06-12-2012, 12:43 PM
Analysis of Multi-Carrier PWM Methods for Asymmetric Multi-Level Inverter
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Abstract-
Various topologies and modulation strategies of Multilevel
inverters have been proposed for utility and drive
applications. This paper studies multi-carrier PWM methods for
asymmetric multi-level inverter. Phase disposition, phase
opposition disposition and alternative phase opposition disposition
methods are discussed considering switching frequencies,
spectrum of the output waveform and the use of inverter state
redundancies.
INTRODUCTION
In recent years, there is a growing demand for high voltage
conversion systems capable of providing high output voltage
signals and having good spectral performance and easy control.
Examples of such systems are FACTS devices VSC HVDC
transmission, AC drives, and active filters [1-2]. To overcome
the semiconductor voltage and current rating limitations, some
kind of series and/or parallel connection will be necessary. Due
to their ability to synthesize waveforms with a better harmonic
spectrum and attain higher voltages, multi-level inverters have
been receiving increasing attention in the past few years.
However, the increasing number of devices tends to reduce the
overall reliability and efficiency of the power converter [3-6].
Any inverter that generates more than two output-voltage
levels is called a multi-level inverter. Multi-level inverters are
characterized by the number of different output voltage levels
that can be generated by the inverter. The well-known circuit
topologies are: cascaded H-bridge multi-level inverter, diodeclamped
multi-level inverter and flying-capacitor multi-level
inverter.
ASYMMETRIC MULTI-LEVEL INVERTER STRUCTURE
Up to now, mainly asymmetric cascaded H-bridge inverters
have been investigated [7–9], although the concept can be
applied to other circuit topologies as well [9–11 and 16–18].
Asymmetric multi-level inverters have exactly the same circuit
topology as symmetric multi-level inverters. They differ only
in the rating of capacitor voltages.
The single phase circuit diagram of an asymmetric cascaded
nine-level inverter is shown in Fig.1. Each H-bridge cell has
four possible switching states, of which two results in the same
output voltage. The output voltages are ‘plus the dc-link
voltage’, ‘minus the dc-link voltage’ and ‘zero voltage’ (two
times). The phase output voltage is, of course, obtained by
summing over all cells. Table I lists the inverter states, which
allow bi-directional current flow at a constant inverter output
voltage.
Phase Disposition PWM (PD-PWM)
The phase disposition PWM (PD-PWM) method, as one of
the carrier-based PWM methods, is based on a comparison of a
sinusoidal reference waveform, with vertically shifted carrier
waveforms. The PD-PWM method uses N-1 carrier signals to
generate the N-level inverter output voltage. As it can be seen
in Fig. 3, the carrier signals have the same amplitude, Ac and
the same frequency, fc and are in phase. The sinusoidal
reference wave has a frequency fr and an amplitude Ar. At each
instant, the result of the comparison is decoded in order to
generate the correct switching function corresponding to a
given output voltage level.
Fig. 3 shows the carriers and the resulting PWM signals (with
ma=0.85 and mf=39) which directly yields the gate signals for
the switches S1, S4, S5 and S8 of Fig. 2. Also, this figure shows
the main and sub-inverter output voltages, output voltage and
its harmonic spectrum. As it can be seen in Fig. 3, all inverter
states of the Table I, expect states 1, 3, 6, 8, 12, 13 and 14,
have been used.
CONCLUSION
Multi-level inverter has been widely investigated for highpower
applications in recent years. Various topologies and
modulation strategies have been proposed for utility and drive
applications in the literature. This paper studies multi-carrier
PWM methods for asymmetric multi-level inverter. PD-PWM,
POD-PWM and APOD-PWM methods have been discussed
considering switching frequencies, spectrum of the output
waveform, and the use of inverter state redundancies.
In different carrier-based PWM methods for asymmetric
multi-level inverters, the switching actions are unbalanced
among half bridges. Also, the switching action is a function of
the frequency modulation index and amplitude modulation
index. Comparing different methods from harmonics point of
view, the significant harmonic energy of PD-PWM is
concentrated on the carrier frequency.