Superpower transmission: Economies and limitations of the transmission system of extraordinary length

This paper is a study of the transmission of very large blocks of power for extraordinary distances and has for its purpose the bringing out of the major operating characteristics of such a system, the characteristics which it possesses ivhich are different from those of shorter transmissions and the duties imposed upon generators, transformers, synchronous condensers, switches, etc., primarily as affecting their design. The paper defines a superpower transmission line as a line of great length in which the charging kilovolt-amperes per mile of length is of the same order of magnitude as the reactive kilovolt-amperes developed by the full-load line current passing through the reactance of the line and in which the resistance is small relative to the reactance. Such a line is adapted for economical transmission only for a fairly definite amount of load and any great increase or decrease below this point leads to poor economy or unstability. Since, however, the load appropriate to a given line depends upon the voltage, an appropriate line can be laid out for any reasonable amount of power to be transmitted. In order to secure a definite set of conditions to serve as a specification for determining the performance of generators, transformers etc., all applying consistently to the same system, a typical hypothetical transmission has been assumed, namely, a delivery of 400,000 kw. over a distance of 500 miles over four circuits 220,000-volt at each end. The characteristics of this line are worked out showing the efficiency, condenser capacity required, data on circuit breaker arrangements, protective relays connection to receiving network, provision for spare parts, switching of units and ratings of various apparatus. It is shown that such a system is very sensitive to the receiving end voltage; when this voltage drops there will be a tendency for the generators to run away if the system is not properly laid out. In addition a discussion is given of the effects of various prescribed values for the terminal voltage between 220,000 and 245,000 to show the effect of increasing the voltage 10 per cent. of maintaining the generating end 10 per cent higher than the receiving end and also of stabilizing the middle point with synchronous condensers. The typical hypothetical case chosen shows one layout, it being recognized that other layouts may be chosen. This particular layout is operated without any high-tension switching of live lines. The layout is intended to relieve the duly on circuit breakers and as a matter of fact no breaker can be called upon to interrupt a short circuit of more than 3/4 of a million kilovolt-amperes; this is a very favorable condition and is secured without materially limiting the equalization of the load. The layout connects with the assumed distribution net al a considerable number of points and no large portion of the total delivered power can be concentrated at any one point; this serves to secure a very intimate connection between the network and the transmission, and at the same time prevents any one breakdown, however, complete, from materially disturbing the major portion of the transmission system.