Page 52 - Power Quality in Electrical Systems
P. 52
Voltage Distortion 35
capacitor bank is switched. The ripple in the load voltage is due to the capac-
10
itor current that rings at the resonant frequency of the LC circuit.
Interruption
An interruption is defined as a reduction in line-voltage or current to
less than 10 percent of nominal, not exceeding 60 seconds in length.
Interruptions can be a result of control malfunction, faults, or improper
breaker tripping. Figure 3.13 shows an interruption of approximately
1.7 seconds in length.
Notching
Another common power-quality event is “notching.” Notching can occur
during current “commutation” in single-phase and three-phase rectifiers.
Shown in Figure 3.14a is a three-phase rectifier with the line inductance
L of each phase shown. Note that in the rectifier, if we assume that the
s
rms variation
120
100
Voltage (%) 60
80
40
20
0
0 0.05 0.1 0.15 0.2 0.25 0.3
Time (ms)
150
100
Voltage (%) 50 0
−50
−100
−150
0 25 50 75 100 125 150 175 200
Time (ms)
Figure 3.13 An interruption of approximately 0.17 seconds duration 11 [3.1].
The top trace is the rms line-voltage. The bottom trace is the first 200 mil-
liseconds of the interruption.
[© 1995, IEEE, reprinted with permission]
10
The ring frequency is 1>2p 2LC radians/second, or 1591 Hz.
11
From IEEE Std. 1159-1995, p. 16.
