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Tony S

Rail transformers

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Tony S

Power distribution is predominantly three phase, railways can use three phase for low speed lines but it makes for complicated pick up gear. Switzerland is about the only place it is used for low speed mountain railways.








You can see from the photograph the complicated arrangement of pickup wiring and pantographs to collect two phases, the third phase being the grounded track. 





A single OH wire makes life simpler for the rail operators but presents a problem for the supply companies.





Given that modern single phase 25kV electric locomotive can under full load draw 5MW some way has to be found to spread the load across the three phases. Various arrangements can be used.












Volt drop is a major problem as a train moves further away from the feed in point, to counter this an early method was the boost transformer. This relies on a secondary return line in parallel with the track return. Any voltage difference is corrected up to a point by boost transformers. You can tell the lines using this system by a single cable on small insulators strung between gantries. Volt drop of up to 20% can be experienced by the locomotive.





The west coast line uses autotransformers in a different manner. Power is taken from a lightly loaded OH catenary to feed the other more heavily loaded catenary. LeBlanc and Scott transformers are ideal for this type of line due to the 10.4kV difference between catenaries.






At some point there will be a break between feed in substations.





Using boost transformers average losses are 2.4%, auto transformers 1.8%. Compare with the third rail 650V DC system with losses of 15%, 25kV wins hands down.

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