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STATCOM and SVC are kind of power electronic based devices that
are used widely in industry to regulate the output voltage continuously and
discretely by absorbing or giving reactive power to it [27].
UPS is a well-known power electronic based device that can sense fre-
quency and voltage unbalances and supply the sensitive load with a pure
sinusoidal waveform with a fixed frequency that is generated by the included
converters. From the physical point of view, UPSs could be categorized into
two main groups, static UPSs that are power electronic based and dynamic
UPSs that are based on some rotary elements, such as flywheels. The main
drawback of the static UPSs is the requirement of large energy storage
though it has to feed the whole load in the case of unbalance or blackout.
Based on the application point of view, UPSs could be classified into three
main groups: offline, line-interactive, and online. Researchers are trying to
develop a method to use UPSs as APFs to actively compensate the unba-
lances instead of feeding the whole load; this will overcome the storage
device problem of these kinds of UPSs [28,29]. For a more detailed compari-
son between different types of UPSs and their application, it is worth
referring to Ref. [14].

4.3.3 Transition condition, a bridge between conventional
and smart electrical systems
The operation and control of conventional electric power systems due to the
increasing use of energy storage systems and deployment of variable genera-
tion technologies mostly as DG will become more complicated. On the other
hand, power quality has always been a challenge in conventional electric
power systems, and it is expected to become a vigorous task due to the strict
standards and the ever increasing of variety of loads in upcoming modern-
ized systems. Smart electric power systems discussed so far are the promis-
ing solution to overcome these challenges and difficulties arising from aging
infrastructure, demand growth, and of course power quality issues.
But the problem with smart grids is that their development is faced with
numerous economic, technical, and legal barriers, and it is not a sudden one-
day event. In other words the time needed to equip conventional power sys-
tems with smart devices is much longer than the changes brought by the new
conditions of power systems.
These conditions, which could be called “transition conditions,” lead to
more challenges in the operation and control of power grids. This “transition
conditions” also affect the quality of power delivered to the consumer.
Conventional power quality enhancement methods are no longer efficient,
and the network has not reached the degree of intelligence that is needed for
solving power quality problems.
This problem and the variety of power quality requirement by the con-
sumer will further increase the challenges that utilities will face. Modern