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Overview of Single-Phase Grid-Connected Photovoltaic Systems 43

on power converter advancements in single-phase grid-connected PV systems for residential appli-
cations, which will be detailed in Section 3.3. First, demands from grid operators and consumers for
single-phase PV systems are introduced in Section 3.2. In order to meet the increasing demand, the
general control structures of single-phase grid-connected PV systems are discussed in Section 3.4
before the conclusion.

3.2 DEMANDS FOR GRID-CONNECTED PV SYSTEMS
The grid-connected PV systems are being developed at a very fast rate and will soon play a major
role in power electricity generation in some areas [22, 23]. At the same time, demands (require-
ments) for PV systems as shown Figure 3.2 are increasing more than ever before. Although the
power capacity of a PV system currently is still not comparable to that of an individual wind turbine
system, similar demands for wind turbine systems are being transitioned to PV systems [18, 21]
since the number of large-scale PV systems (power plants) is being continuously increased [24].
Nevertheless, the demands for PV systems can be specified at different levels. At the PV side,
the output power of the PV panels/strings should be maximized, where a DC–DC converter is com-
monly used, being a double-stage PV system. This is also known as maximum power point tracking
(MPPT). In this case, the DC voltage (DC-link voltage) should be maintained as a desirable value
for the inverter. Moreover, for safety (e.g., fire), panel monitoring and diagnosis have to be enhanced
at the PV side [25]. At the grid side, normally a desirable total harmonic distortion (THD) of the
output current should be attained (e.g., lower than 5% [26]) for a good power quality. In the case
of large-scale PV systems with higher power ratings, PV systems should not violate the grid volt-
age and the grid frequency by means of providing ancillary services (e.g., frequency regulation).
Additionally, PV systems have to ride through grid faults (e.g., voltage sags and frequency varia-
tions), when a higher PV penetration level becomes a reality [18, 21, 27–33].
Since the power capacity per generating unit is relatively low but the cost of energy is relatively
high, there is always a strong demand for high efficiency in order to reduce the cost of PV energy and
also to optimize the energy yield. With respect to efficiency, the power electronics system (includ-
ing passive components) accounts for most of the power losses in the entire PV system. Thus, pos-
sibilities to meet the efficiency demand include using advanced semiconductor devices, intelligent
control, and power-lossless PV topologies. Transformerless PV technology is an example, and trans-
formerless PV inverters can achieve a relatively high conversion efficiency when the isolation trans-
formers are removed [11, 26]. However, minimizing the ground current in these transformerless

Power electronics system
Photovoltaic (PV) (power converters)
panels Power grid

°C DC = AC~
P pv P g
Q g

MPPT High efficiency Power quality (THD ) i
DC voltage/current Temperature management Voltage level
Panel monitoring and High reliability In the case of large scale:
diagnosis Monitoring and safety Freq.–watt control
Forecast and prediction Islanding protection Volt–var control
(mission profiles) Communication Fault ride through

FIGURE 3.2 Demands (challenges) for a grid-connected PV system based on power electronics converters
(DC, direct current; AC, alternating current; P pv , PV output power; P g , active power; Q g , reactive power).