Comparative Study of NPC and Cascaded Converters Topologies

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Introduction
The interest for multilevel converters comes from their
capacity to provide higher power, to generate good quality of
waveforms, to operate at low switching frequency with low
loss of energy and the low effort on statics devices [1], [2].
Among multilevel converters topologies, there are the Neutral
Point Clamped (NPC) converter, the cascaded H-bridge
multilevel converter and the imbricated cell multilevel
converter [1], [3].


Theoretical Analysis
A. NPC Converter
Three phase NPC converter with five levels of voltage is
shown on Figure 1. Each phase is made of eight controlled
switches which are unidirectional in voltage and bidirectional
in current and six maintaining diodes connected along the
continued bus. The converter is powered by a DC source V,
connected to four capacitors of equal values.
Our study is limited to a leg of NPC converter with five
levels. The voltage values vao are determined for different
operating conditions and the switching sequences of the statics
devices are presented. Five switching sequences can be
identified:


III. Generalization
A. Multilevel Voltage NPC Converter
The study of any multilevel NPC converters is derived
from the analysis of three level and five level NPC converters.
Figure 5 shows the configuration of a leg of the multilevel
NPC converter.



IV Principle of Control
Many methods to generate pulse width modulation
(PMW) signals for multilevel converter have been reported
during the last two decades. There are two pulse width
modulation technologies: the interceptive modulation and the
space vector modulation (SVPWM) [5]. Recently, with the
development digital technology, the SVPWM (Space Vector
Pulse Width Modulation) is widely used, not only because of
its easily implementation, but also for the quality of signals
obtained by this method, in particular low displacement
current factor [5] [6]. The major inconvenience of this method
is that, the control algorithm to generate control signals are
becoming increasingly complex for converters with voltage
levels more than three [5].


V. Simulation
Simulation study was carried out for two three phase
topologies: the three levels and the five levels converters. The
phase shifted (PS) PWM control method was used for power
switches. The input voltage source is 110V, the load is a of a
resistance R =1 W in series with an inductance L = 0.75 H .
The power factor is cosj = 0.80 .