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216 CHAPTER 6 Flexible Power Control of Photovoltaic Systems

(A)

(B)

(C)

FIGURE 6.7
Different grid-connected photovoltaic inverter structures: (A) module inverters connected
to a common dc-bus, (B) string/multistring inverter applied in single- or three-phase
systems (residential and commercial applications), and (C) center inverters for
commercial or utility-scale applications (high power, e.g., 100 kW).

small-scale PV system (e.g., modular PV converters, string/multi-string inverters),
the PV voltage is usually lower than the required dc-link voltage because of the
limited number of PV panels connected in series. In this case, a two-stage power
conversion is normally required. The first conversion stage is a dcedc conversion,
whichisemployedtostepupthe PV voltage to matchthe minimumrequireddc-
link voltage for grid-connected applications (e.g., dc-link voltage of 450 V for
230 V ac grid). In the case of large-scale PV system, the PV voltage is usually
high enough to connect to the grid without the need of the voltage amplification.
Therefore, a single-stage conversion can be employed, where the dcedc conver-
sion stageisnot required. Theabsence of thedcedc conversion stage can reduce
the power loss introduced by the dcedc converter, which is an important aspect for
large-scale PV plants.
In this chapter, the single-phase string inverter configuration is used as an
example as a result of its popularity. An example of the two-stage grid-connected
PV system with an LCL filter is shown in Fig. 6.8. Moreover, to achieve a higher
efficiency, transformer-less PV inverter topologies are favorable. However, the