Page 334 - A Comprehensive Guide to Solar Energy Systems

Chapter 16 • Small-Scale PV Systems Used in Domestic Applications 339

clude grounding, DC injection protection and electromagnetic interference limitations.
Furthermore, the standards also recommend additional protection features against so-
called islanding. Therefore, grid-tied PV inverters also monitor the operating status of a
grid continuously, and disconnect when the voltage or frequency exceeds certain bounds,
which is known as anti-islanding. hence the situation of having active live islands of the
AC grid is avoided.
It can be noted here that the stand-alone PV inverters can have very similar electrical
characteristics reference to grid-tied inverters but without an anti-islanding feature. Fur-
ther features may also be present in stand-alone inverters to work seamlessly with other
nano-grid supplies (such as batteries, small-scale wind turbines, and diesel generators)
that may be available in local grids. It is envisaged that due to a large scale penetration of
small-scale PV systems and their negative impact on power quality, the future PV systems
are likely to accommodate battery systems and offer localized isolated grid operation in
the case of a blackout. however, this will require faster and safer communication tools to
be developed and integrated inside like PV inverters. In addition, technological changes
in power electronics, like the development of wide band gap material based switching
devices (SiC and GaN), will offer circuit operation at higher voltages, frequencies, and tem-
peratures, eliminating up to 90% of the power losses. This will also allow new inverter to-
pologies and soft switching with increased power density (highly desirable in small-scale
PV systems) to be implemented.

16.3.2 Commonly Used Grid-Tied Converter Topologies

As it is illustrated in block diagram forms in Fig. 16.2, the primary aim of the converter
circuits in PV systems is to deliver the maximum power to the load side (stand-alone or
grid connected) for timely utilization of solar energy. Although the control aspects may
vary due to the characteristics of the input power, well-known standard power electronic
circuit topologies are also utilized in PV systems, include DC–DC converters (primarily
for voltage gain and MPPT) and an inverter (to connect to AC grid or to local AC load, in
single or three phase forms). Although such circuits are also suitable in wind generation
applications, their basic features will be explained here to link to the distinct characteris-
tics of a PV source.
Grid-connected inverters can be classified into two based on their commutation type
(1) line-commutated and (2) self-commutated. The switching frequency of line-commu-
tated inverters is controlled from the line current usually involves thyristor switches and
are rarely used due to the requirements of commutation circuits, auxiliary devices, and
filtering elements which all increase size and complexity and reduce efficiency.
The self-commutated inverters, however, have better and much flexible control
options as they utilize metal oxide semi-conductor field effect transistors (MoSFeTs)
and insulated gated bipolar transistors (IGBTs) as switching devices, and can also
come in two different forms (Fig. 16.5) (1) voltage-source (VSI) and (2) current-source
inverters (CSI).