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Z:\D Working Folder\D2 Reports\D2.3 Technical Notes\TN14 Power Supplies for Light Rail, Metro and Heavy Rail Systems.doc

Technical Note No 14Technical Note No 14Technical Note No 14Technical Note No 14

Power Supplies for Light Rail, Metro and Heavy Rail SystemsPower Supplies for Light Rail, Metro and Heavy Rail SystemsPower Supplies for Light Rail, Metro and Heavy Rail SystemsPower Supplies for Light Rail, Metro and Heavy Rail Systems

1111 Light Rail SystemsLight Rail SystemsLight Rail SystemsLight Rail Systems

1.11.11.11.1 Power SupplyPower SupplyPower SupplyPower Supply

Light Rail systems normally have a dc traction supply. Modern practice has the traction supply tothe vehicle provided by an overhead line system. The positive is provided via a contact wire andthe negative return is via the wheels and the rails.

1.1.11.1.11.1.11.1.1

Overhead LineOverhead LineOverhead LineOverhead Line

In dense urban street running areas common practice has a single positive contact wire in the airto minimise visual impact. If the single contact wire does not provide sufficient cross section tocarry the traction current parallel positive feeders are installed in ducts at the trackside.

In extra urban areas a contact and catenary system can be visually acceptable, and could befurther reinforced aerial parallel feeders.

The overhead line systems can be supported from:-

Buildings This is preferred where possible as it reduces street furniture, but does requirethe co operation of planners and owners of the buildings, etc.

Combined lighting columns This is the second choice. The lighting columns have to be

purpose designed to carry the loads, and special earthing arrangements imposed. This is areasonable proposition but does require the cooperation of the local authority, etc.

Dedicated overhead line structures The simplest solution but do tend to clutter theurban environment.

1.1.21.1.21.1.21.1.2 Traction substationsTraction substationsTraction substationsTraction substations

The traction substations are typically distributed at spacing of approximately 2 km in urbanareas. The spacing of the substations is dependent upon the tram service headways, operationalspeed and the cross section of the over head line.

The substations normally have a high voltage (HV) supply that is sourced locally from the local

electricity company and in the UK this is at 11 kV. The electricity company is normally able toprovide connections from different areas of their system so that common mode failures areavoided. This certainly is the case for Manchester Metrolink, Nottingham and the Sheffield SuperTram in the UK. This is acceptable as the tram service levels are about 12 trams per hour perdirection and above ground.

When the service levels become very dense up to 50 trams per hour per direction then the loadsare high and need to be very secure. This is when a dedicated tramway HV network will berequired.

The substations contain transformer rectifiers and will vary in size, small systems with singleunits and 12 minute headways will typically have 600 kW units, while busier systems such asManchester have 1.2 MW transformer rectifiers. I would estimate that a system such as theplanned Cross River tram in London may have up to 2.5 MW transformer rectifiers. The size of the

transformer rectifiers is determined by vehicle performance, service headway and geography. Thetransformer rectifiers are the heaviest item weighing up to 14 tonnes and occupy an area up to3 m2.

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The substation will typically have an HV switchboard, a dc switchboard, one or more rectifiersand often an auxiliary transformer. There are a number of ancillary items for control, etc.

The substations, if located on the surface on an ideal site, occupy an area of about 15 x 6 m. Theyneed road access and space to off-load the plant and should be located as close to the track aspossible. Ideally the substation will be on the surface, but it is also reasonable to put themunderground or in another structure. One has been planned for the basement of a departmentstore in Liverpool, and Dublin has two below tram stops. Many are planned to be incorporatedinto new building developments. The preferred design is dependent on the surroundings, and thesubstation buildings can have pretty much any architectural finish to make them blend moreeffectively into the surroundings, the more innocuous the better from the security and vandalismpoints of view. It is also common practice to construct substations in re-locatable buildings,which have the advantage that they can be manufactured off-site and tested. This is quick,inexpensive and reliable, but they do not enhance their surroundings, however they can bescreened by the use of suitable architectural finish. Network Rail has used stainless steel as theconstruction material on all of the Southern Region Power Upgrade substations.

In the design of the power system consideration must be given to redundancy, the minimumrequirement will be N-1, i.e. there shall be no observable decrease in functionality of the tramservice in the event of the first equipment failure.

1.1.31.1.31.1.31.1.3 DuctingDuctingDuctingDucting

A duct system is normally provided on each side of the tram track: one side of the track isnormally dedicated to power and the other to signalling, communications and control.

The power side will carry HV power cables, pilot cables (for control, protection and inter-tripping), dc positive parallel feeders (where used), dc negative reinforcement cable (where used),and stray current collection cables.

The ducting on the signal, control and communications side, will have cabling for the tramlocation systems, SCADA for the stops (for ticket machines, public address and general control),there will be cables relating to the Urban Traffic Control System, and there will be video servicefor CCTV. The low voltage supply cables may be run in this duct run or the power run.

The ducting system often has to negotiate its way round existing services. Therefore it tends toend up under the track, which is acceptable as long as the inspection pits and access are locatedoutside of the swept path of the vehicle as there may be a need for emergency access duringtraffic hours.

Service diversions have to be considered when planning the tramway. It have to be consideredthat all services not part of the tramway cross the track at approximately 90 degrees as once theyare under the track they cannot be accessed from above. There are also preferences for choices ofmaterials for pipes, etc.

2222 Metro SystemsMetro SystemsMetro SystemsMetro Systems

The type of metro system discussed here is the heavy end of light rail, such as the LondonUnderground or the Paris Metro. These have two principal areas: below ground and open areas.This is an important distinction as this directly affects safety.

On a dense metro system the power supply has to be secure. The primary inputs have a reliabilityat a level of N-2, which means that one item of plant can be taken out of service formaintenance, say, and a second item of plant can fail on a fault, and under such conditions therewill be no observable disruption to the train service or station services.

To achieve this, the primary power supply will be sourced from primary distribution substations,

e.g. at 132 kV, (which is typical in the UK). There will be a minimum of three incomingconnections from different parts of the grid network, which does not mean that there would bethree bulk supply points. In one system we have recently designed, two connections were madeto different busbar sections at one primary distribution substation, which are normally suppliedfrom different parts of the grid network, hence reducing the likelihood of common mode failure,

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and the third connection was made to another primary distribution substation at the other end ofthe railway.

Bulk Supply Points (BSPs) vary in rating depending on the system being supplied, but for examplethe East London Line is 10 km in length and will have 16 trains per hour per direction. There aretwo BSPs, which between them have three 60 MVA transformers. The current design loading is35 MVA. Allowance has been made for some load growth, but other factors were considered, inparticular the electricity company keeps strategic spares of this size of transformer. This isimportant as they have a long lead time.

The London Underground System has at present four BSPs and will introduce a fifth one soon tocater for the growing loads. These sites are all parallel redundant and are rated between 90 MVAand 120 MVA each on a secure basis.

These BSPs are very large buildings, one that we are currently designing starts at -10 m and thetop of the building is at +16 m on account of the restricted nature of the site, being approximately75 m long and 15 m wide.

Metro systems typically have their own private HV distribution networks that supply the tractionpower substations, station power supplies and ventilation and pumping sites.

The stations, critical vent sites and important installations will have a second supply from thelocal electricity company for essential loads. This does go some way to back up the HV network ina minor electrical outage. These supplies are typically backed up by some form of uninterruptiblepower supply (UPS). This is vital underground for supply of life safety systems, such as lightingand communications. It is totally unacceptable to black out a station, as it is likely that there willbe panic, which my lead to multiple injuries. London Underground have UPS supplies thattypically last 30 minutes, but they have their own independent power station which is operatedto support the emergency loads. The requirement for the supply of emergency power is that itshall be sufficient comfortably to evacuate everyone from the stations, tunnels and trains as a

minimum.The traction supplies tend to be 750 V dc, but there are some variations with systems such asTyne and Wear (UK) and Dublin (Eire) operating at 1500 V dc. On the 750 V dc systems thesubstations are typically spaced at 4 km. They typically contain:

An HV switchboard

A number of transformer rectifier units

A dc switchboard

A number of auxiliary transformers

A low voltage switchboard

SCADA control equipment

The number and rating of the transformer rectifiers is dependent upon the rating of the trains,formation, passenger loading, the train service headway, the geography of the railway andauxiliaries, in particular air conditioning. In outer suburban areas there are typically twotransformer rectifiers, in suburban areas this increases to three and in areas of dense service orwhere the substation supplies for more than one line or branch it is not uncommon to find up sixtransformer rectifiers.

Sizes vary: Dublin has transformer rectifier units rated at 4 MW, the older London Undergroundlines have 1.5 MW units and the upgraded lines have 2.5 MW units. These transformer rectifiersoccupy at space of a 3 m cube and weigh up to 15 tonnes.

There will normally be a least one or two auxiliary transformers. These will be rated at a standard

size of 500kVA if they are supplying substation domestic loads and loads like the signallingsystem. However they will rapidly increase in size if they are supplying the underground stationsand the ventilation and/or air conditioning plant. Modern small stations such as the modernised

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Angel in London have transformers sized at 1 MVA, and large stations such as majorinterchanges can see transformers sized at 2.5 MVA or more.

Within the metro stations there will be a level of redundancy as it is totally unacceptable to havedisruption of the train service on the first substation fault. If a second fault were to occur, thenthe expectation would be that a train service could continue but it would be degraded operation,e.g. low speed of operation through the affected area and increased intervals.

The traction substation buildings tend to be quite large often two stories and they have a cablebasement. A minimum size would be about 12 x 25 m, and they can be a lot larger. Thesesubstations are typically above ground, however there are many examples, where they have beenplaced underground in the basements of new developments such, shopping centres (Liverpool),hotels and banks (London).

Most of the HV cabling will be installed trackside, however it is possible that some may be routedalong the highway such as HV cables to the BSPs and others between different metro lines.

In the planning of a city the design should allow for a separate substation for the depots. It isgood practice to supply the depot separately from the main line for stray current protectionreasons.

3333 Intercity RoutesIntercity RoutesIntercity RoutesIntercity Routes

3.13.13.13.1 Traction Power SuppliesTraction Power SuppliesTraction Power SuppliesTraction Power Supplies

Modern practice within Europe over the past 50 years has seen the electrification of many of themain line routes, inner and outer suburban and inter-city routes with overhead line systems.Most of these systems have selected an operating voltage of 25 kV ac.

In recent times high speed line upgrades such as the UK West Coast Main Line and the new-buildChannel Tunnel were electrified at 25-0-25 kV.

The substations for the 25 kV systems are normally supplied direct from the transmission systemat 132 kV or higher. Again as above, the spacing of the substations varies with the service levelsand train types. But typical spacings are 10 km for urban areas, increasing to 20 30 km forextra-urban sections.

These substations contain the following items of plant:-

Possible extra high voltage switchgear (often this is located at the Grid site)

The 132/25 kV transformer(s)

The 25 kV ac switchgear

SCADA control equipment.

These sites tend to be more open.

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