Why can the regenerative brakes of the Oslo Metro only share energy with other trains if they are "nearby"?
Is the resistance in the wires along the track making it not worth it?
That will be one factor. The article states that each set has 12 x 140 kW motors giving a total of 1680 kW (1.68 MW) for each train set. The system is 750 V DC and, unusually, uses third-rail in some sections and overhead lines in others. At those power levels currents in the order of 2000 A will be involved so line resistance certainly becomes an issue. Line resistance may also be a factor in circuit-breaker operation and trip times and place further constraints on the maximum length of a section.
Another factor to remember is that the power stations (basically transformers / rectifiers / filters and circuit-breakers) will be spread out along the line with sectional isolators between each power station. In this case the current can't flow from one section to the next. I suspect that this is the real reason for the "nearby" constraint.
Couldn't the energy be fed back into the grid instead?
It could, but it would require inverters to convert DC to AC and these wouldn't be cheap at those power levels and the duty cycle (the amount of regeneration time involved) may not make them worthwhile.
Additional information.
- OS MX3000.
Acceleration in the range 0 to 40 kilometers per hour (0 to 25 mph) is limited to 1.3 meters per second squared (4.3 ft/s2). In this phase, the fully loaded train uses 5.0 kiloampere.
So, 5000 A max current per train. I can't find any resistance tables for steel rails so I can't provide an estimate of the voltage drop per km.
For obvious reasons, any railway network is divided into isolated sections and each of those is powered separately from the medium or high voltage grid through its own transformer, circuit breaker and switch.
Two trains within the same section can share power directly. Trains in different sections can only do so through the grid. Since the Oslo Metro uses DC and rectifiers are usually one-way, power sharing through the grid is not available and therefore limited to trains within the same section.
The image below shows a section isolator in an AC overhead line. The sections are powered by different phases of the three-phase high voltage grid for load balancing.
image source
Electric railway guy here.
Long distance propagation
I have seen 600V trolley wire dip to only 200V four miles from the substation under heavy ~300A load from a single articulated car. (4/0 wire, 107 mm2, rails as return).
Third rails are a great deal beefier, but subway trains are a great deal heavier. Typically third rail shoes are fused at 400 amps (per shoe, and not every shoe is in contact at once) with as many as 8 cars. Oslo runs big articulated cars that are electrically 3 cars.
If the regenerated electricity passes a substation, it's even more at disadvantage.
I mean the subway train could push its regenerated power any distance if it's willing or able to increase voltage without limit. Unregulated, DC motor regen can act like an old, inductive constant-current source, increasing voltage until current flows. Burning up much of it in transmission losses would be fine, it's "free energy". However it hits limits of a) onboard equipment (not least, insulation strength in motors), and b) the third rail. BART aimed to have a 1000 volt third rail, but found the worst case scenario of rain on brake dust caused spectacular flash-overs even in their temperate climate. They backed down to 900 volts but it is still troublesome. Oslo is already at 750, not much headroom.
Really, to regenerate productively, there needs to be a train nearby already pulling the voltage down and able to gobble up those amps.
Regen onto grid
This is hard, not least because a couple megawatts of power injected for a few seconds isn't all that useful to the grid.
Also, DC-AC regen itself is hard, with large silicon inverters required at every substation.
In the Golden Age, rotary converters were perfectly capable of efficient DC-AC regen (in fact, they had circuits to prevent accidental regen, e.g. a substation's local grid having a brownout, causing it to be backfed from another substation via the trolley wire). Electric railways had more of their own AC power distribution. And third rail voltage was only 600V, so more headroom. However, the cars were not capable of it: subway trains were very simple back then, with only 7-12 wires on the inter-car control lines.
Rotary converters were abolished just as soon as mercury-arc rectifiers became available, and even those were gone by the time of the first regen cars.
I don't expect any resurgence in rotary converters (more's the pity, since they are dog simple, actually correct power factor in the local grid, and may be competitive since they are simple). So it comes down to complex, large inverters. Given the limited financial gain from selling power back, only very advanced (high R&D) systems like BART are dipping their toes into grid regen from DC.