Multilevel voltage source converters are emerging as a new breed of power converter options for high-power applications. Here, staircase voltage wave is generated from several levels of dc capacitor voltages. the voltage unbalance between different levels is the major disadvantage of multilevel converters. voltage clamping or capacitor charge control are the main techniques used to solve this. The following types of multilevel voltage source converters are described here:
a)diode-clamp: The diode-clamp method can be applied to higher level converters.
b) flying capacitors
c) cascaded-inverter with separate dc sources.
The operating principle, features, constraints, and potential applications of these converters are described in this article.
A multilevel converter typically generates a sinusoidal voltage from several levels of voltages, typically obtained from capacitor voltage sources. A three-level converter, or neutral-clamped converter contains two capacitor voltages in series and uses the center tap as the neutral. two pairs of switching devices in series is presented in case of each phase leg. By increasing the number of steps, the staircase waveform nearly resembles the sinusoidal waveform with reduced number of harmonics.

DIODE-CLAMP MULTILEVEL CONVERTER
An m-level diode-clamp converter typically consists of
m - 1 capacitors on the dc bus and produces m levels of the phase voltage.

MULTILEVEL CONVERTER USING FLYING-CAPACITORS
The voltage level defined in the flying-capacitor converter is similar to that of the diode-clamp type converter. The phase
voltage of an m-level converter has m levels including the
reference level, and the line voltage has (2m - 1) levels.

MULTILEVEL CONVERTER USING CASCADED-INVERTERS WITH SEPARATE DC SOURCES
extra clamping diodes or voltage balancing capacitors is avoided here. Each SDC is associated with a single-phase full-bridge inverter. The ac terminal voltages of different level inverters are connected in series.

Get the report here:
http://filesonicfile/24224779/Multilevel Converters.pdf