29-06-2012, 01:01 PM
A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules
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INTRODUCTION
PHOTOVOLTAIC (PV) power supplied to the utility grid
is gaining more and more visibility, while the world’s
power demand is increasing [1]. Not many PV systems have
so far been placed into the grid due to the relatively high
cost, compared with more traditional energy sources such as
oil, gas, coal, nuclear, hydro, and wind. Solid-state inverters
have been shown to be the enabling technology for putting
PV systems into the grid.
The price of the PV modules were in the past the major
contribution to the cost of these systems. A downward tendency
is now seen in the price for the PV modules due to a massive
increase in the production capacity of PV modules. For example,
the price per watt for a PV module was between 4.4 7.9
USD in 1992 and has now decreased to 2.6 3.5 USD [2].
The cost of the grid-connected inverter is, therefore, becoming
more visible in the total system price.
SPECIFICATIONS, DEMANDS, AND STANDARDS
Inverter interfacing PV module(s) with the grid involves two
major tasks. One is to ensure that the PV module(s) is operated
at the maximum power point (MPP). The other is to inject a
sinusoidal current into the grid. These tasks are further reviewed
in this section.
Demands Defined by the Operator
The operator (the owner) also has a few words to say. First of
all, the inverter must be cost effective, which is easily achieved
with similar circuits as these used in today’s single-phase
power-factor-correction (PFC) circuits and variable-speed
drives (VSDs). However, the user also demands a high efficiency
over a wide range of input voltage and input power since
these variables are defined in very wide ranges as functions
of solar irradiation and ambient temperature. Fig. 2 shows
the average irradiation during a normal year in Denmark
(Northwestern Europe) [15]. The figure shows that most of the
potential energy is available in the range from 50 to 1000 W/m
of irradiation.
Transformers and Types of Interconnections
As stated earlier, some inverters use a transformer embedded
in a high-frequency dc–dc converter or dc–ac inverter, others
use a line-frequency transformer toward the grid and, finally,
some inverters do not include a transformer at all (see Fig. 6).
The line-frequency transformer is regarded as a poor component
due to increased size, weight, and price.
Modern inverters tend to use a high-frequency transformer.
This results in entirely new designs, such as the printed circuit
board (PCB) integrated magnetic components [36].
AC MODULES
The ac Module is the combination of one PV module with a
grid-connected inverter [see Fig. 3(d)]. According to the above
discussion, the inverters should be of the dual-stage type with
an embedded HF transformer. Reviews of ac module inverters
are given in [25]–[35]. Next follow some classical solutions for
the ac module inverters. The results from the literature survey
are compiled in Table II.
The topology shown in Fig. 9 is a 100-W flyback-type inverter
[37]. The circuit is made up around a single-transistor flyback
converter, with a center-tapped transformer. The two outputs
from the transformer are connected to the grid, one at a time,
through two MOSFETs, two diodes, and a common filter circuit
[37]. The flyback converter can, in this way, produce both
a positive and a negative output current.