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 outstanding 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. This fact, together with the inherent tendency of synchronous machines to “stiffen” under sudden applications of load, makes it possible to rely on the usual vibrating-type voltage regulator working on the field of the exciter to provide the necessary field change. It is brought out that the maximum load that a system can carry under steady conditions at normal voltage can be suddenly thrown on, and the voltage regulator, with the assistance of the factors mentioned, will provide the necessary excitation. Voltage regulators are practically a necessity where it is desired to approach, under operating conditions, the ultimate maximum power of the system. Transient load changes that occur on the usual system, such as throwing on or off load, cutting in or out transmission circuits, etc., can be easily taken care of, providing such changes do not exceed the steady state limits of the system. The effect of short circuits depends upon their nature, whether three-phase or single-phase, and upon the location and duration. This subject is discussed briefly and the conclusion drawn that successful operation can be obtained under usual short-circuit conditions if adequate relaying is provided. The possibility is discussed of increasing the limit of power transmission by improving the apparatus and the characteristics of the transmission circuits, and it is pointed out that no great development may be expected from any scheme yet proposed regarding a modification in line characteristics. With reference to the apparatus, it is possible to make some changes in the design of synchronous machines tending to “stiffen” them, such as higher saturation, larger air-gap, etc., but in general no radical improvement may be expected here that does not materially increase the cost and decrease the efficiency of the machine. Attention is turned therefore toward such schemes of regulation, or compensation, of the synchronous apparatus as would increase the maximum power. Among these is the use of reactors for locally controlling power factor and thus too the field excitation of the more important synchronous machines. However, the possible additional power thus obtained is limited, and, as it now appears, other methods which have greater promise will be resorted to. The use of a mercury-arc rectifier in the alternator field circuit seems to have great possibilities. By varying the field current in rigid proportion to the armature current, a very significant degree of compensation of the armature reaction is obtained — about 50 per cent, which is, of course, equivalent to almost doubling the inherent capacity of the generator. While this scheme is not yet in practical form, its efficacy has nevertheless been demonstrated in factory tests, and it is regarded by the authors as one of the most promising developments at this time.