174 BULL SYSTEM TECIIXICAL JOCRXAL 



process and reducing the investment per kilowatt of generating ca- 

 pacity. This has led to the development of larger and larger generat- 

 ing units. A single shaft unit of 160,000 kw. capacity and a triple- 

 element unit of 208,000 kw. capacity are in operation. The latter 

 consists of one high pressure and two low pressure turbines with their 

 respective generators. Single shaft units of 200,000 kw. capacity are 

 under construction and it seems probable that the trend in the future 

 will be toward even larger units of both types. This trend toward 

 larger units instead of the equivalent in small units has resulted in 

 improved wave shape but otherwise does not directly affect coordina- 

 tion except in so far as it may reflect the general trend toward larger 

 concentrations of power with the accompanying tendency to increased 

 magnitude of system abnormals. 



Another definite trend in the power industry, but one which is not 

 of importance from the standpoint of coordination, is the increasing 

 use of automatic devices to replace manual operation. Complete 

 automatic operation is being practised to some extent in hydroelectric 

 generating stations and is widely practised in substations of various 

 types. The trend is definitely toward wider use of automatic devices 

 and new types and applications of such devices are being constantly 

 developed. 



In view of the remarkable development and rapidly multiplying uses 

 of thermionic tubes and related devices in other fields, and the theoreti- 

 cally potential applications in the power art, the question will doubtless 

 be asked as to the trend of their application in the power field. How- 

 ever, other than application for current rectification, such as in railway 

 work, it cannot be said that progress has advanced to the point of 

 establishing a trend. 



Those trends in power system development which are more directly 

 concerned with matters of coordination are discussed in the following. 



System Voltages.— Referring to Table III, it is of interest to note 

 that the rate of increase of transmission line mileage, as a whole, is 

 lagging behind the rate of growth of both installed generator capacity 

 and electricity production. P'urthermore, mileages of the higher 

 transmission voltages, 220 kv., 132 kv., 110 kv. and particularly 66 kv., 

 are growing at a faster rate than the group average. These compari- 

 sons reflect the increasing utilization of the higher voltages with the 

 greater circuit capacities they provide. As power industry growth 

 requires the handling of larger l)locks of power and as greater distances 

 between sources and markets are encountered, the development and 

 use of circuits and apparatus to transmit at voltages higher than the 

 220 kv. initiated in 1923 must be expected as an economic necessity. 



