Hi, I know that on a railway system with power supply to the 3rd rail, when say the train goes over a crossing even though the trains power supply pick up shoe loses contact with the rail it doesnt matter as there is more than one shoe on the train so it always gets power, so no risk of losing all power if it stops on the crossing (unlikely).And also it doesnt matter if one shoes on one power supply section and another on the section behind as its all 750V DC (im in England down south). And the 2 power supplies cant conflict as its all smooth DC.
But what about the same problem on a 25Kv AC 50Hz overhead power line on most other parts of UK rail track? If the gap between 2 sections is too big and the train stops in the dead zone it will become stuck and if you put 2 pickups on the pantograph to get around this, then there could be times where one pickups on one overhead power supply section, and the other pickup on another section, now because its AC this time, there is bound to be times where one section of overhead line is out of phase with a different section, as it all ones off the national grid which is 3 phase and the phases need to be balanced out with say the yellow phase feeding maybe every 3rd section of the overhead line and so on for the other 2 phases.
surely either the train will at times pick up power from both sections on the changeover from one section to the next and hence have 2 out of phase AC currents leading to a different overall voltage, and designing the system another way with a big gap between sections and only one pickup on the pantograph there will be dead sections which will be hard to get over if the trains unlucky and stops on them.
How do they get around this problem? 25Kv's good for long distance lines as theres less power losses at high voltages over long distance.
cheers.
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