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Three-Phase Photovoltaic Systems: Structures, Topologies, and Control 69

PV string DC/DC DC/AC Filter Grid

PWM PWM PV inverter Hardware

Grid
MPPT Modulator sync.
PV side Grid-side
measurements measurements
V DC Current Anti-islanding
control control
Low-level
PV interface Grid interface control

Aux. Power
Grid operator services Grid stability quality
command High-level
Power system integration control

FIGURE 4.1 Three-phase grid-connected PV system, showing the hardware layer from the PV generator
(PV string) to the grid; in the bottom the two control loops—Low-level control mainly related to the hardware,
high-level control related mainly to the power system.

Finally, grid integration aspects with focus on grid code requirements and PV-specific functions
are presented, followed by conclusions.

4.2 PV INVERTER STRUCTURES
Depending on the size of the PV plant, one can talk about the three different structures, as presented
in Figure 4.2. Central inverters (CI) are usually connected to several parallel-connected PV strings,
thereby having a PV array made up of several thousands of PV panels connected to the DC link of
the inverter. This solution would offer the cheapest €/kW price for the inverter, but the MPPT will
not be optimum for all the PV strings and the annual energy production (AEP) will suffer. CI can be
found for power ranges from 100 kW to 2 MW size in a modular design, where 4–10 inverters are
connected in parallel to the same DC input and the same AC output. In this way, based on the actual
input power, only the required number of inverters will operate, thereby improving the efficiency of
the entire PV system.
The tracking efficiency of PV systems can be improved by connecting each PV string to a sepa-
rate DC input, either by having string inverters or by having a multistring inverter with several DC/
DC converters and a common DC/AC stage. These inverter structures are also called minicentral
inverters (MCI). With the MCI structure, each PV string has an individual MPPT that changes
the working point of the PV string based on the individual atmospheric conditions, leading to an
increased AEP and energy yield. The string topology structure is used within a power range of
1–10 kW. The multistring topology structure is also used in case of MCIs up to power ranges of
60 kW. This way MW-sized PV plants can be designed with a modular approach, making PV plant
design flexible and simple to build. In case of plant extension or maintenance, only a single PV
section needs to be disconnected, while the rest of the PV plant is still actively connected to the
network, thereby reducing the unnecessary energy loss to a minimum.
To further improve the AEP of PV systems, one could use module-integrated converters (MICs).
In this configuration, each PV panel has its own converter, thereby optimizing the tracking of