Talk
Tuesday 13th December 2016 – The Thameslink Project – Electrical Challenges
M Sigrist MSc, BSc(Hons), CEng, MIET
Principal Design Engineer, Thameslink Programme, Network Rail.
The Thameslink Programme (TLP) has been an active project for the past twenty years or so, and went through a number of development phases before full design and construction, started in 2007.
The main objectives of the programme were to:
• Reduce overcrowding on the Thameslink and other commuter services, including London Underground;
• Reduce the need for interchanging between main-line and LUL train services;
• Provision of new cross-London services, so improving public transport accessibility in South East England;
• Facilitate the flow of passengers to and from St Pancras International station;
• Support the introduction of the new Class 700 Rolling Stock.
Figure 1 – Extent of the Thameslink Routes
The facilitate the construction works to enable these objectives, the programme was split into a number of Key Output stages to spread the costs associated with the works, but also allow more time to develop a complete construction strategy for the re-development of London Bridge station. The original proposal had been to complete all works, in all areas, at the same time over a four year period, which was thought to be too disruptive.
The programme was therefore split into the following Key Output (KO) stages:
• KO0 – enabling works for KO1;
• KO1 – Introduction of 12 Car Trains and service level increase to 16 trains per hour in the Core Area;
• KO2 – Introduction of New Rolling Stock and service level increase to 24 trains per hour in the Core Area;
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1.1 KO1 Main Objectives
Following a short phase of enabling works, KO1 was the first main milestone required to allow longer 12 car train operations between Bedford and Brighton by December 2011. The phase of works also required an improvement in the service capacity up to 16 peak train paths per hour in the Core Area between Blackfriars and St Pancras stations.
To enable the increase in train length and service frequency there were a number of works identified that need to be completed, these were:
• Station rebuilds to allow longer Trains (Farringdon, Blackfriars);
• Platform Extensions (Numerous!) on the Midland Main Line;
• Resignalling the Core Area (Kentish Town to Loughborough Junction) to allow a higher service level;
• Revised Track Layout at Blackfriars station, to streamline services from London Bridge;
• 25kV OLE Extension and move AC/DC interface from Farringdon to Blackfriars, to allow for operational
recovery of trains that fail to changeover between traction systems;
• Traction Power Reinforcement for 25kV & 750V DC to allow for the longer 12 Car trains that were to operation
on the MML (AT system) and Brighton Main Line
All the works required to enable the change of longer trains and services for KO1 were completed on time during 2011.
1.2 KO2 Main Objectives
To provide the final Thameslink service the main objective of this stage is to give further improvement to the train service capacity in the Core area of up to 24 peak train paths per hour. In addition the connection through to the East Coast Main Line (ECML) from the Midland Main Line (MML) was required to be completed.
To enable the further increase in service frequency there were a number of works identified, these were:
• Station rebuild (London Bridge) to allow better access to the services and connections to London Underground, this also supported the plans to redevelop the London Bridge Area;
• Revised Track Layout for the London Bridge Area to streamline the Thameslink services through the London Bridge area to the Core Area tunnels;
• Implementation of ATO in the Core Area, to provide an enhanced train control to reinforce the ability to deliver a 24 train paths per hour service level;
• Power Reinforcement for 25kV to allow 12 Car operation on the ECML to support the new longer trains on the Route (this work was coordinated with and delivered by the East Coast Power Supply upgrade programme)
• Power Reinforcement for the 750V DC Thameslink Routes, mainly further works on the Brighton Main line to support the new longer trains on the Route;
• Main Depot Facilities at Three Bridges and Hornsey to allow for maintenance of the new Rolling Stock;
• Stabling Facilities at various sites including Brighton, Horsham, Cricklewood, Peterborough, Cambridge.
Currently the majority of the implementation works above have been completed, and of those still outstanding, all of the remaining works are programmed to be completed on time for the main KO2 milestone of December 2018.
2. Development of the Electrification Scope
The original electrification scope was based on studies and traction system design modelling that was undertaken by the original Thameslink 2000 project team during the 1990’s. However the programme was then delayed for a number of years when the Transports and Works Act (TWA) was not initially accepted.
During the earlier 2000’s, the Thameslink Programme was restarted and a number of key factors had to be re-considered from an electrification prospective, these were:
• Finding a suitable location of a 25kV Grid Intake point in the St Pancras / Kings Cross Area (originally referred to as Regents Canal), as no powers given in the TWA;
• Changes / amendments to the assumptions used for timetabling and rolling stock characteristics (i.e. for DC was high current operation required?), these affected the original Thameslink 2000 traction system design;
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• Delivery Programme was amended from a flat delivery (all TLP with TWA permitted timescales of approximately 3 years) to Service level outputs (KO1 & 2) which then staged the power demands into two instead of just one.
These factors meant that all the previous traction system design had to be reassessed as many of the original assumptions had changed.
The largest area of change from the original electrification scope was the introduction of the 25kV Autotransformer proposal for a section of the Midland Main Line (MML) between Borehamwood and Kentish Town.
This solution was selected as there were a number of factors driving the decision, these were:
• No Location for the 25kV Grid Intake point in the St Pancras / Kings Cross Area could be found and agreed with National Grid. This would have also needed a public enquiry to gain local council acceptance as the proposed options were outside the Thameslink TWA powers
• The Programme given at the time for the new Intake Point was approximately 2015 (date given in 2007) and would miss the KO1 delivery date and delay KO2, which was then a 2015 milestone;
• In addition the estimated cost at the time exceeded the budget by approximately 300%, with all additional risk of land procurement and any public enquiry being the responsibility of the Thameslink Programme.
As the main driver for the programme, in terms of having a suitable solution was the timescale, due to the introduction of longer trains were required at KO1, therefore the Autotransformer (AT) system was selected and developed to reinforce the MML. Like the proposal of new intake point at Kings Cross, this was mainly due to making sure that the 25kV system voltage at City Thameslink, the most southerly part of the 25kV system on the Thameslink route, remained constant when loaded, for both the KO1 and 2 timetable changes. However with the AT system, the main intake could now be located further away from London.
Figure 2 – Diagram showing the 25kV feeding changes at Borehamwood due to the AT system
3. Electrification Works
3.1 Midland Main Line Upgrade
The traction reinforcement scope defined for the Midland Main Line was to install a 25-0-25kV Autotransformer system between Borehamwood to Kentish Town which is approximately 20km of the route, this included:
• Provision of a new Grid Connection at Elstree, this is the nearest National Grid substation to Borehamwood (see figure 3);
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• Replacement of three traction Feeder & Sectioning locations on the existing railway at Borehamwood (see figure 2), Grahame Park and Kentish Town (see figure 4);
• Installation of two new Autotransformer feeders, which run on each side of the railway that are located on the Overhead Structures;
• Installation of a return screen conductor, to provide an enhanced level of protection against lineside electro- magnetic induction (EMI) from the enhanced 25kV system;
• Removal of the booster transformers and the return current conductor as this function is replaced by the AT system and the return screening conductor.
Construction works for the AT system started in 2009 and initial commissioning started in May / June 2011 and was available for the train service change with longer trains and KO1 deadline in December 2011.
Figure 3 – One of the National Grid transformers at Elstree required to support the AT system
Figure 4 – Kentish Town lineside section locations
3.2 DC Routes Upgrade
As well as the upgrades to the north of London on the 25kV system, the routes south of London to destinations such as Brighton, also had to be upgraded due to the new and longer trains. So various traction reinforcement works were undertaken, basically to either install or replace various part of the DC traction System that the traction system design indicated would become overload under the worst case feeding, these works included:
• Upgrade of ten substations (Farringdon, Southwark, Cannon Street, Brockley, Coulsdon North, Great Lake Farm, Gatwick, Three Bridges, Ifield, Pangdean), where works in either the transformers / rectifiers or DC circuit breakers were required, due to higher loads, see figure 6;
• Conversion of one Track Paralleling (TP) Hut to a Substation (London Bridge), which involved cutting into an adjacent HV feeder cable and providing new HV switchgear, transformer / rectifiers and DC circuit breakers;
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Installation of one new Substation (Patcham), this was to reduce the loading at an adjacent substation and to improve the electrical protection on the DC electrical sections in the area;
Installation of four new Track Paralleling Huts (Gipsy Hill, Clayton Tunnel North and South, Tinsley Green), where basically the DC electrical sections are paralleled via DC circuit breakers to improve the electrical section voltage and assist with load sharing;
Completion of outstanding Power Supply Upgrade works associated with HV Feeder cable (F3061) – Three Bridges to Keymer. Completing this work gave an increase in availability of the HV distribution system as this is an alternative feed to maintain HV supplies to the railway if there are HV outages due to faults or maintenance;
Upgrade of the Electric Track Equipment (ETE) positive and negative cabling from Brighton to Farringdon along the main and diversionary routes through Tulse Hill, which consisted on doubling all DC cables on all tracks, to improve the load capability of the cabling, see figure 7
Figure 5 – Substation Upgrade works at Gatwick
Figure 7 – Typical ETE works showing upgraded hookswitch
Dual Electrified Area works
While the upgrade works on the 25kV AC and 750V DC systems were challenging and complex, probably the most technically challenging was the changes brought about by extending the Dual Electrified Area (DEA) at Farringdon station in the Core area of the Route.
While there had been an AC / DC interface in this area since the late 1980’s when the route was originally reopened, the Dual Electrification Area (DEA) was only in the Farringdon station area. The associated contactor equipment, used to prevent stray DC currents going into the earthed 25kV AC system, was located just south of Farringdon station within Snowhill tunnel although this was only configured for trains with a maximum of eight carriages, see figure 8. With the increase to twelve carriage trains the AC / DC interface contactors were not suitable for the longer and high frequent services.
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Figure 8 – Layout of the original AC / DC interface and DEA in the Farringdon Station Area
In addition to the AC / DC interface only being able to operate with eight car trains, Farringdon station itself could only accept eight car trains. To overcome this restriction, the platforms were lengthened which meant removing the railway lines to Moorgate. Although by removing these lines it also removed the means of recovering trains that failed to changeover from AC to DC, as the Moorgate line were used to recover these trains.
To overcome this operational restriction the 25kV overhead was extended to City Thameslink Station to the south of Farringdon. This meant that extending the DEA and relocating the AC / DC interface south of the City Thameslink station and clear of the limit of 25kV overhead line, see figure 9.
Figure 9 – Layout of the extended DEA and AC / DC interface in the Blackfriars Station Area
In addition to the 25kV Overhead Line and relocation of the AC / DC interface, it was also necessary to decommission at existing DC traction substation that was located under Blackfriars station. This was due to the re-construction of the station above and the refurbishment of Blackfriars bridge a new DC traction substation was developed that integrated the DC contactors required for the AC / DC interface into the substation design and building. The new combined AC / DC interface and DC traction substation called Ludgate Cellars, was built and installed between City Thameslink and Blackfriars stations.
Figure 10 – Ludgate Cellars combined AC / DC interface and DC traction substation
3.3.1 AC / DC Contactors
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In terms of the AC / DC interface this consists on a series of contactors that are located within Ludgate Cellars, with a number of Insulated Rail Joints (IRJs) in the return rail which are aligned to gaps in the conductor rails. The basic premise is for contactors, with the IRJs and conductor rail gaps, is to always maintain separation between the AC traction (earthed) and the DC traction (floating) return systems. If this is not achieved that DC stray currents could flow towards the earthed 25kV system and anything else that system is connected too, such as stations etc. To make sure that the contactors open and closed at the correct times to guarantee system separation, it is essential to know precisely where the train is as it passes through the AC / DC interface. The separation is achieved by using the signalling track circuits to control the operation of the contactors, so that contactors can keep the required open point on the positive and negative return systems and also maintain a traction supply to the train at all times. Figure 11 below shows example of a 12 car train passing through the AC / DC contactors where the train is located within the contactor zone and the open point is in the process of changing over from the front of the train (C3) to the rear (C1).
Figure 11 –AC / DC interface contactor arrangement
Following the train position shown in figure 11, as the train moves forward the contactor C2 then opens and C1 closes, with the final state being C3 open and C2 / C1 closed and leaving the AC / DC interface ready for another train to enter.
An additional issue that needed to be considered with the AC / DC interface control was the introduction of the European Train Control (ETCS) overlay that is required in the same vicinity to allow for a robust 24 train per hour service. While the trains are detected via the track circuits, in some case the limit of movement of the trains can move beyond fixed signalling to a virtual point called a Block Marker. The positioning of the block markers had to be coordinated with the allowable positions of the trains passing through the AC / DC interface. This coordination is shown in figure 11 below.
Primarily this was to prevent a train that had intentional stopped in the interface sections and then allowing a train to move into the same electrical sections crossing an open contactor, that would likely result in damaging the IRJs as the wheels cross the insulated sections allow traction current to bridge the gaps.
Figure 12 – Signalling Plan showing coordination of block markers (brown) and AC / DC interface equipment (blue)
3.3.2 25kV Overhead Line Extension
In addition to the creation of a new AC / DC interface at Blackfriars, the 25kV overhead line extension was also required to take the AC electrification to City Thameslink to allow the operational requirements to be met.
One of the largest concerns for the overhead line was the existing Victorian soffit heights in Snowhill Tunnel as these were exceptionally low and special electrical clearance assessments were required to confirm if it was possible to install the overhead line system through these tunnels.
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Figure 13 – 25kV Electrical Clearance issues in Snowhill tunnel between Farringdon and City Thameslink Stations
The reason the electrical clearance assessments were carried out was that solutions to increase the available height for the Overhead Line Equipment (OLE) like lowering the track or changing the soffit, were not practical in this location due to other constraints. So every effort was made to see if the OLE could be installed without changing any infrastructure.
3.3.3 DC Stray Currents
One of the other considerations of extending the DEA between Farringdon and City Thameslink Stations was the thought that implementing a longer DEA would lead to an increase in the levels of DC stray current in the area.
This was a known issue along the route as DC stray currents had first been observed when the railway line was reinstated during the first Thameslink Project during the late 1980’s. The DC stray currents are produced due to the rail to earth voltages created by the DC traction system to the south and when these are connected, via the running rails, to the earthed 25kV return system this allows DC current to flow to earth before returning to the source of supply in the south.
In the original Thameslink scheme this created the need for the DC separation contactors located in Snowhill Tunnel, to mitigate the flow of DC currents that at the time had been observed in the region of 160A.
Following the installation of the new DC contactor separation system at Ludgate Cellars, various monitoring has been undertaken to review the levels of DC stray currents. It should be noted that mitigation can only be provided as the stray currents cannot be removed completely due to the presence of the DC traction system.
These tests undertaken have shown that while the stray currents can be present when the two traction systems (AC & DC) are connected together, the levels are comparable to those seen during the original Thameslink project in the late 1980’s and that the new contactor system still provides suitable separation. This can be seen in the following graph in figure 14 of one of the tests undertaken when the AC / DC system was put into Bypass.
Figure 14 – Stray Current testing showing level of stray current when the AC/DC interface is in Bypass
The Bypass condition for the AC/DC interface had to be tested to confirm the levels, as this operating mode had to be in place to cover either faults on the equipment or if longer than normal trains were required to traverse the interface. The peaks in the graph are when all contactors are closed, which connects the AC & DC systems together and the sections in
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between the peaks are when the contactors are working normally and providing the correct level of separation. From these tests we have also shown that the levels of DC stray currents are similar to those previously recorded and peaks of between 150-200A have been recorded. Below is a diagram, figure 15, showing a summary of the results seen through the DEA, this includes both the rail to earth voltages which drive the current and the approximate values of conducted DC currents seen in the DEA.
Figure 15 – Typical values of rail to earth voltages & stray DC currents while the AC/DC interface is in Bypass
3.3.4 Insulated Rail Joint (IRJ) rail head arcing
One of the unforeseen issues of the new AC / DC interface was the level of rail head arcing seen on the IRJs that provide the separation in the running rails, see figure 16
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Figure 16 – Typical example of IRJ rail head arcing
The arcing seen on the IRJs initially was not known about and was only revealed when following the originally commissioning a track circuit failed, due to the insulation barrier of the IRJ, breaking down and the two rails effectively joining.
Following a number of investigations, the cause of the arcing was identified as the inductance of the long DC negative return cables that run from the IRJs back to the main contactors located within Ludgate Cellars. These cables are approximately 500 to 600m in length, so as the wheel crosses the IRJ, with the contactor closed at Ludgate Cellars, current flows through the wheel across the IRJ. However as the wheel starts to break the connection over the IRJ, the DC current tries to maintain the path through the wheel, rather than changing to flow through the cables to the conductors due to the cable inductance, see figure 17 below.
Figure 17 – Simplified diagram showing the arcing when the wheel crosses the IRJ
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There have been a number of proposed solutions considered to remove or reduce the arcing to an acceptance level to allow normal maintenance levels to be achieved. These solutions have included:
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Relocation of the complete main contactors closer to the IRJ locations, not consider as insufficient space due to Blackfriars Station;
Use of capacitors to counteract the effect of the cable inductance, not considered due to the high currents involved and the estimated size of capacitance required as insufficient space due to Blackfriars Station Developed of Thyristor technology to provide a localised switch around the IRJ, not considered due to the high currents involved and time required to develop the solution;
Use of a simple mechanical device to divert the arc onto a sacrificial plate adjacent to the IRJ. This solution was trialled and worked successfully in reducing the arcing, but unfortunately could not be made robust enough to cater for the continuous number of wheel passes from each train and thus could not be considered as a long term solution.
Use of a simple contactor slaved off and connected in parallel to the main contactors and located as close to the IRJs as position as possible. This solution was successfully trialled and a full scheme will be commissioning into service before the Thameslink full timetable is implemented, see figure 18 below.
Figure 18 – Trial Slave Contactor located adjacent to the IRJs used for AC / DC interface separation
Other Electrification Works
On top of all the main electrification works to either upgrade the existing traction supply capability or provision of new functionality, such as the AC / DC interface, there have been various works that have either modified or provided changes to the electrification equipment. These changes are not necessarily associated with the provision of more power, but more a reconfiguration of the railway to allow the railway service enhancements; it is these enhancements that then impact on the existing traction supply arrangements such that they have to be re-aligned to the new track or station layouts.
All for these works were carried out in parallel to the main upgrade works and have included:
• Platform extensions to allow longer trains that have affected overhead lines masts or contact wire positions;
• Stabling facilities and the provision of new or modified traction supplied equipment;
• New Depot facilities and the provision of supplies to enable traction and depot supplies;
• Fit out of the new Canal Tunnels between the Midland Main Line (St Pancras) and the East Coast Main Line;
• London Bridge area remodelling to allow of the revised track layouts and for the station rebuild.
While these works are not primarily electrification there is an impact in the electrification infrastructure. A good example of this is the new railway connection between the Midland Main Line and the East Coast Main Line through the Canal Tunnels, where new 25kV overhead line equipment was required.
In this case the 25kV system did not consider of a normal contact / catenary wire, but a conductor beam that holds the contact wire in place, see next page
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Figure 19 – Conductor Beam Neutral section located within Canal Tunnel
Other works include the re-development and re-build of London Bridge station. The stage works for this part of the project have been ongoing over the past three years and are due of for completion around Christmas 2017. These particular works have meant continual change on all the railway systems including the electrification infrastructure associated with the conductor rails, return bonding, substation equipment and remote control equipment (Supervisory Control and Data Acquisition – SCADA).
Figure 20 – SCADA screen used for remote control that has to mirror the operational railway through the stageworks
5. Summary
The electrical challenges on the Thameslink Project have varied considerably from designing and specifying the requirements for upgrading the power availability, to resolving unforeseen technical issues such as rail head arcing.
Throughout all of these electrical engineering challenges, the main programme milestones of KO0 and KO1 have been met. In addition the final milestone of KO2 for December 2018 is still on target to be achieved.
