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 (expressed as a percentage of the fundamental). Various means, usually filters, inductors,
or capacitors, are used to reduce the effect of harmonics caused by loads on the system and on
other loads.
8.2.4.4 Frequency. The system frequency regulation is generally determined by the
serving utility. Most utilization equipment will operate satisfactorily over a frequency range of
+/- 5 percent, however, most electronic equipment requires a tighter range of +/- 0.5 to 1 percent.
This degree of frequency regulation is easily achieved by the use of utility grids in the continental
United States. Systems fed from isolated generation sources or those located in certain foreign
countries may experience frequency variations outside the range required by electronic
equipment. These conditions may then require special static power supplies for such sensitive
electronic equipment.
8.2.4.5 Voltage Unbalance. Voltage unbalance on multi-phase systems is usually limited
to a maximum of 3 percent. Unequal distribution of single-phase loads will increase the amount
of voltage unbalance. Whenever possible, effort should be made to distribute the load evenly
between the phases. Excessive phase voltage unbalance can cause excessive heating to
three-phase devices such as motors. Additionally, high ripple may be observed in some
three-phase AC/DC power supplies if the voltage unbalance is too high.
8.2.5 Operational Improvements. System configurations should be designed and installed
with provisions for future expansion, or possible changes in the system configuration, and
improvements to the system equipment. Operational improvements may be required resulting
from increased load on the system or the addition of a different type of utilization equipment
(which may require a different voltage or frequency to operate properly or a better quality supply
than that available). In these situations, the system must carry more load than originally
anticipated in the design. Increasing the distribution voltage(s) and increasing transformer
capacity by the addition of cooling fans are two methods used on existing equipment to modify
and successfully fulfill the new load requirements. The improvement in system power factor
may release additional system capacity for utilization to serve new loads, rather than being
wasted on the reactive power requirements of the existing system configuration.
8.2.5.1 Increasing System Distribution Voltage. This technique is often used on overhead
pole line type distribution systems to increase system capacity. The system must have
transformers that are capable of operating at the new voltage. This method, therefore, is usually
limited to systems consisting of many single-phase transformers operating in three-phase banks
which can be reconnected on their primary side from delta to wye without change to the
transformer's operating voltage. The system voltage is generally increased by a factor of the
square root of three (e.g., from 2400 V to 4160 V), the transformers are reconnected, and the new
load is added onto the line at new transformer locations. of course, the existing transformers
cannot carry additional load, but the individual distribution circuit's load capacity has increased
by the square root of three, or 1.732. If excessive voltage drop is a problem, then the same
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