| dc.description.abstract | Multilevel voltage source inverters have become a widely accepted and cost-effective
power converter technology for applications requiring high-power medium-voltage
control. The demand of power level requirement has reached operational limits of megawatt
range. Multilevel inverters (MLI) find applications in power transmission and distribution
systems like HVDC which are connected with high voltage network lines and controlled
ac drives operating at medium voltage levels. For low voltage applications, most prevalent
topology which dominates industrial drives is conventional two-level inverter. With
state of the art semiconductor technology, self-commutating converters with arrangement
of several low voltage devices, help achieving voltage ranges till hundreds of kilovolts.
Apart from high voltage operational capability, advantages like power quality control,
better electromagnetic compatibility, lower switching losses, keep multilevel inverters a
class above the conventional two-level inverters. In order to attain good waveform quality,
the inverter needs to switch at very high frequencies. The harmonics appear only at
switching frequency sidebands, which can be easily filtered externally. But, considering
large voltage stress handled by the devices in two-level inverter and large switching loss
in the devices degrade the efficiency of system substantially. Specific to applications like
medium voltage drives, the major issues on electromagnetic interference, device stress,
harmonic performance, and dv/dt control are mostly addressed by employing multilevel
inverters. Most popular multilevel inverter topologies are neutral-point clamped inverters,
flying capacitor inverters, and cascaded H-bridge inverters. These basic MLIs are
further used to obtain hybrid multilevel inverters generating more number of voltage
levels. Other applications of multilevel inverters include photovoltaic, hydel and wind
energy systems, energy storage and management systems, electric vehicle applications,
traction drives etc.
As a 24-sided polygon is closer to a circle than a hexagon or a 12-sided polygon, the
above presented schemes generate high quality motor phase voltage waveforms without
using any external filters. A physical sine-wave filter can be completely relaxed for such
variable speed drive applications, and the dynamic performance is never compromised
since the filtering action is performed by switched capacitors. The topologies and modulation
techniques presented are optimized for low switching frequency operation of large
voltage blocking inverters and shifting relatively higher frequency switchings to low voltage
cascaded H-Bridge inverters. Above all, single DC source operation can bring down
the cost and complexity of the system drastically enabling easier back to back operation
for drive. Also, such schemes can be directly driven from battery operated systems in
electric vehicles without any passive sine filters. With all the mentioned advantages, the
proposed drive schemes are highly suitable for high performance, medium voltage drive
applications. | en_US |
