Page 47 - Power Quality in Electrical Systems

30 Chapter Three

rms variation
120
115
Voltage (%) 110
105
100
95
90
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Time (ms)
(a)
150
100
Voltage (%) 50 0

−50
−100
−150
0 25 50 75 100 125 150 175 200
Time (ms)
(b)
5
Figure 3.7 Voltage swell. The top trace is the instantaneous rms value of the
voltage. The bottom trace is the line-voltage [3.1].
[© 1995, IEEE, reprinted with permission]
Voltage “Swell”
A “swell” is the converse of the sag, and is a brief increase in the rms line-
voltage. Shown in Figure 3.7 is a voltage swell caused by a line-to-ground
fault.

Impulsive “Transient”
An impulsive “transient” is a brief, unidirectional variation in voltage, cur-
rent, or both on a power line. The most common sources of impulsive tran-
sients are lightning strikes (Figure 3.8). Impulsive transients due to
6
lightning strikes can occur because of a direct strike to a power line, or from
magnetic induction or capacitive coupling from strikes on adjacent lines.
Note that in this case, the maximum amplitude of the transient cur-
rent is ~23 kiloamps and the duration of the transient current is tens
of microseconds. The frequency and amplitude of lightning-induced
transients vary geographically, as shown in Figure 3.9.

5
From IEEE Std. 1159-1995, p. 19.
6
Also called lightning “strokes.”