On 22 October 2011 Balfour Beatty Rail, working alongside Balfour Beatty Engineering Services Traction Group, installed the Paisley Gilmour Street Track Side Cabin as part of the Paisley Corridor Improvement Project in Scotland.

To look at, this trackside cabin is no different from many others on the UK Rail infrastructure. However, inside it is a different story. Hidden within this unit is the state of the art Balfour Beatty Rail Tracfeed Air Insulated Switchgear (AIS) that is being trialled at this site.

The Tracfeed AIS has been designed specifically to meet the requirements of 25kV railway applications and is derived from conventional 3-phase switchgear. It is common for 25kV switchgear used for railway applications to be insulated with SF6 (sulphur hexafluoride) gas and this equipment is known as Gas Insulated Switchgear (GIS).

However, since the Kyoto Protocol came into force on 16 February 2005, industry across the European community and other industrialised countries has been committed to reducing the green house gas emissions that cause damage to the ozone.

SF6 is one of the six main gases identified in the protocol. Balfour Beatty Rail’s AIS therefore provides Network Rail, and the railway industry in general, with a valuable alternative means of achieving environmental and sustainability policy objectives.

Air insulated busbar chamber

Additional benefits of the Balfour Beatty Tracfeed switchgear, when compared to its gas insulated switchgear equivalents, is that the busbar chamber is also air insulated thus eliminating the need for 24 /7 monitoring of the insulating gas pressure. Particularly in the winter months, a drop in temperature can cause low-gas-pressure alarms to be activated. These are always a cause for concern as, if the gas leaks out of gas insulated switchgear, the bus bars will fail as the insulation is lost. Of course the leaking gas will also cause an environmental incident.

Air insulated switchgear is also generally easier to extend for future capacity increases when compared with gas insulated equivalents. There is no need to de-gas and then re-seal and re-pressurise the bus bar chamber if extending the system. The new Balfour Beatty Rail air insulated system is modular, metal clad and extendable. Each switchgear panel consists of a bus bar compartment, a combined cable connecting and circuit breaker high voltage compartment, a circuit breaker truck, an integrated pressure relief channel and a low voltage compartment. The low voltage compartment is located at the top of the operating side of the panel and houses the protection relays and control equipment.

The vacuum interrupter is mounted on a retractable circuit breaker truck which is located inside the high voltage compartment. By rolling the circuit breaker truck in or out, a gap in the main current path is created or closed, thereby performing the function of a disconnector switch. Rails located in the panel guide the truck when it is moved between the disconnected or operating positions. The fixed contact system to the bus bar is protected against direct shorts whilst in the retracted position by an automatically operated shutter system.

Internal faults involve arcs and would lead to a pressure increase in the affected panel. To prevent this, an integrated pressure relief channel runs along the top of all of the panels to vent any excess pressure and protect against the mechanical or thermal effects of such arcs.

Easy maintenance

An important feature of the Balfour Beatty TracFeed TAC switchgear is that all equipment can be accessed from the front and the circuit breaker truck can also be withdrawn from the panel completely for maintenance.

Outgoing circuits are directly earthed by earthing switches mounted on the cubicle’s steel structure by means of insulators. These switches are motor operated by a spring drive with the capability of emergency hand operation and are interlocked with the corresponding disconnectors. A separate, fully rated electrical earth connection is provided directly to the structure.

Importantly the new switchgear is fully compatible with most protection devices and SCADA systems.

The complete trackside cabin was assembled in Scotland at Balfour Beatty Rail Engineering Services’ factory at Huntly Road, Glasgow. Situated close to the project, this provided an excellent environment for the Network Rail and Balfour Beatty engineering teams to develop and deliver this innovative new design which is a first in the UK. Balfour Beatty provided a “one stop shop” to Network Rail for the design, manufacture, integration, installation and testing of the equipment as part of the overall Paisley Corridor Improvement project.

Tunnels too

The fitting of electrification equipment within the spatial constraints of the UK rails civil infrastructure, some of which originates from Victorian times, has been a perennial problem for electrification engineers. Balfour Beatty Rail has, over a number of years, developed special techniques, expertise and products that allow the most complex tunnel electrification projects to be successfully completed.

The introduction of a new scissor crossover into the Midland City Line at Midland Road situated under the heart of the City of London within the Kings Cross North Tunnel, is a case in point. A special wiring configuration had to be developed to allow the crossover to be integrated into the existing 25kV electrification equipment in the tunnel. The new crossover allows trains using the St Pancras sub surface station to be turned around when an overhead line isolation is in place at the southern end. This project was undertaken as part of an operational upgrade associated with the Thameslink Improvement Programme.

Design evaluation

During the design phase of the project a number of possible options for electrifying this new piece of infrastructure were evaluated against operational performance, constructability and whole life costs. The analysis established that, for this particular project, a reduced-height semi-flexible conductor-based wiring system demonstrated the greatest cost efficiency for delivery while meeting the performance specification and construction programme requirements.

Physically fitting the electrification equipment into the confined tunnel profile, while ensuring conformance to mechanical and electrical clearance standards, was the predominant challenge of this project whilst at the same time ensuring electrical independence of the main through roads.

A clearance study, using various CAD design tools and numerical analysis was undertaken to assess the chosen OLE support configuration and ensure that the necessary electrical and mechanical clearances were maintained. This exercise considered the positioning of supporting equipment in conjunction with the vehicle and pantograph gauges that operate on this route. It involved 3D modelling of the wiring configuration and additionally, following numerical analysis, a number of 2D cross section slices were developed through the area of the crossover depicting the wires and pantographs in their relative operational positions.

Electrical sectioning was achieved by employing two 25 kV Section Insulators on the crossover wires. A special arrangement was chosen for this application to improve the along-track positioning of the equipment while ensuring that electrical clearances to the pantographs passing on the through lines were maintained.

The crossover wires for the scissors are auto-tensioned using spring tensioning devices which comply with the spatial envelope available and were easily mounted to the tunnel soffit.

Dynamic stability

To provide dynamic stability, the overhead line equipment arrangement directly above the crossover was supported vertically at the high load points of the section insulator. In addition, supports were added to the opposing side of the crossover to counteract any imbalance. This support configuration allowed the system to be adjusted on site to achieve equilibrium and a level contact wire profile which provides efficient current collection at the contact wire pantograph interface.

The structural integrity of the tunnel surface was tested at pre-construction phase to verify that the new electrification equipment could be introduced onto the existing civil infrastructure.

Construction commenced in June 2011. The Balfour Beatty Rail construction team used two 24 hour weekend track possessions for the installation of the support equipment, tensioning devices and bonding of the new equipment. A third 58 hour weekend possession was used to run the new contact and catenary wires for the crossover and the installation of the two new section insulators. Verification of the installation prior to section proving was achieved by manufacturing a track-mounted crucifix gauge that had been uniquely modified to incorporate the electrical clearance and kinetic vehicle tolerances.

The key to the success of this technically complex and challenging project was the depth of experience within the Balfour Beatty Rail engineering and construction teams that worked closely and in harmony with Network Rail’s Thameslink project team. The result was a successful project, delivered on time and to budget.

Doug Lee, Balfour Beatty Rail Programme Director, National Electrification, stated that, ‘We are committed to innovation and technical development to ensure that Network Rail’s objectives for the future electrification of the UK rail network are fully achieved in a cost effective manner.’

Written by Steve Cox and Barry Calder for the rail engineer