A Single-Stage PV Module Integrated Converter Based on a Low-Power Current-Source Inverter
Abstract—
This paper presents a transformerless three-phase
inverter designed for the integration into a special type of a
photovoltaic (PV) module, which is capable of providing an output
voltage of several hundred volts. The chosen topology, a currentsource
inverter, features a single-stage power conversion system
that directly feeds into the grid. The principle operation and control
is described, and a modified modulation strategy is proposed
to attenuate common-mode currents. A robust and highly efficient
laboratory prototype of a 250-W module integrated converter has
been implemented and tested. Its compact and flat design allows
the direct attachment to the PV module.
Index Terms—Alternating current (AC) module, current-source
inverter (CSI), photovoltaic (PV) power systems, pulsewidthmodulated
(PWM) inverters.
I. INTRODUCTION
FOR THE grid connection of photovoltaic (PV) energy
sources, several PV modules are typically wired in series
strings. These strings can be paralleled and tied up to a central
inverter station or can be directly interfaced by separate (multi-)
string inverters. However, both configurations allow limited
modularity, and the string or even the whole array can only
be operated at a single maximum power point (MPP). As a
consequence, the series connection of PV modules could lead to
certain MPP mismatch losses due to manufacturing tolerances
and nonoptimal conditions such as partial shading [1], or alignment
in different angles. For this reason, several contributions
proposed the application of dc–dc converters attached to each
PV module as individual MPP tracking (MPPT) units [1], [2].
A further step is associated with the integration of the whole
dc–ac inverter unit into each PV module. The module integrated
converter (MIC) concept performs power processing without
accessibility to dc connections. Thus, the whole unit can be
regarded as a PV “ac module” connected in parallel to the
public grid [3]. PV ac modules may become a future trend in
PV system technology [4], particularly for building integrated
applications in urban areas. AC modules could promote the
Manuscript received October 25, 2007; revised April 9, 2008. This work
was supported by the European Commission through the integrated project
Photovoltaic Module with Integrated Power Conversion and Interconnection
System under Contract TREN/04/FP6EN/S07.34959/503123.
B. Sahan, A. N. Vergara, N. Henze, and P. Zacharias are with the Institute
for Solar Energy Technology, University of Kassel, 34119 Kassel, Germany
(e-mail: bsahan[at]iset.uni-kassel.de).
A. Engler is with Liebherr-Elektronik GmbH, 88131 Lindau, Germany.
Digital Object Identifier 10.1109/TIE.2008.924160
Fig. 1. Principle MIC systems. (a) Single-phase multiple-stage converter and
input decoupling. (b) New approach: three-phase single-stage converter.
reduction of installation cost because no dc-specific equipment
(e.g., dc cabling, connectors, fuses) is necessary. Furthermore,
only conventional ac installation expertise is required, and system
planning is made easier due to the high level of modularity.
In terms of safety, ac systems are less affected by electric arcs
and nearby lightning.
However, for all that, specific inverter production cost tends
to increase with lower power ratings [5], and, yet, MICs could
become really competitive when many small units are mass
produced (economy of scale), and integrated system designs are
applied. Despite these distinct system advantages, MICs have
no market share at present [5]. Existing devices often suffer
from high cost or poor efficiency, and reliability is a major
concern. Considering that maintenance procedures would be
quite complex, a long useful life span needs to be ensured
to be comparable to one of the PV modules (> 20 years).
This is challenging because the electronic circuit is exposed to
harsh environmental conditions such as high peak temperature
stress and thermal cycling. Another constraint is that MICs have
practically the same requirements to grid compatibility (e.g.,
safety, harmonics, electromagnetic compatibility) as traditional
inverters [3], but this discussion is out of the scope of this paper.
The following sections present recent work on a new approach
for a PV MIC as introduced in [6]. The chosen topology,
a pulsewidth-modulation (PWM) current-source inverter (CSI),
features a single-stage power processing system that directly
feeds into the three-phase grid. Latest experimental results will
be discussed in Section IV.


Download full report
http://googleurl?sa=t&source=web&cd=4&ve...550678.pdf&ei=GXoyTqfiCI3JrQetlMzVCw&usg=AFQjCNGjKHEW5uYfvFZ7qUtCw1H2GcwnJQ&sig2=7TVD8ePIzoAjeayOkEKYyQ