Regenerative braking on the third rail DC network

Regenerative braking on the third rail DC network

15 Aug 2008

Train operators Southern and Southeastern have become the first train operators in the country to introduce regenerative braking on the third rail DC network. After almost two years of planning and testing, the first Class 375, 377 and 376 Electrostar trains are now returning electricity back into the rail system when braking, allowing other trains to draw on that energy for power.

This represents the culmination of an intensive 18 month project delivered in a model cross-industry partnership between Southern, Southeastern, Network Rail, and train manufacturer Bombardier, with project management from Booz & Company. Southern and Southeastern are both operated by Govia, the partnership between the Go-Ahead Group and Keolis.

Gerry McFadden, Engineering Director Southern explains the work being carried to achieve significant energy savings by regenerative braking.

The Regenerative Braking process

When a train brakes regeneratively, its kinetic energy is converted to electricity that can be reused. Those of you with a rudimentary knowledge of electricity will appreciate that this reuse is reasonably straightforward in the case of AC rolling stock, but ’tricky’ in the case of DC.

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 life is not so simple for the DC rolling stock

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.

Real and legitimate concerns

A very large proportion of the country’s rolling stock is south of the River Thames running on a massive DC system which is very, very hungry for energy. There has been a long-held reluctance to embark on regeneration programmes because of reasonably held concerns by infrastructure engineers that there are real issues of safety and asset management to be addressed. These concerns have, until now, blocked any progress.

There were worries that surges in voltage would damage older rolling stock and that equipment, run from the common DC supply, would speed up. There were real and legitimate concerns that, as a consequence of running regenerative trains, the ability of the 3rd rail system to protect itself - to trip out if there were faults – would be compromised.

The mechanism for this effect goes like this: If there is a short circuit in an electrical section, then the surge of current is detected in the substation and circuit breakers cut off the supply. The fear was that the current from a train regenerating electricity might feed the short circuit directly, so masking the effect of the short circuit. The substation would not ’see’ the problem and so the circuit breakers would not trip out. The initial problem would then persist long enough for secondary and extensive damage to occur.

There was a solution to this problem some years ago. It was known as inter-tripping and had been installed in a small area of Kent. It fell into disrepair and now no longer exists. To fit the entire network with the facility would be prohibitively expensive.

ATOC initiative

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.

Resolving the issues

The surge theories were challenged when the real nature of modern train braking was analysed. A 12 car train doesn’t use electric braking for its whole cycle. Once it gets to low speed, electric braking blends into friction braking for the final touch by the driver. During electric braking the train control software can efficiently blend the levels of regenerative and rheostatic braking, providing a safe, efficient and smooth brake.

The short circuit masking problem was also rigorously analysed. A very detailed risk assessment by Network Rail modelled the actual potential fault scenarios. Once they started to work through on a section by section basis, it was found to be very unlikely that a train would be in a particular location regenerating at the precise time as a fault occurring – and these faults don’t actually happen that often either.

There is a very long list of ’ifs’ and this reduces the risks significantly. This piece of work quantified the issue, and enabled informed decisions to be taken. Lots of theoretical models of 3rd rail receptivity say that on a line with only a few trains, such as Southern’s South Coast service, there would be very little receptivity as compared with an urban area. However in real life, there is ’tons of receptivity’, perhaps because the system is so heavily interlinked that energy is redistributed over a very large area.

Initial results with just a couple of units return economies easily in double figures – perhaps even a 20% energy recovery. There need to be more tests to reassure London Underground (LUL) in those areas where the electrical supply is common.

The original plan was to physically separate LUL sections - but in the light of practical experience, this may be revisited. The effect on voltage may not be that high, as the network is such a big sink.

Testing the theories

Early experiments took place between Dorking and Horsham. There used to be large resistor banks available in BR days. But many of these bulky container vehicles ’disappeared’ and those that did survive had asbestos complications. However, the train planners and operators in Southern came to the rescue.

Through some excellent possession planning and really very complex short term train planning we managed to plan into a ’wheels free possession’ sufficient rolling stock to act as the passive load. We parked them in possessions, turned all the heaters on and opened the doors. This was done night after night for weeks on end until we had exhausted all of the required tests.

And we managed to get all the trains into the possession at the end of the days operation, then back to their stabling locations at the end of testing ready for passenger service the next morning. This was a really very difficult train operating challenge that was delivered brilliantly by the Southern operators.

Bombardier’s software engineers came over from Sweden and took time from their already crammed schedule to assist. Network Rail helped with train paths and possession management. Drivers were scheduled for nights and the whole exercise was completed in time for start of services each morning over a period of eight weeks.

So, in the background, power engineers have been making huge progress to reclaim energy that has hitherto been lost. The original projection of a 5% saving may well be proved very pessimistic.

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