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4. Power Converter Technology and Control for PV Systems 215

FIGURE 6.6
System diagram of the grid-connected photovoltaic (PV) system, where the PV arrays are
the source of input power and power converters are employed to enable the control of the
PV system and the integration of the grid, whose control strategy is imposed by the grid
requirements.

In the state-of-the art technology, there are three mainstream system configura-
tions of grid-connected PV systems, as summarized in Fig. 6.7 [41,42]. The adap-
tation of each configuration is mainly dependent on the applications and the power
ratings. For example, a small-scale PV system usually employs module PV con-
verters shown in Fig. 6.7A because of the small volume and high scalability.
The module PV converter enables individual MPPT tracking at each PV panel,
which is the advantage of this configuration due to the increased energy yield.
Nevertheless, this configuration requires a dcedc converter with high conversion
ratio, as the PV voltage of the module is usually small because of the limited num-
ber of panels, to be able to be connected to the ac grid through the inverter. At the
same time, high efficiency needs to be attained over a wide range of operation,
which is a challenge of this system configuration. For a medium/large-scale PV
systems, string, multistring, or central inverters shown in Fig. 6.7B and C are
more promising because of their overall high conversion efficiency. In this case,
the PV power can be directly fed to the ac grid without or with a dcedc converter
with a small conversion ratio, as the PV voltage is usually high enough because of
the large number of PV panels. As the PV utilization is still at a residential appli-
cation (e.g., rooftop system), the string and multistring inverters are dominating in
market, and the single-phase connection is more often to see [39]. In fact, the string
inverter becomes more and more popular in the recent years because of its high
scalability, high reliability, and simple installation. This system configuration is
expected to be more adopted in the large-scale PV system as a replacement of
the central inverter [43,44].
Another way to classify the PV system configuration can be done by consid-
eringthe number of powerconversionstagesemployedinthe system.For a