One system which was used in the early days of large scale digital computing was the AC equivalent of the battery-across-the-supply-line. A local engine-driven generator was floated across the electrical supply to the computer and was kept running during threatening weather. A reverse-power relay was included in the system so that, in the event of loss of the main utility power to the computer, the dedicated generator and the computer would be separated from the faulty power line without dropping the computer load. The engine-driven generator would provide a sort of electrical lifeboat. Serious questions arose about whether the added complexity of the system was justified by any increase in reliability. Modern systems are more effective and economical.
There are two major types of UPS in use, one using stored kinetic energy (flywheel) and the other using stored chemical energy (battery). The inertia-driven system uses a flywheel which provides energy to an alternator whenever the source of the main driving power is lost. A number of schemes exist for doing this, differing principally in the way the driving power is delivered to the system when all is well. Because the flywheel gives up energy only by giving up some of its speed, the flywheel UPS systems are suitable only for riding through interruptions of short duration (during reclosure operations or transfer switch operations), typically less than 30 cycles or one-half second. Battery-supplied inverters, on the other hand, or flywheel systems which use a DC motor and battery are limited only by the size of the battery and can continue for a relatively long time.
The operating procedures scheduled for use during a power interruption affect the necessary size of a UPS. Frequently, it is possible to defer the completion of an operation if an orderly shutdown procedure has been followed after a disturbance. In this case, the UPS is needed for only the duration of the shut-down sequence. If, however, the operation must be continued for a long time, as when the computer is a major part in the emergency operating system, the UPS must be larger and store more energy. This would use some sort of auxiliary power system for support like a diesel generator.
