22-03-2013, 03:29 PM
A Cascade Multilevel Inverter Using a Single DC Source
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Abstract
A method is presented showing that a cascade
multilevel inverter can be implemented using only a single DC
power source and capacitors. A standard cascade multilevel
inverter requires n DC sources for 2n + 1 levels. Without
requiring transformers, the scheme proposed here allows the use
of a single DC power source (e.g., a battery or a fuel cell stack)
with the remaining n−1 DC sources being capacitors. It is shown
that one can simultaneously maintain the DC voltage level of the
capacitors and choose a fundamental frequency switching pattern
to produce a nearly sinusoidal output.
INTRODUCTION
A cascade multilevel inverter is a power electronic device
built to synthesize a desired AC voltage from several levels of
DC voltages. Such inverters have been the subject of research
in the last several years [1][2][3][4][5], where the DC levels
were considered to be identical in that all of them were either
batteries, solar cells, etc. In [6], a multilevel converter was
presented in which the two separate DC sources were the
secondaries of two transformers coupled to the utility AC
power. In contrast, in this paper, only one source is used
without the use of transformers. The interest here is interfacing
a single DC power source with a cascade multilevel inverter
where the other DC sources are capacitors. Currently, each
phase of a cascade multilevel inverter requires n DC sources
for 2n+1 levels in applications that involve real power transfer.
In this work,
EXPERIMENTAL RESULTS
A three-phase wye-connected cascaded multilevel inverter
using 100 V, 70 A MOSFETs as the switching devices [13]
was used to carry out the experiments. A power supply was
used as the DC power source with Vdc = 25 V and a 8
mF capacitor was used as the second DC source. The realtime
controller was implemented on an FPGA chip with an
8 μ sec time step where the switching angles as a function
of the modulation index were stored in a lookup table. The
multilevel converter was connected to a three phase induction
motor whose nameplate data is 1/3 hp, 1.5 A, 1725 rpm and
208 V (RMS line-to-line at 60 Hz).
CONCLUSIONS
A cascade multilevel inverter topology has been proposed
that requires only a single DC power source. Subject to
specified constraints, it was shown that the voltage level of the
capacitors can be controlled while at the same time choosing
the switching angles to achieve a specified modulation index
and eliminate harmonics in the output waveform.