Overhead Catenary Cables to Charge Electric Lorries.

How would you deal with leakage/unbalance currents to avoid the vehicle metalwork rising above normal earth potential especially if one pantograph lost contact with it's wire? 'Traditional' trolley buses operate at 600-750V DC apparently without problems. Would higher voltages and AC cause bigger problems?

The gap between the wires must be greater than the width of the pantograph which restricts the width of the pick up head and hence the allowable amount of sideways deviation. The trolley bus type solution is only suitable for relatively low speeds.

I did a bit of a 'thought experiment' along these lines a while ago - it was actually thinking about the possibility of re-introducing electric trolley buses but given the increase in congestion on modern roads looking for a way of allowing the bus to connect to the overheads automatically when it was 'in lane' but still being free to move out of lane (and run from small batteries) whenever the need arose. One idea to come out of that was of a 'segmented pantograph' (think something lile the commutator on a universal motor, but perhaps herring-bone shaped to allow smooth transitions of the overhead wire between segments) and two or even three a.c. overhead lines. The lines need only be a few segments apart to prevent shorts (so preventing direct wire-to-wire shorts due to wind movement would more likely be the limit). Each segment would then be connected via a pair of rectifiers (big diodes) - like the first half of a bridge rectifier - to provide unsmoothed d.c. to the vehicle. An automated system would then look to lower the pantograph smartly whenever it was drifting out of lane, and re-connect automatically when the bus was steady under the wires again. By providing a 3-phase supply (or balanced single phase) and by disconnecting the vehicle from the pantograph contacts when anything other than all phases were present, leakage or capacitive coupling currents should mostly cancel out.


  - Andy.

How would you deal with leakage/unbalance currents to avoid the vehicle metalwork rising above normal earth potential especially if one pantograph lost contact with it's wire? 'Traditional' trolley buses operate at 600-750V DC apparently without problems. Would higher voltages and AC cause bigger problems?

The gap between the wires must be greater than the width of the pantograph which restricts the width of the pick up head and hence the allowable amount of sideways deviation. The trolley bus type solution is only suitable for relatively low speeds.

I did a bit of a 'thought experiment' along these lines a while ago - it was actually thinking about the possibility of re-introducing electric trolley buses but given the increase in congestion on modern roads looking for a way of allowing the bus to connect to the overheads automatically when it was 'in lane' but still being free to move out of lane (and run from small batteries) whenever the need arose. One idea to come out of that was of a 'segmented pantograph' (think something lile the commutator on a universal motor, but perhaps herring-bone shaped to allow smooth transitions of the overhead wire between segments) and two or even three a.c. overhead lines. The lines need only be a few segments apart to prevent shorts (so preventing direct wire-to-wire shorts due to wind movement would more likely be the limit). Each segment would then be connected via a pair of rectifiers (big diodes) - like the first half of a bridge rectifier - to provide unsmoothed d.c. to the vehicle. An automated system would then look to lower the pantograph smartly whenever it was drifting out of lane, and re-connect automatically when the bus was steady under the wires again. By providing a 3-phase supply (or balanced single phase) and by disconnecting the vehicle from the pantograph contacts when anything other than all phases were present, leakage or capacitive coupling currents should mostly cancel out.


  - Andy.