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 1.6.8 Uninterruptible Power Supply Systems. The UPS system includes all mechanical and
electrical devices needed to automatically provide continuous, regulated electric power to critical
loads during primary power system disturbances and outages. During normal conditions, the
UPS system receives input power from the primary source and acts as a precise voltage and
frequency regulator to condition output power to sensitive loads. During disturbance or loss of
the input power, the UPS draws upon its stored-energy source to maintain the regulated output
power. The stored energy source is usually sized to supply the UPS load for several minutes,
until emergency or the normal input power is restored, or until the loads have undergone an
orderly shutdown. There are two basic uninterruptible power supply systems: the rotary
(mechanical stored-energy) system and the static (solid-state electronic system with
storage-battery).
1.6.8.1 Rotary (Mechanical Stored-Energy) Systems. Upon loss of input power, rotary
systems deliver uninterruptible power by converting the kinetic energy contained in a rotating
mass to electric energy. These systems provide an excellent buffer between the prime power
source and loads that will not tolerate fluctuations in voltage and frequency. Many types of
systems are in use, but since static equipment has been used to replace rotary systems in the past
ten years, only one configuration will be described.
The rotating flywheel no break system is shown in Figure 1-15. An induction motor is driven
from the utility supply and this motor is directly coupled to an alternator with its own excitation
and voltage regulating system. Coupled directly to the motor generator set is a large flywheel
with one member of a magnetic clutch attached to the flywheel. The other half of the clutch is
connected to a diesel engine or other prime power. Upon loss of alternating current input power,
the generator is driven by energy stored in the flywheel until the engine can be started and drive
the generator and flywheel. The voltage regulator maintains the voltage and, with proper
selection of components to minimize the start and run times of the diesel engine, the frequency
dip can be kept to approximately 1.5 to 2 Hz. Thus with a steady-state frequency of 59.5 Hz, the
minimum transient frequency would be from 57.5 to 58 Hz. The time for the diesel to start,
come up to speed, and assume the load would normally be from 6 to 12 seconds.
1.6.8.2 Static (Solid-State Electronic Circuitry) Systems. The basic static UPS system
consists of a rectifier, battery, and DC-to-AC inverter. Static systems are very efficient power
conversion devices. The advantages of static systems are stable operation, frequency unaffected
by load changes, excellent voltage regulation, and fast transient response. These systems
normally operate at 480Y/277 V or 208Y/120 V, 3-phase, 60 Hz input voltage and provide an
output of 480Y/277 V or 280Y/120 V. Typical output specifications are: voltage regulation of +1
percent and frequency regulation of +0.001 percent. The ratings of these systems range from 50
VA to more than 1200 kVA. A UPS system can be designed with various combinations of
rectifiers and inverters to operate in a nonredundant or redundant configuration.
(a) A nonredundant UPS system is shown in Figure 1-16. During normal operation,
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