15-12-2012, 01:59 PM
Multilevel Current Waveform Generation Using Inductor Cells and H-Bridge Current-Source Inverter
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Abstract
This paper presents a new circuit configuration of
single-phase multilevel current-source inverter (CSI). In this new
topology, a basic H-bridge CSI working as a main inverter generates
a multilevel current waveform in cooperation with inductor
cells connected in parallel as auxiliary circuits. Each inductor cell
is composed by four unidirectional power switches with an inductor
across the cell circuit. The inductor cells work by generating
the intermediate level of the multilevel current waveform with
no additional external dc-power sources. A simple proportionalintegral
controller is applied to control the intermediate-level
currents of the multilevel output waveform. A five-level and a ninelevel
pulsewidth-modulation inverter configuration, with chopperbased
dc-current sources, are verified through computer simulations.
Furthermore, an experimental prototype of a five-level CSI
is set up and is tested. The results show that the test circuit works
properly to generate the multilevel output-current waveform with
low output harmonics by using small size of inductors without any
additional external dc-power sources, which proves feasibility of
the proposed strategy.
INTRODUCTION
RECENT development of high-performance semiconductor
power switches such as MOSFETs and insulated-gate
bipolar transistors (IGBTs) increases the research interest in
high-power converters, such as multilevel voltage-source inverters
(VSIs) and its dual, multilevel current-source inverters
(CSIs). Multilevel inverters have the capability to deliver
higher output power with lower dv/dt or lower di/dt and with
less-distorted output waveforms, resulting in reduction of electromagnetic
interference (EMI) noise and size of an output filter
[1]–[3]. In distributed-power-generation application, as most
renewable energy sources, such as photovoltaic systems, deliver
dc power; the generated power must be converted to ac power
and is fed into the grid through grid-connected inverters [6]–[8].
Various international standards, like IEEE-1547, IEEE-929, and
EN-61000-3-2, impose requirements on the inverter’s outputpower
quality, i.e., harmonic currents and total harmonics distortion
(THD) of the output current.
DC-Current Source
In the proposed multilevel CSI, the dc-current source is indispensable.
In order to test the proposed multilevel CSI, the
dc-current source is obtained by employing a chopper with a
smoothing inductor (Li ) connected with the H-bridge CSI. The
chopper consists of a controlled switch (QC ) that regulates the
dc current flowing through the smoothing inductor as the dc
input current ILi . A free-wheeling diode (DF ) is used to keep
continuous current flowing through the smoothing inductor.
PWM Technique and Inductor Cell Control
In order to achieve a lower distortion of the output-current
waveform, a PWM technique is applied. In this paper, a levelshifted
multicarrier-based sinusoidal PWM technique is employed
to generate gate signals for the CSI power switches and
to obtain the PWM current waveforms [18], [19]. A schematic
control diagram, including the current controller of the chopper
and the inductor cell for the five-level CSI, is shown in Fig. 8.
The control circuit of the inductor cell functions to control
the operation modes, i.e., the charging, the discharging, and the
circulating modes, of the inductor cell Lc . The current flowing
through the inductor cell ILc is kept constant. It generates the
intermediate-level currents based on the output-current waveform
of the H-bridge CSI. A PI regulator is applied to zero the
error between the detected current flowing through the inductor
cell and the reference current to obtain stable and balanced
intermediate-level currents. The amplitude of the inductor cell
current is half of the dc input current ILi. The output of the PI
regulator is modulated by a triangular carrier to generate the
control signal i[0], determining the operation mode of the inductor
cell. In case of the nine-level CSI, the control circuit of
the second inductor cell is similar to the first inductor cell mentioned
earlier. The difference is only the reference value of the
second inductor cell current ILc 2 , which is quarter of the dc input
current. Therefore, for anM-level CSI, if the dc-current source
is assumed to have amplitude I, the current flowing through the
Nth inductor cell ILc is as expressed in (2).
Filter Capacitor
It is necessary to connect a capacitor across the load, because
the inverter works as a current source and the load usually has an
inductive component. The capacitor also functions to filter the
harmonic components, e.g., switching harmonic components, of
the PWM multilevel output current [17]. The harmonic components
of the PWM current will flow through the filter capacitor
Cf . In general, using a higher switching frequency with its constraints,
and using the higher level number of the output current,
a smaller size of filter capacitor can be achieved.Aproper choice
of the filter capacitor is also important to minimize the heat in
the filter, such as capacitors having small equivalent series resistance
(ESR).
SIMULATION RESULTS
In order to examine the proper operation of the proposed
multilevel CSI topology, a five-level and a nine-level CSI configurations,
as shown in Figs. 4 and 5 with chopper-based dccurrent
source, were tested by using computer simulation with
a PSIM software. The test parameters are listed in Table II.
Fig. 10 shows a computer simulation result of the proposed
five-level CSI, where the five-level current, load current, the
current flowing through the filter capacitor, the dc input current,
and the inductor cell current waveforms are presented. Fig. 11
shows some transient waveforms in the start-up of the proposed
five-level CSI, i.e., a five-level PWM current, a load current,
a dc input current, and an inductor cell current with the same
circuit parameters, as defined in Table II. An excellent transient
characteristic of the chopper and the inductor cell controllers
can be confirmed, as shown Fig. 11.
CONCLUSION
In this paper, a new configuration of multilevel CSI, which
employs inductor cells as auxiliary circuit, has been proposed.
The inductor cells are connected in parallel with the main
H-bridge CSI to generate multilevel output-current waveforms
without additional external dc-power sources. The following
are some advantages that can be obtained using the proposed
multilevel CSI topology compared with other topologies.
1) Compared with the conventional two-level power converter,
the proposed multilevelCSI can generatemultilevel
output-current waveform with less distortion by connecting
a single or more inductor cells across the H-bridge
CSI. It results in a smaller di/dt produced by the circuit.
Furthermore, a smaller size of the output capacitor filter
can be used to filter the harmonic components of the output
current.