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W.B. Johnston

Engineering Manager, Hawker Siddeley Rail Projects Ltd

It is unlikely for an electrified railway system with such diverse elements as traction, power supply, signalling and telecommunications that there will be a "perfect" system. What may show tremendous advantages to the traction engineer may well create a situation within the system which is completely unacceptable to his sign alling, telecommunication or power supply counterpart. The introduction of power electronics for traction drives about 15 years -ago brought about new possibilities for energy efficient drives. Modern semiconductor technology gives the traction engineer the possibility of achieving, on an a.c. electrified railway, what is for him an ideal mechanism: a fast acting, notchless control of tractive effort. The control of tractive effort could be achieved by a single bridge (either fully or half controlled) operating over the full secondary  voltage of the transformer. However, engineers from other disciplines may well object to the high magnitude of harmonic currents, poor power factor and gross supply voltage distortion.

Increasing the number of series bridges to two, four or even more may show advantages to power supply, telecommunication and signal engineers, but from a financial and spatial consideration it is not an attractive solution for the traction engineer. It is therefore essential that the design and interface parameters are co-ordinated to ensure the compatibility of each element in optimising the system design, taking into account not only the financial factors involved, but also the important, though less easily quantified factor of practicability.

The problems of compatibility are not unique to the NIMT Project but are similar to those of an a.c. electrified railway which has signalling and telecommunication networks either directly linked or in close proximity to the running rails and obtains power from a catenary system which is coupled via a transformer to a high voltage grid network.