The Anonymous Widower

Regenerative Braking On A Dual-Voltage Train

Yesterday, I found this document on the Railway People website, which is entitled Regenerative Braking On The Third Rail DC Network.

Although, the document dates from 2008, it is very informative.

Regenerative Braking On 25 KVAC Trains

The document says this.

For AC stock, incoming power from the National Grid at high voltage is stepped down by a transformer. The AC power is transmitted via OHL to the trains. When the train uses regenerative braking, the motor is used as a generator, so braking the axle and producing electrical energy. The generated power is then smoothed and conditioned by the train control system, stepped up by a transformer and returned to the outside world. Just about 100% of regenerated power is put back into the UK power system.

But I have read somewhere, that you need a 25 KVAC overhead electrification system with more expensive transformers to handle the returned electricity.

Regenerative Braking On 750 VDC Trains

The document says this.

After being imported from the National Grid, the power is stepped down and then AC power is rectified to DC before being transmitted via the 3rd rail. Regenerated Power can not be inverted, so a local load is required. The power has to be used within the railway network. It cannot be exported.

So the electricity, is usually turned into heat, if there is no train nearby.

The Solution That Was Applied

The document then explains what happened.

So, until such time as ATOC started to lobby for a change, regenerative DC braking was going nowhere. But when they did start, they soon got the backing of the DfT and Network Rail. It takes a real combined effort of all organisations to challenge the limiting assumptions.

In parallel, there were rolling stock developments. The point at which all the issues started to drop away was when the Infrastructure Engineers and Bombardier, helped out by some translating consultants (Booz & Company), started to understand that new trains are really quite clever beasts. These trains do understand what voltage the 3rd rail is at, and are able, without the need to use any complicated switch gear – just using software, to decide when to regenerate into the 3rd rail or alternatively, use the rheostatic resistors that are on the train.

Effectively, the trains can sense from the voltage if the extensive third-rail network can accept any more electricity and the train behaves accordingly.

As most of the electric units with regenerative braking at the time were Bombardier Electrostars, it probably wasn’t the most difficult of tasks to update most of the trains.

Some of the Class 455 trains have recently been updated. So these are now probably compatible with the power network. Do the new traction motors and associated systems use regenerative braking?

This document on the Vossloh-Kiepe web site is entitled Vossloh Kiepe enters Production Phase for SWTs Class 455 EMU Re-Tractioning at Eastleigh Depot and describes the updating of the trains. This is said.

The new IGBT Traction System provides a regenerative braking facility that uses the traction motors as generators when the train is braking. The electrical energy generated is fed back into the 750 V third rail DC supply and offsets the electrical demands of other trains on the same network. Tests have shown that the energy consumption can be reduced by between 10 per cent and 30 per cent, depending on conditions. With the increasing cost of energy, regenerative braking will have a massive positive cost impact on the long-term viability of these trains. If the supply is non-receptive to the regenerated power, the generated power is dissipated by the rheostatic brake.

So thirty-five year old British Rail trains now have a modern energy-saving traction system.

Has The Solution Worked On The Third-Rail Network?

The Railway People document goes on to outline how they solved various issues and judging by how little there is about regenerative braking on the third-rail network, I think we can assume it works well.

One Train, Two Systems

If you have a train that has to work on both the 25 KVAC and 750 VDC networks, as Thameslink and Southeastern Highspeed trains do, the trains must be able to handle regenerative braking on both networks.

So is there a better way, than having a separate system for each voltage?

In Do Class 800/801/802 Trains Use Batteries For Regenerative Braking?, I investigated how Hitachi’s new Class 800 trains handle regenerative braking.

A document on Hitachi’s web site provides this schematic of the traction system.

Note BC which is described as battery charger.

The regenerative braking energy from the traction motors could be distributed as follows.

• To provide power for the train’s  services through the auxiliary power supply.
• To charge a battery.
• It could be returned to the overhead wires.

Hitachi’s system illustrates how using a battery to handle regenerative braking could be a very efficient way of running a train.

Hitachi’s diagram also includes a generator unit or diesel power-pack, so it could obviously fit a 750 VDC supply in addition to the 25 KVAC system on the Class 800 train.

So we have now have one train, with three power sources all handled by one system.

What Has Happened Since?

As the Hitachi document dates from 2014, I suspect Hitachi have moved on.

Siemens have produced the Class 700 train for Thameslink, which is described in this Siemens data sheet.

Regenerative braking is only mentioned in this sentence.

These new trains raise energy efficiency to new levels. But energy efficiency does not stop at regenerative braking.

This is just a bland marketing statement.

Bombardier are building the first batches of their new Aventra train, with some Class 345 trains in service and Class 710 trains about to enter testing.

Nothing has been said about how the trains handle regenerative braking.

But given that Bombardier have been experimenting with battery power for some time, I wouldn’t be surprised to see batteries involved.

They call their battery technology Primove and it has its own web site.

There is also this data sheet on the Bombardier web site.

Class 387 Trains

There is another train built by Bombardier, that is worth investigating.

The Class 387 train was the last and probably most advanced Electrostar.

• The trains have been built as dual-voltage trains.
• The trains have regenerative braking that works on both electrification types.
• They were built at around the time Bombardier were creating the Class 379 BEMU demonstrator.
• The trains use a sophisticated propulsion converter system called MITRAC, which is also used in their battery trams.

On my visit to Abbey Wood station, that I wrote about in Abbey Wood Station Opens, I got talking to a Gatwick Express driver about trains, planes and stations, as one does.

From what he said, I got the impression that the Class 387/2 trains, as used on Gatwick Express, have batteries and use them to keep the train and passengers comfortable, in case of an electrification failure.

So do these trains use a battery to handle the regenerative braking?

How Big Would Batteries Need To Be On A Train For Regenerative Braking?

I asked this question in a post with the same name in November 2016 and came to this conclusion.

I have a feeling that using batteries to handle regenerative braking on a train could be a very affordable proposition.

As time goes on, with the development of energy storage technology, the concept can only get more affordable.

Bombardier make a Primove battery with a capacity of 50 kWh, which is 180 mega-Joules.

So the braking energy of what mass of train could be stored in one of these batteries?

I got these figures.

• 100 mph – 180.14 tonnes.
• 110 mph – 148.88 tonnes.

What is the mass of a Class 387 train?

This is not available on the Internet but the mass of each car of a similar Class 378 train averages out at 32 tonnes.

Consider these points.

• A Class 387/2 train, has 219 seats, so if we assume each passenger and baggage weighs eighty kilograms, that adds up to 17.5 tonnes.
• As the Class 387 trains have a maximum speed of 100  mph on third-rail electrification, it would appear that a Primove 50 kWh battery could handle the braking energy.
• A Primove 50 battery with its controller weighs 827 Kg. according to the data sheet.

It all looks like using one of Bombardier’s Primove 50 batteries on a Class 387 train to handle the regenerative braking should be possible.

But would Bombardier’s MITRAC be able to use that battery power to drive the train in the most efficient manner? I suspect so!

If the traction layout is as I have outlined, it is not very different to the one published by Hitachi in 2014 on their web site for the Class 800 train.

Conclusion

Hitachi have got their traction layout right, as it can handle any number of power sources.

 

 

October 26, 2017 - Posted by AnonW | Energy Storage, Transport/Travel | Aventra, Class 387 Train, Class 800 Train, Hitachi, Regenerative Braking