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

This presentation compares the Two-Level, Three-Level, and Modular Multi-Level Converters. It explains the advantages and disadvantages of each type, including voltage levels, construction, efficiency, and harmonic performance. The presentation also includes references for further study.

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.

 Advantages
 Allows compensation of three-phase load currents that are
unbalanced in four wire system.
 Disadvantages
 Poor voltage balancing on its DC capacitors.
 Uneven distribution of losses between devices.
 The excessive necessity of clamping diodes during higher voltage
levels.

Modular Multi-LEVEL
CONVERTER
MMC has 6 valves that each
have a connection from one
terminal of ac to one terminal of
DC. The difference occurs where
the MMC valves have separate
voltage source control as
opposed to two-level converters
where the ICBT valves are
connected in series controlled
with high voltage.

 Every valve of MMC has independent sub-
modules converters each having a storage
capacitor. Every sub-module has a connection
to two IGBT’s in series to midpoint capacitor
connection and one terminal of the capacitor
connected externally.

 A capacitor would then have a connection to
the circuit or bypassed with dependence to
which IGBT is on. Therefore, every sub-module
is an independent two-level converter that
generates the voltage of value Usm or zero.
Having a suitable connection of these sub-
modules in series could facilitate the synthesis
of stepped waveform voltage with a close
approximation to the sine wave of low
harmonic distortion level.

Advantages
 Separate sources of DC required for each valve.
 Possible voltage output is greater than twice the DC
sources value.
 The process is cheap and quick.
 High efficiency in the switching of the fundamental
frequency.
 Applicable to back-to-back inter-connection due to
common phase DC bus sharing.
 Voltage level balancing is possible due to available
phase redundancy.

 Disadvantages
 In the transmission of real power, there is poor
switching utilization and efficiency.
 Increased number of capacitors makes the
converter more bulky and expensive.
 Complicated control in voltage level tracking
across the capacitors.

 Comparison
 The two-level converter has 2 discrete levels of voltage while the three-
level converter has three and the MMC has more than three discrete
voltage levels.
 The level of difficulty in construction and obtaining increases from two-
level through to MMC with the increase in IGBT valves.
 Efficiency also increases from the two-level converter through to MMC
with the increase of harmonic performance.
 The MMC has greater difficulty in tracking capacitor operation than both
two-level and three-level converters. Also, the increased number of
switches makes the MMC difficult to operate.
 The MMC has separate voltage source control of its IGBT connected in
series compared to the other two whose IGBT connections are in parallel.

References
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Systems, Mönchengladbach: John Wiley & Sons, 2011.
 C. Maurizio, P. Marcello and V. Gianpaolo, Power Converters and AC Electrical Drives
with Linear Neural Networks, Bielefeld: CRC Press, 2017.
 W. Bin, High-Power Converters and AC Drives, Hamburg: John Wiley & Sons, 2007.
 L. Yushan, A.-R. Haitham, G. Baoming, B. Frede, E. Omar and C. L. Poh, Impedance
Source Power Electronic Converters, Frankfurt: John Wiley & Sons, 2016.
 L. Fang and Y. Hong, Advanced DC/AC Inverters: Applications in Renewable Energy,
Bielefeld: CRC Press, 2016.
 Q. Li and J. Chen, Advanced Materials And Energy Sustainability - Proceedings Of The
2016 International Conference On Advanced Materials And Energy Sustainability
(Ames2016), World Scientific, 2017.
 S. Kamran, H. Lennart, N. Hans-Peter, N. Staffan and T. Remus, Design, Control, and
Application of Modular Multilevel Converters for HVDC Transmission Systems,
Bremen: John Wiley & Sons, 2016.
 L. Leonard, Electric Power Generation, Transmission, and Distribution, Third Edition,
Bonn: CRC Press, 2016.
 A. Sergio, S. A. and I. Maria, Multilevel Converters for Industrial Applications,
Bremen: CRC Press, 2016.