In the penetration of renewable energy basedIn the penetration of renewable energy based

In recent years, electricity market liberalization and
environmental concerns factors that contribute to the renewed interest in
distributed generation (DG).  In
liberalized markets, it is important to adapt to the changing economic environment
in the most flexible way. DG technologies in many cases provide flexibility
because of their small sizes (J.Driesen et al 2006). At present, environmental
policies are probably the major driving force for the demand for DG.
Environmental regulations force players in the electricity market to look for
cleaner energy solutions. Furthermore, most government polices aiming to
promote the use of renewables also results in an increased impact of DG
technologies as renewables. There has been an immense interest in incorporating
DG into distribution networks in recent years. DG definitions used in the
literature (Ackermann et al 2001) have proposed the following general
definition for DG: Distributed generation is an electric power source connected
directly to the distribution network or on the customer side of the meter.
During the last decade the penetration of renewable energy based DG units has
increased significantly. Wind and solar are the natural renewable power choices
for many countries and they have successfully integrated into existing
electricity infrastructures. DG units bring many new challenges for power
system operators (T.Aziz et al 2011) needing to integrate these units into
existing distribution system. These challenges includes: bidirectional power
flow, less inertial generation and unregulated / intermittent behaviors. These
challenges mean that power quality is one of the key issues and DG units have a
significant impact on the system and equipment in both steady state operation
and in dynamic operation. They affect reliability, instability and safety for
both customers and electricity suppliers. Because of the close proximity of the
DG units to loads and to their controllers in medium and low voltage networks,
dynamic interaction is a more complex issue than it is in large systems.The
impact of DG on power quality,there are typically two majors concerns(Thomas
Ackermann et al 2002),namely voltage flicker and harmonics.In distribution
network  the most common cause of flicker
is a rapid variation of load current,however, not only load variation s cause
flicker but also DG can directly or indirectly contribute to voltage
flicker.The main causes of flicker are (i) starting of a large DG unit (ii)
sudden and large variations of the output of DG (iii) Interaction between DG
and the voltage controlling equipment of feeder.It is widely recognized that
the presence of non-linear components of power systems manifests in the appearance
of harmonics.The presence of harmonics in a power system is undesirable for a
number of reasons,some of which are (i) Harmonics increases power loss in both
utility and customer equipment (ii) Sometimes harmonics may provoke malfunctionally
of sensitive load or control equipment (iii)

Harmonics having significant magnitutes can cause a
reduction of life time of motors,tranformers,capacitor banks and some other
equipment.Power electronics devices,as used for DG,might can cause harmonics.If
DG was not properly interfaced with the distribution network,its presence may actually
reduce the overall reliability of the distribution grid,here all the aforementioned
apples:harmonics,flic

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ker and voltage regulation.Hence assessing and responding to
power quality issues are important tasks for accommodating the higher penetration
of DG units and for the secure operation of the electricity system.