UFC 3 -520-01
June 10, 2002
POWER SYSTEM QUALITY
This chapter provides a technical basis for power system quality as a design
consideration and explains different methods of solving power quality problems. Unlike
other electrical design requireme nts, power quality design solutions are very dependent
on the types of transients and disturbances that can and will occur in power systems.
Also, power quality solutions often involve a certain level of compromise between the
electrical system design and the design of the end -use equipment. In many cases, it
will be easier to provide protection and power quality design features to specific
equipment rather than generically throughout the facility. Nonetheless, power quality is
an issue that should be addressed at the facility level in order to be certain that the
electrical distribution system is designed properly for the anticipated disturbances and
Voltage and current surge transients caused by lightning or switching
operations are discussed in Chapter 11. Surge protection is not considered a power
quality issue, but instead is considered a part of electrical system protection. Power
quality design considerations focus on 1) longer-term degradation of the ideal sinusoidal
voltage and current caused by harmonic distortion or unbalanced system operation and
2) power system disturbances.
The principal source of a steady-state voltage unbalance is unbalanced
single-phase loads o n a three-phase system. Voltage unbalance is particularly
important for three-phase motor loads. ANSI C84.1 specifies a no -load service
entrance voltage unbalance of less than 3 percent to avoid motor overheating or failure.
High efficiency motors are p articularly susceptible to unbalanced voltages; these motors
have a lower negative sequence reactance which causes higher negative sequence
currents during unbalanced voltage conditions. Unbalanced voltages can also be
caused by blown fuses on one phase o f a circuit or single phasing conditions.
At the design stage, evaluate the loading on each phase and balance the
loads as well as possible. As part of acceptance testing , monitor the degree of
unbalance and make corrections if necessary.
The degree to which a system is unbalanced can be evaluated by
symmetrical components. The ratio of the negative or zero sequence component to the
positive sequence component is used as an indicator of the percent unbalance:
V2 - V1
Percent Unbalance =