20-10-2012, 01:52 PM
Control of a Fuel Cell Based Z-Source Converter
Control of a Fuel Cell Based Z-Source Converter.pdf (Size: 1.04 MB / Downloads: 46)
INTRODUCTION
RECENT technological advances in small generators,
power converter technology, and energy storage devices
have provided a new opportunity for distributed generation systems
(DGS). The DGS can offer improved service reliability,
better economics and a reduced dependence on the local utility.
Environmental friendly emerging technologies such as wind
turbines, hydro turbines, photovoltaic cells, and fuel cells have
also encouraged a more decentralized approach to power delivery
due to environmental regulations regarding greenhouse gas
emission [1]–[6]. Especially, the fuel cells can offer the prospect
of supplying the world with safe, clean, efficient, sustainable
electrical energy because they use hydrogen as a fuel and can
continuously generate electric power as long as hydrogen and
oxygen are available. Furthermore, the fuel cell development
has been accelerated by investment of the automotive industry
since it has an interest in clean energy for transportation.
CONFIGURATION OF Z-SOURCE CONVERTER FOR FUEL CELL SYSTEMS
The Z-source converter is based on a new concept different
from a conventional dc-to-dc or dc-to-ac power converter [20].
In the conventional three-phase voltage source inverter (VSI),
the shoot-through in which both power switches in a leg are at
once turned on must be avoided because it causes a short circuit.
The traditional three-phase VSI has eight switching vectors
that consist of six active vectors (V1–V6) and two zero vectors
(V0, V7). On the other hand, the Z-source converter has one
more vector (i.e., the shoot-through zero vector) besides eight
switching vectors. The Z-source converter utilizes the shootthrough
to directly step up a dc source voltage without a boost
dc/dc power converter. Thus, a boosted voltage rate depends on
total duration (Ta) of the shoot-through zero vectors over one
switching period (Tz).
CONTROL SYSTEM DESIGN
a block diagram of the control system proposed
for the fuel cell based Z-source converter. As shown in this
figure, the overall system has three feedback controllers and an
asymptotic observer. In the following sections, design of each
controller will be described in detail.
CONCLUSION
The paper studies system modeling, modified space vector
PWM implementation and design of a closed-loop controller of
the Z-source converter employing L–C passive components. An
R–C circuit model is used to realize slow dynamics caused by
a chemical/electrical response of the reformer and stack of the
fuel cells, and a voltage–current polarization curve of a cell is
also considered. A space vector pulse-width modulation technique
is modified to realize the shoot-through zero vectors that
boost the dc-link voltage.Furthermore, a new control algorithm
is presented which can guarantee a fast, no-overshoot current
response, a zero steady-state voltage, and low Total Harmonic
Distortion (THD).
First of all, the Z-source converter does not need power devices,
voltage/current sensors and a DSP controller as part of
a dc/dc boost converter. As a result, when comparing the Zsource
converter with dc/dc boost converter plus voltage source
inverter (VSI), it has some advantages such as increased efficiency,
enhanced reliability, and lower cost at the expense of
the added complexity in control. Especially, the efficiency can
be improved in the range of several percent over the conventional
dc/dc boost converter plus VSI because of no switching
and conduction losses due to power switches of the dc/dc boost
converter. Also, the total system cost can be reduced by tens of
percent because the power devices, sensors and DSP controller
to boost dc-link voltage are removed.