20 TRACTION TRANSFORMERS

 TRACTION TRANSFORMERS

Traction transformers are used to provide single phase supplies for train overhead catenary pick-up systems and, 

since the late 1950s in the UK, these have operated at a nominal voltage of 25 kV AC. 

These transformers have ratings which vary between about 5 and 18 MVA and 

were initially ONAN cooled with impedances of between 8 per cent for the smallest units up to 12 per cent

for the largest. 

Later units were provided with mixed cooling, typically ONAN/OFAN/OFAF to give ratings as, for example, 18/20.5/26.5 MVA. 

In addition, all sizes of transformers are required to have the capability to provide a cyclic output of 133.3 per cent of rated load in an ambient temperature of 30ºC for 8 hours followed by 16 hours at 60 per cent load.

This is to cater for the situation of an

outage of an adjacent unit. The transformers normally take their supply from two phases of the 132 or 275 kV transmission networks and feeder stations are located adjacent to the rail tracks at 40 or 50 km intervals.

This section describes the design and constructional features of the transformers used to provide traction supplies in the UK, but the general principles are applicable to transformers used to provide these supplies in many parts of the world.

The duty of traction supplies transformers is a particularly onerous one in that, although their loading may be only intermittent, they are subjected to rapid and repeated load current fluctuations taking them from zero to twice full-load current and with an incidence of system short-circuits which may be as high as 250 per year of varying magnitude up to full fault current. 

In terms of the electromechanical stresses applied to the transformer windings, this duty is very similar to that described for arc furnace transformers in the previous section and hence, the constructional features of the windings and their clamping arrangements are also very similar to those of arc furnace transformers.

Traction transformers, from the earliest times of the use of AC supplies, have had to withstand the additional duty resulting from the high harmonic content of the load current. 

This is a problem common to all transformers supplying rectifier loads.

HVDC converters as well as ordinary rectifier transformers.

With the most recent AC traction systems utilising Insulated Gate Bi-polar.

Transistor (IGBT) drive mechanisms, the nature of the harmonics problem

has become somewhat different.

The IGBT system is switched at a high frequency, 4 kHz in some systems used in the UK, so that the harmonic frequencies appearing are in the 70th to 80th range. 

These harmonics unfortunately excite particularly severe harmonic voltages on the catenary system, typically 5 kV at 4 kHz, so the LV windings of the traction supplies transformers must be designed to withstand these voltages superimposed onto their normal

working voltage.