20-04-2012, 02:53 PM
Z-Source Inverter
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INTRODUCTION
THERE EXIST two traditional converters: voltage-source
(or voltage-fed) and current-source (or current-fed) converters
(or inverters depending on power flow directions). Fig. 1
shows the traditional three-phase voltage-source converter (abbreviated
as V-source converter) structure. A dc voltage source
supported by a relatively large capacitor feeds the main converter
circuit, a three-phase bridge. The dc voltage source can
be a battery, fuel-cell stack, diode rectifier, and/or capacitor. Six
switches are used in the main circuit; each is traditionally composed
of a power transistor and an antiparallel (or freewheeling)
diode to provide bidirectional current flow and unidirectional
voltage blocking capability. The V-source converter is widely
used. It, however, has the following conceptual and theoretical
barriers and limitations.
Z-SOURCE CONVERTER
To overcome the above problems of the traditional
V-source and I-source converters, this paper presents an
impedance-source (or impedance-fed) power converter (abbreviated
as Z-source converter) and its control method for
implementing dc-to-ac, ac-to-dc, ac-to-ac, and dc-to-dc power
conversion. Fig. 3 shows the general Z-source converter
structure proposed. It employs a unique impedance network (or
circuit) to couple the converter main circuit to the power source,
load, or another converter, for providing unique features that
cannot be observed in the traditional V- and I-source converters
where a capacitor and inductor are used, respectively.
EQUIVALENT CIRCUIT, OPERATING PRINCIPLE, AND
CONTROL
The unique feature of the Z-source inverter is that the output
ac voltage can be any value between zero and infinity regardless
of the fuel-cell voltage. That is, the Z-source inverter is a
buck–boost inverter that has a wide range of obtainable voltage.
The traditional V- and I-source inverters cannot provide such
feature.
INDUCTOR AND CAPACITOR REQUIREMENT OF THE Z-SOURCE NETWORK
For the traditional V-source inverter, the dc capacitor is the
sole energy storage and filtering element to suppress voltage
ripple and serve temporary storage. For the traditional I-source
inverter, the dc inductor is the sole energy storage/filtering
element to suppress current ripple and serve temporary storage.
The Z-source network is a combination of two inductors and
two capacitors. This combined circuit, the Z-source network is
the energy storage/filtering element for the Z-source inverter.
The Z-source network provides a second-order filter and is more
effective to suppress voltage and current ripples than capacitor
or inductor used alone in the traditional inverters. Therefore,
the inductor and capacitor requirement should be smaller than
the traditional inverters. Detailed design guide and formulas of
the Z-source network will be presented in a near future paper.
A brief discussion is given below in terms of physical sizes
and requirements.
CONCLUSIONS
This paper has presented an impedance-source power
converter for implementing dc-to-ac, ac-to-dc, ac-to-ac, and
dc-to-dc power conversion. The Z-source converter employs a
unique impedance network (or circuit) to couple the converter
main circuit to the power source, thus providing unique features
that cannot be observed in the traditional voltage-source and
current-source converters where a capacitor and inductor are
used, respectively. The Z-source converter overcomes the
conceptual and theoretical barriers and limitations of the traditional
voltage-source converter and current-source converter
and provides a novel power conversion concept.