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TOSHIBA G9000 UPS White Paper
© 2008 TOSHIBA INTERNATIONAL CORPORATION – INDUSTRIAL DIVISION Page 6
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UPSG9000WP081201
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This improvement in waveform synthesis results in considerable
size/weight reduction and, consequently, space savings since a multilevel
topology requires a smaller filter to obtain a clean sinusoidal waveform.
Some simulation results demonstrate the difference between the
conventional and the multilevel converter in terms of attenuation of
high-frequency components. Figure 8 shows the inverter current and its
harmonic spectrum (fs: switching frequency) for, respectively (from top to
bottom):
n
a conventional inverter with a 0.05 p.u. filter reactor;
n
a multilevel inverter with a 0.05 p.u. filter reactor;
n
a conventional inverter with a 0.10 p.u. filter reactor.
Note that, to match the current ripple and harmonic spectrum performance
seen in a three-level converter, a two-level topology requires a larger
reactor. Therefore, three-level based structures have a competitive
advantage in terms of size/weight reduction.
Inverter Current
Inverter Current
Inverter Current
1
x
fs 2
x
fs 3
x
fs 4
x
fs
1
x
fs 2
x
fs 3
x
fs 4
x
fs
1
x
fs 2
x
fs 3
x
fs 4
x
fs
Harmonic Spectrum
Harmonic Spectrum
Harmonic Spectrum
2- LEVEL INVERTER
REACTOR INDUCTANCE 0.05 p.u.
3- LEVEL INVERTER
REACTOR INDUCTANCE 0.05 p.u.
2- LEVEL INVERTER
REACTOR INDUCTANCE 0.10 p.u.
FIGURE 8
Inverter current and
its harmonic
spectrum for
various circuit
configurations:
2-level with a 5%
reactor (blue),
3-level with a 5%
reactor (green) and
2-level with a 10%
reactor (red).
Compactness is an
inherent feature of
multilevel converters.
FIGURE 7
Line-to-line voltage
in a 3-level inverter