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CHB Inverter Fed Drive Source: Toshiba - General Electric
H-bride power cells
Topic 6
Multilevel Cascaded H-Bride (CHB) Inverters

Lecture Topics
H-Bridge Inverter
CHB Inverter Topologies
Phase Shifted PWM Level Shifted PWM
PWM Scheme Comparison
Multilevel CHB Inverters

To increase inverter operating voltage without devices in series
To minimize THD with low switching frequencies fsw
To reduce EMI due to lower voltage steps
Why Use Multilevel Inverters?
Switching frequency for high power converters:
fsw = 60Hz ~ 1000Hz
Multilevel CHB Inverters

Per-Phase Diagram
Multilevel Inverter Topologies

H-bridge Power Cell
H-Bridge Inverter

H-bridge Power Cell
Typical Industrial Applications
Five-level CHB inverter
H-Bridge Inverter

Bipolar Modulation
Bipolar PWM:
vAB from -Vd to +Vd
or from +Vd to -Vd
H-Bridge Inverter

Bipolar Modulation (FFT)
H-Bridge Inverter

Unipolar Modulation (1)
Two modulation waves
vm and vm-
One carrier wave vcr
Unipolar PWM:
vAB from 0 to +Vd
or from 0 to -Vd
H-Bridge Inverter

One modulating wave: vm
Two carrier waves: vcr and vcr- Unipolar Modulation (2)
H-Bridge Inverter

Five-Level CHB Inverter
Converters in cascade, but no switching devices in series.
Complementary Switch pairs:
S11 and S41;
S31 and S21;
S12 and S42;
S32 and S22;
CHB Inverter Topologies

Output Voltage and Switching Status (five-level)
Waveform of vAN is composed of five voltage levels: 2E, E, 0, -E, and -2E
CHB Inverter Topologies

Seven- and Nine-Level Inverters (Per phase diagram)
CHB Inverter Topologies

Unequal dc Bus Voltages
CHB Inverter Topologies

Unequal dc Bus Voltages (Two-cell seven-level topology)
CHB Inverter Topologies

Carrier Based PWM – Phase Shifted
# of voltage levels: m = 7
# of carriers: mc = m - 1 = 6
Phase shift: 360° / mc = 60°
Carriers for H1 bridge:
vcr1 and vcr1-
Carriers for H2 bridge:
vcr2 and vcr2-
Carriers for H3 bridge:
vcr3 and vcr3-
m = 7
Phase Shifted PWM

Inverter Waveforms (7-level, phase shifted)
Switching occurs at
different times fsw(device) = 60 mf = 600Hz
Inverter phase voltage
levels: 7
Low EMI
Line-to-line voltage
levels: 13
Close to a sinusoid
Low THD
Phase Shifted PWM

FFT (7-level, phase shifted)
Lowest harmonics:
around 2mf
Lowest harmonics:
around 6mf
Containing triplen
harmonics
No triplen harmonics
Equivalent fsw(inverter)
= 60(6mf ) = 3600Hz
Phase Shifted PWM

Harmonic Content (7-level, phase shifted)
Phase Shifted PWM

Carrier Based PWM – Level Shifted
# of voltage levels: m = 5
# of carriers:
mc = m - 1 = 4
IPD provides the best harmonic profile.
IPD
APOD
POD
Level Shifted PWM

# of voltage levels:
m = 7
# of carriers:
mc = m - 1 = 6
fsw(device):
- not equal to fcr, and
- not the same for all switches.
Device conduction angle:
- not equal.
Necessary to swap switching
pattern.
Gating Arrangement (7-level)
Level Shifted PWM

Inverter Output Voltages (seven-level)
m = 7
vAB close to a sinusoid
Low THD, low EMI
fsw(inv) = fc = 3600Hz
Switching occurs at
different times
fsw(device) = fcr /mc =
600Hz (avg)
Level Shifted PWM

Prototype at Ryerson
(Seven-level CHB Inverter)
Level Shifted PWM

Measured Waveforms (IPD, 7-level)
Inverter phase voltage vAZ
Line-to-line voltage vAB
Level Shifted PWM

PWM at Low ma
- Phase shifted PWM:
THD = 96.7%
- Level shifted PWM:
THD = 48.8%
At ma = 0.2:
PWM Scheme Comparison

Total Harmonic Distortion (THD)
PWM Scheme Comparison

Summary
PWM Scheme Comparison