First a little history. Whilst the UK is currently still in the throes of whether it can afford to electrify the current railway, electrification started in the early 20th century.
DC – Direct Current
AC – Alternating Current
The North Eastern Railway used 600v DC for its Tyneside railway in 1904. Through the years different voltage systems were used by the various private operators on different routes. Bury to Holcombe Brook used 3,500v DC in 1913, the much-missed Woodhead route through the Pennines used the 1, 500v DC system. Finally, post World War 2 and through the BR Modernisation plan, British Rail settled on the 25kV AC system which we use today. Although amazingly, at the time, money and time were still being spent on converting some lines to 1500v DC!
But just how do our electric trains get their electricity? It arrives at the trains via the OHL system and Pantograph.
Starting at the power stations, National Gird produces electricity at 25kV AC and then steps up the voltage to either 400,000V, 275,000V, or 132,000V AC. This supply arrives at the railway feeder stations that are located along the West Coast Main Line, from Euston up to Glasgow, Liverpool, and Manchester. These stations step down the voltage to 25kV AC that the trains use.
The reason the National Grid steps the voltage up for transmission around the Country is to reduce energy losses.
By increasing the voltage, power losses are significantly reduced. The higher the voltages the smaller the conductors that are needed, therefore smaller (cheaper and lighter) cables are required.
The electricity from the different feeder stations to the railway cannot be mixed as this would cause problems with the National Grid (bypassing their switching systems and causing interference). To overcome this, neutral sections are inserted in the overhead lines, between the feeder stations. The neutral section itself can be anything from 100 metres to 400 metres in length.
These sections are also useful for maintenance, where Network Rail can isolate the sections of the OHL that the maintenance teams need to work on instead of having to turn off miles and miles of OHL.
The neutral sections can be distinguished when traveling on the train if you are in coach C or H on the Pendolino. There is a "thud" when the train enters and leaves a neutral section above your head where the ceiling is noticeably lower that the rest of the carriage. You'll also notice the air conditioning switching off. The onboard batteries keep the lights and other systems on.
The "thud" you can hear is what is known as the VCB (Vacuum Circuit Breaker) operating. This piece of equipment sits on the roof near the pantograph. It is a large on/off switch (like your circuit breakers at home). It is designed to remove the power from the train when it enters the neutral section and reinstates power to the train once out of the neutral section.
An example of arcing (a flash or spark) is shown below:
The VCB protects the pantograph and traction equipment from damage when the train has the power supply removed. If this piece of equipment wasn't there, as the train enters a neutral section (a dead section) an arc would be drawn from the live OHL to the pantograph.
This would damage the carbon strips on the pantograph head, the contact wire, and/or the traction systems on the train.
And when the pantograph is dropped (either in the maintenance depot or when switching ends for return journeys), without opening the VCB, as the pantograph drops away from the live contact wire, a stream of very hot plasma is produced as the electricity jumps from the contact wire to the pantograph.
This arcing can also be witnessed in normal operation on a cold day when there is ice on the contact wire. Looking out the window near where the pantograph is, you would see lots of flashing. Ice is a very effective insulator and will disrupt the electricity flowing from the contact wire, thereby causing a lot of sparking (arcing).
This arcing during icy weather will damage the carbon strips on the pantograph at a far greater rate. It is unavoidable but a couple of projects historically were tried to improve the situation.
Historically the first train out on a wintery day was known as the ice breaker and would have a strengthened carbon head fitted to the pantograph to remove the ice. However, the trade-off was that there was increased wear to the contact wire thereby accelerating its replacement requirement. Additionally, there was a project that looked into heating the overhead lines (OHL) but that was deemed to be too expensive (imagine the cost of heating all the copper wire in the UK!).
The location of the VCB on a Pendolino is shown below:
But how does the train know when it is entering a neutral section and when to remove the electric feed from the contact wire?
This is achieved using a magnet on the track and a receiver mounted on the train.
On the Pendolino, there are four receivers, two on each Pantograph car.
The system is known as the APC (Automatic Power Control). An example of an APC receiver that is mounted on the Pendolino train is shown below:
For the system to work with all electric train types that run on the Railway, the positioning of both the receiver and magnet is mandated by Network Rail.
A magnet is placed shortly before the neutral section on the side of the track. As the train approaches the neutral section, the APC receiver detects this and will send a signal to the incoming power circuit breaker to open the VCB to remove the power to the traction system/ancillaries.
As the train exits the neutral section there is another trackside magnet that the APC receiver detects. This sends a signal to the power circuit breaker to close the VCB and restore power.
For this process to work, the system utilises the two poles of a magnet, north and south.
The trackside magnets have the "south" pole facing up. The APC receiver is in "north" pole mode. As the train enters the neutral section the receiver crosses the “south” facing magnet. On detecting the south pole, the receiver switches from "north" to "south" which is the signal for the VCB to open. Once this operation has happened, the receiver reverts to "north" effectively resetting the system to be ready for the next operation.
On leaving the neutral section and the receiver detects the "south" magnet again. The receiver switches from "north" to "south" which is the signal for the VCB close.
After the VCB is closes, power to the train systems is not restored in one hit. Whilst the traction is restored first, the auxiliaries are enabled approximately 10 seconds later in order not to overload the system. If everything came on at once there could be a big inrush of current with the potential to overload/damage the electrical systems.
The device that has the hardest time on the train is the pantograph. It needs to operate in all weathers and speeds and deal with varying heights on the contact wire, all whilst keeping constant contact with the contact wire. The Pendolino's pantograph is shown overleaf.
The pantograph 'head' is shown below. You can see where the two carbon strips are located
The contact wire is installed to a specific tension. And being metal (copper) the tension will vary depending on temperature. The wire will expand when the temperature is hot and vice versa in the cold. To counteract this there are weights installed along the OHL to ensure the correct tension is maintained irrespective of temperature. During very hot weather speeds are reduced to try and stop any damage to the OHL (bring the wires down).
To ensure the pantograph maintains a constant contact with the contact wire the pantograph applies a specific force. This force is maintained at around 91 Nm.
An added complication for the Pendolino fleet is the tilt function. The pantograph must remain level, irrespective of the tilt angle. A clever system moves the pantograph in the opposite direction to the tilt.
A close-up of the system on the Pendolino is shown below:
Through the trains' tilting system, a signal is sent to the electric motor that then operates the belt drive to move the assembly left or right. The springs help in the positioning of the pantograph.
The frame sits on the wheels that are shown in the photograph and moves along the curved track, left and right when the train is tilting.
Hopefully, that gives you a good overview of how the electricity gets to the train, what happens (and why we have) neutral sections. Also in how we keep the pantograph level on a tilting train.
