White City, pictured during an Engineering Possession.
These notes on London's Underground Railways are taken from training material prepared by Ford Electronics Limited, with permission. Ford Electronics produce Tunnel Telephone systems for use on underground railways. This description applies to most lines, but there are local differences.
Introduction
At turnouts, crossings, expansion gaps, isolator switches and section gaps the conductor rails are broken. Ramps at the start and end of conductor rail section lift the train collector shoes onto the rail or lower the shoes from the rail. The length of the gap depends upon the track feature. At turnouts and crossings, the gap is variable. Expansion gaps are normally 1 metre, provided every 246 metres on the surface and every 606 metres underground. Isolator switch gaps are normally 1 metre and Section gaps 15 metres.
The outer conductor rail is normally about 450 volts positive with respect to earth and the inner conductor rail in between the running rails is normally about 180 volts negative with respect to earth. The d.c. power is not directly earthed and the actual voltage to earth on each conductor rail will depend upon the insulation resistance. During wet weather on exposed lines, around 300 volts on each conductor rail is possible. Intentional indirect earthing through Bleed Resistors (see below) is provided at certain locations to establish the conductor rail to earth voltages during non-leaky conditions.
Source of traction power
Originally, London Underground produced its own power at generating stations like Lots Road, which no longer exists. Nowadays, power is taken from the National Grid at a number of sites at either 11kV or 22kV 50Hz, but London Underground currently retains the capability to generate power at a peak-lopping gas turbine installation at Greenwich, which is only used as emergency supply if the grid fails. As necessary, this high voltage a.c. power is distributed to Traction Sub Stations. To provide the necessary supply security Traction Sub Stations frequently have alternative sources, usually at 11kV 50Hz.
Traction Sub Stations
Transmission losses at 630 volts d.c. are relatively high so, to avoid excessive voltage drop, Traction Sub Stations (TSSs) have to be located quite close together. Originally, Traction Sub Stations were staffed but they are now remotely controlled from a power control room at a central London site.
At each Traction Sub Station, the incoming three-phase high voltage supply is transformed down and rectified (using fan-cooled semiconductor rectifiers) to provide the earth-free d.c. supply for the conductor rails. A typical rating for a single rectifier unit is 1500kW. Substations are normally provided with two or more rectifiers. Often, one rectifier will power the two roads in one direction, a second will power the two roads in the other direction. Circuit breakers allow individual roads to be discharged.
The conductor rails are divided into sections extending from one Traction Sub Station to the next. Each section is double-end fed with d.c. from rectifiers at both Traction Sub Stations, to further minimise voltage drop, particularly when more than one train is in a section. A simplified diagram of the arrangement is shown in Figure 2.1 below.
The practical arrangement of a typical Traction Sub Station with two rectifiers is illustrated in Figure 2.2 below.
The substation has two d.c. busbars linked or isolated by a coupling breaker. Each rectifier and each road supplied is associated with a circuit breaker. At most TSSs, the coupling breaker is normally closed so that both rectifiers and all four roads are connected together to minimise voltage drop.
Track Paralleling Huts
At Track Paralleling Huts, the conductor rails are broken to form a section gap, but normally contactors are closed to connect together all the positive conductor rails and all the negative conductor rails. Again, the aim is to reduce the voltage drop as a number of rectifiers can contribute current to each section. The arrangement is shown in Figure 2.3 below.
Sectionalisation
As described above, at most Traction Sub Stations the coupling breaker is normally closed so that the conductor rails are effectively continuous from section to section. Since the conductor rails are not directly earthed, an earth fault from one side of the supply to earth (for instance, on a traction motor or collector shoes) does not prevent the system from operating. But a second earth fault on the other side of the supply will cause the overcurrent protection to operate. If all the coupling breakers were closed, a second earth fault could propagate a shutdown along the whole length of the line, involving a large number of passengers.
To limit this fault propagation, London Underground introduced a technique called 'Sectionalisation' where a number of traction sections are connected together (to help minimise voltage drop) but the coupling breakers are intentionally open at the Traction Sub Stations defining the ends of that Sectionalisation section.
Bleed Resistors
In the absence of leakage currents to earth from the conductor rails, the voltage of each conductor rail with respect to earth is defined by a potential divider of bleed resistors added at each end of the Sectionalisation Section from each conductor rail to earth. The conventional values are 220 ohms (positive rail) and 110 ohms (negative rail).
Related articles in this Blog
Tunnel Telephone System
In tunnel sections of the system, a Tunnel Telephone system is provided to facilitate traction discharge and voice communication between the driver and Line Controller. Traction trip and speech is provided over two bare copper alloy wires carried, one above the other, on pairs of porcelain insulators supported on metal brackets fixed to the tunnel wall, usually on the right-hand side in the normal direction of travel. The insulators are generally provided every 6 metres along the tunnel so as to keep the wires about 115mm apart. The wires are positioned so as to be accessible by leaning from the driving cab window of a train. The general appearance of a single-bore deep tunnel is shown in Figure 2.4 below.
Where it is necessary for the tunnel wires to be carried across to the opposite tunnel wall, Over Track Crossings (OTX) are provided using cable. Where different sections of tunnel wire need to be interconnected, or equipment introduced, cabling is used and test boxes with sliding disconnection links may be provided to facilitate fault-finding.
All my posts on London Underground can be found here.
This topic is continued at London Underground - Traction Power Distribution (2).
[Bleed resistor values transposed, link to part (2) added 11-Apr-2021]
