The concept of utilizing multiple small voltage levels to perform power conversion
was patented by an MIT researcher over twenty years ago. Advantages of this
multilevel approach include good power quality, good electromagnetic compatibility
(EMC), low switching losses, and high voltage capability. The main disadvantages
of this technique are that a larger number of switching semiconductors are required
for lower-voltage systems and the small voltage steps must be supplied on the dc
side either by a capacitor bank or isolated voltage sources. The first topology
introduced was the series H-bridge design. This was followed by the diode clamped
converter which utilized a bank of series capacitors. A later invention
detailed the flying capacitor design in which the capacitors were floating rather
than series-connected. Another multilevel design involves parallel connection of
inverter phases through inter-phase reactors
.
In this design, the semiconductors
block the entire dc voltage, but share the load current. Several combinational
designs have also emergedsome involving cascading the fundamental topologies.
These designs can create higher power quality for a given number of
semiconductor devices than the fundamental topologies alone due to a multiplying
effect of the number of levels.
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