Stability Characteristics of Alternators

During the past few years, stability characteristics of systems using long lines have been discussed at considerable length, but not so much attention has been given to the characteristics of the load. This paper shows that power limits may be reached with very short lines and certain classes of load. The characteristics of several classes of load, such as motors of various kinds with constant shaft output, variable impedance loads (synchronous converters for railways), constant impedance, and miscellaneous combinations, are discussed as they affect the stability of the generator. The criterions for the stability of an alternator as developed by this paper are ``short-circuit ratio,´´ saturation, power factor of the load, and character of the load. A series of curves and a formula for the minimum allowable value of short-circuit ratio as a function of saturation and power factor are proposed for general purpose alternators which may be called upon to deliver power to any of the various classes of load. These curves are derived from characteristic curves of typical machines. It is not intended that these curves shall be used for generators which supply power over long transmission lines, as the characteristics of these lines may require considerably higher values of short-circuit ratio, and the generator must be specially designed to meet the individual requirements.