Dissertação
Estratégias de modulação para conversores multiníveis em cascata sob faltas
Fecha
2012-01-20Registro en:
CARNIELUTTI, Fernanda de Morais. New modulation strategies for cascaded multilevel converters. 2012. 135 f. Dissertação (Mestrado em Engenharia Elétrica) - Universidade Federal de Santa Maria, Santa Maria, 2012.
Autor
Carnielutti, Fernanda de Morais
Institución
Resumen
Multilevel converters are being increasingly employed nowadays, specially in mediumand
high-voltage industrial applications. Even though these converters are able to
synthesize output line-to-line voltages with a high number of levels, close to a sinusoidal
waveform, their modulation is more complex than the one for two- and three-level
converters. In this context, this dissertation proposes new modulation strategies for
multilevel converters, specifically symmetrical and asymmetrical cascaded multilevel ones,
composed of many full-bridges, or power cells, per phase. If the converter has one
or more faulty cells, they can be bypassed and the converter can continue to feed the
load, increasing the process reliability. However, the converter phase voltages must be
modified so as to keep the output line-to-line voltages balanced. With the objective
of proposing modulation strategies that allow the cascaded multilevel converters to
satisfactorily operate under these conditions, an extensive bibliographical review of the
existing modulation techniques has been carried out. The carrier-based modulation
approaches were studied first. It could be noticed that all these strategies belong to a
larger set of solutions for the obtention of the converter modulating phase voltages. This
set is derived in this work, resulting in a generalized geometrical modulation strategy for
symmetrical and asymmetrical cascaded multilevel converters with any number of levels
and operating under normal or faulty conditions. As the faulty cells are restrictions for
converter operation, for each fault condition the region that contains all the possible
converter common-mode voltages, that compensate for the loss of cells, is derived. The
choice of a common-mode pertaining to this set allows the entire converter synthesis
capability to be explored. The modulating voltages are the sum of the reference and the
common-mode voltages, maximizing the amplitudes of the output line-to-line voltages.
For asymmetrical cascaded multilevel converters, the voltages synthesized by the highervoltage
cells are restrictions for the operation of the lower-voltage ones. Concerning
the Space Vector (SV) modulation, it was derived only for the asymmetrical cascaded
multilevel converter. The higher-voltage and lower-voltage cells switch, respectively, with
low frequency by the choice of the nearest vector to the reference, and with high frequency,
by the choice of the three nearest vectors to the reference, in one switching period. The
voltage synthesized by the higher-voltage cells is subtracted from the reference, resulting
in the new reference for the lower-voltage cells, and so successively, until the cells with the
lowest voltages. A specific switching sequence is defined off-line for each sector of the SV
diagram. The algorithm is carried out in a modified αβo coordinate system, resulting in
switching vector with only integer entries. The choice of the switching vectors considers all
the possible redundancies in abc coordinates. At last, simulation and experimental results
Abstract
that prove the good performance of the proposed modulation strategies are presented.