Page 42 - Power Quality in Electrical Systems

Chapter
3

Voltage Distortion

In this chapter, we shall discuss distortion of the line-voltage
sine waves. Distortion to the line-voltage waveforms can be
caused by transient or continuous disturbances. Examples of
transient disturbances include lightning, motor starting and
stopping, clearing faults, and other harmonic-generating
occurrences. Throughout the chapter, we use PSPICE
simulation results to illustrate examples of voltage distortion.
A chart summarizing voltage distortion and its causes is
shown at the end of this chapter.

Voltage Sag
1
A voltage sag is an event where the line rms voltage decreases from
the nominal line-voltage for a short period of time. Figure 3.1 shows an
80 percent sag with a duration of a few 60-Hz cycles. This type of vari-
ation can occur if a large load on the line experiences a line-to-ground
fault, such as a short in a three-phase motor or a fault in a utility or
plant feeder.
In Figure 3.2a, we see a circuit with a line supplying an electric motor.
Note that the line impedances cause a voltage drop when currents are
drawn from the line. When the motor is energized, the motor current
I causes a voltage drop to other loads in the system at the point of
m
common coupling (PCC). Figure 3.2b displays a voltage sag due to a large
motor starting, such as a pump or air-conditioner motor. Note that when
an induction motor starts, it can draw very high currents until the rotor

1
The IEEE term sag is a synonym to the IEC term dip.

25
Copyright © 2007 by The McGraw-Hill Companies, Inc. Click here for terms of use.