3 ms·

The tube already does regenerative braking. This trial is about better regenerative braking. Currently, the trains return power to the track when braking, but

The tube already does regenerative braking. This trial is about better regenerative braking. Currently, the trains return power to the track when braking, but if the track is already at capacity the remainder is dumped via resistors on-board the train. If I understand correctly, this new substation can sense when this is about to happen, take that extra current and feed it through an inverter to supply it back to the power grid. Some more details are here: http://eandt.theiet.org/news/2015/sep/tube-brake-energy.cfm http://eandt.theiet.org/news/2015/sep/tube-brake-energy.cfm

lucaspiller 11y ago
I don't fully understand the physics behind it, but how exactly does can the track be 'at capacity'? Isn't the track just fed from the power grid, so if the demand on energy on the track is lower, won't it just take less energy from the grid?
- sandworm101 11y ago
  It's like the voltage regulator on your car. The car's alternator is pumping out power and the regulator is topping up the car's 12v system (actually 13.9). The regulator doesn't want to create an over-voltage situation and so dumps any extra power to a resistor, turning it into heat. On an electric train, instead of a battery the regulator is pushing power back to the third rail. Once the rails max voltage is reached, when it is at capacity, everything else is turned into heat via resistors on the train.
- Animats 11y ago
  The London Underground uses 630 VDC traction power. Power distribution is 11KV/22KV 50Hz, converted to DC at substations every few stations along the right of way.[1] 630V is too low a voltage for long distance transmission without big losses, so distribution is at a higher voltage. AC to DC conversion is with rectifiers, which are one-way; they can't make AC from DC. With inverters, that's possible. Inverters are basically big switching power supplies. This is just an inverter installation at a substation. It's probably bi-directional, converting AC to DC or DC to AC as required. The whole transit system isn't one big DC circuit; it's in sections of a mile or so. Regenerative braking can only power trains in the same section, unless traction power can be up-converted back into the AC system. If more trains in a section are braking than accelerating, the excess power has to be dumped somewhere, usually into big iron resistance grids that waste it as heat. Using inverters eliminates that energy dump. [1] http://www.trainweb.org/tubeprune/tractioncurr.htm http://www.trainweb.org/tubeprune/tractioncurr.htm

lucaspiller 11y ago

I don't fully understand the physics behind it, but how exactly does can the track be 'at capacity'? Isn't the track just fed from the power grid, so if the demand on energy on the track is lower, won't it just take less energy from the grid?

sandworm101 11y ago
It's like the voltage regulator on your car. The car's alternator is pumping out power and the regulator is topping up the car's 12v system (actually 13.9). The regulator doesn't want to create an over-voltage situation and so dumps any extra power to a resistor, turning it into heat. On an electric train, instead of a battery the regulator is pushing power back to the third rail. Once the rails max voltage is reached, when it is at capacity, everything else is turned into heat via resistors on the train.
Animats 11y ago
The London Underground uses 630 VDC traction power. Power distribution is 11KV/22KV 50Hz, converted to DC at substations every few stations along the right of way.[1] 630V is too low a voltage for long distance transmission without big losses, so distribution is at a higher voltage. AC to DC conversion is with rectifiers, which are one-way; they can't make AC from DC. With inverters, that's possible. Inverters are basically big switching power supplies. This is just an inverter installation at a substation. It's probably bi-directional, converting AC to DC or DC to AC as required. The whole transit system isn't one big DC circuit; it's in sections of a mile or so. Regenerative braking can only power trains in the same section, unless traction power can be up-converted back into the AC system. If more trains in a section are braking than accelerating, the excess power has to be dumped somewhere, usually into big iron resistance grids that waste it as heat. Using inverters eliminates that energy dump. [1] http://www.trainweb.org/tubeprune/tractioncurr.htm http://www.trainweb.org/tubeprune/tractioncurr.htm