A System Out of Step and Its Relay Requirements

This is a study of the current, voltage, and power values throughout a simple transmission system when machines are operating out of step. The investigation was made to determine the proper relay protection for some large frequency changers which supply power to an electrified railroad. The object was to find means for protecting machines that had already fallen out of step or, if possible, to prevent them from falling out due to overload. The treatment of the problem is geometric, probably the easiest method to comprehend. The assumed conditions have been kept as simple as possible, but it is hoped that the method of treatment is complete enough to permit anyone interested in the subject easily to apply it to more complex problems. It is to be particularly observed that the common method of assuming that a system consists of pure reactance leads to errors that are noticeable on 25-cycle systems where the reactance is low. As a by-product of the study of machine protection, it was discovered that the behavior of different types of line-sectionalizing relays under out-of-step conditions can be easily predetermined and proper adjustments made to obtain a more desirable performance of such relays.