This article traces the development of 25 kV AC power supply delivery to the overhead line equipment (OLE) on the UK railway network since 1956. Its aim is to uncover the underlying design issues that govern the delivery of power supplies to the OLE and highlight the critical design drivers that influence future design principles of switching stations on the railway. The term "switching stations" is used throughout this article to describe all types of 25 kV switching arrangements.

Overview of Power Supplies Feeding Arrangement

A series of feeder stations installed strategically along an electrified route are used to feed 25 kV, 50 Hz, single-phase power to the OLE. These feeder stations are, in turn, connected to the electricity supply industry high voltage network. The primary connection voltages include 132 kV, 275 kV and, lately, 400 kV for use on automatic transformer systems. Figure 1 shows a typical feeding arrangement diagram. The track, return current systems and switches are omitted for clarity.

The OLE is divided into main electrical sections separated by neutral sections at feeder stations and mid-point track section cabins/locations. Bus-section circuit breakers are installed across the neutral sections. The feeding system is radial, with the normal arrangement being feeds from each feeder station out to the open bus section at the mid-point track section cabin. Radial feeding is necessary because the supplies from feeder stations are from different phase pairs and different power networks, thus precluding paralleling. During abnormal feeding, however, this open point may be moved using the bus section circuit breakers.

For operational and maintenance reasons, the OLE is further divided into electrical subsections at track section cabins or intermediate track section cabins/locations where installed. The OLE sectioning at these locations is achieved through insulated overlaps between mechanical tension lengths in the OLE. Typically, dual supply circuits are installed at feeder stations to safeguard the security of power to the track. These circuits are usually derived from different phases and, for added security, perhaps from different parts of the electricity supply industry network.

Background

Railway switching station design goes back to the early days of 25 kV AC electrification, which started in the UK in 1956 for the route between Euston, Manchester and Liverpool via Birmingham. The change to electrification involved the installation of 76 switching stations. The design concept was to house all high voltage switchboards in brick buildings together with associated ancillary equipment (e.g. supervisory control and data acquisition systems (SCADA), battery and charger heaters, low voltage (LV) panels for domestic power supplies, and changeover panels for signalling supplies). The type of switchgear used was K11 oil-filled circuit breakers. The feeds to the OLE were via cables.

Figure 1: Typical Feeding Arrangement Diagram

In 1959 the Colchester-Walton and Clacton line was electrified, but this time using small oil-filled type circuit breakers. Again, the choice of housing was a brick building and the feed to the OLE was through cables. Between 1967 and 1973, the Glasgow-Gourock-Wemyss Bay line was electrified using outdoor oil-filled switchgear K11-W. The delivery of power to the OLE was through weatherproof bushings and the connection to the OLE was achieved by aerial bare-copper conductors.

After completion of the Euston, Manchester and Liverpool electrification scheme in 1966, equipment suppliers introduced the vacuum circuit breaker for 25 kV AC single-phase use. The British Railway Board had always taken an active interest in simplifying the switching station design and lowering the maintenance burden, and the use of vacuum interrupters obviously reduced the maintenance work when compared with oil-filled equipment.

The first vacuum circuit breaker equipment was installed at Camden track section cabin for evaluation. This trial proved to be successful and has led to vacuum circuit breakers being used in metal-clad housing with roof bushings on subsequent electrification schemes, which included the Weaver Junction to Glasgow electrification in the mid-1970s and the East Coast Main Line in 1984-1991. In both cases, the delivery of power to the OLE was through bare aerial feeds. The switching stations design development profile is depicted in Figure 2.

Figure 2: Railway Switching Stations Design Status

In 1963, Kennedy & Donkin (which joined PB in 1998) provided support to the British Railway Board for calculating touch and step potentials and has provided many feasibility studies regarding the 25 kV 50 Hz electrification in the UK. (Touch and step potentials are calculated to ensure that under normal operating conditions no harmful voltages will appear in switching stations compounds, and there is no danger of system workers and users getting electrical shocks because the switching station is not properly earthed. Touch and step potentials are very important design safety parameters for electrified sites. We were very active in the electrification design activities and, in fact, providing the majority of technical papers written on electrification.

Switching Station Designs

Traditionally, the switchgear market has been led by the electricity supply industry, which had used both indoor and outdoor switchgear successfully on its infrastructure. The railway switchgear market segment is not normally large, so a "quantity dilemma" was experienced by manufacturers who were unable to justify the cost of producing small batches of units specifically for the railway market. In fact, toward the end of the 1980s some suppliers reported that the future production of the vital vacuum interrupters might be disrupted.

Because most vacuum circuit breakers installed on the railway were approaching their 20-year design life, this situation had caused some concern to equipment maintainers. Railway authorities are slowly addressing the issue by the use of design/build and maintain projects that include manufacturing partners and alliances to deal with long-term equipment replacements and the new technology available in the marketplace.

Figure 3: Approximate diagram of 25kV Switching Station Brick Building

Indoor Switching Stations

Indoor switchgear had benefited from key features such as the ability to place the isolate facilities and the earth relevant circuit breakers inside, precluding the need to go out onto the track environment. The key features of this design include the following:

Housing electrification equipment in brick buildings has served the power distribution discipline well. These buildings have survived more than 35 years. There are, however, several disadvantages of the brick buildings:

It should be recognised that this method of delivery was an answer to 1950s power supplies constraints. These have changed now, and the advent of new technology and materials, such as glass reinforced plastic, provide an alternative equipment housing material.

Metal-Clad Switching Stations

During the implementation of metal-clad switchgear design in the early 1970s, a critical decision was taken to transfer the isolation and earthing facilities from indoor environment to the track environment by installing track isolators and earthing switches on the OLE. This change has saved space in the switching stations but has transferred the problems related to space constraints to the OLE. Consequently, more overhead system design was needed and more equipment was added to the OLE, making the catenary system more congested (Figure 4).

Figure 4: Metal-clad Switchgear Arrangement

It was possible to introduce vacuum switchgear on the railway and to place the isolation switches on the OLE because of the introduction of the vacuum circuit breaker, which resulted in integrating the switching station more with the OLE. The consequences of these developments have been new maintenance and operational constraints on either the OLE or the switching station. It was no longer possible to isolate and earth individual circuit breakers at mid point track sectioning cabins. Instead, a complete bus bar section and its associated circuit breakers had to be isolated and earthed to make it safe to work on one circuit breaker only. Moreover, at intermediate track sectioning cabins, the complete switching station had to be taken out of service in order to maintain any one circuit breaker. This was achieved through the operation of by-pass switches located on the OLE.

In spite of the shortcomings due to the isolation and earthing difficulty, this type of switching station remains most popular because of ease of construction and dismantling, and ease of transportation to site.

Structure Mounted Outdoor Switchgear Station

The initial concept of the outdoor delivery system was to install the outdoor switchgear on the OLE structures themselves. This conceptual design was most innovative as it departed significantly from past practice and cleverly made use of existing OLE infrastructure to support the switchgear, thus minimising takings of land along the trackside. Figure 5 depicts the conceptual approach for this type of power feeding arrangement.

Figure 5: Overhead Line Structure Mounted Outdoors Switchgear

Although this design was innovative, it met with a lot of resistance for the following reasons:

• Two maintenance functions were unified by incorporating the structure-mounted outdoor switchgear (SMOS) into the OLE system. Consequently, the end users of the equipment were unable to formulate an integrated maintenance strategy to deal with this change. Arguably, this can be thought of as an organisational problem rather than a design concept deficiency.
• It is particularly difficult to superimpose the SMOS onto an operational railway during the construction process because of safety and operational constraints (e.g. red zone working or complete block to traffic).
• Isolation and earthing procedures for OLE and SMOS are incompatible. Again, this problem has more to do with the inability of various departments to work with each other rather than design weakness.
• Lack of familiarity and training of the end users produced negative feelings and militated against the SMOS concept.
• SMOS is regarded as visually intrusive. Strenuous efforts are required to make the SMOS aesthetically acceptable, particularly in densely populated areas.

Incremental design development took place to mitigate the perceived visual shortfall in the above arrangement. The result was the use of outdoor switchgear, but this time entirely detached from the OLE system by making use of feeder cables to deliver power to the track. Figure 6 shows the latest approach adopted on various locations on the electrified track.

Future Development

In the near future, railway systems in the UK will have high-speed profiles as, for example, the 224 kph (140 mph) service on the West Coast Main Line that is planned for 2005. Notice, however, that this speed is moderate in comparison with others in the world's high-speed train race. Accessing the track to undertake isolation and earthing activities will become more hazardous during train running hours and more costly.

In privatised railways, the commercial realities put perceptible pressure on the designer to improve the functions and features of switching stations to meet the new operational environment. Moreover, in order to provide on-time service to their passengers, train operating companies will demand sustainable power delivery at the point of need and they will contractually levy a charge against railway infrastructure companies that cause delays to their services by switching station or OLE failures.

Figure 6: Fenced Compound Structure Mounted Outdoors Switchgear

Further "disentangling" of the switching stations from the OLE will become necessary. In addition, a reduced need for maintenance staff to be on or about the line will become a safety imperative that must be built into the switching station design process. The challenge to the designer is to contrive design concepts that lower the asset owner costs whilst raising the efficiency of their asset.

In pursuit of cost reduction and a higher level of efficiency in power supply delivery systems, railway infrastructure companies may consider it worthwhile and economically justifiable to agree with the electricity supply industry to forward integrate the railway switching stations with the electricity supply industry interconnected system. This approach may benefit from the dominant power of the electricity supply industry in the switchgear marketplace. Alternatively, the electricity supply industry may choose to lease the switching stations from key manufacturers and obtain guarantees that ensure effective servicing of the switching stations.

Both approaches will stimulate the electricity supply industry and the manufacturers to provide value-added solutions to the switching station design problems, and allow railway authorities to focus on the mechanical aspect of the OLE, which is far more vulnerable to failure than the power supplies equipment. Finally, another future switching station design type that is worthy of research is the possibility of using mobile switching stations as opposed to fixed-location stations.