3 6 2 ELECTRICAL ENGINEERING 



line is of copper, and in the course of the run rivers and railways are crossed. Hence the 

 engineering difficulties of electric transmission of power at 100,000 volts over distances of 

 100 or 200 miles are now largely overcome. In Great Britain with a few exceptions a trans- 

 mission pressure of 6000 volts is now usual in large coal-steam supply stations. It is trans- 

 mitted as two or three phase alternating current and may be then transformed down to low 

 pressure alternating or to 500-600 volt direct current for traction purposes. 



A good example of a recent coal-steam electric power station is the Dunstan Power 

 Station at Dunstan-on-Tyne for supply of power in Northumberland and Durham. The 

 station is laid out for 6 generating units of 10,000 kilowatt capacity, 3 of which have been 

 already installed. The engine units are steam turbines of the A.E.G. impulse type and 

 Brown-Boveri-Parsons type, and drive alternators giving a three phase current at 5,750 

 volts and 40 periods. The feature of the station is the care with which the switchboard gear 

 has been designed. This is contained in a separate fire-proof house 150 yards away from the 

 engine room, and contains the oil switches and controlling gear for governing the distribution. 

 The boilers are provided with automatic stokers and the steam is superheated to 570 F. 

 It is intended to supply electric energy at low rates for metallurgical and chemical operations. 



The problem of railway electrification for other than short suburban railways and 

 with the aim in view of main line electrification is now attracting the attention of 

 engineers. There are four competitive systems 



1. The low pressure direct current (D.C.) system, working at 500-600 volts. 



2. The high pressure D.C. system working at 1,200-2,000 volts. 



3. The single phase alternating system. 



4. The polyphase (3-phase) alternating system. 



System I has abundantly proved its applicability for electric urban traction and short 

 urban or interurban railway lines, as for instance in the Metropolitan Railway (London) 

 and in innumerable cases in urban and interurban tram lines all over the world. The low 

 voltage used limits the economical use to a few miles unless coupled with the employment of 

 transformer stations. Thus in the case of the Metropolitan Railway the energy is trans- 

 mitted by alternating current at 5,000 or 6,000 volts from the generating station to transformer 

 stations along the line and there changed to D.C. current at 500 volts. 



The great contest at present is between the single phase alternating system and the direct 

 current system. Single phase traction motors having commutators, called respectively 

 compensated series and compensated repulsion motors, have now been invented, and these 

 can be accelerated and reversed with the same ease as continuous current motors. The 

 A. C. motors are, however, about half as large again as the D.C. motors for the same power 

 output and the switching arrangements are heavier, and the motor has generally to be asso- 

 ciated with a transformer. One difficulty with the A.C. motor is the sparking at starting; 

 but the advantage of being able to transmit and pick up current from the overhead trolley 

 wire at high voltage and transform down on the locomotive to low voltage is very great. 



Several railways are now operated with single phase line voltages of 11,000 volts at 25 

 periods per second; for example the Rock Island Southern Railroad 52 miles, Denver and 

 Interurban Railroad 55 miles, and the Hoosac Tunnel on the Boston and Maine Railroad, 

 and also the New York, New Haven and Hartford Railroad, all in the United States. The 

 single phase system has also been adopted in England for the electrified section of the London 

 and Brighton Railway, London Bridge to Victoria, and for the Heysham-Morcambe line of 

 the Midland Railway. In Germany engineering opinion inclines to the single phase system. 

 The Prussian Chamber of Deputies has recently voted two million marks for electrifying the 

 Bittcrfeld-Dessau railway, and this is part of a scheme for electrifying a large number of 

 German state railways with the single phase alternating system. On the other hand some 

 railways in the United States which installed this system have now changed over to the high 

 tension direct current system (1,200 volts). There are some 10 or 12 lines electrified on this 

 latter system in the United States, the largest of which (60 miles) is the Washington, Balti- 

 more and Annapolis Railroad. The capital costs are said to be slightly less than for the 600 

 volt D.C. system and the operating difficulties are no greater. In Italy and parts of Switzer- 

 land the three phase alternating current system seems to be preferred. The success of the 

 Valtelina line on this system, and that of several similar north Italian lines, has encouraged 

 an extension, and a Parliamentary appropriation of 12 million pounds sterling has been made 

 for electrification of Italian main line railways on the 3-phase system. The greater complica- 

 tions of the overhead line involving a double trolley wire and those of the arrangements for 

 speed regulation have boon arguments against the 3-phase system. 



The future of railway electrification lies between the single phase alternating and high 

 tension direct current systems for main line work. The horse-power output per ton of 

 electrical equipment has been estimated at II H.P. for D.C. as against 6 H.P. for A.C. 

 motors. The operating costs for A.C. systems are certainly higher than for D.C. For 

 suburban lines and in competition with electric tramways railway electrification is necessary 

 and advantageous, but for lung main lines steam locomotion will continue to hold its own. 



