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 the prime power and rectifier supply power to the inverter, and also charge the battery which is
floated on the direct current bus and kept fully charged. The inverter converts power from direct
to alternating current for use by the critical loads. The inverter governs the characteristics of the
alternating current output, and any voltage or frequency fluctuations or transients present on the
utility power system are completely isolated from the critical load. When momentary or
prolonged loss of power occurs, the battery will supply sufficient power to the inverter to
maintain its output for a specified time until the battery has discharged to a predetermined
minimum voltage. Upon restoration of the prime power, the rectifier section will again resume
feeding power to the inverter and will simultaneously recharge the battery.
(b) The nonredundant UPS system reliability can be improved by installing a static
switch and bypass parallel with the UPS as shown in Figure 1-17. When an inverter fault is
sensed, the critical load can be transferred to the bypass circuit in less than 5 milliseconds. The
static bypass adds about 20 percent to the cost of a nonredundant system, but is much more
reliable.
(c) In the redundant UPS system shown in Figure 1-18, each half of the system has a
rating equal to the full critical load requirements. The basic power elements (rectifier, inverter,
and interrupter) are duplicated, but it is usually not necessary to duplicate the battery since it is
extremely reliable. Certain control elements such as the frequency oscillator may also be
duplicated. The static interrupters isolate the faulty inverter from the critical bus and prevent the
initial failure from starting a chain reaction which might cause the remaining inverter to fail.
The static bypass switch can also be applied to the redundant system.
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