02-08-2011, 02:11 PM
Abstract
When no transformer is used in a grid-connected photovoltaic
(PV) system, a galvanic connection between the grid and
the PV array exists. In these conditions, dangerous leakage currents
(common-mode currents) can appear through the stray capacitance
between the PV array and the ground. In order to avoid
these leakage currents, different inverter topologies that generate
no varying common-mode voltages, such as the half-bridge and
the bipolar pulsewidth modulation (PWM) full-bridge topologies,
have been proposed. The need of a high-input voltage represents
an important drawback of the half-bridge. The bipolar PWM full
bridge requires a lower input voltage but exhibits a low efficiency.
This letter proposes a new high-efficiency topology that generates
no varying common-mode voltage and requires the same low-input
voltage as the bipolarPWMfull bridge. The proposed topology has
been verified in a 5-kW prototype with satisfactory results.
Index Terms—DC–AC power conversion, photovoltaic (PV) systems,
transformerless inverter.
I. INTRODUCTION
GRID-CONNECTED photovoltaic (PV) systems, particularly
low-power single-phase systems (up to 5 kW), are
becoming more important worldwide. They are usually private
systems where the owner tries to get the maximum system profitability.
Issues such as reliability, high efficiency, small size
and weight, and low price are of great importance to the conversion
stage of the PV system [1]–[3]. Quite often, these grid-connected
PV systems include a line transformer in the power-conversion
stage, which guarantees galvanic isolation between the
grid and the PV system, thus providing personal protection. Furthermore,
it strongly reduces the leakage currents between the
PV system and the ground, ensures that no continuous current
is injected into the grid, and can be used to increase the inverter
output voltage level [1], [2], [4]. The line transformer
makes possible the use of a full-bridge inverter with unipolar
pulsewidth modulation (PWM). The inverter is simple. It requires
only four insulated gate bipolar transistors (IGBTs) and
has a good trade-off between efficiency, complexity and price
[5].
Due to its low frequency, the line transformer is large, heavy
and expensive. Technological evolution has made possible the
implementation, within the inverters, of both ground-fault detection
systems and solutions to avoid injecting dc current into
the grid. The transformer can then be eliminated without impacting
system characteristics related to personal safety and grid
Manuscript received September 11, 2006; revised November 1, 2006. This
work was supported in part by the Spanish Ministry of Education and Science
under Grant DPI2003-08887-C03-03. Recommended for publication by Associate
Editor J. Shen.
The authors are with the Dpto. Ingenier a Electrica y Electronica, Universidad
Pública de Navarra, Campus Arrosadía, 31006 Pamplona, Spain (e-mail:
roberto.gonzalez[at]unavarra.es; jesus.lopez[at]unavarra.es; pablo.sanchis@unavarra.
es; luisma[at]unavarra.es).
Digital Object Identifier 10.1109/TPEL.2007.892120
integration [1], [4], [6]–[8]. In addition, the use of a string of
PV modules allows maximum power point (MPP) voltages large
enough to avoid boosting voltages in the conversion stage. This
conversion stage can then consist of a simple buck inverter, with
no need of a transformer or boost dc–dc converter, and it is simpler
and more efficient. But if no boost dc–dc converter is used,
the power fluctuation causes a voltage ripple in the PV side at
double the line frequency. This in turn causes a small reduction
in the average power generated by the PV arrays due to the variations
around the MPP. In order to limit the reduction, a larger
input capacitor must be used. Typical values of 2 mF for this
capacitor limit the reduction in the MPPT efficiency to 1% in a
5-KW PV system [8]. However, when no transformer is used, a
galvanic connection between the grid and the PV array exists.
Dangerous leakage currents (common-mode currents) can flow
through the large stray capacitance between the PV array and
the ground if the inverter generates a varying common-mode
voltage [1], [4].
A topology that generates no variable common-mode voltage
is the half-bridge family of inverters, with two, three or more
levels [1], [4], [8], [9]. The main drawback is the need of highinput
voltages (greater than, approximately, 700 V for European
applications), which involves the use of either a large PV string
or a previous boost dc–dc stage [4], [9]. The full-bridge topology
requires half of the input voltage demanded by the half-bridge
topology, that is, around 350 V for European applications. In
order to avoid a varying common-mode voltage, the full bridge
has to be modulated with bipolar PWM, a modulation strategy
that leads the converter to a low efficiency and a high current
ripple [5].
This letter proposes a new topology that generates no varying
common-mode voltage, requires the same low-input voltage as
the bipolarPWMfull-bridge topology, and achieves a higher efficiency
and a lower current ripple in the inductor. The topology
consists of six switches and two diodes and can be an advantageous
power conversion stage for transformerless grid-connected
PV systems.
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