27-03-2012, 03:58 PM
Natural Balancing of Flying Capacitor Voltages in Multicell Inverter Under PD Carrier-Based PWM
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INTRODUCTION
RECENTLY, both industries as well as academia have
shown significant interests in the flying capacitor multicell
inverter (FCMI) topology and its various applications are
explored [1]–[9]. A single-phase five-level multicell inverter,
shown in Fig. 1, is considered in this paper. In Fig. 1, the switches
S11–S41 are complementary to S1–S4 , respectively. Each switch
pair Sk , Sk 1 (k = 1–4) represents a single cell, which can be
called cell-k. Each switch represents an insulated gate bipolar
transistor (IGBT) with an antiparallel diode. Vdc represents the
dc-link voltage. The capacitors C2 , C3 , and C4 are called flying
capacitors and their voltages VC2 , VC3 , and VC4 are controlled
to be regulated at 3Vdc/4, Vdc/2, and Vdc/4, respectively.
PREFERENTIAL CELL-SELECTION-BASED NATURAL
BALANCING SCHEME
The scheme presented here, like the previous one, uses the
conventional PD carrier patterns to achieve natural balancing of
FCMI. However, the control switching signals are not generated
directly by the outputs of the comparators comparing dam and
SHUKLA et al.: NATURAL BALANCING OF FLYING CAPACITOR VOLTAGES IN MULTICELL INVERTER UNDER PD CARRIER-BASED PWM 1689 the carriers, unlike in the previously presented scheme. The tasks of voltage level selection and switching event distribution are decoupled in this scheme.
V. EXPERIMENTAL RESULTS
To experimentally verify the afore-presented natural balancing
approaches under PD PWM, a prototype of a single-phase
five-level IGBT-based multicell inverter is built in the laboratory.
The overall structure of the experimental setup
CONCLUSION
This paper presents two improved modulation approaches
to achieve natural balancing of FCMI under PD carrier-based
PWM scheme. The PS scheme offers natural balancing in a
straightforward manner. However, it is not possible to achieve
natural balancing with PD scheme, which is harmonically superior
among the carrier-based modulation schemes, by using
the direct modulation principle. Previous attempts for achieving
natural balancing under PD scheme have used reduced magnitude
trapezoidal carrier waveforms to compare against the
modulation signal.