Transmission stability: Analytical discussion of some factors entering into the problem

The subject of stability has been much discussed lately, because it has an important bearing on future large power developments. In the early stages of a large program, such as the proposed superpower program, good engineering and common-sense dictate that each step should be very carefully considered from all points of view, since a blunder or failure to give proper weight to some important factor, such as stability, might set back the development program for many years. A brief historical review of the subject of stability follows; for those who are not familiar with “static” stability there is a review of the subject in the Appendix. A criterion of stability is suggested based on present operating conditions, namely, that for reliability each unit of the superpower shall be at least equal to the best that has heretofore been obtained with similar power systems. The necessity of a careful study of the characteristics of all machinery connected to the transmission line is pointed out. The necessity of proper inherent characteristics in generators and synchronous condensers is emphasized, and particular stress is laid on the necessity of proper volt ampere characteristics both inherent and with the exciter. The action inside a generator during the transient following a change in load is discussed; it is pointed out that the true field is a resultant due to several magnetomotive forces in addition to that of the field circuit; the combined effect is a marked tendency to self-excitation, and inherent self-excitation would take place if it were not for the damping effect of resistance in the different circuits. A brief review of other factors entering into the problem is given. These factors comprise inertia of moving parts, mechanical torque, speed of relays, circuit breakers, etc. The difficulty of correlating all these quantities is pointed out, and a basis on which it is practical to make computation is suggested. Those who have not studied the subject of stability are recommended to read the Appendix before proceeding with the subject of transient stability. The subject of transient stability is opened with a definition of stability of a power system. The elements of the problem are discussed in some detail. The problem is one of obtaining the conditions of equilibrium, taking into account mechanical or applied torque, electrical or counter torque, inertia torque and damping factors, in addition to the electrical characteristics of the system. The action of a generator under suddenly applied load is discussed in some detail. The “transient” stability of a simple system is discussed, use being made of a new diagram known as the power angle diagram which may be derived from the circle diagram as obtained for static stability. Three diagrams are required for the simple investigation but the method may be elaborated to include all the factors affecting the problem including the characteristics of governors, exciter systems, and so forth. The difference between the problems of switching operations, load swings and short circuits is pointed out. In the last case the effect of different values of ground resistance is discussed at some length and also the effect of length of time before circuit breaker opens. The necessity of obtaining reliable data on ground resistance with faults is stressed. Throughout this paper the essentiality of delivering the necessary kilovolt-amperes to the line either by adequate exciter systems or by proper modification of machine characteristics in order to maintain a high order of stability is insisted on. It is pointed out in the Appendix that while inherently compensated generator, synchronous condensers, etc., are future possibilities, our main concern is the problem of getting the most out of present day designs as our present day problems depend on these and not on something that may be commercially developed five years from now. Consequently the conclusions refer to means that may be made quickly available and the two most important are: a. Improved inherent regulation of machines. b. Increased speed of excitation.