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The Journal of the New York Museum of TransportationSUMMER 2007 |
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WHAT’S A SUBSTATION? With our trolley operation now in full swing every Sunday, and with one of the men responsible for our substation—Dick Holbert—featured in this issue’s “Volunteer Spotlight”, it seems a good time to explain the purpose of this major new facility at NYMT. In the early days of the electric power industry, it was discovered that direct current (DC) electricity, while lending itself to simple, rugged motors for trolley power, could not be transmitted efficiently over long distances. Alternating current (AC) proved better for that, and transmitting at high voltage helped even more, but AC motor control was more complex. As the industry evolved from small, local power companies that were close to their customers, to larger, centralized firms, efficient transmission over long distances grew in importance. The concept of a substation was developed to provide the necessary transition from the high voltage AC to the lower voltage DC required for trolley operations. The museum’s substation provides this necessary transition, as our trolleys require the 600 volt DC power that was established as a standard over 125 years ago…but the transition is accomplished through the technology of today. Dick has provided us with a “Trolley Substation Overview”, and we’ll draw on that document for this “tour” of our facility. Our substation was built through a cooperative effort of NYMT and RGVRRM volunteers, and funded through donations, NYMT funds, and accumulated gift shop profits. Throughout the design and construction phases, all required inspections and approvals were obtained, both from local government and from Underwriters Laboratories. The facility provides 600 volts DC power at a maximum load of about 600 amperes. Input to the substation is a 480 volt AC, 400 amp, 3-phase, 4 wire wye service from National Grid (formerly Niagara Mohawk), and the line from the power pole to the building is under ground. First in line for the incoming AC power is a 300 KVA (think 300,000 watts!) Isolation Transformer, with a Delta primary to Wye secondary. The Wye common point is “floating”, as required for the Rectifier which is next on the list. A Circuit Breaker set to trip at 350 amps guards the line into the Rectifier in case of high current overload. This Circuit Breaker has a Trip Circuit which provides for remote trip capability. The Trip Circuit includes an Emergency Trip Button outside the substation door, a Trip-and-Lockout Switch on the DC Control Panel, and an Interlock Switch on the Overhead Grounding Switch. Our Rectifier was built for us by the late Fred Perry, and consists of EMD diesel locomotive rectifier diodes and fuses. A 3-fan cooling system on the Rectifier is protected by Motor Sentinel overload protection. The Rectifier changes the alternating current to direct current. From the Rectifier, the negative ground line passes through a current metering shunt (in order to monitor the current) and is connected to the track by two 350MCM underground cables. At the rail connection point, there are two 5/8” copper-clad ground rods, 8 feet long, with cadwelded connections, thus assuring a reliable ground. |
The positive 600 volt DC output from the Rectifier goes to the DC Control Panel, where a 1000 amp Ribbon Fuse in a transit car shoe fuse enclosure protects against backfeed of foreign voltage from outside (for example if a National Grid power line were to fall on the rails during a storm). This Ribbon Fuse also provides a means to disconnect the Rectifier for maintenance. From the Ribbon Fuse, the 600 volt DC connects to a large Knife Switch, rated at 1600 amps, that permits us to ground the overhead line. The center of this single-pole, double-throw switch is connected to two 350 MCM cables which pass underground to feed the overhead wire. The bottom of the switch is grounded. Except when the overhead is powered for trolley operation or other work, the switch is locked in the grounded (down) position as an absolute safety precaution. The switch is also fitted with a plastic safety cover which must be in place to engage and interlock when the switch is in the energized (up) position. A System Monitoring Panel is located outside the door of the Substation, with indicator lights arranged in a schematic of the substation to show switch and voltage status. Keeping the door to the substation closed and locked, with very limited access among designated volunteers, is an important part of the plan to assure electrical safety, which has been fundamental to the design and operating plan for the substation. So, the basics are: (1) purchased 480 volt AC power, (2) Isolation Transformer, (3) Circuit Breaker with Emergency Trip, (4) Rectifier to convert alternating current to direct current, (5) negative DC line to the rails, (6) positive DC output to Ribbon Fuse for backfeed protection, (7) Knife Switch to ground the overhead wire, (8) positive line to the overhead wire above the tracks, and (9) a System Monitoring Panel. There’s more: Inside the substation are numerous indicator lights and meters to monitor conditions at various points in the system, and there are additional transformers to provide 120 volt AC power for substation room lighting, heating and ventilation, and to energize the Substation Control system. The latest feature of this facility is a remote control station located in the main barn close to the trolley boarding area. Staffed by one of the trolley crew when in operation, this feature of our system permits control of power to the overhead without having to maintain constant presence of a Substation Operator. In the Substation, a 480 volt 3-phase Contactor permits control of the overhead system from inside the Substation as well as via a low-voltage (24 volt DC) control circuit from the remote control station. Our new Substation is truly state-of-the-art, and we’re proud of the quality workmanship and attention to safety that’s been built in at every step in its creation. It will serve us well for many years as we extend our trolley line and bring back the interurban era for our visitors. Congratulations to Dick Holbert, Jim Johnson, Charlie Harshbarger, and all the others responsible for completing this critical part of our future!
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HEADEND is published four times a year by the New York Museum of Transportation, © 2007. All rights reserved. No portion of this newsletter may be reproduced in any form without written permission from the publisher. www.nymtmuseum.org |
Editor -
Jim Dierks |