Thyristors in Railway Traction How can their effects be measured?

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An ideal method of predicting reliably the effects of a new thyristor equipment still on the test bed (especially on AC electrified railways) uses the 2031 Narrow Band Analyzer. The technique consists in going out and recording both the traction supply current (at the train) of an existing traction unit — not necessarily a thyristor vehicle — and the corresponding induced signal in the associated light-current trackside circuits. Recordings should be made at all the locations of interest, including at least one with the vehicle close to the substation (where its behaviour will be similar to testbed behaviour when it comes to harmonics). Back in the laboratory, the recordings are fed into a 2031 Narrow Band Analyzer, which is used to evaluate and display differencespectra between the traction current and induced signal. These spectra are in effect transfer-functions of attenuation, from catenary this with advantage. The next step each location by the new thyristor to S & T circuit, against frequency. is to record and analyze the traction traction unit. Since all the trackside components current of the new thyristor equip- are strictly linear, the spectra re- ment under test on the test-bed. The reason this method is more present the absolute characteristics This analysis will produce quite a suited to AC electrification applicaof the electrification system at good picture of the "harmonic signa- tions is that conventional diode rollthose locations, independent of the ture" of the equipment — and by ing-stock generates most of the hartype of vehicle used. The spectra subtracting from it the difference- monies likely to occur, but at differmay be plotted very quickly on the spectra obtained previously, either ent level from thyristor equipment. Type 2308 X-Y recorder for subse- visually or digitally (with the help of The 2031 Analyzer has a 70 dB dyquent use. If a desktop calculator the calculator), it is possible to see namic range (with a 9dB overload with a reasonable size store is avail- what level of each harmonic might margin), so it can readily display the able, the spectra may be fed into be induced in the S & T circuits at amplitudes even of very weak harmonics. It might appear that if the signal to be analysed has been recorded on a tape-recorder with 44dB dynamic range (i.e. the Type 7003), most of this benefit is lost. However, the noise on the tape is mainly spread out uniformly over the 12 kHz of its bandwidth, and if we effectively pass this through a narrow-band filter — as the 2031 does when it performs harmonic analysis — the noise level in any given narrow band falls dramatically, making it entirely possible to extract individual harmonics 30 dB (one-thirtieth) below the level of broadband noise. Example of primary current waveform and psophometrically-weighted frequency spectrum, recorded just after the start of conduction of the second rectifier bridge 14
Log the tape recordings A tape recorder always records in real time, and a sensible and systematic commissioning team who record all the tests they carry out, on the test-bed and on the track, will end up with a lot of tape. To remain useful over a period of months or years, this tape must be systematically logged — and on the basis that one picture is worth a thousand words, it is much more useful to produce a sequence of plots of what is going on on each tape, than to write out a long description on the tape box. Briiel & Kjaer manufacture graphic hard-copy units for both of the principal duties likely to be encountered in this kind of work. For timevarying functions such as traction current, train speed, and psophometric current, three level recorders are available, Types 2306, 2307 and 2309. For plotting two related variables such as harmonic amplitude against harmonic frequency, or . . Extracts from tape recording of thyristor locomotive trial, logged using a Type 2307 Level Repsophometric current against corder. The upper trace records RMS pantograph current over a period of about 2 minutes, and Speed, an X-Y recorder, lype ^oUo, the lower trace records current in one armature during the same period. Note the application of is available. The latter is also better electric rheostattc braking (zero pantograph current) at the end of the record (because of its paper size) for making permanent records of time-waveforms, for subsequent comparison. the train, these pulses may be used Type 2306: To some extent, of course, either to regulate the paper throw in pro- * Fully portable battery operation, kind of instrument can be used for portion to distance travelled along from built-in dry or rechargeable both purposes. the track, instead of logging the sig- NiCd cells, or operation from senal at a uniform time rate. parate mains supply such as The advantage of the B&K level Type 2808 or 6—12V DC recorders is that they may be used All the B&K level recorders write 500mA max. for recording DC or AC signals, with disposable fibre pens on contin- * Weight only 3,5 kg (7,7 lb), disince they all incorporate true RMS uous calibrated paper from a roll. mensions only 180 x 245 x detectors to convert an incoming The Types 2306 and 2307 also of- 1 10 mm (7,1 x 9,6 x 4,3 in) AC signal into a pen deflection. The fer the option of writing with sap- * Built-in meter to monitor supply 2307 also has peak and average de- phire styli on wax-coated paper. condition tectors as alternatives. All recorders These techniques result in high-con- * 8 fixed paper speeds 0,01 to also provide for logarithmic record- trast, instantaneous records requir- 30 mm/s, plus external TTL ing to give constant resolution over ing no subsequent processing. pulse train or voltage ramp (usthe whole dynamic range. If linear Other features of the recorders are ing Type WB 0228 paper drive recording is required of AC signals set out below. unit) (such as traction current on AC e l e c - * 4 writing speeds 1 6 mm/s to trified equipment), it is necessary t o 2 5 0 mm/s use an external measuring i n s t r u - * Can interface with Type 4426 ment (such as the psophometer i n N o i s e Level Analyzer for, e.g. staits unweighted mode) in c o n j u n c - t i s t i c a l analysis of telephone intion with the Types 2306 a n d t e r f e r e n c e 2 3 0 9 . * Remote paper stop/start The Type 2306 and 2309 have their paper throw controlled by stepping motors. If speedometer or tachometer pulses are available on Type 2306 Portable Level Recorder 15
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Thyristors in Railway Traction How can their effects be measured?

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