Page 340 - A Comprehensive Guide to Solar Energy Systems

Chapter 16 • Small-Scale PV Systems Used in Domestic Applications 345

complexity of the previous topologies, it is necessary to remove the central DC–AC con-
verter that ties everything to a common point. The topology in Fig. 16.10D can eliminate
the central inverter by accommodating multiple miniature (PV panel-rated) inverters
that operate in isolation from each other and directly connected to the AC grid. hence,
each micro inverter tracks the MPP of a PV panel and eliminates power mismatch issues
in series strings. one of the main drawbacks of micro inverters (excluding the higher
system cost) is the conversion efficiency, which is reduced due to the requirement to
step up the low voltage of a single PV panel to the DC-link voltages required to interface
the mains grid.

16.5 Issues on PV Systems and Cell and Module Level
Failures

Generally, any PV system consists of three main components such as modules and arrays,
central wiring point, and converter/inverter. There are internal wirings (inside a rigid and
weather resistance frame) in a PV module and main wiring connecting PV panels in an ar-
ray form to a central point to connect to power electronic circuits (DC/DC and/or DC/AC
converters). This forms the final stage both in a stand-alone or grid-tied system. Therefore
the issues and faults can also be classified around these three main components of a PV
system, in addition to the integrated system-related issues.

16.5.1 Shading

In practice, to achieve higher voltage and current levels (hence powers) both in stand-
alone or grid-tied applications, series and/or parallel-connected PV modules are used. In
such arrangements, however, there may be power mismatches (nonuniform PV power dis-
tribution) between PV modules.
The major power mismatch in practical systems is primarily due to nonuniform shad-
ing by trees and clouds, and other factors such as dirt built up, manufacturing mismatch,
aging, and angling (usually a permanent effect due to roof positions). It should be noted
here that nonuniform shading conditions actually occur between PV cells of a PV module
itself. however, due to the physically close proximity of the PV cells to each other in the PV
module itself, it can be assumed that any of the conditions that would affect one cell would
likely affect the whole module.
Although different inverter systems are adapted in practice (Fig. 16.10) to reduce or
eliminate power mismatches in PV arrays while increasing reliability and reduce cost,
shading still needs a special attention as it has a significant under-powering effect.
Since shaded (or under-powered) PV modules will dissipate power (due to their internal
resistances) from the nonshaded/over-powered modules, causing them to heat up and cre-
ate hot spots in a PV array, as a common practice, bypass diodes are connected in parallel
with each PV module in an array (Fig. 16.11A). Bypass diodes simply protect system as well as
reduce energy loss due to the internal resistance. Fig. 16.11B Illustrates variation of the power