energies

Review
Electric Power Quality through Analysis and Experiment
Gabriel Nicolae Popa

Department of Electrical Engineering and Industrial Informatics, Politehnica University of Timis, oara,
5 Revolution Street, 331128 Hunedoara, Romania; gabriel.popa@fih.upt.ro; Tel.: +40-254207541

Abstract: The quality of electrical energy is of particular importance for power engineering. This
study presents an analysis of articles made in the Special Issue “Analysis and Experiments for
Electric Power Quality”. As techniques and technology advance, electrical consumers and equipment
become more sensitive to disturbances in the electrical network (in particular, low- and medium-
voltage). It can lead to costly outages and lost production, which affect productivity. The analyzed
articles present interesting technical studies made on industrial and nonindustrial consumers, of
low- and medium-voltage, from the point of view of the quality of electricity. Voltage and current
harmonics, voltage sags and swells, interruptions, unbalance, and low power factor will lead to
higher electricity bills, overloading, and rapid aging of electrical networks and electric equipment.
The power quality depends not only on the supplier but also on all consumers connected to the same
power network; some can cause disruptive influences in the supply network, affecting the operation
of other consumers. Ensuring the power quality of industrial and nonindustrial applications is
an objective difficult to achieve.

Keywords: electric consumers; industrial applications; power quality

1. Introduction
Electrical energy is used by a wide variety of consumers, from the industrial ones,
which are fewer, but of high power, to the domestic ones, characterized by low power, but
Citation: Popa, G.N. Electric Power
very numerous, used both in the urban and rural environment. Electromagnetic disturbance
Quality through Analysis and
is any electromagnetic phenomenon that can degrade the performance of an electrical,
Experiment. Energies 2022, 15, 7947.
electronic, or radio device, and a consumer, equipment, or system can adversely affect life
https://doi.org/10.3390/en15217947
or inert matter [1].
Academic Editor: Abu-Siada Ahmed Electromagnetic disturbances can be classified according to several criteria [2–4]:
Received: 9 October 2022
By frequency:
Accepted: 23 October 2022 - Low-frequency disturbances (refer to signals with a frequency below 1 MHz);
Published: 26 October 2022 - High-frequency disturbances (signals with a frequency above 1 MHz);
Publisher’s Note: MDPI stays neutral According to the mode of propagation:
with regard to jurisdictional claims in - Disturbances conducted through the network conductors (including currents and
published maps and institutional affil- voltage differences);
iations. - Radiated disturbances (in the air), through electric and magnetic fields;
By duration:
- Permanent or sustained disturbances (affecting analog electronic circuits);
Copyright: © 2022 by the author. - Transient, random, or periodic disturbances (affecting digital electronic circuits).
Licensee MDPI, Basel, Switzerland. The origin of electromagnetic disturbances can be both in the electrical network
This article is an open access article (e.g., incidents or wrong maneuvers, and defects) and in the consumer’s electrical network.
distributed under the terms and
Currently, ensuring the power quality of electricity has become an increasingly complex
conditions of the Creative Commons
task of major interest to both electricity suppliers and consumers [3,5,6]. The power quality
Attribution (CC BY) license (https://
concerns have been guiding the following major issues:
creativecommons.org/licenses/by/
4.0/).

Energies 2022, 15, 7947. https://doi.org/10.3390/en15217947 https://www.mdpi.com/journal/energies

Energies 2022, 15, 7947 2 of 14

- To increase the yield in the production, transport, and use of electricity, power electron-
ics have been introduced on a large scale to control the energy conversion processes,
and electronic equipment have been introduced to control the power factor;
- The complexity of energy systems and the mutual influences between them
and users, as well as between consumers connected to the same power system, are
constantly growing;
- The amount of nonlinear electrical equipment, generating electromagnetic distur-
bances, has grown at an impressive rate in recent decades;
- Modern electrical equipment is more sensitive to the decrease in the power quality
of electrical energy because they include sensible electronic devices and control sys-
tems based on microprocessors/microcontrollers, whose operating characteristics are
affected by electromagnetic disturbances from the electrical network;
- Consumers have become more aware and better informed about the impact that
different electromagnetic disturbances have on electrical equipment and technological
processes and, as a result, they ask suppliers to provide them with electricity at the
contracted electrical quality parameters.
Two essential aspects must be followed when supplying electricity. On the one hand
is the quality of the product (refers to the technical parameters, such as voltage amplitude,
frequency, harmonics content, and symmetry of three-phase systems), and on the other
hand is the quality of the service (refers to the continuity of the supply and refers to
short/long interruptions and safety in supplying) [3]. The most important types of electrical
consumers that cause electromagnetic disturbances are [7–12]:
- Nonlinear consumers, for example, modern household appliances, electric tools,
electric arc furnaces, and electromagnetic induction furnaces, which absorb a nonsinu-
soidal current, whose harmonics, passing through the harmonic impedances of the
electrical supply, lead to harmonic voltages on the bars;
- Unbalanced consumers, for example, electric arc welding equipment, public lighting,
and interurban electric traction, which absorb currents of different amplitudes on the
three phases and, passing through the upstream impedances of the electrical network,
cause voltage asymmetry on the power bars;
- Consumers with variable loads produce voltage fluctuations on the power bars (for
example, power mill, mechanical-processing-equipment-driven electrical, and starting
large power motors; electric arc furnaces and spot welders cause flickering).
The electromagnetic disturbances that directly influence the electrical network and,
therefore, the power quality of the electricity supplied to consumers, are the low-frequency
conducted ones (with frequencies up to 9 kHz at the most). This category includes [3,4,6]:
- Variations in the frequency of the supply voltage;
- Variations in the supply voltage;
- Gaps and interruptions (short and long) of voltage;
- Harmonic distortion (harmonics and inter-harmonics);
- Voltage fluctuations/flicker;
- Asymmetries;
- Temporary over-voltages and transient phenomena;
- The continuous component in the applied voltage curve;
- Signaling voltages;
- Voltages induced by low-frequency.

2. A Short Review of the Contributions in Special Issue “Analysis and Experiments for
Electric Power Quality”
This volume contains the successful invited and peer-reviewed submissions [13–22] to
a Special Issue of Energy “Analysis and Experiments for Electric Power Quality” on the
subject area of power quality.
Energies 2022, 15, 7947 3 of 14

Power quality is covered on two main subjects: the development of power quality
indices and the detection, analysis, and correction of electrical disturbances.
This Special Issue of Energies, “Analysis and Experiments for Electric Power
Quality”, published outstanding contributions on electric power quality, in low- and
medium-voltage applications:
- Harmonics;
- Blackouts;
- Under- and over-voltage;
- Sags and swells;
- Unbalance;
- Flickers.
A variety of engineering solutions are available to eliminate or reduce the effects of
electric power quality problems and it is a very active area of innovation and development.
The articles presented different power quality problems in power systems and had brief
ideas about their solutions with comparative studies.
The articles made for the Special Issue “Analysis and Experiments for Electric Power
Quality” had the following topics: unbalance [13]; short-term voltage variation indices [14];
passive filters [18]; active power filters [15,16]; hybrid active filters [22]; residual current
devices at high frequency [17]; voltage sags and short interruptions [19]; photovoltaic
integration using neural networks [20]; autonomous power supply [21].
Article [13] presented a study of the balancing of single-phase electrical consumers in
a three-phase system, and improving the power factor in low-voltage power substations in
residential and educational areas. Industrial electrical consumers are usually three-phase
(with three or four wires) and high-power, with voltage and current unbalance being at
a low level; consumers in the residential and educational sectors are usually single-phase,
in large numbers; unbalanced voltages and, especially, currents are important. To perform
the study, experiments were carried out in the laboratory and the low-voltage electrical
power substation, before and after balancing the single-phase electrical consumers per
phase, on workweek and weekend days. It was found that after balancing the electrical
consumers by phases, the current unbalance in the three-phase system was reduced and
the power factor was improved by using single-phase capacitive electrical consumers (for
example, personal computers, which are in large numbers in such sectors) distributed
equally on all phases.
Following a uniform distribution and balancing of the electrical consumers
(e.g., single-phase consumers from classrooms, offices, and libraries) among the three
phases, the measurement data were taken in the power substation of the residential and
educational sectors. All of the nonlinear consumers (particularly single-phase) connected
at various locations throughout the low-voltage network have the effect of deforming the
voltage and current waveforms from point of common coupling (PCC).
As a result, the balanced consumers are connected to a network of unbalanced con-
sumers, turn into active power unbalance consumers, report a higher amount of power
consumption than is necessary, and consequently perform worse overall. Additionally,
supply conductor losses (personal technological usage) rise. Unbalanced consumers are to
blame for these losses, but the power systems support the growth.
Balance was achieved by using capacitive electrical consumers (e.g., PCs) and uni-
formly distributing electrical consumers among the three phases (within the technical
possibilities). Additionally, we improved the power factor without using fixed capacitor
banks or a power factor controller with capacitor banks connected to the PCC of the power
substations, and we were able to achieve the relative balance of the current and voltage,
respectively. Additionally, the unbalance of the voltage was only impacted little by the kind
of electrical consumers and the amplitude of the supply voltage, although the unbalance of
the current, PF, and DPF was significantly altered [13].
Determination and knowledge of short-term voltage variation indices are important
for the power quality of electricity. In paper [14], a new methodology was proposed
Energies 2022, 15, x FOR PEER REVIEW 4 of 14

Energies 2022, 15, 7947 Determination and knowledge of short-term voltage variation indices are 4 of importan
14
for the power quality of electricity. In paper [14], a new methodology was proposed for
determining indices of voltage variation over a short period. The variables that best de
scribe
for the short-term
determining indicesvoltage disturbance
of voltage werea short
variation over established,
period. as
Thewell as thethat
variables clusters
best that al
low more
describe theadequate
short-termdefinition of the basic
voltage disturbance values
were for theas
established, indices.
well as To
the implement
clusters thatthe new
method,
allow morereal data from
adequate 19 distribution
definition of the basicsystems ofthe
values for a national energy
indices. To company
implement were used
the new
method, real data from 19 distribution systems of a national energy company
The study (including the proposed flowchart—Figure 1) can be useful as a basis for mak were used.
The
ing study (including
regulations the proposed
regarding flowchart—Figure
short-term 1) can be
voltage variation useful and
indices as a basis for making
establishing clusters of
regulations regarding short-term voltage variation indices and establishing clusters of
electricity distribution systems.
electricity distribution systems.

Figure1.1.Flowchart
Figure Flowchartofof
thethe
proposed methodology
proposed [14]. [14].
methodology
Figure 1 provides an overview of the suggested methodology. Industrial users with
Figure
sensitive 1 provides
loads an overview
suffer significant financialof the suggested
losses because methodology.
of voltage sags.Industrial
The criteriausers with
sensitive loads suffer significant financial losses because of voltage
for recommending limits will vary in the future. Accordingly, it is thought that the most sags. The criteria for
recommending
appropriate limits
technique to will
use isvary in the future.
to construct a uniqueAccordingly, it is for
base impact factor thought that the most ap
each distribution
propriate
system technique
by the to use is to that
system performance construct
it most aclosely
unique base impact
matches. factor
This work, for presented
which each distribution
asystem
methodology
by the for the establishment
system performanceofthat the it
base
mostimpact factor
closely that wasThis
matches. utilized
work,in the
which pre-
computation of the index
sented a methodology forthat
theregulates
establishmentvoltageofsags, is in impact
the base line with the goals
factor of the
that was utilized in
electrical industry in this context.
the computation of the index that regulates voltage sags, is in line with the goals of the
Offshore oil and gas platforms are large consumers of electricity and represent impor-
electrical industry in this context.
tant industrial applications of power quality (Figure 2), in which power quality indices
Offshore oil and gas platforms are large consumers of electricity and represent im-
are low (especially current distortion and low power factor). In the design of article [15],
aportant industrial
selection applications
and evaluation of power
tool for active power quality
filters (Figure 2), in which
(power electronic power
devices quality
made with indices
are low (especially current distortion and low power factor). In
silicon and, in particular, with silicon carbide, taking into account the reduced number the design of article [15]
a selection
of components, andtheevaluation
power losses,toolandfor filter
active power
size) filters (power
was proposed. electronic
For active power devices
filters made
with silicon and, in particular, with silicon carbide, taking into account the reduced num
ber of components, the power losses, and filter size) was proposed. For active power filters
Energies 2022, 15, x FOR PEER REVIEW 5 of 14
Energies 2022, 15, x FOR PEER REVIEW 5 of 14
Energies 2022, 15, 7947 5 of 14

used in these industries, size and weight are critical constraints in offshore applications.
used
At insame
thein these industries, size and weight are critical constraints
at twoin offshorevoltage
applications.
used thesetime, compensation
industries, size andstrategies
weight arearecritical
presented,
constraints different
in levels,
offshore applications.
Atidentify
to the samethe time, compensation
best solutions for strategies
improving are
thepresented, at two
deformation different
regime and voltage levels,
increasing the
At the same time, compensation strategies are presented, at two different voltage levels,
to identify
power the best solutions for improving the deformation regime and increasing the
factor.
to identify the best solutions for improving the deformation regime and increasing the
powerfactor.
power factor.

Figure 2. Diagram of a typical power grid of an offshore oil and gas platform (SAPF—Shunt Active
Figure2.
Figure
Power 2.Diagram
Diagram
Filters) of aa typical
[15].of typical power
power grid of an offshore oil and gas platform (SAPF—Shunt Active
Active
PowerFilters)
Power Filters)[15].
[15].
In isolated power networks, such as those found on oil and gas platforms, where poor
Inisolated
In isolatedpower
powernetworks,
networks,suchsuch asas those
those found on oil and gas gas platforms,
platforms, where
where poor
poor
power quality necessitates reactive and harmonic compensation, this paper demonstrated
powerquality
power qualitynecessitates
necessitates reactive
reactive and
and harmonic compensation, this paper demonstrateddemonstrated
that SAPF can be a workable option. Based on several factors, including the SAPF connec-
thatSAPF
that SAPFcan can be
be aa workable
workable option. Based on several factors, factors, including
including the
theSAPF
SAPFconnec-
connec-
tion point, losses, passive components, power quality, and semiconductor type, a SAPF
tion point,
tion point, losses,
losses, passive
passive components,
components, power quality, and semiconductor type, a SAPF
pre-selection tool was created.
pre-selectiontool
pre-selection toolwas
wascreated.
created.
The power quality of electrical energy is also important in medium-voltage electrical
Thepower
The powerquality
quality ofof electrical
electrical energy
energy is also important in medium-voltage electrical electrical
networks, in conditions where nonlinear electrical loads are connected to the network.
networks,
networks, in in conditions
conditions where nonlinear electrical
electrical loads
loads are
are connected
connected to to the
the network.
network.
Article [16] applied the decision tree method for choosing and dimensioning active power
Article
Article[16]
[16]applied
appliedthe the decision
decision tree
tree method
method for choosing and dimensioning active active power
power
filters, which represents a method of improving the power quality. An analysis was made
filters,
filters,which
whichrepresents
representsaa method
method of of improving
improving the power quality.quality. An analysis was made made
of the number and location of active power filters so that energy losses and investment
of
ofthe
thenumber
number and and location
location ofof active
active power
power filters
filters so that
that energy losses and investment
investment
costs are minimal, under the conditions of permanent monitoring of the total harmonic
costs
costs are
areminimal,
minimal, under
under thethe conditions
conditions of of permanent
permanent monitoring of the total harmonic
distortion of
distortion of voltage
voltage inin the
the nodes
nodes ofof the
the medium-voltage
medium-voltage network.
network. In In these
these applications,
applications,
distortion of voltage in the nodes of the medium-voltage In these applications,
the
the decision tree method allows the selection of the optimal solution (Figure 3).
thedecision
decisiontreetreemethod
methodallows
allows the
the selection
selection ofof the
the optimal
optimal solution
solution (Figure
(Figure 3).
3).

Figure3.3.Schematic
Figure of recurring
Schematic of recurring part of decision
part of decision tree
tree used for the
used for optimization of
the optimization of active
active power
power filter
filter
(APF)
(APF) placement
placement [16].
[16].
(APF) placement [16].
Energies 2022, 15, x FOR PEER REVIEW 6 of 14

Energies 2022, 15, 7947 6 of 14

This study presented the issue of harmonic filter allocation optimization in terms of
lowering powerpresented
This study losses and theAPF
issueexpenses. It was
of harmonic highlighted
filter allocation that the decision
optimization tree ap-
in terms
proach
of lowering power losses and APF expenses. It was highlighted that the decision treefilter
is well-known in many research fields but has not yet been used for power
allocation
approach isinwell-known
medium-voltage
in many networks.
research fields but has not yet been used for power filter
Residual
allocation current devicesnetworks.
in medium-voltage are protective devices found in almost all low-voltage net-
Residual
works, being current
common devices are protective
nowadays devices
[17]. Their found in
importance inalmost all low-voltage
preventing net- in
electric shocks
works, being common nowadays [17]. Their importance in preventing electric
the case of indirect contact is known and, in the case of direct contact, can be considered shocks in
theadditional
as case of indirect contactPulse-width-modulation-controlled
protection. is known and, in the case of direct contact,
powercan be considered
electronic converters
as additional protection. Pulse-width-modulation-controlled power electronic
(Figure 4) produce ground fault currents made up of high-frequency components (can be converters
(Figure 4) produce
tens of kHz). ground
The usual fault currents
residual currentmade
devicesup are
of high-frequency
not designed tocomponents (can be
detect high-frequency
tens of kHz). The usual residual current devices are not designed to detect high-frequency
currents, and they are ineffective. In article [17], an analysis of residual currents with fre-
currents, and they are ineffective. In article [17], an analysis of residual currents with
quencies up to 50 kHz was made on residual current devices. As expected, most residual
frequencies up to 50 kHz was made on residual current devices. As expected, most residual
current devices (especially F and B types) do not work under the conditions in which they
current devices (especially F and B types) do not work under the conditions in which they
are testedaccording
are tested accordingtotothe
thestandards.
standards.Limitations
Limitationsof of
thethe standards
standards in force
in force were
were identified
identified
and proposalsfor
and proposals fortheir
theirimprovement
improvement were
were made.
made.

Figure 4. A
Figure 4. A variable-speed
variable-speeddrive
drivecircuit
circuitproducing
producing residual
residual currents
currents of high-frequency
of high-frequency components;
components;
RCD—residual
RCD—residual current device; iD—residual current; PWM—pulse width modulation [17]. [17].
current device; iD—residual current; PWM—pulse width modulation

Utilization
Utilizationof ofresidual
residualcurrent
currentdevices
devices hashasbecome
become mandatory
mandatory for for
industrial as well
industrial as well
as modern
as moderndomestic
domesticapplications.
applications.This Thisdevice’s
device’s main
main goal is to
goal is protect users
to protect fromfrom
users electric
electric
shock in
shock in the
the event
event of of direct
direct or
or indirect
indirect contact.
contact. ForForsuch
suchdevices,
devices, exposure
exposure to to residual
residual cur-
currents with high-frequency components poses the greatest challenge.
rents with high-frequency components poses the greatest challenge. In certain situations, In certain situations,
it is possible that the device will not trip at the anticipated level, meaning that electric shock
it is possible that the device will not trip at the anticipated level, meaning that electric
protection may not be guaranteed.
shock protection may not be guaranteed.
Power electronics have a special impact on low- and high-power electric drives [18].
Power electronics have a special impact on low- and high-power electric drives [18].
In many of today’s electric drives, the load is variable, and the static frequency converter-
In many of today’s
motor assembly is aelectric
stronglydrives, the load
nonlinear is variable,
element and the
with direct static frequency
implications converter-
on reducing
motor
the power assembly
qualityisofa electricity,
strongly nonlinear element
e.g., flicker with direct
and distortion implications
of voltage waveforms.on reducing
Power the
power
capacitor batteries can be used to reduce the voltage drop, and well-sized passive Power
quality of electricity, e.g., flicker and distortion of voltage waveforms. filters ca-
pacitor
can be usedbatteries can be
to reduce theused to reduce
current deformingthe regime.
voltage Using
drop, capacitor
and well-sized
batteriespassive filters can
and passive
be used
filters to reduce
(Figure the iscurrent
5), there deforming
the possibility thatregime. Usingmechanical
with a small capacitor batteries
load of the and passive
motor,
filters (Figureand
over-voltages 5), there is the
electrical possibility
resonance that withmay
phenomena a small
occurmechanical
between the load of the
passive motor,
filter
and the power transformer used to supply the drive. Thus, the capacitors
over-voltages and electrical resonance phenomena may occur between the passive filter in the application
mustthe
and have a performing
power transformer dielectric
usedto toincrease
supply their lifetime.
the drive. In article
Thus, [18], an analysis
the capacitors and
in the applica-
design
tion were
must made
have regarding the
a performing harmonic
dielectric to filters
increaseused in such
their applications.
lifetime. In article [18], an analysis
and design were made regarding the harmonic filters used in such applications.
5, x FOR PEER REVIEW 7 of 14
Energies 2022, 15, 7947 7 of 14

Figure 5. Single line drawing of passive filter [18].

Figure 5. Single line drawing of passive filter [18].
In this study, it was proven that a passive harmonic filter system in the mill motor
In this study,
driveit system
was provenwith anthat
idealacapacity
passive harmonic
could filter system
offer a cost-effective in thethat
solution mill motor
simultaneously
drive system with an ideal capacity
compensated for reactive could
power offer
andaabsorbed
cost-effective solution
harmonics. The that simultane-
following topics were
also covered: attenuation of harmonic voltage
ously compensated for reactive power and absorbed harmonics. The following topicsby current divider Hid; key components
were also covered:of the filter capacitor
attenuation and SR design;
of harmonic voltage harmonic filter bank
by current setting
divider Hid; parameters.
key compo- Voltage
fluctuation characteristics and voltage harmonics were measured to confirm the harmonic
nents of the filter capacitor and SR design; harmonic filter bank setting parameters. Volt-
filter’s performance.
age fluctuation characteristics
Voltage gapsand voltage
(voltage drops harmonics were value)
below a certain measured to confirm
and short voltage the har-
interruptions,
monic filter’s performance.
from industrial applications in both low- and medium-voltage networks, are important
Voltage gaps (voltage of
components drops below
the power a certain
quality value) that
of electricity anddeserve
short voltage interruptions,
to be studied more deeply. In
article [19], a specific questionnaire was made for the
from industrial applications in both low- and medium-voltage networks, are important field study, for industrial consumers
(an analysis was made of 33 companies from 12 distinct types of industrial activities) con-
components of the power quality of electricity that deserve to be studied more deeply. In
nected to medium-voltage networks. The study carried out led to an important contribution
article [19], a specific questionnaire was made for the field study, for industrial consumers
to the analysis of voltage gaps and short interruptions of medium-voltage in industrial
(an analysis wasapplications,
made of 33which companies
completesfrom the12 distinct types
knowledge in this of industrial
area activities) con-
of power quality.
nected to medium-voltage The survey networks.
results The study
allowed forcarried out led to an
the quantification of important contribu- by
the losses experienced
tion to the analysis of voltage
33 small- gaps and short
and medium-sized interruptions
businesses of medium-voltage
with an average of 349 employees, in indus-
distributed
across 12 different business sectors, and all connected
trial applications, which completes the knowledge in this area of power quality. to medium-voltage networks (11.9 kV
and 13.8 kV). The average cost per incident was USD 7364.75 and the average cost per in-
The survey results allowed for the quantification of the losses experienced by 33
terrupted kW was USD 6.72. The objectives of this study were achieved, and it significantly
small- and medium-sized
benefited the businesses with aninaverage
electricity industry particular.of As
349a recommendation
employees, distributedto continue across
this line of
12 different business sectors,itand
investigation, all connected
is proposed to includetomore
medium-voltage
activity segments networks
and increase(11.9thekV and size
sample
13.8 kV). The average cost pernow
of the segments incident was USD
being studied 7364.75
(food industry,and the average
furniture, mining,cost perand
stones inter-
granites,
rupted kW was USD 6.72. The objectives of this study were achieved, and it significantly the
oil). The success rate of responses to the survey form was 47.1% when considering
initial sample
benefited the electricity of 70 firms
industry and 33 responses;
in particular. however, when accounting
As a recommendation to continue for this
the sample
line of
59 companies due to the withdrawal of 11 companies, the rate improved to 55.9% [19].
of investigation, it is proposed to include more activity segments and increase the sample
Currently, the emphasis is on the production of green electricity using photovoltaic
size of the segments nowwind
systems, being studiedetc.
turbines, (food
When industry, furniture,systems,
using photovoltaic mining,the stones
outputand gran-is dc,
voltage
ites, oil). The success rate of responses to the survey form was 47.1% when
and to transform it into ac (used most often by consumers), power inverters are used, which considering
the initial sample of 70
have an firms
impactand 33 responses;
on the power quality. however,
In workwhen [20], anaccounting
analysis was for the sample
made of the neural
control (recurrent artificial neural networks of nonlinear
of 59 companies due to the withdrawal of 11 companies, the rate improved to 55.9% [19]. autoregressive type with external
Currently, the emphasis is on the production of green electricity using photovoltaic
systems, wind turbines, etc. When using photovoltaic systems, the output voltage is dc,
and to transform it into ac (used most often by consumers), power inverters are used,
which have an impact on the power quality. In work [20], an analysis was made of the
 Energies 2022, 15, 7947 8 of 14
Energies 2022, 15, x FOR PEER REVIEW 8 of

input) applied to power inverters with the aim of estimating the harmonic behavior in
photovoltaic systems. Following the acquired and measured data, it was found that the
that the neural network (NARX networks, Figure 6) captures the dynamics of the syste
neural network (NARX networks, Figure 6) captures the dynamics of the system to control
andcontrol
to reduce and reduce
harmonic harmonic distortion.
distortion.

Figure6.6.Architecture
Figure Architectureof configured NARX
of configured networks:
NARX (a) serial–parallel
networks: architecture
(a) serial–parallel (open loop);
architecture (open loop
(b)
(b)parallel
parallelarchitecture (closed
architecture loop)loop)
(closed [20]. [20].

It was found that integrating the PV system power obtained through electronic in-
It was found that integrating the PV system power obtained through electronic i
verters into the PCC has an impact on the sinusoidal waveform of current in the electrical
vertersgrid.
supply intoThetheestablishment
PCC has an impact onefficient
of a highly the sinusoidal
pattern inwaveform of current
terms of execution timesin and
the electric
supply grid. The establishment of a highly efficient pattern in
computational resources as a result of modeling the dynamic and nonlinear behavior of terms of execution tim
and signal
that computational
using NARX resources
networks asproduced
a result ofanmodeling the dynamic
MSE of 0.0067 with respectandtononlinear
the actual behavi
behavior of the signal, demonstrating the high performance
of that signal using NARX networks produced an MSE of 0.0067 with respect of the neural network. Whento the actu
employing the closed-loop NARX to anticipate the results, an MSE of 0.0094
behavior of the signal, demonstrating the high performance of the neural network. Whe was obtained,
demonstrating the model’s viability and demonstrating a significant correlation between
employing the closed-loop NARX to anticipate the results, an MSE of 0.0094 was obtaine
inputs and error values.
demonstrating the model’s viability and demonstrating a significant correlation betwee
In terms of the type and volume of data that can be managed, the resultant model
inputs and
exhibits error values.
remarkable flexibility, enabling representation and prediction of the behavior of
In terms
the system under ofinvestigation
the type and volume
over extendedof data that canand
time periods beunder
managed,
diversethe resultant mod
operating
exhibits remarkable
situations. The resulting flexibility,
algorithm enabling
can be used representation and prediction
to create real-world of thefor
or virtual systems behavior
reducing or controlling harmonic disturbances that impact electrical
the system under investigation over extended time periods and under diverse operatingrids.
Ensuring
situations. Thetheresulting
continuity of electricity
algorithm cansupply
be used is an important
to create area of power
real-world quality.
or virtual systems f
An extremely important field of research has been represented by electrical energy stor-
reducing or controlling harmonic disturbances that impact electrical grids.
age systems that can be used together with uninterruptible power sources to ensure the
Ensuring the continuity of electricity supply is an important area of power qualit
continuity of the electrical energy supply. In article [21], an analysis was made of flow
An extremely
batteries important
(vanadium fieldbatteries)
redox flow of research
thathas
can been represented
provide by electrical
long-term electricity energy storag
in critical
systems thatThe
installations. canadvantage
be used together with uninterruptible
of flow batteries is that they can be power sources
designed fromto ensure the co
indepen-
tinuity
dent of the
blocks. Theelectrical energy supply.
study presented a detailedInexperimental
article [21], an analysis
analysis of a was maderedox
vanadium of flow batte
flow
ies (vanadium redox flow batteries) that can provide long-term electricity inused
battery (VRFB, Figure 7), especially from the point of view of the electrolyte critical insta
(electrolyte
lations. The with the addition
advantage of of hydrochloric
flow batteriesacid).
is that they can be designed from independe
blocks. The study presented a detailed experimental analysis of a vanadium redox flo
battery (VRFB, Figure 7), especially from the point of view of the electrolyte used (electr
lyte with the addition of hydrochloric acid).
FOR PEER REVIEW 9 of 14
Energies 2022, 15, 7947 9 of 14

Figureof
Figure 7. Block diagram Block
7. the UPSdiagram
systemofbased
the UPS
on system based
10 kW/30 on 10
kWh kW/30 kWh
vanadium vanadium
redox redox flow
flow batteries
[21]. batteries [21].

A study of the characteristics of the cells with a change in the electrolyte pumping rate
A study of thewascharacteristics
conducted, and aof the hydraulic
VRFB cells with a change
system in the electrolyte
was developed. pumpingthat
It was demonstrated
rate was conducted, andina the
a change VRFB hydraulic
electrolyte pumping system
rate hadwas developed.
little impact on theItpower
was demonstrated
and efficiency of the
that a change in the electrolyte pumping rate had little impact on the power and Aefficiency
stack (10%), while the stack was running in one of its operational modes. VRFB-based
UPS electrical circuit and an associated control algorithm were suggested. After researching
of the stack (10%), while the stack was running in one of its operational modes. A VRFB-
the dynamic characteristics of the UPS in the VRFB charge/discharge modes, diagrams
based UPS electrical
were circuit
offered. and an associated
The demonstrated VRFBcontrol
could algorithm
function with were suggested.
a 1.5-fold overloadAfter
without
researching the dynamic
efficiency characteristics
degradation and with of the UPS
a rise in the VRFB
in efficiency without charge/discharge
significantly reducingmodes,
capacity,
demonstrating
diagrams were offered. the electrolyte’s VRFB
The demonstrated strong compatibility
could function with with
the device.
a 1.5-fold overload
Matrix converters are used more and more often. If they are connected to ordi-
without efficiency degradation and with a rise in efficiency without significantly reducing
nary electrical loads, they produce harmonics of the order of kHz and even tens of kHz.
capacity, demonstrating thepresented
Article [22] electrolyte’s
a studystrong
(through compatibility
simulations and with the device.
experiments) on a hybrid power
Matrix converters are used
filter (Figure 8) thatmore and more
is controlled often.
by a fuzzy If they
system are connected
to reduce to ordinary
very-high-frequency signals.
electrical loads, they produce harmonics of the order of kHz and even tens of kHz. Article
The hybrid power filter had a low total harmonic distortion for the fundamental voltage
(50 Hz).(through
[22] presented a study A comparative analysis and
simulations was performed
experiments) with aon PI-controlled
a hybrid powerhybrid filter
filterthat
demonstrated the superior performance of the fuzzy-system-controlled hybrid filter in
(Figure 8) that is controlled by a fuzzy system to reduce very-high-frequency signals. The
terms of compensation speed, accuracy, total harmonic distortion of supply current, and
hybrid power filter hadintegrity
overall a low total
of the harmonic distortion for the fundamental voltage (50
matrix converter.
Hz). A comparative analysis was performed
High-frequency harmonics in with
supplya PI-controlled hybridand
lines heat transformers filter that and
motors demon-
interfere
with metering and telecommunications equipment,
strated the superior performance of the fuzzy-system-controlled hybrid filter in terms ofas well as protective relays. Inter-
ference with hospital and laboratory settings and measurement tools is the worst-case
compensation speed, accuracy, total harmonic distortion of supply current, and overall
scenario. There is a need to safeguard delicate loads and equipment that is not designed
integrity of the matrix
to handleconverter.
high frequencies, particularly harmonic frequencies, as high-frequency trans-
mission applications are expanding globally and the issue of using the power lines as
communication lines is increasing. After active filter activation, the suggested HAPF only
received a response for less than half a cycle. The right control approach for producing the
compensational currents is the basis of the HAPF. The hysteresis control approach, which
 [22] presented a study (through simulations and experiments) on a hybrid p
(Figure 8) that is controlled by a fuzzy system to reduce very-high-frequency si
hybrid power filter had a low total harmonic distortion for the fundamental v
Energies 2022, 15, 7947 Hz). A comparative analysis was performed with a PI-controlled hybrid 10 of 14filter th
strated the superior performance of the fuzzy-system-controlled hybrid filter i
compensation speed, accuracy, total harmonic distortion of supply current, a
was used inof
integrity this
thestudy, placesconverter.
matrix restrictions on the ability to adjust the switching frequency of
the HAPFs [22].

Figure
Figure 8.8.Proposed
Proposed hybrid
hybrid activeactive power
power filter filter
(HPAF) (HPAF) architecture.
architecture.

3. Challenges on Power Quality

As electromagnetic disturbances affect both the economic and functional parameters of
the electricity supplier and the consumers, appropriate power quality of electricity requires
their joint actions [1,5,6]. In this regard:
- The electricity supplier must monitor the level of electromagnetic pollution of the
electrical network and establish acceptable levels for different types of disruptive
emissions of consumers, so that all equipment connected to the electrical network may
have normal operating conditions;
- The electricity user is responsible for keeping the emissions they generate at the
common connection point limited, below the limits specified by the electricity supplier.
It is also responsible for drawing up studies and choosing methods to limit the
emission of electromagnetic disturbances.
The most obvious defects determined by the power quality of electricity at consumers
are interruptions and voltage gaps, in which the voltage increases or decreases for a short
time. The transport and distribution systems of electricity can cause electromagnetic distur-
bances, lightning, wind, ice deposits, etc. The negative impact of voltage gaps (typically
0.3 s at most) on the power quality of electricity supplied to consumers is particularly
important and depends on the type of gaps (accounting for the percentage in which the
voltage drops, but also the duration of the gaps), as well as on the acceptability curves of
different classes of electrical energy equipment. Security in the supply of electricity is a very
important aspect of the power quality for large consumers because interruptions in the sup-
ply of electricity cause great damage to users [1,2]. At present, the securing of the electricity
supply to consumers is achieved with the help of classic devices of automatic activation of
the reserve, usually powered by two independent energy sources. Power-outage-sensitive
and critical consumers must be equipped with uninterruptible power supplies to ensure
power continuity in the event of a power outage.
Due to the size, weight, and cost advantages, switch mode power supplies have been
used almost exclusively for all types of low- and medium-voltage power consumers. Practi-
cally, switching sources are present in almost all household and industrial appliances, such
as computers, monitors, laptops, electronic ballasts for fluorescent lamps, and induction
furnaces. These types of consumers show the highest harmonic distortion of the current
absorbed from the network.
Harmonic pollution also causes negative effects on equipment in electrical networks [8].
Thus, additional losses occur, which reduce the efficiency of the electrical energy trans-
formation, the lifetime of the equipment, and the functioning of the equipment, and their
operating regimes are negatively influenced (this effect is manifested in power transformers
Energies 2022, 15, 7947 11 of 14

and electronic equipment). A high power factor reduces reactive power, reduces electrical
energy losses, and increases electrical-energy-carrying capacity. If the waveform of the
current is nonsinusoidal, then the power factor is lower relative to the power factor of the
fundamental, with a higher value of the total harmonic distortion.
The methods of limiting the deforming regime can be divided into three groups:
passive filters, active filters, and hybrid filters [3,9]. Each option presents its advantages
and disadvantages, so the choice of a certain type of filter requires a careful analysis of
the efficiency of all types of filters for the specific situation, under the conditions of the
respective electrical network configuration. Electrical filters can be used in medium- and
low-voltage electrical networks, but must be used with caution in order not to create
unwanted resonances, and possibly additional losses, in the electrical network. In practice,
the combined solution of power factor improvement and electrical filtering of current
harmonics can be used for deforming electrical consumers. The solution of filtering at
each electrical consumer separately, although it is a more expensive solution, is a more
effective solution in the long term than filtering, with high power filters, in the point of
common coupling.
In three-phase networks, when using nonlinear consumers, the load capacity of three-
phase transformers is reduced. Electric motors fed by static frequency converters experience
additional thermal stresses, inadequate ventilation, and strong mechanical stresses (with
direct implications on the life of the motor). The load capacity of electrical cables is reduced
when using nonlinear and phase-unbalanced electrical consumers. The most requested
conductor may be the neutral conductor if it is designed improperly.
When choosing and adjusting protection and switching devices, the deformation
regime must be taken into account [5]. An incorrect adjustment of the protections causes
their untimely actuation, and the de-energization of electrical consumers, even during
their normal operation. In the deforming mode, when measuring voltages and currents,
measuring devices (e.g., multi-meters) of the true RMS type must be used to ensure the
correct measurement of these quantities. Sometimes, in practice, it is possible to reach
a measurement even 40% lower than these quantities, if inappropriate measuring devices
are used.
The operation in a distorting mode and the irrational consumption of reactive power
lead both to penalizing the consumer, due to noncompliance with the technical norms in
force, and to the ageing of the consumer’s electrical equipment.
In the case of sinusoidal regimes, the solution adopted for reactive power com-
pensation consists of the use of capacitor batteries [6]. Knowing the active daily load
curves and the reactive daily load curves allows the modification of some technological
parameters, but also the appropriate connection of the capacitor batteries and the regula-
tion of the reactive power according to the inductive reactive power requirement of the
electrical consumers.
In electrical installations, the symmetry of electrical consumers on phases is difficult
to achieve, especially in the case of single-phase ones, different in power and sometimes
fed by switching power sources.
The negative effects of the asymmetry of the supply voltages consist of [3]:
- Heating in the three-phase rotating electric machines, due to the additional losses
introduced by the negative and zero sequence currents, which pass through the
windings of the machines;
- High-frequency pulsating torques in rotating electric machines, which represent para-
sitic inverse braking torques; thanks to these torques, vibrations appear, which increase
(in the case of fluctuating asymmetries);
- Negative influences on telecommunications lines;
- Reducing the reactive power provided by the capacitor banks, and, implicitly, the
power factor.
Currently, the power systems do not have systems for measuring the additional
circulation of electricity and, therefore, the related losses, conditioned by the presence
Energies 2022, 15, 7947 12 of 14

of asymmetries [11]. To be able to highlight the energetic effects of the circulation of

asymmetric powers, it would be sufficient that, in parallel with the current means of
measuring powers and energies, the means of measuring the circulation of asymmetric
powers and energies should also be provided, because they are kept separately. For this
purpose, some counters equipped with filters of symmetrical components (negative and
zero) would be sufficient. As unbalanced receivers can give, but also receive, the energy of
asymmetry, the meters should be designed with the double-way operation, having one dial
for one direction and another for the other direction.
Another direction of research refers to the different strategy control of equipment
(e.g., for dynamic voltage restorer [23]) used in power quality.
Due to topographical and/or utility investment constraints, there may be several
isolated microgrids in remote locations. The configuration, energy sources, and types of
loads present problems for the isolated microgrids that affect power quality. Organizing
these microgrids into many clusters depending on their relative locations and connecting
them may also have other advantages including increased dependability, stability, and
cost-effective operation [24].
Accumulating low-power sources in a microgrid with solar and wind power plants is
a realistic way to improve the efficiency of distributed generation and renewable energy
sources [25]. These systems have a lot of characteristics, including a lot of semiconductor
equipment and bi-directional power flows. Therefore, ensuring the necessary power quality
indicators is a crucial responsibility.
Stand-alone microgrids are those that can run independently from the bulk power
supply or the national grid. Microgrids are typically connected to the national grid or
a bulk power supplier. Power quality is the main concern for both standalone and micro-
grids connected to the national grid. For many years, research has been conducted to find
a solution and raise the standard of power in microgrids, as renewable energy sources
(including solar, wind, and fuel cells) are frequently connected to microgrids to meet local
consumer demands and to lower operational costs. The microgrid will have a reactive
voltage problem as a result of the renewable energy sources’ failure to supply the microgrid
with reactive power instead of real power [26–28].

4. Conclusions
Ensuring the power quality in household and industrial applications is a complex and
difficult objective to achieve. It is found that the disturbances that occur in the operation of
energy systems affect practically all the characteristics of the voltage and current: shape,
frequency, amplitude, interruptions, voltage gaps, flicker, symmetry (in the case of three-
phase systems), and continuity of electricity supply.
The power quality, unlike other sectors of activity, therefore depends not only on the
supplier, but also on all consumers connected to the same power network; some of them
can cause disruptive influences in the supply network, affecting the operation of other
consumers connected to the same network.
Solving electricity quality problems requires the assessment of the quality of the
supplied energy, by monitoring the voltages and currents in the point of common coupling,
but also by monitoring the electromagnetic disturbances introduced by the consumers. The
distorting and nonsymmetrical regime is a difficult process that requires a detailed analysis,
both at the level of suppliers and consumers of electricity.
Short and, especially, long power interruption affects electrical consumers (in partic-
ular, industrial ones). Ensuring the continuity of electricity supply is perhaps one of the
most important research directions in power quality. Currently, there is a tendency toward
the use of microgrids that have renewable energy sources (e.g., solar, wind, and fuel cells).
The use of renewable energy sources will determine new challenges.
Energies 2022, 15, 7947 13 of 14

Funding: This research received no external funding.

Acknowledgments: The author is grateful to the Energies MDPI Publishing House for the invitation
to act as guest editor of Special Issue "Analysis and Experiment for Electric Power Quality"—Energies
and thanks the “Energies” editorial office for their kind cooperation, patience, and committed
engagement. I would like to give special thanks to my colleagues Eng. Angela Iagăr and Eng. Corina
Maria Dinis, for interesting discussion on the main topics and, also, for analysis of the submitted
articles at this Special Issue “Analysis and Experiment for Electric Power Quality”, Energies MDPI.
Conflicts of Interest: The author declares no conflict of interest.

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