Page 132 - Decision Making Applications in Modern Power Systems

Power quality issues of smart microgrids Chapter | 4 97

filters [14]. Since the early 1970s, passive filters, a combination of capacitors
and inductances, have been used in parallel or series to filter-defined harmo-
nics. When it is installed in parallel with loads, it will make a detour for har-
monic currents, by setting the inductance and capacitor values in a way that
it has high impedance in fundamental frequencies and very low impedance
facing desired harmonic frequencies to absorb the harmonic current. These
devices are installed in series with a load to prevent harmonic currents from
entering the load. Although passive filters are cheap and simple in structure,
they have the drawback of need for redesign for each new case. The filters
need to be tuned for a specific harmonic to act correctly and may lead to
overvoltage at the end of line during low power demand. Passive filters are
used in some specific applications nowadays despite their drawbacks for
their simplicity and cost-effectiveness; it is worth noting that most of these
applications are some hybrid applications of passive filters to reduce the
costs and increase the total reliability of the system. To overcome the draw-
backs of the passive filters, APFs were developed to compensate and
improve power factor, to compensate current harmonics, unbalance and
flicker, and to regulate voltage. APFs have been used in several applications
with several topologies and control methods, and detailed comparison
between different APFs and their application has been done in Ref.
[14,18,19]. APFs could be divided into two main groups, shunt APFs and
series APFs. Shunt APFs are used in parallel to compensate current harmo-
nics by adding the harmonic current with the same magnitude and with 180
degrees phase difference to have nearly sinusoidal grid current. It could also
be used to compensate reactive power if a proper control method is applied
[20]. It is worth mentioning that improving the operation of control methods
is an interesting ongoing topic about these devices [21]. Series APF is used
in series with harmonic loads to compensate harmonic voltages by adding
harmonic voltages with the same magnitude but with the opposite phase. The
main drawback of these devices is that it could be in the same power rating
with the load, so for high-power applications, it will be an expensive and
unaffordable solution. To reduce the cost of using high-power APFs, hybrid
power filters are an appropriate alternative that has the advantage of both
active and passive power filters at the same time. Defining new applications
for hybrid power filters has been popular for the last decade; another
research field in the area of hybrid power filters is to find improved control
strategies to enhance the performance of these hybrid power filters
[14,22 24]. Fig. 4.2 shows a group of first- and second-generation PQI
devices.

4.3.2 Second generation of power quality improvement devices

The second generation of PQI devices includes the most popular group of
PQI devices used in electricity grid up to now. These devices are not as cost