Multiple Span Overlaps......................................................................................................73

6.8 Turnout Wiring .................................................................................................................74

6.8.1 Low Speed Tangential Method..........................................................................................74

6.8.2 Cross Contact Method.......................................................................................................74

6.8.3 Cross-Droppered Cross Contact Method ..........................................................................75

6.8.4 High Speed Tangential Method .........................................................................................76

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6.8.5 High Speed Three Wire System ........................................................................................76

6.9 Other Electrical Break Devices.......................................................................................76

6.9.1 Section Insulator................................................................................................................76

6.9.2 Neutral Sections ................................................................................................................78

6.10 Mechanical Clearances ...................................................................................................80

6.11 OLE structures.................................................................................................................80

6.11.1 Single Cantilever................................................................................................................82

6.11.2 Double Cantilever ..............................................................................................................83

6.11.3 Back to Back Cantilever.....................................................................................................84

6.11.4 Twin Track Cantilever........................................................................................................85

6.11.5 Portals................................................................................................................................86

6.11.6 Headspans.........................................................................................................................87

6.11.7 Spanwire Portals................................................................................................................88

6.11.8 Bridge and Tunnel Supports..............................................................................................88

6.11.9 Anchors..............................................................................................................................91

6.12 OLE Foundations .............................................................................................................94

6.12.1 Planted Mast Foundations.................................................................................................94

6.12.2 Side Bearing Concrete Foundations..................................................................................94

6.12.3 Mass Concrete Foundations..............................................................................................94

6.12.4 Piled Foundations..............................................................................................................94

6.12.5 Gravity Foundations...........................................................................................................95

6.12.6 Rock Foundations..............................................................................................................96

6.12.7 Attachment to Other Infrastructure ....................................................................................96

6.12.8 Basic Design Ranges ........................................................................................................97

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6.13 OLE Assemblies Overview .............................................................................................98

6.14 OLE Geometry................................................................................................................100

6.14.1 Vertical Limitations...........................................................................................................100

6.14.2 Horizontal Limitations ......................................................................................................102

6.14.3 Load Limitations...............................................................................................................105

7. OLE DESIGN AND CONSTRUCTION PROCESSES ....................................................106

7.1 Process Overview ..........................................................................................................106

7.2 Form EA and Form EB Processes ...............................................................................106

7.3 Design Documentation..................................................................................................106

7.3.1 Major Feeding Diagram...................................................................................................107

7.3.2 Section Diagram and Switching Instructions ...................................................................108

7.3.3 Wire Run Diagram...........................................................................................................109

7.3.4 OLE Layout Plan..............................................................................................................110

7.3.5 OLE Cross Section ..........................................................................................................111

7.3.6 OLE Bridge Drawing........................................................................................................111

7.3.7 Bonding Plan ...................................................................................................................112

7.3.8 Dropper Tables................................................................................................................112

7.3.9 Bill of Quantities...............................................................................................................113

7.3.10 Overhead System Design................................................................................................113

7.3.11 Testing & Commissioning Plan........................................................................................113

7.3.12 Operation & Maintenance Manuals .................................................................................113

7.4 Checking Process..........................................................................................................113

7.5 Design Licensing ...........................................................................................................114

7.6 Basic Design Ranges ....................................................................................................116

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7.6.1 GE/MSW Range ..............................................................................................................117

7.7 Mark 1 Range ................................................................................................................. 118

7.7.1 OLEMI Range..................................................................................................................119

7.7.2 UK1 Range ......................................................................................................................120

7.7.3 Auto Transformer Range .................................................................................................121

7.7.4 Other Assemblies ............................................................................................................121

7.8 Construction Methodology ...........................................................................................122

7.9 OLE Maintenance...........................................................................................................123

7.10 Types of UK Equipment ................................................................................................125

7.11 Interfaces with Other Subsystems...............................................................................125

7.11.1 Permanent Way...............................................................................................................125

7.11.2 Civil & Structural ..............................................................................................................125

7.11.3 Signalling.........................................................................................................................125

7.11.4 Telecomms......................................................................................................................125

7.11.5 Electrical & Mechanical Services.....................................................................................126

7.11.6 Operations .......................................................................................................................126

7.11.7 Highways.........................................................................................................................126

7.11.8 Environment.....................................................................................................................126

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Table of Figures

Figure 1: A shortened TGV train takes the world rail speed record on 3 April 2007........................17

Figure 2: 6.7kV AC OLE on the London, Brighton and South Coast Railway; circa 1910...............20

Figure 3: The Sheffield Manchester route via Wath, electrified with 1500V DC OLE...................21

Figure 4: 1500V DC at Gidea Park on the Great Eastern; this was converted, first to 6.25kV ACand then 25kV ..........................................................................................................................22

Figure 5: Mark 1 25kV AC, WCML, London Euston........................................................................23

Figure 6: Track damage after 1955 high speed run; France ...........................................................24

Figure 7: 0 series Shinkansen; Japan .............................................................................................25

Figure 8: Extreme gradients on the TGV; Tonnerre, France ...........................................................25

Figure 9: APT tilting on neutral section tests; Murthat, WCML, UK.................................................27

Figure 10: Eurostar in preparation for record breaking run; Medway Viaduct, UK.......................... 28

Figure 11: Typical feeding arrangements for AC OLE.....................................................................35

Figure 12: Sectioning arrangements for Perturbation Crossovers ..................................................38

Figure 14: Booster Transformer arrangement for OLE....................................................................41

Figure 16: Auto Transformer arrangement for OLE.........................................................................43

Figure 17: Typical feeding arrangements for DC OLE.....................................................................45

Figure 18: Detection of a fault .........................................................................................................46

Figure 19: ECR display screens; Melbourne, Australia ...................................................................47

Figure 20: Step potential .................................................................................................................50

Figure 21: Touch potential...............................................................................................................51

Figure 23: Principal of secondary insulation....................................................................................53

Figure 24: Principal of permanent earthing and buffer sections......................................................54

Figure 25: A pantograph on test; Old Dalby, UK .............................................................................55

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Figure 26: Standard UK pan profile .................................................................................................56

Figure 27: Differential wind force on pantograph.............................................................................57

Figure 28: Typical Contact Wire Cross Section...............................................................................59

Figure 29: Contact Wire Strength against Conductivity...................................................................60

Figure 30: Overhead contact bar; Paris, France .............................................................................61

Figure 31: Lightweight shedded polymeric 25kV insulator; WCML, UK ..........................................62

Figure 32: Polymeric 25kV rod tension insulator; Stone, UK...........................................................63

Figure 33: 25kV shedded porcelain tension insulator formed of 3 cap & pin sections; Norton Bridge,UK ............................................................................................................................................63

Figure 34: 25kV shedded porcelain post insulator; WCML, UK.......................................................64

Figure 35: 25kV porcelain switching insulators with shed protectors; Norton Bridge, UK ...............64

Figure 36: Tramway OLE ................................................................................................................66

Figure 37: Stitched tramway OLE....................................................................................................66

Figure 38: Simple catenary OLE .....................................................................................................67

Figure 39: Presagged simple catenary OLE....................................................................................67

Figure 40: Stitched simple catenary OLE ........................................................................................68

Figure 41: Compound catenary OLE...............................................................................................68

Figure 42: Fixed termination OLE....................................................................................................69

Figure 43: Auto tensioned OLE .......................................................................................................69

Figure 44: Uninsulated zero span overlap.......................................................................................70

Figure 45: Insulated zero span overlap ...........................................................................................71

Figure 46: Uninsulated single span overlap .................................................................................... 72

Figure 47: Insulated single span overlap.........................................................................................72

Figure 48: Uninsulated three span overlap......................................................................................73

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Figure 70: MPA arrangement for cantilevers...................................................................................92

Figure 71: Typical MPA arrangement for portals; Norton Bridge, UK..............................................92

Figure 72: Planted mast; WCML .....................................................................................................94

Figure 74: TTC with gravity pad; Aveley Marsh, UK........................................................................96

Figure 75: Pull-off single cantilever .................................................................................................98

Figure 76: Push-off single cantilever ...............................................................................................99

Figure 77: Height and stagger for OLE..........................................................................................100

Figure 78: Typical contact wire profile (y axis exaggerated).......................................................... 101

Figure 79: Determining minimum stagger .....................................................................................102

Figure 80: MSO, blowoff, stagger effect and MTO ........................................................................103

Figure 81: Typical MFD detail........................................................................................................107

Figure 82: Typical section diagram detail ......................................................................................108

Figure 83: Typical wire run diagram detail.....................................................................................109

Figure 84: Typical layout plan detail ..............................................................................................110

Figure 85: Typical cross section detail ..........................................................................................111

Figure 86: Typical composite bonding plan detail .........................................................................112

Figure 87: Typical basic design drawing .......................................................................................116

Figure 88: GE OLE; Stratford, UK .................................................................................................117

Figure 89: Mark 1 portal; Norton Bridge, UK .................................................................................118

Figure 90: APT under Mark 3a headspans; Winwick Jct, WCML UK............................................119

Figure 91: UK1 overlap portals; Millmeece, UK.............................................................................120

Figure 92: Auxiliary Feeder on CTRL section 1; Ashford, UK ....................................................... 121

Figure 93: OLE construction; Temple Mills, UK.............................................................................122

Figure 94: OLE Maintenance; Stafford, UK ...................................................................................123

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1. PURPOSE

The purpose of this advisory note is to give an introduction to Overhead Line Electrificationsystems for railways. This document covers all types of railway and Overhead Line Equipment;all developments are covered, together with examples of UK systems.

2. SCOPE

This document applies to all OLE for tram systems, light or heavy rail, low speed or high speed.

Discipline Applies?

Overhead Line Equipment (OLE) 9

Electric Traction Equipment (3rd/4th rail) 8

Mechanical & Electrical Systems 8

3. DEFINITION OF TERMS

Term Definition

High speed Speeds above 200kph

Heavy rail Traditional railway systems; as opposed to light rail and tramsystems

Overbridge A bridge over the railway

Underbridge A bridge under the railway

All other terms are defined in the body text.

4. BASICS OF OLE

4.1 What is OLE?

Overhead Line Equipment (OLE) is a system used to deliver continuous electrical energy to astationary or moving train. It is also known in the UK as OHL or OHLE. In Europe & the US, it isknown as Overhead Catenary System (OCS), and in New Zealand, as Overhead Wiring System

(OWS). The generic term for the system is Overhead Contact Lines.

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This document will use OLE, as it is the preferred term in the UK.

4.2 Unique Features of OLE

Unlike other power transmission systems, OLE is required to transmit high power (up to ~10MVA per train) to a load at a distance of several miles, which may be stationary or moving atup to 574kph1. The contact wire is therefore a twin system; it functions as both powertransmission mechanism and sliding contact with the train.

Figure 1: A shortened TGV train takes the world rail speed record on 3 April 2007

The key requirement for any OLE system is to provide continuous power at the train. For this tohappen there must be continuous contact between OLE and the pantograph (see section 6.1).Loss of contact leads to degradation of energy transfer and unwelcome damage to the contactwire and pantograph.

OLE is a very exposed system, and is vulnerable to:

climate (especially wind, snow and ice);

wildlife (particularly birds);

1 The current railway speed record is held by a French TGV unit, which reached 574.8kph on 3 April 2007 travellingunder modified and super-tensioned (40kN) 31kV OLE. See http://en.wikipedia.org/wiki/TGV_world_speed_recordfor details.

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pollution;

vandalism.

It must be capable of withstanding frequent fault conditions without degradation of performance.The system tends to be constrained by other railway infrastructure, particularly in the UK whereit has been retrofitted to railways dating from the 19th century, which were not built with OLE inmind.

Due to the continuous contact requirements, the contact wire position is paramount. There is noredundancy in this part of the system; a second contact wire is economically and practicallyunsound. If contact wire strays outside position limits, the pantograph will usually damage asignificant length of the OLE.

OLE is therefore both an electrical and mechanical system, and the requirements of each must

be balanced in the design.

4.3 Advantages and Disadvantages of the System

The key advantages of OLE systems over train-borne traction (e.g. diesel, gas turbine) can besummarised as:

Flexibility of energy source;

Reduced emissions;

Concentration of emissions at single source;

Lower energy usage through regenerative braking;

Lower rolling stock maintenance costs;

Greater reliability leading to smaller fleet requirements;

Reduced noise.

Additionally, OLE has the advantage over conductor rail transmission system at high speeds;the conductor rail is limited by current collection requirements to about 160kph with currenttechnology.

Set against this are the disadvantages of the system:

High capital cost of installation;

Lack of redundancy in contact wire;

Management of safety risks from high voltages;

System is vulnerable if badly designed.

Because of the high capital cost of OLE, it has historically been difficult to gain funding for newelectrification schemes; especially in the UK. Therefore the OLE designer should at all stagesseek to minimise the installation costs, while balancing this against the Reliability, Availability,Maintainability and Safety (RAMS) criteria for the system.

4.4 RAMS

RAMS analysis is a technique used to optimise the performance of a system. The ideal is toreach, but not exceed, the required levels of Reliability, Availability, Maintainability and Safety

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using the most cost-effective means over the life of the system the lifecycle. This takes intoaccount not only the capital cost of the system installation, but the maintenance and operationcosts over the lifecycle.

4.4.1 Reliability

It is essential that an OLE system be reliable, as measured in mean time between failures. Thereliability should be set at a level that is the same, or better than, the other railway systems atthat location. Reliability is especially important for OLE, as it is critical for electric train service.

For instance, financial constraints at the time of the East Coast Mainline (ECML) electrificationmean that both the electrical supply and the mechanical support arrangements are less reliablethan the other systems. The ECML is subject to frequent and serious service delays due totraction supply failure, and this can lead to a reduction in the credibility of electric traction

systems in general particularly in the eyes of those financing the system.

4.4.2 Availability

Availability is a measure of the amount of time the system has to be taken out of service forroutine maintenance. Poor design leads to more frequent maintenance requirements, and loweravailability.

For instance, poor choice of contact wire material can lead to increased wear; this in turn meansthe wire must be replaced more frequently, necessitating longer periods out of service.

4.4.3 Maintainability

OLE is an exposed system and is subject to wear and damage from a variety of causes. It isessential that the ability to access the equipment for maintenance is built into the design.

High maintenance items should be readily accessible. For instance, manually-operatedswitching sites should be placed near access points, and configured to cause minimumdisruption to services.

4.4.4 Safety

The electrical and mechanical energy contained within OLE can cause serious injury anddeath if not control led. It is the designers role under the Construction Design & Management(CDM or CONDAM) regulations, to ensure that safe construction, operation, maintenance anddecommissioning of the system is fully integrated with the design. While it is impossible toachieve absolute safety, the risks inherent in the system must be analysed, and anyunacceptable risk reduced to an acceptable level. For instance, placing live OLE adjacent to aschool playground fence creates an unacceptable risk. The addition of a suitable screen at thislocation reduces the risk to a degree known asALARP (As Low As Reasonably Practicable).

4.5 Development of OLE systems

The following sections give an overview of the history of OLE development. For a more detailedlist of UK builds, see APPENDIX III.

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4.5.1 Electric Beginnings

The first OLE systems were used with passenger trams in the last years of the 19th century.

These generally consisted of a simple single wire (trolley) system, suspended from poles andbuildings, and fed at a low voltage. This was preferred to the previous 3rd rail systems, whichhad safety implications for on-street running.

The first thirty years of the 20th century saw these principles extended to mainline systems asthe advantages of OLE over 3rd rail became clear. Due to the increasing distances covered andthe I2R losses encountered, voltages were increased. At the same time, more sophisticatedsuspension systems were required to maintain good current collection at increasing linespeeds.

Figure 2: 6.7kV AC OLE on the London, Brighton and South Coast Railway; circa 1910

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Experimental AC schemes were implemented for Lancaster to Heysham (1908) and LondonVictoria to London Bridge (1909) schemes, both at 6.7kV, 25Hz AC. AC motor technology wasnot developed at this time, necessitating complex train-borne rectification equipment; this was

not yet reliable, so AC did not make any further headway until after World War Two.

On Tyneside, the Newport Shildon line, which featured heavy coal trains running over steepgradients, was electrified with 1500V DC OLE in 1915.

4.5.2 Mainline DC Growth

The problems with AC, coupled with the transmission limitations of DC current, meant OLE wasonly used for suburban and freight systems; heavy electrical loads and short distances meantDC OLE made economic sense. In the UK, 1500V DC OLE was agreed in the 1930s as thenational standard. The Sheffield to Manchester (via Wath) route, which required very heavy coal

trains to be hauled over the steep gradients of the Derbyshire peaks, was authorised forelectrification in 1939. However World War Two brought this (and all other electrificationschemes in Europe) to an abrupt halt.

These recommenced after the war, but at a much reduced rate; the railways priority wasrebuilding their battered infrastructure rather than funding new schemes. The Wath scheme waseventually completed in 1952: this turned out to be a pyrrhic victory, as within 6 years the DCstandard was obsolete. The line survived until 1981, by which time it was an isolated system.

Figure 3: The Sheffield Manchester route via Wath, electrified with 1500V DC OLE

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Unlike the UK, where overhead electrification proceeded only falteringly, the rest of Europeinstalled a large amount of 1500V DC in the pre- and post-war years, and much of this networkstill exists.

4.5.3 AC Developments

The 1950s saw increased interest in AC OLE; this was driven by the emergence of reliableindustrial frequency AC technologies in the electricity supply industry. This meant that highvoltage, long-distance AC transmission and by inference, inter-city OLE systems was nowfeasible. Across Europe, the 1500V DC standard was dropped in favour of 25kV at 50Hz AC; inthe UK this was approved as the standard for future schemes in 19562.

The Lancaster to Heysham route, which had pioneered HV AC OLE in 1908, was convertedfrom 25Hz to 50Hz in 1951 to serve as a test bed for industrial frequency supply. These tests

confirmed the choice as the right one. A test scheme was installed between Colchester andClacton in 1959. Various types of OLE were trialled, including simple and stitched, butcompound was chosen as giving the best current collection at speed3.

It was initially assumed that 25kV AC systems would require substantial electrical clearances toexisting infrastructure; in particular, it was felt that 275mm clearance would be required forbridges. In the UK this would not be possible without reconstruction work, particularly for manybridges in the vicinity of large stations. For these areas a reduced voltage of 6.25kV wasproposed; trains would be dual-voltage and switch between them on the move as necessary.

Figure 4: 1500V DC at Gidea Park on the Great Eastern; this was converted, first to 6.25kV AC and then 25kV

2Electric Railways, 1880 1990, Michael C Duffy, The Institution of Electrical Engineers, 2003, p321

3Paper; A D Suddards, T H Rosbotham, T B Bamford

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Experience on the lines out of Liverpool St, where the 1500V DC lines were converted to6.25kV AC, showed that there was excessive caution in the standard clearances. Reduced andSpecial Reduced clearances were added (see section 5.9), so that 25kV could be adopted

throughout. The West Coast Mainline (WCML) electrification was the first large scale 25kVscheme in the UK; it was first proposed with 6.25kV sections, but was implemented fully at25kV. The dual voltage locomotives which had been built for the route were modified as singlevoltage machines4. The existing 6.25kV areas were then converted to 25kV throughout.

The first phase of the West Coast scheme was extremely successful in operational terms; itbrought about a step change in service speed, and revived an image of high speed rail travellast seen in the 1930s; this came to be known as the sparks effect.

In Engineering and financial terms, however, the scheme was less successful. The project ranover budget, and when British Rail (BR) proposed a rolling programme of mainline electrification

schemes, the Ministry of Transport made it clear that costs would have to come down.

BR responded with a wholesale overhaul in the design systems for OLE. The heavy, bespokeportal arrangements of the West Coast equipment were shelved, in favour of a new, lightweight,modularised, headspan-based metric system (the Overhead Line Equipment Master Index, orOLEMI see section 7.7.1). This system, initially known as Mark 3, was further developed asMark 3a; in this form it was used on the second phase of West Coast from Weaver Junctionthrough to Glasgow in 1974.

Figure 5: Mark 1 25kV AC, WCML, London Euston

4.5.4 High Speed Lines

4Electric Railways, 1880 1990, Michael C Duffy, The Institution of Electrical Engineers, 2003, p323

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Elsewhere in Europe, the possibilities for higher speedpassenger trains using electric traction began to be explored.The French state railway, SNCF, began a series of

experimental runs in the 1950s, culminating in a record-breaking run reaching 326kph in March 1955. This used amodified 1500V DC system, with the line voltage increased to1900V by means of mobile substations5. The record stooduntil 1981.

The tests showed the obstacles to be overcome if speedsover 300kph were to become the norm. Frictional heatcaused the pantographs to collapse; track damage was sogreat that derailment was only narrowly avoided.

Figure 6: Track damage after 1955 high speed run; France

5Electric Railways: 1880 1990; Michael C Duffy; The Institution of Electrical Engineers; p389

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Japan was the first country to build anelectrified mainline railway from scratch. TheTokyo - Osaka Shinkansen ('New Trunk Line')opened in 1964. This was segregated fromexisting lines, and used 25kV 60Hz AC OLErather than the 1500V DC used elsewhere inJapan. The line had no level crossings andwas designed for continuous high speed withlinespeeds of up to 210kph. Following thesuccess of this build, the Shinkansen networkhas spread, with higher speeds being attained the latest trains run at 300kph.

Figure 7: 0 series Shinkansen; Japan

France continued to develop their high speed system, and the Train Grande Vitesse (TGV)concept was born. This would use dedicated high speed lines, high powered trains and a 50kVATx system (see section 3.4). Gradient profiles would be steep, since the high power availablemeant that expensive civil engineering works would be minimised.

Figure 8: Extreme gradients on the TGV; Tonnerre, France

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The first TGV line between Paris and Lyon (TGV Sud-Est) opened in 1981. Since then,additional lines have been opened, and the TGV concept has been exported to Germany (asthe ICE), the US (theAcela) and the UK (the Channel Tunnel Rail Link).

4.5.5 UK Developments

In the UK, the oil crisis and recession of the 1970s brought a further squeeze for railwayfinances. Having completed the West Coast in 1974, BR had intended to electrify the GreatWestern (GW) mainline between London and Bristol. However, finance was not forthcomingand so BR turned to the High Speed Train (HST) concept. This pushed diesel traction design tothe limit to produce a 200kph fixed length diesel train capable of sustained high speed running.

The HST held the world speed record for a diesel train, reaching 232kph on 12 June 1973; thiswas not surpassed until 2002. The project was so successful that the HST build was extendedto provide higher speeds on the East Coast Mainline (ECML) and Midland Mainline (MML). Thiseffectively stalled the mainline electrification program in the case of ECML, by 10 years; MMLis yet to be fully electrified.

Despite the success of HST, it was recognised that diesel technology had reached its limit, andthat for higher speeds, OLE traction was needed. Furthermore, the financial squeeze meant thatthese speeds would have to be attained on existing infrastructure.

This led to the development of theAdvanced Passenger Train (APT) the worlds first tilting

train. By using tilt, the train was able to achieve speeds of up to 255kph on the existing curvesof the WCML.

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Figure 9: APT tilting on neutral section tests; Murthat, WCML, UK

The APT used a number of novel technologies in addition to tilt. Articulated bogies were used,where carriage ends sat on a single bogie, thus improving ride quality. Two stage hydraulic/airbrakes were used to improve braking performance. The pantograph was linked by chains to thebogie, thus countering the tilt of the train body.

On 20 December 1979 an APT took the UK speed record from HST, reaching 259 kph.

The complexity of the train proved to be its undoing. Major teething problems were encounteredwhen the train entered service in 1981, and this was compounded by some ill-advised press

runs leading to bad publicity, and the worst weather seen in years that winter. By 1984 BR wereon the point of solving the technical problems, but political backing for the project hadevaporated and funding was stopped.

APT was ultimately a failure of political will rather than technology. The lessons learned weretaken by Italian train builders, who developed the Pendelino concept, successfully usedthroughout Europe and now, ironically, sold back to the UK as the West Coast Pendelino.

Electrification proceeded in the UK through the 1980s, albeit on smaller schemes such as StPancras Bedford and Colchester Ipswich. BR was finally given the go-ahead in the mid1980s to electrify the ECML, and this was completed in 1991. However, budgetary constraints

meant the OLE on this scheme was under-powered and prone to dewirement under extremewindspeeds.

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Infill schemes continued in the 1990s, with Cambridge to Kings Lynn, Carstairs to Edinburghand London to Heathrow Airport all completed. A significant milestone was the opening of theChannel Tunnel, operating at 160kph with OLE. However privatisation has broken the link

between infrastructure capital cost and train maintenance cost which was vital to justify theinitial cost electrification schemes. The splitting of rolling stock procurement, rolling stockoperation and infrastructure ownership led to a huge increase in diesel procurement, as noparty would benefit from the whole life advantages of electric traction.

By the early years of the 21st century the only major schemes in progress were the West Coastupgrade, and the Channel Tunnel Rail Link (CTRL), which finally brought true high speed(300kph) running to the UK. Section 1 of CTRL opened in 2003, and section 2 into London wasopened in 2007.

On 30 July 2003, a Eurostar test train took the UK rail speed record from the APT, reaching

334.7kph (208mph) on section 1 of CTRL.

Figure 10: Eurostar in preparation for record breaking run; Medway Viaduct, UK

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4.6 Categories of OLE System

The parameters of the OLE must be matched to the system to which it is to be applied. OLEsystems may fall into one of four broad categories.

4.6.1 Tram Systems

Trams are mass transit systems, used to move large volumes of people over relatively shortdistances at relatively low speeds (up to 80kph), usually in and out of urban centres. Thesesystems feature on-street running, tight radius curves, steep gradients, short headwaysbetween trams and line of sight driving (i.e. no signalling except at highways interfaces). Theyare usually of post-war vintage.

Tram OLE system design is driven by the need to ensure the safety of the public, and by themany interfaces with buildings and highways. The systems are low voltage (usually 750V DC)and are often split into on-street and off-street equipments; the former being characterised byhigh contact wire, fixed termination tramway (see 6.5) and support from buildings, and the latterby a more conventional system with catenary and auto-tensioning. Support assemblies are verylight, and secondary insulation is used to prevent stray currents (see 5.10.2) from enteringburied services.

4.6.2 Light Rail Systems

Light rail systems are a step up from trams. They are also mass transit systems, situated in and

around urban centres, but they do not feature on-street running, and share many of thecharacteristics of heavy rail, such as fixed signalling. Speeds are usually below 120kph.

For these systems, supply voltages are higher (often 1.5kV DC), and the OLE is often fixedtermination, with simple catenary (see section 6.5). Structures and assemblies are lightweight,and headspans (see section 0) are often used. The Tyne & Wear Metro is an example of such asystem.

4.6.3 Mainline Systems

Mainline systems form the bulk of the OLE railway route mileage worldwide. These systems are

mainstream traditional railways; speeds may be anywhere up to 200kph, and traffic may beheavy and frequent, with a mix of passenger and freight. The railway may date from Victoriantimes, the OLE having been superimposed at a later date.

Standard supply voltages are 1.5kV and 3kV DC, and 25kV 50Hz AC (standard for all systemssince the 1960s). OLE is either simple or compound; assemblies are heavier, and portal (seesection 6.11.5) or headspan structures may be used.

4.6.4 High Speed Systems

Mixing passenger services at speeds above 200kph with slower moving freight is notpractical or safe. For this reason, high speed systems are usually dedicated to passengerservices; the high power available often means steep gradients are used, reducing constructioncosts. These lines are usually less than 40 years old, and built with OLE standard clearances.

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The standard supply voltage is 25kV 50Hz AC, usually with a transmission voltage of 50kV andan Auto-Transformer system (see section 5.5.2). Good current collection becomes paramount;OLE is either sagged simple (see section 6.5), stitched simple or compound. Assemblies are

lightweight; structures are a mix of portals and cantilevers (see section 6.11).

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5. ELECTRICAL PRINCIPLES

5.1 Supply voltages and currents

5.1.1 Transmission and Supply Voltages

Transmission Voltage is the voltage at which energy is transmitted to the trains location. SupplyVoltage is the voltage at which the train is supplied with energy.

For the majority of systems, the transmission and supply voltages are equal. However, somehigh speed lines use a higher transmission voltage to avoid excessive heat losses and thusprovide more power at the train.

A variety of supply voltages are used around the world, due to a combination of historical andoperational factors. 750V DC is the de facto standard for tram systems, and is chosen tominimise safety issues in public areas. 25kV 50Hz AC is used for the majority of new mainlineand all new high-speed builds. Most countries (with the notable exception of the UK) have alegacy network of 1500V DC.

It should be noted that the supply voltage is not a single constant value; I2R losses, magnitudeof load in section and other factors affect the supply voltage at the train. For instance, below arethe allowable voltages for UK 25kV AC systems6;

System

Voltage

Description

25 kV Nominal system voltage

29 kV Train equipment should be able to operate without suffering damage ifthe voltage rises to this level for 5 minutes

27.5 kV Maximum voltage at which train equipment should operate continuously

24 kV Average voltage for use in train performance calculations

20 kV Minimum constant current voltage value

16.5 kV Minimum voltage in normal operation. If the voltage falls below thisvalue it should not be possible to initiate regenerative braking

14 kV Minimum voltage at which a train should continue to operate for notmore than ten minutes without being damaged. Also the voltage belowwhich regenerative braking should cease.

6RT/E/C/27010 Compatibility Between Electric Trains and Electrification Systems, Issue 1, November 1997,

Para. 2.2

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SystemVoltage

Description

12.5 kV Minimum voltage at which a train should continue to operate for notmore than two minutes without being damaged. Equipment energisedfrom the overhead line need not continue to operate if the voltage fallsbelow 12.5kV, but should not be damaged.

Each system will have a line voltage performance specification in this way; the OLE systemdesign is tailored to meet this.

5.1.2 Supply Current

The supply current is obviously dependent upon the train power characteristic and the numberof trains in section at any one time. ForLow Voltage (LV) DC systems, the supply current isrelatively high;

Train Type Train Current Draw

750V DC three car tram ~ 1100A

750V DC train ~ 3000A

1500V DC train ~ 1500A

For AC system, the higher voltage available means lower supply currents;

Train Type Train Current Draw

25kV AC passenger train ~ 200A

25kV double headed freight train ~ 500A

Of paramount importance is the maximum fault current, that is, the maximum current which willflow under fault conditions. The entire OLE system must be designed to withstand many suchfaults over the lifetime of the equipment, without degradation of the components. For UK 25kVsystems, the maximum fault current is 6kA: for some high current systems, an increased level of12kA is proposed.

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5.2 AC Supply Principles

DC systems have historically been constrained to lower supply voltages (up to 3kV) due to the

expense and availability ofrectification equipment. Since AC systems do not require rectificationequipment, they can use higher voltages than DC. This leads to much lower volt drop, and sofeeder stations can be further apart than for DC systems for a standard 25kV feeding system,feeder stations are 40 to 60km apart. This eliminates the requirement for a separate HV feedingnetwork.

Supplies have traditionally been obtained at each feeder station from the 132kV DistributionNetwork Operator(DNO - also known as a Regional Electricity Company or REC). 25kV isobtained through 132/25kV transformers supplied by the DNO. These are often duplicated togive supply security orredundancy. These transformers are usually procured by the railway butowned and maintained by the DNO. They may be sited at a DNO compound alongside therailway feeder station, or sited at a remote DNO site with 25kV cabling between the two.

More recent installations take their supplies from the 400kV Electricity Supply Industry (ESI also known as National Grid Company or NGC) system to limit load imbalance (see below).

The single phase supply taken from the 3 phase system at each feeder station creates anunbalanced load (or phase imbalance) on the supply authoritys system. The supply authorityhas contractually agreed limits with its customers on the total imbalance. The railway can oftenbe the biggest single contributor to this imbalance.

Therefore the supply authority will generally impose an overall limit for the railway contributionto the imbalance. UK limits are 1.5% on the grid or DNO; and 0.5% contribution by the railway.

To help limit the imbalance, adjacent feeder stations use different phase combinations; e.g.feeder 1 uses red-blue, feeder 2 uses blue-yellow, feeder 3 uses yellow-red. Direct connectionof these adjacent systems is prohibited for electrical reasons, so a short section of dead OLE a neutral section is used to keep the phases apart. Trains shut off power before the neutralsection, usually by means of an automatic trip, and coast through the neutral section before thepower is tripped on again (see section 6.9.2).

The OLE between feeder stations is electrically split into sections and subsections to allow foremergency feeding and maintenance. Each running line is electrically separate from the others.Sectioning is maintained at intermediate locations called Track Sectioning Cabins (TSCs).

These are the equivalent of the Track Paralleling Hut (TPH) on a DC railway. The midpoint TSC so called because it is midway between feeder stations carries a neutral section whichkeeps the phases at adjacent feeder stations apart. Each feeder station also has a neutralsection. This means that the phase split may be moved up and down the railway in emergencyfeeding conditions.

A typical sectioning arrangement is shown overleaf. Typical spacings for 25kV classic feedingare:

Feeder Station to Feeder Station from 40 to 60 km;

Feeder Station to midpoint TSC - approx 24km;

Feeder Station to TSC - approx 11km.

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Railway Electrification & Power EREPE Handbook

ngineering: Introduction to Overhead Line Electrification

Page 34 August 2008

Spacings are determined by the traffic to be handled, the train performance requirements andthe electrical characteristics of the overhead and supply systems. Such considerations result inan optimum spacing which it is not often possible to achieve, and shorter sections are often

used to locate the feeder stations at strategic points such as junctions or route intersections.Feeder Stations are usually situated in close proximity to grid substations in order to avoid thedisadvantages of long feeders.


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Figure 11: Typical feeding arrangements for AC OLE


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5.3 AC Supply Equipment

5.3.1 AC Transformers

AC transformers are used to step voltages down from 132kV or 400kV to the supply voltage.They are typically of a conventional oil-filled naturally-cooled design. Off load tap changing of2 and 5 percent is normally provided to allow the output voltage to be adjusted, buttransformers with remotely controlled on-load tap changing are sometimes installed to allow thisadjustment to be made in service. Transformers are generally supplied in standardised sizes:

15 MVA/600A;

10 MVA/400A;

7 MVA300A;

5 MVA/200A.Where AC supplies are derived from networks operating at voltages lower than 66kV, (e.g.33kV or 11kV) the transformers are usually purchased by the railway infrastructure owner.

5.3.2 Auxil iary Transformers

Auxiliary supplies are often taken from the OLE at a Feeder Station or TSC, either to supplylocal Low Voltage (LV) equipment or as a backup to other supplies. Auxiliary supplies can befor:

Signalling Supplies (typically 650V or 400V);

Battery charging (typically 110V); Operation of switchgear;

Lighting;

Heating.

These supplies are derived from the traction supply by means of step down transformers.

5.3.3 AC Circui t Breakers

Circuit Breakers are designed to allow the supply to be interrupted during fault conditions or forroutine maintenance. They must be capable of closing and opening (making and breaking) boththe normal operational currents (load current) and the much higher currents experienced during

a fault (fault current). They must be able to do this many times over their life withoutexperiencing degradation of the contacts. Of particular importance is the ability to quicklyextinguish the arc which forms as the electrical contacts move apart.

AC circuit breaker technology has advanced significantly in the last 50 years, and this isreflected in the range of circuit breaker types on the railway. In historical order of installation,they are:

Oil insulated;

Air insulated;

Vacuum insulated;

Sulphur Hexafluoride (SF6) insulated.

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Oil Circuit Breakers (OCBs) were used for installations until the 1970s. Oil provides a goodelectrical insulator and will extinguish the arc quickly while dissipating the heat generated.

However, they are heavy and bulky, and are not able to clear faults quickly, while repeatedoperations contaminate the oil with carbon deposits which further degrade performance,meaning regular maintenance is required.

Vacuum Circuit Breakers (VCBs) were first used in the 1970s. Their simplified mechanicalarrangement means they were thought to be more reliable than OCBs, giving improvedinterrupting capacity, increased contact life, and requiring less maintenance. They are alsosignificantly quieter and smaller than OCBs.

Sulphur Hexafluoride (SF6) breakers were introduced because the vacuum in VCBs was provinghard to maintain. Initially SF6 was used for both insulation and arc-breaking purposes, but it wasfound that under arcing conditions the gas breaks down into acidic elements which damage thebreaker.

More recent SF6 designs have used the gas for insulation only, with a vacuum used forextinguishing the arc. However SF6 is extremely environmentally damaging and the search is onfor a suitable replacement. Recent developments have looked at the use of resin, or even amodern form of the oil-filled circuit breaker.

5.3.4 AC Cables

Incoming supplies from the DNO are typically delivered to the railway feeder stations through400-500mm2 two-core concentric pressure cables. The same type of cable is also used whereconnections are required between railway feeder stations. Generally the cables are of the oil-filled type, with some being gas-filled. This latter type has the advantage of a lowerchargingcurrent, and is favoured for tunnel use. Connections from the switchgear to the 25kV overheadcontact system are usually formed of 25kV single core solid type cables.

5.4 AC Section ing Principles

OLE systems are generally divided into electrical sections, allowing sections of OLE to beisolated during planned maintenance or emergency situations. Sectioning is carefully chosen togive the ability to isolate any OLE section, while keeping the resultant train diversion around theisolation as short as possible.

Crossovers are provided, typically every 3 - 5 miles, to allow trains to transfer from the normalrunning line to the wrong direction line underperturbation conditions. Subsections bridged byisolators are provided to allow the OLE to be isolated at a fault. The train then runs wrongdirection around the isolated subsection. Isolators have traditionally been manual, requiringswitching on site, but remote-operated motorised isolators are increasingly used.


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Figure 12: Sectioning arrangements for Perturbation Crossovers


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5.5 AC Feeding and Immunisation Methods

5.5.1 Classic feeding arrangement

The first OLE systems used the OLE to transmit power to the train, and one or more runningrails to return current to the supply point. In AC systems this was found to be unsatisfactory, dueto the large electromagnetic (EM) field created around the OLE. This induces a voltage in anyLV cables in the vicinity of the OLE, and is a particular problem for safety-critical signallingcables.

A partial solution was found withthe introduction of the ReturnConductor(RC) system. The RC

is a conductor which runs parallelto the OLE, at approximately thesame height, and positioned onthe lineside. The RC is bonded atregular intervals to the runningrail, and provides a measure ofcurrent sharing. Because thecurrent flows in the oppositedirection to that in the OLE, and itat equal height, the two EM fieldstend to cancel at ground level.However, the fields are not equal,

and there is still the potential forinterference.

The answer lay in transferring allthe return current from the rail tothe RC. This was achieved bymeans ofBooster Transformers(BTs). A BT is a 1:1 ratio

transformer; the primary isconnected across the OLE in

such a way that traction current is forced through it. The secondary is connected across a break

in the RC. The current in the primary induces an equal and opposite current in the RC; thiscurrent is drawn from the rail at a bond connection midway between BTs called the midpointconnection (MPC).

This system ensures that all return current moves from rail to RC and so provides a largemeasure of signalling immunisation. Booster Transformers are located every 3 miles, and OLEoverlaps are used as a convenient point for a break in the OLE. For this reason signals shouldnot be located near to Booster Transformers, as there is a risk of OLE burnout when apantograph comes to a stand shorting out the BT (see section 6.7.2).

The Booster Transformer arrangement (also known as classic feeding) is more efficient inreducing interference than RCs alone, but is more expensive and introduces greater losses in

Figure 13: Booster transformers; Whitmore, UK

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the traction system. It has been used widely across Europe and in the UK, but there remain alarge number of legacy RC only routes.

The BT arrangement is shown overleaf.


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Figure 14: Booster Transformer arrangement for OLE


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5.5.2 Auto Transformer Feeding Arrangement

There are practical limits to the amount of power that a BT system can deliver; I2R lossesincrease, and there is a limit to the amount of OLE cross section which can be provided withoutdriving wire tensions to impractical values.

In particular, the drive for higher speeds led to the requirement for a system with much higherpower availability. TheAuto Transformer(ATx) system was pioneered first at Philadelphia in theUS in the early 20th century, using 36kV transmission and 12kV supply. It was then used on theShinkansen at 60kV:30kV, before being introduced in Europe for TGV routes at 50kV:25kV.This is now the favoured ratio for new high speed lines, and the detail below is based on this.

The ATx system is a 50kV AC

transmission system, which is whyhigher power levels are available.Since power is proportional to V2,there is more power available in theclassic feeding system.Alternatively, the same power canbe delivered with fewer feederstations spaced further apart.

The heart of the system is the ATxitself. This is a 1:1 ratio 25kV-0V-

25kV centre-tapped transformer,with the OLE is fed from one half ofthe winding at +25kV, and anAuxiliary Feeder(AF) fed from theother half at -25kV (actually anti-phase to the OLE). The running rail is connected to the 0V centre tap. The AF carries out thesame immunisation role as a return conductor, which is not required in an ATx system.

Delivery to the train is at 25kV as normal a key requirement of any 50kV system, which mustinterface with traditional systems and trains. Approximately half the train current comes directlyfrom the feeder station via the OLE; the other half comes from current induced in the OLE bythe 25kV half of the ATxs.

The additional 25kV AF conductors create additional design challenges; 25kV clearances mustbe maintained for AF to earth, but also 50kV clearances for AF to OLE. This can be a particularproblem through limited clearance overbridges and stations.

The ATx system is widely used on European high speed systems, and in the UK it is installedon the CTRL and is being piloted on WCML to overcome the problem of a saturated classicfeeding system.

The ATx arrangement is shown overleaf.

Figure 15: Simplified AT Feeding


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Figure 16: Auto Transformer arrangement for OLE


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5.6 DC Supply Princ iples

For the historical reasons outlined in section 5.2 DC overhead transmission is generally at lowervoltages (3kV or lower). This means the system suffers from a large volt drop as a percentageof the supply voltage, and substations must be placed close together (typically 4km apart). Thecost of providing a direct feed from the DNO at each location would be prohibitive, so DCrailways usually have a dedicated HV trackside feeder system to provide power to thesubstations. These rings are typically at 66, 33, 22 or 11kV, fed from a 132kV grid infeed. TheHV supply is then transformed down and rectified at each substation to provide power to therailway. The rectifier is fed with all 3 phases, meaning there is no imbalance on the DNO supplyor requirement for neutral sections.

5.7 DC Sectioning Principles

Switching is carried out at intermediate Track Paralleling Hut locations. These help keep thesystem impedance down by paralleling all tracks together.

A typical sectioning arrangement is shown overleaf.


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Figure 17: Typical feeding arrangements for DC OLE


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5.8 Protection, Monitoring and Control

5.8.1 Fault Protect ion

OLE systems are vulnerable to a large number of faults. These faults can cause currents to flowthat are much larger than those caused by normal operation; these would cause considerabledamage if allowed to flow unchecked. To prevent this damage a protection system is used toclear faults by opening the circuit breakers feeding into the section.

Any system of protection must:

Be sufficiently sensitive to detect a fault in its early stages;

Be absolutely reliable in operation the simpler and most robust the design the better;

Discriminate between currents fed to faults within the section being protected and currentpassing through to a fault in another section.

OLE is split into sections that are fed from one (AC) or both (DC) ends. Each feed is routedthrough a circuit breaker. Attached to this by means of a current transformer(CT) and VoltageTransformer(VT) is a relay which looks continuously for faults, by measuring the impedance ofthe section that it is feeding. This is known as impedance ordistance protection. Fault currentwill usually flow through several circuit breakers between the fault and the feeder station. Thisgives the opportunity to provide time delayed backup protection. For instance, UK heavy railOLE has three zones of protection: zone 1 is instantaneous and is set to the impedance of the

initial section less a calculation tolerance; zone 2 sees approximately 70% of the next sectionand has a small time delay; and zone 3 sees all of the next section with a larger time delay.

Consider a system consisting of three series sections, each protected separately and capable ofisolation by a circuit breaker at the feeding end:

Figure 18: Detection of a fault

The fault at F is a section fault relative to Section C, but a through fault relative to Sections Aand B. Thus the protective devices on Sections A and B should not trip their respective circuitbreakers, whilst the protection on Section C should open its circuit breaker.

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If circuit breaker C does not clear the fault within the specified time, then the protection onsection B will cause circuit breaker B to trip. Similarly, circuit breaker C will act if circuit breaker

B does not.

In addition to impedance protection, overcurrent and under-voltage protection may be provided.These systems will protect the OLE against overload and trains against undervolts respectively.

5.8.2 Control and Monitoring

The circuit breakers at feeder stations and TSCs are under the control of the Electrical ControlRoom (ECR). This is a central control centre which supervises operation and maintenance ofthe OLE. A telecommunication system known as Supervisory Control and Data Acquisition(SCADA) is used to monitor and control circuit breakers remotely. The SCADA system polls

each feeder station in turn, interrogating the state of each circuit breaker. Any change in state oralarm is relayed back to the ECR. Similarly the ECR can send an instruction to a particularcircuit breaker to open or close in the event of a fault or maintenance. The ECR Operator(ECRO) is able to monitor and control the whole system from a set of display screens at acentral terminal. Motorised switches under ECRO control may also be provided at key locationsif fast perturbation management is required.

Figure 19: ECR display screens; Melbourne, Australia

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5.9 Electrical Clearances

It is essential for safety and reliable operation to keep all live parts of OLE a sufficient distancefrom other infrastructure, so that flashoveris prevented.

For this reason two sets of clearances are defined. The static electrical clearance is theclearance which must be achieved under static conditions. The passing electrical clearance isthe clearance which must be maintained for a short duration as the train passes.

These clearances are set for a particular system voltage. It is usual to have more than one levelfor each clearance in recognition of the different circumstances which may apply.

For instance, UK 25kV AC standards7 define four levels of clearance which may be allowed;

Enhanced Clearances should be used wherever possible; Normal Clearances should be used where enhanced clearances cannot be attained;

Reduced Clearances can only be used with the consent of the infrastructure owner whennormal clearances cannot be attained;

Special Reduced Clearances can only be used with the consent of the safety authority whenreduced clearances cannot be attained.

Enhanced Normal Reduced SpecialReduced

Static 600mm 270 599mm 269 200mm 199 150mm

Passing 600mm 200mm 199 150mm 149 125mm

Where enhanced clearances cannot be provided, such as at low overbridges, a section ofcontact wire replaces the catenary. This minimises the chance of wire stranding in the event ofa flashover. This cable is known as contenary.

In addition to electrical clearances, minimum safety clearances must be maintained to thoseareas accessible to public and staff. These safety clearances are considerably more onerousthan the equivalent electrical clearances a given system. For instance, the normal minimumsafety clearance for 25kV AC lines is 2.75 metres.

It is only possible to reduce this clearance if some form of protective barrier or screen isprovided.

It should be noted that the term live parts includes the pan itself. Due to the position and widthof the pan, this can often be the most extreme part of the live envelope; it is important to

7GE/RT8025 Electrical Provisions for Electrified Lines; Railway Safety & Standards Board; Para. B4.6.2

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consider this in the design of signals and other infrastructure which have maintenance accessplatforms.

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5.10 Earthing and Bonding

Traction earthing and bonding is the term used to describe the arrangements for ensuring a lowimpedance return path for traction current and fault current.

The system is designed to:

Provide a low impedance path for return current;

Allow faults to be detected and cleared quickly;

Keep the rail potential within limits;

Eliminate touch potentials and step potentials.

Step potentials arise during fault conditions, or when current is allowed to flow to earth. Duringthese conditions, the system earth electrode may be subject to a rise in potential. This will createa potential gradient in the surrounding ground, the potential reaching true earth or zero at somedistance from the earth electrode. Step Potential is the potential difference between a personsfeet caused by this potential gradient.

Figure 20: Step potential

Touch potentials arise where a metal service connected to another earth system (e.g. DNOearth) is adjacent to OLE. In this situation, a person may be able to simultaneously touch thetwo earth systems (e.g. an OLE mast and an equipment cabinet). The two earths may be atdifferent potentials, and so a current will flow between them. This potential is the TouchPotential.

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