04-10-2012, 02:15 PM
SWITCH-GEAR CONTROL-GEAR & RECTIFIER MANUFACTURING DIVISION
SWITCH-GEAR CONTROL.docx (Size: 1.18 MB / Downloads: 476)
INTRODUCTION
This page describes the most recent developments of electric train power equipment including the latest IGBT controlled 3-phase Alternating Current (AC) motors and the new permanent magnet motor.
AC AND DC DIFFERENCES
To understand the principles of modern traction power control systems, it is worth a look at the basics of DC and AC circuitry. DC is direct current - it travels in one direction only along a conductor. AC is alternating current - so called because it changes direction, flowing first one way along the conductor, then the other. It does this very rapidly. The number of times it changes direction per second is called the frequency and is measured in Hertz (Hz). It used to be called cycles per second, in case you've read of this in historical papers. In a diagrammatic representation, the two types of current appear as shown in the diagram above left.
From a transmission point of view, AC is better than DC because it can be distributed at high voltages over a small size conductor wire, whereas DC needs a large, heavy wire or, on many DC railways, an extra rail. DC also needs more frequent feeder substations than AC - the ratio for a railway averages at about 8 to 1. It varies widely from one application to another but this gives a rough idea. See also Electric Traction Pages Power Supplies.
AC LOCOMOTIVES WITH DC DRIVES
This diagram (above) shows a simplified schematic for a 25 kV AC electric locomotive used in the UK from the late 1960s. The 25 kV AC is collected by the pantograph and passed to the transformer. The transformer is needed to step down the voltage to a level which can be managed by the traction motors. The level of current applied to the motors is controlled by a "tap changer", which switches in more sections of the transformer to increase the voltage passing through to the motors. It works in the same way as the resistance controllers used in DC traction, where the resistance contactors are controlled by a camshaft operating under the driver's commands.
Before being passed to the motors, the AC has to be changed to DC by passing it through a rectifier. For the last 30 years, rectifiers have used diodes and their derivatives, the continuing development of which has led to the present, state-of-the-art AC traction systems.
THE THYRISTOR
The thyristor is a development of the diode. It acts like a diode in that it allows current to flow in only one direction but differs from the diode in that it will only permit the current to flow after it has been switched on or "gated". Once it has been gated and the current is flowing, the only way it can be turned off is to send current in the opposite direction. This cancels the original gating command. It's simple to achieve on an AC locomotive because the current switches its direction during each cycle. With this development, controllable rectifiers became possible and tap changers quickly became history. A thyristor controlled version of the 25 kV AC electric locomotive traction system looks like the diagram here on the left.
DYNAMIC BRAKING
Trains equipped with thyristor control can readily use dynamic braking, where the motors become generators and feed the resulting current into an on-board resistance (rheostatic braking) or back into the supply system (regenerative braking). The circuits are reconfigured, usually by a "motor/brake switch" operated by a command from the driver, to allow the thyristors to control the current flow as the motors slow down. An advantage of the thyristor control circuitry is its ability to choose either regenerative or rheostatic braking simply by automatically detecting the state of receptivity of the line. So, when the regenerated voltage across the supply connection filter circuit reaches a preset upper limit, a thyristor fires to divert the current to the on-board resistor.
THE GTO THYRISTOR
By the late 1980s, the thyristor had been developed to a stage where it could be turned off by a control circuit as well as turned on by one. This was the "gate turn off" or GTO thyristor. This meant that the thyristor commutating circuit could be eliminated for DC fed power circuits, a saving on several electronic devices for each circuit. Now thyristors could be turned on and off virtually at will and now a single thyristor could be used to control a DC motor.
It is at this point that the conventional DC motor reached its ultimate state in the railway traction industry. Most systems now being built use AC motors.
AC MOTORS
There are two types of AC motor, synchronous and asynchronous. The synchronous motor has its field coils mounted on the drive shaft and the armature coils in the housing, the inverse of normal practice. The synchronous motor has been used in electric traction - the most well-known application being by the French in their TGV Atlantique train. This used a 25 kV AC supply, rectified to DC and then inverted back to AC for supply to the motor. It was designed before the GTO thyristor had been sufficiently developed for railway use and it used simple thyristors. The advantage for the synchronous motor in this application is that the motor produces the reverse voltages needed to turn off the thyristors. It was a good solution is its day but it was quickly overtaken by the second type of AC motor - the asynchronous motor - when GTO thyristors became available