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ELECTRIC RAILWAY JOURNAL · Vol. 70, No. 21 · November 19, 1927 · pp. 952-953.
Power for Dorchester Extension
By F. N. Carothers, General Foreman of Substations, Boston Elevated Railway
Abstract of a paper read at meeting of New England Street Railway Club. Boston, Mass., Oct. 27, 1927.
Construction of the Dorchester rapid transit extension necessitated the building of a new automatic substation, located on Dickens Street near Fields Corner. This station furnishes energy not only for the rapid transit line but for the surface cars in this district as well. The equipment consists of two 3,000-kw. shunt-wound converter units with single-phase, outdoor, oil-cooled transformers. Alternating-current energy is transmitted through two 13,200-volt, 25-cycle, three-phase, No. 0000 underground cables, direct from the generating station in South Boston. Two other a.c. cables that will be used as an emergency feed are connected across town to the bus of Eggleston substation. Direct current is distributed to the lines for car service through eleven 1,000,000-circ.mil and seven 2,000,000-circ.mil feeders.
The transformers change the 13,200-volt, 25-cycle power to 440 volts for converter supply, being connected through oil circuit breakers, Y for starting and delta for running. The delta-connected oil switch is connected 30 electrical degrees ahead of the Y-connected oil switch. This gives very smooth operation in changing from starting to running position.
The converters are General Electric six-phase, shunt-wound, commutating-pole, 3,000-kw., 600-volt type. The direct current drops 5 to 6 per cent between no load and full load. They can be started and stopped by means of the supervisory system, but it is customary to allow them to function on load demand. Either converter can be made the leading or trailing unit, by the throwing of the hand-operated sequence switch. The leading unit goes into service through low-sustained trolley voltage on the d.c. bus, and the trailing unit goes into service when the load on the leading unit has reached its capacity for a period of 30 seconds. The units are shut down in the reverse order. The trailing unit drops out when the load has dropped to a point where one machine will safely carry it. The leading unit shuts down when the load remains at about 20 per cent of its capacity for a period of two minutes. At this point the tie feeders with manual stations are sufficient to keep the bus voltage normal. In event of failure of the leading unit to go into service, the trailing unit, after a short timing period, goes into service, and the supervisory notifies the system operator that the leading unit has failed.
The machines are protected from severe overload on the d.c. side by means of the MC-2 air circuit breakers in eleven d.c. surface feeders, and JR-10 high-speed circuit breakers in seven d.c. rapid transit feeders. There are also load-limiting resistors in the positive leads of the machines that will hold them to their commutating capacity, even if the d.c. bus should become grounded. These resistors also give a cushioning effect as the machines go into service. They are located in the basement directly below the converter. This is a departure from the usual practice, but has given no trouble in another station that has been in service for almost two years. The overload trip on the hand-operated d.c. machine circuit breaker has been allowed to remain on the breaker and is not set for any higher current than those of machines of the same capacity at manual stations.
It was thought that for this particular installation it would be better not to use high-speed circuit breakers in either the positive or negative side of the converter, as all d.c. feeders are tied into the system network, and trouble developing on one feeder, causing a blowout on the converter might result in the loss of several manual stations. It was decided, however, to protect the rapid transit feeders with high-speed circuit breakers and to use MC-2 breakers on the surface feeders. At the present time three sections from this station feed the rapid transit lines, two of which consist of two 2,000,000-circ.mil and one of three 2,000,000-circ.mil cables. Each cable has its own high-speed circuit breaker connecting it to the station d.c. bus. In case of a section blowout, it is necessary for all breakers on a particular section to close simultaneously. Otherwise the first breaker to close would try to take the total load of the section, which it could not carry, and would blow out again just about the time the second breaker was closing. This also would blow out, resulting in a pumping action, and not restoring power to the section. Of course if it were possible to set the time delay reclosing relay exactly the same on all breakers of a particular section, this would not happen.
These relays are of the PQ time delay closing and instantaneous dropout type, and cannot be set for absolute accuracy. To get around this feature it was necessary to connect the closing side of all No. 102 relays of each section in parallel, so that the first No. 102 to close would close all circuit breakers of the section. The first No. 102 was set to operate in seven seconds, the second in nine seconds and the third in eleven seconds. This also offers the advantage that should the first relay fail, the second or third relay would operate the breakers. The breakers would not close back on short circuit, for after a breaker opens, a resistance which is connected across the breaker feeds power to the fault, and the short-circuit detector holds it out until the load is such that the section will not be overloaded when it closes.
The actual operation of the reclosing feature is as follows: A circuit breaker blows out; the resistance of 15 ohms, which was shorted out by the breaker, allows some power to feed out on the cable. At the same time a B switch on the circuit breaker starts time delay relay No. 102 to start timing. When this relay has completed its timing and closes its contact, a circuit is completed to the M and N coils of the reclosing relay No. 182, but the torque of this coil is not sufficient to operate the contacts, and must wait for the aid of the A. B. coil that is connected from the feeder to the negative bus. If a grounded cable or third rail exists, the drop across the A. B. coils will be 0, and it cannot give any aid to the MN coil. The consequence is that the circuit breaker cannot reclose and must wait until the trouble has been cleared, or for load to be reduced to a point where the drop across the A. B. coil will allow No. 182 to close its contacts, which recloses the circuit breaker.
In addition to functioning automatically, all pieces of apparatus in this station have supervisory control and indication from the system operator's office. That is, the system operator can start, stop or lock out of service either converter found open, close or lock out any a.c. or d.c. cable, as he desires, provided, of course, that all automatic features function. He cannot start a rotary if it has flashed over, has grounded windings, overheated bearing, or windings, or anything that might cause serious trouble.
The supervisory is of the Western Electric 8A distributor type, and allows system operator to give as many operations as desired, either opening, closing or both, at the same time. After an operation indication is given in five seconds. The position of all pieces of apparatus is checked at five-second intervals. Watt-hour meter readings on both converters, and R. T. feeders are transmitted to system operator's office over this supervisory.
Four No. 22 telephone wires rented from the New England Telephone & Telegraph Company are used from substation to system operator's office. The use of these wires are: One for operation, one for indication, one for synchronizing the distributors and the fourth is a common for all three.
Energy for rapid transit signals is supplied from two 50-kva., 13,200 to 600-volt, 25-cycle, single-phase transformers. Only one is being used at present, the other being held as a spare. The low-tension oil circuit breaker on the signals will reclose three times in case of trouble and then lock out. The supervisory notifies system operator on each operation and finally that is locked out. There are about ten seconds between first and second reclosing, and twenty seconds between second and third reclosing. The lockout only occurs in case of three consecutive blowouts in a space of about one minute.
While this station is completely automatic, it can be changed to manual operation, even with a complete failure of all automatic devices, in a period of about two minutes. For the ultimate extension to Ashmont, another similar station will be required on Beal Street near the Ashmont station. The order for electrical equipment has been placed.
Sources: Electric Railway Journal, McGraw Hill Company, Digitized by Microsoft, Americana Collection, archive.org.
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