Thyristors in Railway Traction How can their effects be measured?
Thyristors in Railway Traction How can their effects be measured?
Thyristors in Railway Traction How can their effects be measured?
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Introduction<br />
Although there are many different<br />
standard voltages and several different<br />
frequencies <strong>in</strong> use for electric<br />
railway traction supplies, all such<br />
systems <strong>be</strong>long to one of two broad<br />
groups — AC or DC.<br />
The earliest electrifications used<br />
DC <strong>be</strong>cause it was cheap and simple<br />
to construct electric motors<br />
which would run directly off the<br />
l<strong>in</strong>eside supply. DC is still widely<br />
used for<br />
railways.<br />
suburban and commuter<br />
absorbs signifi<strong>can</strong>t amounts of en- sier to use <strong>in</strong> AC traction systems,<br />
For railway electrification over ergy and generates a good deal of where the alternations of the supply<br />
long distances, AC is preferred for heat, especially on commuter tra<strong>in</strong>s ensure that it switches off.<br />
the same reason as it is preferred subjected to frequent stopp<strong>in</strong>g and<br />
for the public electricity supply — it start<strong>in</strong>g. Alongside the track are laid<br />
may <strong>be</strong> transmitted at a high vol- cables carry<strong>in</strong>g telephone circuits,<br />
tage (and low current) — and trans- <strong>Thyristors</strong> offer the promise of re- signall<strong>in</strong>g circuits and tra<strong>in</strong> control<br />
formed at the po<strong>in</strong>t of use to a low duced weight, reduced ma<strong>in</strong>tenance circuits. It is a feature of electrified<br />
voltage (and high current). and reduced cost — especially on railways that there is a need to lay<br />
AC systems. Additionally, there is light-current cables for long dis-<br />
It is a feature of electric motors considerable scope for reduc<strong>in</strong>g the tances <strong>be</strong>side electric power cables,<br />
used for railway propulsion that the energy consumption of tra<strong>in</strong>s, both and immunisation aga<strong>in</strong>st <strong>in</strong>duced<br />
voltage applied to the mach<strong>in</strong>e must by elim<strong>in</strong>at<strong>in</strong>g the resistances and currents and voltage <strong>in</strong> the light-cur<strong>be</strong><br />
controlled over quite a wide by permitt<strong>in</strong>g k<strong>in</strong>etic energy from a rent cable is established practice.<br />
range — typically 30:1 — to accom- tra<strong>in</strong> slow<strong>in</strong>g down to <strong>be</strong> chan- <strong>How</strong>ever, thyristor traction usually<br />
modate the slowly chang<strong>in</strong>g speed nelled back to the l<strong>in</strong>eside electric- comes as replacement equipment to<br />
of the tra<strong>in</strong>, and this is where thyris- ity supply for use by the other tra<strong>in</strong>s an exist<strong>in</strong>g railway, with all the<br />
tors are attractive. (regenerative brak<strong>in</strong>g). light-current circuits immunised<br />
Established practice on DC sys- These <strong>be</strong>nefits, which as we shall<br />
only for conventional traction units,<br />
for which the problems are much<br />
terns is to <strong>in</strong>sert variable resist- see have to <strong>be</strong> paid for, arise by vir- less severe.<br />
ances <strong>in</strong> the motor circuits, and re- tue of the property of the thyristor<br />
group motors <strong>in</strong> series and parallel of <strong>be</strong><strong>in</strong>g able to switch very large It is therefore very important at<br />
to achieve this control. On AC sys- currents very rapidly with no mov- an early stage to measure and analterns,<br />
motor voltage may <strong>be</strong> varied <strong>in</strong>g parts. Both on AC and DC sys- yse the audio frequency currents <strong>in</strong>by<br />
select<strong>in</strong>g taps on the trans- terns it controls the traction motor jected by a thyristor traction equipformer.<br />
Both these techniques re- voltage by act<strong>in</strong>g as a variable mark- ment <strong>in</strong>to its supply. To a limited<br />
quire heavy-current switch<strong>in</strong>g-con- space-ratio device, switch<strong>in</strong>g on extent this may <strong>be</strong> done on the testtacts<br />
which must <strong>be</strong> very heavy and off many times per second. It is <strong>be</strong>d. <strong>How</strong>ever, the real test comes<br />
and robust to avoid frequent failure much easier to switch on than off, when the equipment gets on to a<br />
and ma<strong>in</strong>tenance. Series resistance and so thyristors are somewhat ea- tra<strong>in</strong> and out on the track, <strong>be</strong>cause<br />
a traction supply, with all its resonances<br />
and frequency-dependant<br />
characteristics, <strong>can</strong> <strong>in</strong>fluence considerably<br />
the distribution of harmonics.<br />
The ultimate test is the level of<br />
harmonic currents <strong>in</strong>duced <strong>in</strong> the<br />
light-current circuits. Neither of<br />
these phenomena <strong>can</strong> <strong>be</strong> realistically<br />
exam<strong>in</strong>ed on the test <strong>be</strong>d<br />
alone. At the same time, track trials<br />
are difficult and costly to organise,<br />
usually call<strong>in</strong>g for full occupation of<br />
a busy railway, so it is essential<br />
that all the right <strong>in</strong>formation is obta<strong>in</strong>ed<br />
first time round. The methods<br />
descri<strong>be</strong>d <strong>in</strong> the follow<strong>in</strong>g <strong>in</strong>di-<br />
lllustration of the <strong>effects</strong> of a tra<strong>in</strong> with AC thyristor traction control equipment on track cir- Cate how this may <strong>be</strong> achieved.<br />
cuits and telecomms circuits<br />
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