Fundamental Considerations of Power Limits of Transmission Systems

At this time the power limit of transmission lines is a live subject and presents such complications as to require very careful analysis. The paper points out the essential features to be considered in a study of the problem, and calls attention to some out-standing results of an experimental investigation of the subject with a view to clarifying some of the points that have been under discussion in the past two years. It is shown that the problem of stability is not necessarily confined to long-distance, high-voltage transmission, but may be present in any system where the impedance of the transmitting circuit is high compared with the load to be carried. While the impedance of the transmission line and transformers plays an important part in establishing the breakdown point of a system, the characteristics of synchronous apparatus with the method of voltage regulation used are of equal importance. It is shown that the synchronizing power of synchronous apparatus is largely dependent upon the field excitation at the time excess load is applied, that field excitation is determined by the circuit conditions under steady load, and, in order to provide for increase of excitation with increasing loads of considerable magnitude, some automatic means of controlling the field is essential. The rate at which mechanical load in large quantities can be added to a system is limited on account of the necessity of change in angular displacement between the generators and receiving bus; this changing angle-requires relative speed change, which takes time.