06-12-2012, 12:43 PM
An Energy Stored Quasi-Z-Source Inverter for Application to Photovoltaic Power System
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Abstract
The quasi-Z source inverter (qZSI) with battery
operation can balance the stochastic fluctuations of PV power
injected to the grid/load, but its existing topology has a power
limitation due to the wide range of discontinuous conduction
mode during battery discharge. This paper proposes a new
topology of the energy stored qZSI to overcome this disadvantage.
The operating characteristic of the proposed solution is analyzed
in detail and compared to the existing topology. Two strategies are
proposed with the related design principles to control the new
energy stored qZSI when applied to the PV power system. They
can control the inverter output power, track the PV panel’s
maximum power point, and manage the battery power,
simultaneously. The voltage boost and inversion, and energy
storage are integrated in a single stage inverter. An experimental
prototype is built to test the proposed circuit and the two
discussed control methods. The obtained results verify the
theoretical analysis and prove the effectiveness of the proposed
control of the inverter’s input and output powers, and battery
power regardless of the charging or discharging situation. A real
PV panel is used in the grid-tie test of the proposed energy stored
qZSI, which demonstrates three operational modes suitable for
application in the PV power system.
INTRODUCTION
THE worldwide-installed PV power capacity shows nearly
an exponential increase due to decreasing costs and to
improvements in solar energy technology. Power converter
topologies employed in the photovoltaic (PV) power
generation systems are mainly characterized by two-stage or
single-stage inverters [1]-[10]. The single-stage inverter is an
attractive solution due to its compactness, low cost, and
reliability. However, its conventional structure must be
oversized to cope with the wide PV voltage variation derived
from changes of irradiation and temperature. The two-stage
inverter topology applies a boost dc/dc converter to minimize
the required KVA rating of the inverter, and boost the wide
range input voltage to a constant desired output value. But, the
switch in the dc/dc converter will increase the cost and decrease
the efficiency. The Z-source inverter (ZSI) presents a new
single-stage structure to achieve the voltage boost/buck
character in a single power conversion stage, which has been
reported in applications to PV systems [11]. This type of
converter can handle the PV dc voltage variations over a wide
range without overrating the inverter.
CONTROL METHODS AND PARAMETER DESIGN
From this point forward, the new topology in Fig. 2 is taken
into account for application to the PV power system. Like the
conventional qZSI, it also has two independent control degrees
of freedom, namely, shoot-through duty cycle D and
modulation index M, which provide the new topology with the
ability to produce any desired output ac power/voltage to the
grid/load, and simultaneously: 1) control the PV panel output
power (or voltage), and as a result battery power (and SOC) is
defined/controlled; or 2) control the battery power, and as a
result the PV panel output power is controlled.
Either the PV panel power or the battery power can be
controlled through the duty cycle D, and the inverter output
power can be controlled by the modulation index. When the
duty cycle D is used to control the MPPT of the proposed
energy stored qZSI-based PV power systems in Fig. 2, the
resultant two control strategies are shown in Figs. 9 and 14.
EXPERIMENTAL RESULTS
In order to confirm the principle and functions of the new
energy stored qZSI, as well as the two proposed control
methods, a set of experiments are carried out by: 1) using
high-voltage-level voltage source in series a resistor to simulate
the PV panel, which verifies that the proposed system can be
used to high voltage. For this test, an LC filter and an R-load (a
standalone load) are connected to the output of the inverter. 2)
The low-voltage-level grid-tie and the real PV panel are
employed to demonstrate the energy stored qZSI-based PV
system’s functions, such as the MPPT, the grid-tie power
control and battery charging/discharging.
CONCLUSION
In this paper, a novel topology for an energy stored qZSI was
proposed to overcome the shortcoming of the existing solutions
in PV power system. Two strategies were proposed to control
the new circuit topology, and their design methods were
presented by employing a small signal model and Bode plot. To
experimentally validate the proposed energy stored qZSI and
two control methods, a bench was built with tests under
different operating conditions.
The theoretical analysis, simulations and experimental
results presented in the paper, clearly demonstrate that the
proposed energy-stored qZSI and the suggested two control
methods, can effectively track the maximum power point in the
PV module, and inject the active/reactive power into the grid by
the inverter, independently, as well as control the battery power
flow. These are key features in most PV power systems,
proving the suitability of the proposed solution for this type of
applications.