Page 88 - Power Quality in Electrical Systems

Harmonic Current Sources 71

Six-pulse
Y/Y rectifier
i a
a
b I L
c
L
Six-pulse
LOAD
∆/Y rectifier
R

(a)

i (t)
a

t

(b)
Figure 5.9 A twelve-pulse rectifier. (a) The circuit. (b) The line current for
phase-a for L/R period of line frequency.
than in the six-pulse case. This is because the 12-pulse topology elimi-
nates the 5th, 7th, 17th, and 19th harmonics, leaving the 11th, 13th,
23rd, and 25th harmonics.

High-Frequency Fluorescent Ballasts
High-frequency fluorescent ballasts (Figure 5.10) are a rich source of high-
frequency voltage and current harmonics. The line waveform is rectified
by a power-factor corrected boost converter. The rectified DC voltage
powers a high-frequency inverter that generates the high voltage needed
i s
Power
factor
v s correction C BUS Inverter Lamp
boost
Figure 5.10 High-frequency switching fluorescent ballast with a power
factor correction front end.