Practical aspects of system stability

During the past few years there has been much discussion regarding the behavior of long transmission lines under transient conditions, such as flashovers, short circuits, arcs and grounds which would tend to make them unstable, but unfortunately this discussion has been largely theoretical due to the absence of any actual operating data upon which to base assumptions. It has only been recently that an opportunity has been afforded to make field tests on one of the two existing 220-kv. systems and the results of such a series of tests made on the system of the Pacific Gas and Electric Company are presented in the paper. This is the first instance where tests of this nature have been attempted and the lack of proper testing equipment proved a serious handicap. It was necessary to develop a special high-speed oscillograph wattmeter, a high-speed oscillograph filmholder and a pilot generator. Moreover the technique of testing was developed so that it was possible to secure oscillographic records 200 mi. apart by telephone signal. The tests established the following important facts: 1. System stability as a problem is inextricably entangled with operating economics, and cannot be handled solely as a problem in design, except for very simple cases. 2. For any adequate conclusions to be reached much more fundamental data is necessary. 3. Requisite equipment for obtaining such data is not now available. 4. Studies of models and artificial transmission lines are not adequate because too little is known about the relative importance of the several factors to allow intelligent duplication. 5. Proper relay equipment and action is vital. 6. Oil-switch operation is an important factor. 7. Only a certain part of the stored energy of a system is available in any given case of trouble. 8. Operating distribution of excitation current is one of the major problems.