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Regenerative braking boosts green credentials

02 Jul 2007

Amid a growing focus on environmental issues, British operators are embracing regenerative braking as a means of saving energy and reducing maintenance costs. Roger Ford explains the benefits of the technology, and acknowledges the challenges standing in the way of wholesale adoption

SWITCHING traction motors to run as generators, thereby generating electric current creating a braking force at the wheels, is an application dating back over 60 years. With direct current traction motors the changeover was technically simple and the current generated could be employed in two ways, known as rheostatic and regenerative braking.

Rheostatic braking is a long-established technique. The current from the traction motors is dissipated in banks of resistors. This type of braking became particularly valuable on heavy haul diesel locomotives running on routes with extensive down grades. With DC motors the electric brake can be set up to maintain constant speed, reducing the need for friction brakes. Substantial savings in brake maintenance can be achieved.

In regenerative braking the current is returned to the overhead line or third rail. Until relatively recently, regenerative braking was mainly employed on DC electrified lines. However, braking is dependent on the ability of other trains on the same route to accept the current. This is known as receptivity and is affected by a number of variables, including location, traffic density and line voltage.

On a busy suburban service at peak times receptivity can be as high as 15%. On a line with long sections between stations and low service frequency it can fall below 5%. One method of improving receptivity on DC electrified systems is to provide energy storage at substations. This can be provided with capacitors, the technology of which is continuing to improve. However, the cost and complexity may not be justified, since poor receptivity is often a function of low service frequency.

Implementing regenerative braking on existing DC electrified lines also has a cost penalty. Because of the high currents involved, up to 4 000 A on a 750 V DC line, there is a risk that current from a regenerating train could prevent the protection equipment in substations from detecting an earth fault in the supply. The solution is to link the protective systems of adjacent substations, known as inter-tripping. Modern trains can also provide additional protection through their control electronics.

Benefits of AC traction

On AC electrified lines, the introduction of power electronics made regenerative braking with DC traction motors technically feasible. However the take-up of this potential was limited. One example was the freight locomotives supplied as part of the 25 kV AC electrification in New Zealand. In general, rheostatic braking remained the standard specification.

With the change from direct current to three-phase asynchronous traction motors, the potential benefit from regenerative braking swung significantly in favour of AC electrification as the complete traction package of transformer, solid state inverters and traction motors became reversible.

As a result regenerative braking can return electric power to the distribution grid. On 25 kV AC electrified lines this can be the national supply system. On systems electrified at 16 2/3 Hz, which have a separate secondary supply network for the railway, receptivity in the railway grid is generally very high.

Effectively AC electrification now provides unlimited receptivity allowing electric braking to be the normal operational mode. In Britain policy on the AC network has been to modify the power supply equipment to allow all trains capable of regenerative braking to operate in this mode, with work due to be completed this year.

Implementing regenerative braking on an existing fleet highlights the potential savings. In the case of c2c's fleet of Bombardier Class 357 EMUs, monitoring over the first two weeks following the introduction of regenerative braking showed an immediate energy saving of 15%.

With receptivity no longer an issue, substantial energy savings can be achieved on inter-city routes in addition to the long-standing application on suburban services and metros. Regenerative braking on the 200 km/h Pendolino fleet on the West Coast Main Line has been enabled since shortly after the fleet's entry into service. Based on several years' experience energy savings of 17% are claimed.

The Pendolino is able to achieve this high figure because of its distributed power configuration with motored axles along the train. The quoted energy recovery is double that achieved in Germany using the original ICE with end power cars. Today the trend in high speed trains is to distributed power, the latest example being the French AGV, although surprisingly, France's high speed network, as well as the Channel Tunnel Rail Link in Britain which follows French electrification practice, is not equipped to accept regenerative braking.

Brake wear savings

While growing environmental concerns have resulted in the promotion of energy recovery as the principal benefit of regenerative braking, there are also long-standing operational and technical advantages. In particular, when regenerative braking is the primary mode, demand on friction brakes is reduced significantly.

As an example of this, the first three-phase drive trains in Britain to operate on the 25 kV network were supplied for commuter services in Birmingham and Manchester characterised by frequent station stops. With regenerative braking enabled, the Class 323 fleet's disc pad life was around 18 months. When the electric braking was switched off while traction current collection issues under icy conditions were investigated, pad life fell to 18 days. While this was an extreme example, the role of regenerative braking in reducing both down-time and maintenance costs is clear.

Technically, growing reliance on regenerative braking places greater emphasis on the interface between traction equipment and brakes. In practice the distinction is becoming blurred as 'smart' braking systems become an integral part of the traction control equipment.

Here the key task is to blend the electric and friction braking systems to maintain the selected retardation rate down to a complete stop. Brake system manufacturers are usually responsible for the blending software which combines the braking effort on the powered and unpowered axles, with the powered axles providing the primary braking effect.

With distributed traction, normal service braking can be all-electric, with the friction braking used only for emergency stops and for bringing the train to a halt. Engineers are now challenging the need for fully-rated friction brakes able to meet normal day-to-day operating duties with electric braking disabled.

Fully-rated friction brakes on a high speed train require triple discs per axle for speeds up to 225 km/h compared with two discs for 200 km/h. The extra discs add both unsprung mass and rotating inertia. In addition to the energy required to accelerate this additional mass, the heavier axles are a factor in increased track forces. The additional callipers and brake rigging also add complexity and cost.

One suggestion is that train friction brakes should be rated to handle a small number of emergency stops with the regenerative brake switched out. In the rare event of an electric brake failure, the train would continue at reduced speed. There is a parallel with aircraft brakes where, in the event of an aborted take-off, the flight is cancelled and the aircraft withdrawn from service for brake replacement.

Overall, electric braking has come of age. In the case of AC electrification, cost and environmental pressures will encourage the optimisation of traction and braking systems of maximise energy recovery. On DC electrified lines, ensuring receptivity remains the greatest challenge.

Le stockage en ligne ou embarqué retient plus l'énergie récupérée

Sur les réseaux japonais, la plupart des automotrices électriques actuellement en service utilisent le freinage à récupération à la fois sur les lignes en pente et lors du freinage normal de service, en partant de vitesses élevées jusqu'à l'arrêt complet. Mais la récupération offre des caractéristiques de freinage inférieures en comparaison avec le freinage rhéostatique et la différence est amplifiée quand le système d'alimentation de la traction n'est pas réceptif au courant qui lui est retourné. Pour faire face à ces problèmes, le stockage embarqué ou en ligne a fait l'objet de vifs débats. Le développement dans un futur proche d'équipements combinant puissance élevée et grande quantité d'énergie avec des pertes acceptables et une longue durée de vie, comme les batteries lithium-ion ou nickel-hydrogène et les condensateurs à double couche, est prometteur pour une exploitation ferroviaire efficace en matière d'énergie

Fest installierte und fahrzeuggebundene Speicher nehmen mehr rekuperierte Energie auf

Die meisten sich bei den japanischen Bahnen im Betrieb befindenden Elektro-Triebzüge haben Rekuperationsbremsen für Gefällsfahrten und für normale Betriebsbremsungen bis zum Stillstand. Aber Rekuperationsbremsen haben schlechtere Bremscharakteristiken im Vergleich mit Widerstandsbremsen, und der Unterschied wird verstärkt, wenn die Stromversorgung nicht in der Lage ist, die zurückgespiesene Energie aufzunehmen. Um diese Probleme abzufangen sind fest installierte oder fahrzeuggebundene Speicher diskutiert worden. Die Entwicklung von Apparaten mit hoher Leistung und Energiedichte und gleichzeitig geringen Verlusten bei einer langen Lebensdauer, wie Lithium-Ionen- oder Nickel-Wasserstoff-Batterien sowie Doppelschichtkondensatoren versprechen einen energie-effizienteren Bahnbetrieb in der nahen Zukunft

La acumulación a bordo y en vía captura cada vez más energía recuperada

La mayor parte de los trenes eléctricos que están ahora en servicio en los ferrocarriles japoneses utilizan el frenado por recuperación, tanto para el frenado de servicio normal como en pendiente, desde la alta velocidad hasta la parada completa. Sin embargo, la recuperación presenta unas características inferiores de frenado en comparación con el frenado reostático y las diferencias aumentan cuando el sistema de alimentación de energía para la tracción no está abierto a la corriente de retorno. Para hacer frente a estos problemas, se ha debatido minuciosamente la acumulación de energía a bordo y en vía. Gracias al desarrollo de dispositivos que combinan una mayor potencia y densidad de energía con unas pérdidas aceptables y una vida útil más larga, como las baterías de níquel-hidrógeno e ión-litio y los condensadores de doble capa, las operaciones ferroviarias tendrán un menor consumo energético en el futuro