Page 50 - Power Quality in Electrical Systems

Voltage Distortion 33

example, we assume there are no lightning arresters or insulation break-
down to limit the transient voltages.

Oscillatory “Transient”
An oscillatory transient is a brief, bidirectional variation in voltage,
current, or both on a power line. These transients can occur due to res-
onances during switching. A circuit capable of exhibiting this phenom-
enon is shown in Figure 3.11a. A power supply bus is shown with the
bus having inductance L. A capacitor bank labeled C is connected at one
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end of the bus. This capacitor bank may be in place, for instance, for
power factor improvement or for voltage sag improvement.
If at some time, we switch in capacitor bank C with the switch as shown,
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a resonant condition is set up between the line inductance and the

R L

Bus
C 1 C 2

(a)
7500

5000

2500
Current (A) 0

−2500

−5000

−7500
8 10 12 14
Time (ms)
(b)
Figure 3.11 Oscillatory transients. (a) Circuit that can produce an oscillatory transient.
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(b) An oscillatory transient [3.1].
[© 1995, IEEE, reprinted with permission]

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From IEEE Std. 1159-1995, p. 14.