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Chapter 1 Introduction
DC/DC
Converter
Inverter
DC
Filter
DC
Grid
AC
DC
Transformer
Energy
Storage
PV Array
Figure 1-6 Components of a grid connected PV systems [13]
The typical configuration of a PV system can be observed in Figure 1-6. Depending on
the number of the modules, the PV array converts the solar irradiation into specific DC current
and voltage. A DC/DC boost converter is used to meet the voltage level required by the inverter.
Energy storage devices can be included in order to store the energy produced in case of grid
support connection. The power conversion is realized by a three-phase inverter which delivers
the energy to the grid. High frequency harmonics that appear due to power semiconductors
switching are reduced by the filter. The power transformer is used only for galvanic isolation
between the PV system and the utility grid [13].
1.1.2 Topologies of grid connected PV systems
In PV plants applications, various technological concepts are used for connecting the PV
array to the utility grid. Further, the existing configurations will be explained [3, 14-17].
Central Inverters
For this architecture, presented in Figure 1-7a, the PV arrays are connected in parallel to
one central inverter. The configuration is used for three-phase power plants, with power ranges
between 10-1000 kW. The main advantage of central inverters is the high efficiency (low losses
in the power conversion stage) and low cost due to usage of only one inverter. The drawbacks of
this topology are the long DC cables required to connect the PV modules to the inverter and the
losses caused by string diodes, mismatches between PV modules, and centralized maximum
power point tracking (MPPT) [3, 14-17].
String Inverters
The configuration presented in Figure 1-7b emerged on the PV market in 1995 with the
purpose of improving the drawbacks of central inverters. Compared to central inverters, in this
topology the PV strings are connected to separate inverters. If the voltage level before the
inverter is too low, a DC-DC converter can be used to boost it. For this topology, each string has
its own inverter and therefore the need for string diodes is eliminated leading to total loss
reduction of the system. The configuration allows individual MPPT for each string; hence the
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