05-02-2013, 12:19 PM
Photovoltaic Stand-Alone Power Generation System with Multilevel Inverter
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Abstract.
This paper proposes a decentralized stand-alone
photovoltaic (PV) system, which presents a set of advantages
when compared to conventional stand-alone PV system. In the
proposed system, the generated energy by the PV arrays is
processed by multi-string MPPT step-up converters, which
assure a maximum utilization of this energy. The storage system
is designed in a decentralized configuration, enabling to achieve
a reduction of maintenance cost of back-up system. Moreover,
the output stage is composed by a multilevel dc-ac inverter,
which enables higher efficiency, low distortion ac waveforms,
low leakage currents and the use of low voltage rating
semiconductor devices.
Introduction
The continuous economic development of many countries
and the environmental issues (gas emissions and the green
house effect) observed in the last decades forced an
intense research in renewable energy sources. Hydro,
photovoltaic (PV) and wind energy conversion are the
most explored technologies due to their considerable
advantages [1]-[2], such as reliability, reasonable
installation and energy production costs, low
environmental impact, capability to support microgrid
systems and to connect to the electric grid [3]. Among
these energy sources the PV is pointed out as one of the
most modular and environmentally friendly technologies.
Therefore, PV systems have been frequently adopted
worldwide, presenting a growth of 45% on the total PV
power installed in 2009 [4] (the largest growth among the
renewable energy sources).
Proposed Stand-Alone PV System
The proposed structure, depicted in Fig. 1(a), can be
classified as a multi-string configuration, where the first stage
is composed by “n” dc-dc converters connected to its
correspondent PV string. Each dc-dc converter output is
connected to a multilevel cell, performing the dc-ac stage.
The back-up system is divided into “n” dc-dc battery
converters (Decentralized Battery Converters – DBC)
managing a small set of batteries, also connected at the
output of the first stage. The arrangement composed by one
PV string, one dc-dc converter, one battery converter and one
multilevel cell will be considered a module of the system.
The dc-dc converters of the first stage must track the
maximum power point of the PV panels. In order to
perform this task, step-up/step-down converters are usually
chosen. However, in the proposed structure, the choice of
the converter will depend on the voltage level of the dc bus,
calculated by means of (1).
The multilevel inverter
The chosen multilevel topology for the proposed system is
composed by cascaded half-bridge cells, where each cell is
connected in the output of the first stage of the module [2],
[12]-[13], as shown in Fig. 2. The multilevel cells are
responsible for producing a specified voltage level, and the
series connection of them will generate the desired
sinusoidal waveform. This topology presents modular
characteristic, simple control and high efficiency due to the
low frequency operation possibility. However, as can be
seen in Fig. 2, only the multilevel cells cannot produce an
ac voltage waveform. Thus, a full-bridge inverter is
necessary at the output of the cascaded multilevel cells,
which operates in the output frequency (50 or 60 Hz).
Operation modes
The operation modes of the proposed structure aim the
power balance under any conditions of the system, and
they are similar to the operation modes of the structure
described in [9]. However, there are special conditions
with more than one possibility. Among these particular
cases, it can be quoted the condition where there are not
enough power generated by the PV panels and the
batteries are fully discharged. In this case, the energy
distribution system could employ a management of
priority loads, reducing the overall demand, or else
shutting down the entire system. These special cases will
not be analyzed in this paper.
Simulation Results
In order to validate the proposed topology, the PV
system was simulated with three strings. Step-up dc-dc
converters (boost) implement the first stage, tracking the
maximum power point (MPP) of the panels. Connected
at the output of the first stage, the multilevel inverter
generates a seven-level waveform. In this stage, a phaseshift
modulation was implemented, reducing the total
harmonic distortion of the output signal. The simulation
parameters are summarized in Table 1.
First, it was simulated a string composed by three PV
panels of 130 W, resulting in 1170 W of total installed
power. Initially, each PV string is producing 390 W at a
radiation of 1000 W/m². In this case, there is excess of
power produced and the batteries are not fully charged
(mode 3). Thus, the bi-directional converters should charge
the batteries (positive current on the batteries) regulating the
energy balancing of the system.
Conclusion
This paper proposed a stand-alone PV system with a
multilevel inverter in the dc-ac stage for medium power
applications. The multilevel inverter presents attractive
advantages when compared to conventional inverters,
such as low harmonic distortion of the output waveform
and employment of low voltage semiconductor devices.
The structure is also composed by converters dedicated
to small sets of batteries, called decentralized battery
system. Moreover, several design possibilities can be
achieved aiming the most efficient and low cost PV
system project. It was also considered an appropriated
charge/discharge method for the batteries aiming to minimize
the maintenance costs of the batteries in stand-alone systems.