Railroad Forums 

Discussion related to commuter rail and rapid transit operations in the Chicago area including the South Shore Line, Metra Rail, and Chicago Transit Authority.

Moderators: metraRI, JamesT4

 #1002242  by Tadman
 
I once read that the NICTD EMUs are capable of regenerative braking, but that energy is dissipated as heat much the same way a diesel does, because the substations cannot handle the backfeed of current. Is this true? What does it take to upgrade the substations to handle such operation? And does this really contribute to lowering the power bill? I know the MILW took good advantage of regenerative braking out west to contribute power to their system from downhill trains. Similarly, the stop/go nature of commuter indicates to me that such would be useful to NICTD and Metra Electric.

Thoughts?
 #1002564  by Nasadowsk
 
Historically, most DC substations could not accept regeneration (the MILW was an odd exception, I don't know technically how they did it). Basically, most recent substations are 12 (or so) pulse rectifiers, and incapable of going bidirectional.

Yes, there really is an advantage to regen - I've heard that the regen into the HEP that NJT's MU fleet does gave them a pretty solid 5 - 7 % drop in overall power use, and I know this is now a standard feature among their locomotives, too (the ALP-46 has NO braking resistors at all). Supposedly Amtrak's next electrics won't have braking resistors, preferring to regenerate into the HEP and /or catenary.

Some DC systems, I think I've heard it claimed of the NYC subway, will attempt to regen into the catenary/third rail, but it'll only be accepted if there's another train on the circuit.

With AC, the only precaution is that the local utility doesn't mind (!) and the substation's protective logic is set up to allow it. It's a lot harder with DC...
 #1002567  by Nasadowsk
 
Oh, yes, I wouldn't be at all surprised if the fleet can regenerate into the head end power, i.e. and MU that's coasting or braking is drawing *no* power whatsoever. It's really easy to do that even on a DC system, and it saves a little bit on the power. Most AC motored DC fed stuff can do this, even if it can't backfeed into the third rail/catenary....
 #1002580  by polybalt
 
All new rapid transit systems allow regeneration into the (DC) supply (trolley or third rail) but do not attempt to convert it back to AC in the substation. This typical regeneration is very effective in reducing power costs because there is almost always a train relatively near to use the current. The only issue is, that to make this system effective, the maximum supply voltage must be allowed to go above that provided by the substation. So on a 750v system, cars are allowed to regenerate power into the supply until the voltage in the supply rises to 900v.

This is only a problem on older systems which can't deal with the higher voltages, either on the cars or on the wayside. For example older systems had strings of light bulbs in series right across the supply voltage. Letting the voltage increase 15% could cause those bulbs to burn out much more quickly. The substations have a simple diode in the feed to the trolley, which prevents backfeed into the substation itself. When I lived along the IC Electric 40 years ago, I think the station lighting may well have been off the trolley voltage? Maybe something like that is the issue on the ME?

BTW as far as I know all new electric railway rolling stock now uses AC motors. (I think the CTA was the last holdout for DC motors in the world) They can all be set up to feed DC power back into the supply, and usually are.
 #1002607  by Tadman
 
Most metra electric cars are still the 1970's cars with straight DC motors. I believe all South Shore cars have been modernized to AC motors.
 #1002954  by byte
 
CTA high-performance stock does not utilize regenerative braking. The new 5000s do, however.
 #1006663  by R36 Combine Coach
 
polybalt wrote:BTW as far as I know all new electric railway rolling stock now uses AC motors. (I think the CTA was the last holdout for DC motors in the world) They can all be set up to feed DC power back into the supply, and usually are.
The CTA 3200s and Metro-North/ConnDOT M-6s were the last DC traction cars built, therefore making 1994 the end of DC traction in North America. GE however still offered the CCM (Computerized Cam Magnetic) propulsion package as an aftermarket installation afterwards.