Quantcast Power Factor Control

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Synchronous motors can also be used to good advantage on large power systems if the 0.8 power
factor design is purchased, rather than the less expensive unity power factor motor. The 0.8
power factor synchronous motor can be used to improve voltage levels on its utilization bus in
the same manner as capacitors. Both devices act as a source of reactive power (VARs) to the
system. Control of the operating voltage range can also be achieved by the use of transformers
with on-load tap changers and line regulators. Both devices use multi-tap devices, in
combination with voltage sensing and control apparatus, to adjust the transformer ratio or
regulator ratio by actively switching taps as the steady-state load changes. These devices are
usually used by utilities in the primary distribution system and provide the final distribution
circuits with a voltage range within the Range A limits of ANSI C84.1. Unless the site
distribution system is unusually large and complex, and the daily load fluctuations quite large,
these devices are not applied to electrical distribution systems on facilities. Power Factor Control. Power factor can be controlled using the same methods as
those used for controlling the voltage. Capacitors and synchronous motors can be switched or
adjusted (either manually or automatically) to achieve a desired power factor. A high power
factor (one close to unity or 100 percent) is generally desirable. When the various feeder and
utilization circuits are operated at a high power factor, voltage drops and losses are minimized
throughout the system. This is one way to achieve more load handling capability. There is more
equipment capacity available for real loads, if the reactive (kVAR) portion of the load is reduced,
since most system devices are current sensitive. Often distribution systems are found operating
at power factors in the 70 percent range. If the power factor can be increased to the 90 - 95
percent range, then the operating load on the equipment can be increased by 25 - 35 percent. It
is, however, frequently undesirable to operate the distribution system in the leading power factor
range. Operating the system with a leading power factor can possibly cause unstable transients.
Since most loads are inductive in nature, it is usually difficult to get the distribution system
power factor into the high 90 percent range. It is even more difficult to achieve unity power
Improved power factor control can release equipment for surplus use or make available
additional load capacity with no change in the system configuration. The cost is usually nominal,
especially when compared to the cost of new transformation equipment, switchgear equipment,
and larger sized cables. Harmonics. Harmonics on the distribution system is an increasing problem
resulting from the growing use of solid-state switching devices in equipment, such as adjustable
frequency drives, rectifier power supplies, high frequency power supplies, uninterruptible power
systems, and arc discharge lamps. ANSI/IEEE 519, Guide for Harmonic Control and Reactive
Compensation of Static Power Converters, contains guidelines on input power sources for the
maximum harmonic content total not to exceed 5 percent and the maximum for any one
harmonic not to exceed 3 percent of the fundamental frequency. Harmonic distortion is
calculated by using the square root of the sum of the squares of each of the harmonic amplitudes


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