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The Technology

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Metro Systems' Power Distribution System

  • The Majority of Bulk Supply Points are provided with two 132/22kV transformers to provide redundancy in the architecture.

  • The power is then distributed at 22KV to a number of large switch houses and sub-stations and a high degree of redundancy is provided in the system with a number of cables feeding each switch house / sub-station.

  • From the switch houses the power is transformed down to 11kV to serve the remainder of the sub-stations. The 11kV Distribution Architecture also has a high degree of redundancy built in, to guard against single point failures.

  • The sub-stations provide traction supplies at 630 or 750V DC, Station & Depot Supplies at 400/230 V AC and some legacy signalling supplies at 600V AC.

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The Traction Supply

The traction supply accounts for circa 85% of metro systems' energy usage and is currently inefficient, with significant energy loss through train braking. The bi-product of this energy loss is the emission of significant amounts of heat into metro system tunnels.

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By far the largest proportion of heat, 50%, comes from the trains slowing down – the process of converting kinetic energy into heat simply by applying the brakes. A metro train pulling into a station will give out about 350kW of heat. 

Regenerative Breaking

To combat this, metro systems have been undertaking line upgrades they have introduced new rolling stock with regenerative braking facilities.

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If used efficiently, regenerative braking facilities reduce the amount of waste energy by providing energy back into the power system for use elsewhere. As less heat is emitted, it also reduces the need for cooling and the associated energy usage.

Efficient use of Regenerative Breaking

Regenerative braking introduced within metro systems has its limitations, as it can only work where trains are braking and accelerating at the same time on the same Traction Section.

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Metro systems are developing strategies to optimise Traction Energy Efficiency.

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The coupling of traction sections increases the likelihood of train braking energy being used to power another train starting. The limited likelihood of these events always coinciding means that inefficiencies will remain in the use of regenerative braking.

 

The DC traction system used in metro systems is a floating (unearthed) system utilising two traction rails. It has been developed this way to minimise the risk of DC leakage currents corroding the metal tunnels. 

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To operate this system safely it has been designed as a sectionalised railway (the line is divided into a number of separate traction sections), with traction earth detection. Coupling sections to increase the efficiency of regenerative braking increases the likelihood of earth faults coming together, and as such reduces the safety of the system.

 

To overcome this issue, metro systems are considering the introduction of automatic earth detection and isolation systems. This has not been implemented on any of the lines that have new train stock with regenerative braking.

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The introduction of inverting sub-stations would allow the braking energy to be released back onto metro's distribution systems. The concept of inverting sub-stations has been trialed, but have not yet been introduced on any lines. The introduction of inverting sub-stations would require capital investment which could be offset by future energy savings.

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A recent line upgrade has introduced trains with regenerative braking. However, the potential savings have not been optimised as sections have not been coupled due to safety concerns related to the detection and isolation of traction faults, and inverting sub-stations were not introduced because of the capital costs.

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The EV Concept

The EV Concept would utilise the capacity released by effective use of regenerative braking and the redundancy in the existing power distribution architecture, to provide fast EV Charging Facilities within existing metro infrastructure and the surrounding areas.

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The system would utilise:

  • Released existing and future capacity, by effective use of regenerative braking

  • The significant night-time spare capacity in the network when traction current is off

  • The redundancy and resilience in the distribution architecture

  • Additional capacity introduced from new external or internal energy generation sources

Potential EV Charging Locations

Fast Charging Distribution could be provided at:

  • Car Parks

  • Bus Garages

  • Public Park and Ride Locations

  • Shopping Centre Car Parks

  • Garage Forecourts

  • Roadside Distribution

  • HGV Haulage Premises

  • Reclaimed land adjacent to the railway.

The Technology Challenge

The technology challenge would be the development of the interface between the existing metro system Power Distribution Systems and the current fast charging technology systems on the market.

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The system would need to manage:

  • A variation in DC voltage 750-1000v at the input

  • The significant fault levels that can be experienced on the traction system

  • Electrical isolation of the two systems so that fault conditions can not be transposed from one system to the other

The Benefits

  • Increases access to fast EV charging, lowering the barriers to EV use in dense urban environments

  • Supports faster roll out of EVs to meet Zero Carbon targets

  • Utilises energy currently wasted on metro systems.

  • Reduces heat emission on metro systems and associated cooling required

  • Reduces CO2 emissions and supports cleaner air in cities

  • Enables metro systems to generate income from the commercial sale of waste energy

  • Brings waste-land adjacent to the rail network into commercial and public use

  • Utilises existing infrastructure at high traffic locations

  • Reduces required National Grid upgrades and future capacity pressures

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