where multiple SPS units are serving a nationwide system of interconnected utilities, because the ability to draw backup power through interarea exchange is diminished as the outage of SPS units propagates across the nation. Another major operating concern is maintaining system stability. System stability is a design as well as an operating problem which is a function of the system characteristics, including both generation and electrical network. System disturbances caused by failure of large components are generally more severe than those associated with small components. Thus failure of an SPS unit or a large transmission line segment associated with an SPS unit presents a substantial stability problem. Consequently, in planning for installation of SPS units, detailed stability studies would have to be performed in order to determine if a suitable system design can be arranged. These studies would have to consider dynamic impacts of an SPS unit under various conditions of system load and scheduled outages. Furthermore, the addition of SPS units to a national power network in lieu of large conventional synchronous machines will result in less system inertia associated with the rotating mass of conventional machines and further complicate the stability problem. Additional operating problems include load following, output level control, reliability, and maintenance scheduling. The value of an SPS unit will be highly dependent on whether or not its output level can be stepped up or down to accommodate changes in power system demand. Additionally, it is highly desirable that an SPS unit be able to rapidly respond to momentary power fluctuations. It is probable that an SPS unit may be a cause rather than a cure for power fluctuations. Because of its size, complexity, and enormous number of components, an SPS unit may be subject to small output fluctuations which as a percentage of the 5 GW rating are not minor from a utility system operational standpoint. Reliability and maintenance requirements are subject to considerable uncertainty; but due to the large number of components involved, the maintenance requirements would be substantial even at a low failure rate.
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