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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Frequency analysis<br />
of thyristor traction<br />
One of the most important data reduction<br />
processes which an<br />
eng<strong>in</strong>eer engaged <strong>in</strong> commission<strong>in</strong>g<br />
or evaluat<strong>in</strong>g new thyristor controlled<br />
roll<strong>in</strong>g-stock is called upon to<br />
undertake is the analysis of the frequency<br />
content of the traction current<br />
or the <strong>in</strong>duced voltages <strong>in</strong> lightcurrent<br />
circuits. The audio-frequencies<br />
which are likely to <strong>be</strong> encountered<br />
<strong>in</strong> the traction supply are as<br />
follows.<br />
On AC electrified railways:<br />
1. the supply frequency and its exact<br />
harmonics, predom<strong>in</strong>antly<br />
odd harmonics<br />
2. slow transients conta<strong>in</strong><strong>in</strong>g a<br />
wide range of frequencies and<br />
some even harmonics of the supply,<br />
caused by traction transformer<br />
<strong>in</strong>rush or wheelslip correction<br />
On DC electrified railways:<br />
1. the chopp<strong>in</strong>g frequency, usually<br />
a few hundred Hz, and its exact<br />
harmonics, predom<strong>in</strong>antly odd<br />
2. the <strong>in</strong>dustrial frequency (50 to<br />
60 Hz) and its harmonics, ma<strong>in</strong>ly<br />
the 3rd, 6th and 12th, result<strong>in</strong>g<br />
from the substation rectifier used<br />
to provide the DC traction supply<br />
3. slow transients conta<strong>in</strong><strong>in</strong>g a<br />
spread of frequencies caused by<br />
l<strong>in</strong>e filter <strong>in</strong>rush at the clos<strong>in</strong>g of<br />
the circuit breakers on the tra<strong>in</strong>,<br />
or wheelslip correction<br />
Arrangement of equipment for plott<strong>in</strong>g spectra of L<strong>in</strong>e Current<br />
As an AC thyristor locomotive accelerates, the level of each harmonic fluctuates cyclically. The<br />
upper figure shows idealized plots, on a l<strong>in</strong>ear amplitude scale, of the levels of the first four odd<br />
harmonics as functions of phase advance, for a s<strong>in</strong>gle rectifier bridge (n = harmonic num<strong>be</strong>r).<br />
The lower figure shows the actual <strong>be</strong>haviour of eight selected harmonics, recorded as functions<br />
of armature voltage, on a logarithmic (dB) amplitude scale, for a two-bridge system, us<strong>in</strong>g a<br />
B & K Type 2031 Narrow Band Analyzer and a Type 2308 X—Y Recorder<br />
On both k<strong>in</strong>ds of electrification systems,<br />
some operators have vehicles<br />
propelled by commutatorless motors<br />
fed withcont<strong>in</strong>uously variable frequencies,<br />
usually obta<strong>in</strong>ed from the<br />
traction supply by thyristor <strong>in</strong>verters.<br />
These equipments <strong>can</strong> <strong>in</strong>ject<br />
currents <strong>in</strong>to the supply at harmonics<br />
of the <strong>in</strong>verter frequency, usu<br />
ally the 6th and its multiples.<br />
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