A couple of comments on BART.



BART was built using the standard General Electric 1200 VDC thrid rail technology. The substation voltage was 1000 volts so the regenrative power from one train would travel to another accelerating train. The regenerated voltage would go to almost 1200 volts. Train traffic has become so heavy on BART that the lower thrid rail voltage is no longer needed, as there is always an accelerating train close by to make use of any regenrated power. The substation output voltage is has been rased over the years to increase capacity so that is now 1200 volts all the time.

The comment on the BART car wire may fall into the catagory of urban legend. BART cars use 2000 volt wire on the 1000 volt circuits. I beleive this is a standard voltage rating for transit car wire. When cars are being built or rebuilt for BART, wire is used in such large quantities that it is made to order. The contracts for this wire is highly competitive. BART has never used diesel locomotive cable as it does not exhibit the flame and smoke properties required at BART for subway operation. BART cars do not have any conduit on them. The 1000 volt power wires are run in cleat blocks aganst the under side of the floor. Lower voltage wires are all run in cable trays.



The transbay tube is not the longest gap between substations, but it is the heaviest load and adequate traction power in the transbay tube is alway a problem. Storage devices that would fit in the transbay tube, such as batteries, flywheels and super cooled magnets have been investigated, but none have ever been installed.

BART runs its own 33,000 volt AC distrubution system to all the traction power substations. Now it also takes some feeds at 115,0000 VAC. There is at least a dual feed to every substation. There were no traction power substations knocked out by the Loma Priata earthquake. The station AC power is taken off feeds from local utilities. In the earthquake, the power was out in downtown San Francisco, so the subway stations were dark, the escalators did not work, and after a few days the train control went dead because the batteries were not getting charged. The trains continued to run, but they did not stop at stations that did not have power.

I'd like to add a little to what Steve posted, in particular his last paragraph. With motors today able to run on commercial frequencies (i.e. 60 hertz here, 50 overseas), there is no reason to use lower frequencies in new construction, such as from New Haven to Boston. The original PRR/RDG/NH electrification is from the time when 11K, 25 cycle (Yes, I'm an old fart-proud of it, too-) was one of the standard ways of doing electrification. To convert that system today would cost mega bucks. The PRR was electrified in the depression with, IIRC, lots of government money; the very same government that says its broke today.

There are several great points about new commercial frequency construction. You do not need any really specialized substation equipment, such as rotary converters to change AC to DC and/or to change frequencies. Relatively simple transformers and breakers can do the job. The local utility can provide power at just about any location on your line.

The construction of overhead also tends to by much lighter. The higher the voltage, the smaller contact wire size you can use. Look at the PRR and NH construction, then look at some of the later US and foriegn installations, the difference if really apparent. Also, the older 11k, 25 cycle lines had their own higher voltage distribution at the top of the cat poles. With commercial frequency, those lines are no longer necessary; again, the local utility comes into play.

While there are certainly large motors that can run on 60/50 Hz, they would be induction motors. These would not be suitable to use as traction motors while running directly on commercial frequency due to their single speed nature. I don't know if there would now be large brush/commutator motors (capable of variable speeds) suitable for 60/50 Hz. Certainly the laws of magnetism and induction etc. have not changed and I don't know if we've learned anything that would change the design of such motors significantly.

What's changed is that since the advent of rectifiers (and thyristors, and choppers etc.) is that there is no longer any need to run AC brush/commutator motors just to gain the advantage of high voltage AC distribution. The rectifiers convert AC to DC for the motors.

Or, in the latest equipment, the AC gets converted to DC which drives inverters that create what I like to call synthetic AC whose frequency is controlled exactly in step with the speed, ramping up and down as needed, so that 3 phase AC induction motors can be used as traction motors. The advantage is simple and very robust motors with barely any wearing parts.

A disadvantage of using commercial frequency power is that traction power distribution is single phase while the utility generates 3 phase. Utilities prefer to have the loads balanced among the phases. In residential areas, where homes are receiving single phase power, the various blocks are divvied up among the three phases. With commercial frequency traction systems, different track segments are divided among the phases in hopes of keeping things more balanced. That requires insulated gaps in the catenary called phase breaks.

In theory it would be possible to convert all three phases of incoming power into single phase power but that would require the sort of conversion equipment at substations not unlike the 25 Hz systems and elimination of that equipment is a primary reason to go with commercial frequency in the first place.

Steve

Thanks for the clarification on BART. That project seems to have generated more than its share of second-hand stories.

There was some discussion in a Los Angeles based transit advocate group about LA Metro buying a set of new light-rail cars for line expansion and replacement of the original Blue Line cars, which are now over 20 years old, and will have passed the 25-year mark by the time new cars are delivered. One transit observer asked whether one or two of the 1989 order of cars might be preserved, presumably at Orange Empire. The question was raised about whether cars designed for 750 volts would run satisfactorily on 600, which is the design voltage for traditional trolleys, and for light rail cars running on "legacy" systems like Boston and San Francisco. My guess is that the cars are fairly tolerant, for example, they're probably designed to stay operable in sections where a substation is off line, and the power comes from a greater than normal distance. OERM has many items of more immediate concern, and the Metro car issue is still at least six years in the future, but tempus doth indeed fugit.

One element of the earlier light rail cars is: what kind of inverters do they have for lighting and other auxiliary functions? When they are finally scrapped, would it be a good move for electric railway preservation groups to buy some of the old inverters and use them to power the lights on old streetcars, and avoid having 600 volts running through old wiring and sockets. This might also eliminate the need for special light bulbs to avoid the hazards of arcs resulting from the use of gas-filled bulbs in streetcars.

This has nothing to do with the BART electrical system, but is a memory from my first ride on BART. I commented to the operator that "This reminds me of the Monorail at Disneyland." I quickly learned that I had made a "faux pas", as the operator replied, with some asperity, "This is a lot bigger and better than Disneyland!" (and visitors to San Francisco are warned never to call a cable car a "trolley".)
(hope this isn't too off-topic)

Light rail sytems usually have a trolley voltage tolerance of +/- 20%. So a 750 volt system can run between 900 to 600 volts. On trolley systems the problem is that the voltage goes down, not up, so systems are usually built with a voltage higher than 750 volts, sometimes as high as 900 volts.

The auxillirary power supply usually put out two voltages, a battery charging DC voltage that also run all the lighting and computer control, and an AC voltage to run the larger motors, HVAC, blowers, air compressor. etc. At BART the auxillirary inverter also manages all the auxilliraty loads, sequencing the motor starts and priority load shedding in case of low traction voltage. These modern power supplies put out a regulated power, so there is very little voltage drop on either the AC or battery circuits. The down side of this is as the line voltage goes down, the current goes up to maintain a constant power output. The current that can be handled is limited by the electronic devices in the inverter, so if there is very much voltage drop it will start load shedding and then shut down altogether to avoid an over current situation.

680VDC might be a bit much for some old trolley gear designed for 550/600V. Pennsylvania Trolley Museum's new (2010) substation takes in 480V 3 phase from a utility owned pad mount transformer. Some of it goes to building services. But the portion that feeds the rectifiers goes thru an additional matching and isolation transformer set to put about 450V into the rectifier. This provides right around 600VDC output.

Late in the last century, South Eastern Pennsylvania Transportation Authority bought some 600 Volts Direct Current welding machines for track repairs. They sometimes burnt out when connected to the trolly wire over the work site. The problem was found to be that the voltage from the substation was about 650 Volts, so that the average in each section of overhead wire would be about 600 Volts!

Here's a question for the trolley museum infrastructure experts: Orange Empire (in Perris CA) has a solid state rectifier plant (that's had a number of upgrades over the years) and a rotary converter from around 1920 as a backup/historical exhibit. Do any other museums use rotating machines rather than or as a backup to solid-state apparatus? We could also add a place for diesel generators, either feeding wire or on small flatcars.

Seashore Trolley Museum and Western Railway Museum both have backup rotary equipment. Shore Line has a rotary unit, but it is not connected and has not been used in many years. Electric City, in Scranton, has a rotary as a static display. We at Pennsylvania Trolley Museum have a rotary in storage. It may end up as a static display or be connected for eventual occasional demonstration use. Since we have dual rectifier units, it it not needed as a backup.

In addition to our rectifier station and rotary converter backup, WRM also has a 400-kw diesel generator for backup.

We have almost completed a second rectifier substation about 5 miles down the line.

For the shop, last year we built a portable substation which can supply 0-600 V DC with an adjustable over-current trip from 5 - 55 amps. It is very handy for testing equipment, and a lot safer than using a "stinger" off the trolley wire.

By "we", we mean the very same fellow who built OERM's standby substation.

The power loss in trolley wire is controlled by Ohm's Law: E=IR. E (voltage drop) = I (current) x R (resistance).
Note that "AC vs DC" is not a factor in this equation. If you converted your 600VDC trolley wire to 600VAC you'd have exactly the same losses in exactly the same places.

The other relevant formula is P=EI. P (power, watts) = E (voltage) x I (current). For any Power, double voltage and you half current. Now plug that into Ohm's Law and see what happens to voltage drop. Now take voltage drop back to P=EI and see what happens to power drop (loss).

Feeder is generally at trolley voltage. It is simply a much larger mass of aluminum to carry that current. Yes, aluminum. Aluminum is twice as good a conductor BY WEIGHT, and 1/8 the cost BY WEIGHT. This makes it a very correct choice for overhead wires the size of your wrist. To rest your confusion, copper is much denser than aluminum and a better conductor by volume, and has better physical properties too.

Feeder ties to trolley every 700 feet or so, so it would be a real mess if it wasn't the same voltage.

McKinney Ave. Transit in Dallas is installing 480v AC motors in the trucks of car 754 as it is being rebuilt. Trolley wire voltage will be reduced and changed to AC current for the motors by an inverter mounted inside the car.









Inverter and other electrical gear will be housed in box which will be under bench in front of car. Left side of photo.

Those look like ordinary industrial 3-phase motors, perhaps optimized for inverter drive (some inverters put out a funky waveform that ordinary motors don't like). Have these motors and inverters been tested for railroad propulsion?

Steve

It is important to keep in mind that street trolley operation usually doesn't keep the motors running for more than a minute or so before shutting down for a stop or traffic. So, motors for interurbans or locomotives could be different but those not suitable for intensive use might be fine for street running.

dave

I think you're looking at the test. Gives whole new meaning to the phrase, "Mixmaster motors." Be interesting to see how successful they are.