Comparison of Two-Level, Three-Level, and Modular Multi-Level Converters

TWO-LEVEL
CONVERTER
It is the simplest and can be
represented as a bridge with 6 pulses
replacing the thyristors with IGBT’s of
anti-parallel diodes. Each phase of the
operation has a voltage switching
between 2 discrete levels of voltages
that correspond to the electric
potential of negative and positive DC
terminals.

 Advantages
 Simple to operate.
 Cheap to construct and obtain.
 Easier voltage tracking across a capacitor.
 Disadvantages
 Leads to an unacceptable production of harmonic distortion levels forcing
use of PWM that improves the distortion.
 Losses due to high on and off switching of the IGBT’s, about twenty times
in a cycle thus reduction of efficiency.

THREE-LEVEL
CONVERTER
Improves the poor harmonics of
the two-level converter
synthesizing the three discrete
voltage levels found in each
phase. The common type is the
diode clamped converter of 4
valves of IGBT's in each phase
having half rotation of the line-
to-line DC voltage with 2 values
of clamping diodes.

 Its DC capacitors are split into 2 branches
connected in series with the values of
clamping diode wired between the midpoint of
the capacitor and three quarter and one-
quarter points of each phase. Positive (1/2)U4
turns on the two IGBT valves on the top and
negative (1/2)U4 turns on the bottom two
valves of IGBT while zero turns on the middle
Valves of IGBT.