Power quality is generally evaluated in terms of harmonic content in
both supply voltage and current.

For an ideal system, harmonics are typically caused by the use of nonlinear
loads found in domestic equipment such as switch-mode power electronics
converters, ballast for fluorescent lamps, computers, televisions, and
other nonlinear loads used in tertiary and industrial applications such
as power electronics operated adjustable speed drives, arc furnaces,
and welding equipment. The nonlinear loads draw nonsinusoidal current
from the network, an important harmonic content of the supply voltage
with regards to the fundamental. The presence of such harmonics in the
system can cause a number of unwanted effects for sensitive electronic
loads such as industrial process controllers, hospital monitoring equipments,
and laboratory measurement devices, and computers malfunction or fail
to operate when connected to an ac line that has high harmonic voltage
content. Also, electric utility transmission and distribution equipment
may be susceptible to ac line harmonics. Furthermore, transmission lines,
motors, and transformers could have higher operating losses; capacitor
banks may fail due to over current, protective relays may not operate
properly.

Generally, at the point of common coupling (PCC), the impact of the
loads on the supply voltage can easily be measured and identified. Two
types of harmonic-producing loads can be characterized at the bus bar
that connects the supply voltage to different loads:

• Current type harmonics-producing loads; these loads are found as diode
rectifiers and phase-controlled thyristor rectifiers feeding sufficient
inductance connected to the dc side.

• Voltage harmonics-producing loads, such as diode rectifiers feeding
sufficient filtering dc capacitors.

These two types of harmonic sources have completely distinctive dual
properties and characteristics.

Based on their natural distinctive properties, both current and voltage
type of harmonic-producing loads have their own suitable filter configurations.

Various mitigation techniques for reducing harmonics in the power system
have been developed with time. Traditionally, passive filters such as
low pass, high pass, band pass, and tuned filters have been used to eliminate
low-order and high-order harmonics, and sometimes tuned filters are used
to attenuate specific harmonics. Moreover, these filters contribute to
the improvement of the power factor (PF), but their bulky size, limited
compensation ability, and susceptibility to resonance with the source
impedance constitute the major drawbacks of the technology.

Other industrial applications use power factor correction (PFC) devices
for reactive power and current harmonics compensation. In these circuits,
switched capacitor banks are typically connected in parallel to current-source-type
loads. Seen from the load side, the capacitance of the PFC and the source
inductor create a parallel resonant circuit. Looking from the source
side, the PFC capacitors and the line inductor represent a series resonant
circuit. To overcome the drawbacks of passive filters integrated with
the PFC equipment, typical active power filter (APF) topologies may be
used [5-6]. They are preferred over the passive filters because of their
filtering characteristics and their capability of improving the system
stability by avoiding possible resonance between the filter components
and the mains impedance.

APFs have been known as an effective tool for harmonic mitigation as
well as reactive power compensation, voltage regulation, load balancing,
and voltage flicker compensation. They can be classified according to
the converter type used (voltage source or current source); the number
of phases (single-phase, three-phase application to three or four wires);
and their topologies, which include shunt, series, hybrid, and unified-power
quality conditioner (UPQC), which is a combination of series and shunt
active filters. Shunt active filters are connected in parallel to electrical
systems and can substantially improve current distortions, reactive power,
load unbalance, and neutral current. It operates by injecting harmonic
current into the utility system with the same magnitudes as the harmonic
generated by a given nonlinear load, but with opposite phases. Unfortunately,
it cannot compensate voltage-source type of nonlinear loads. In fact,
lots of electronic appliances used in power system, such as frequency
converters, switch-mode power supplies, and uninterruptible power supplies
(UPSs) as well as electronic ballasts, etc., have a large filter capacitor
on the dc side of the rectifier circuit.

They intrinsically belong to voltage-source nonlinear type of loads.
The harmonics generated by such voltage-source nonlinear load can effectively
be suppressed by using a series APF. Indeed, series active filters suppress
and isolate voltage-based distortions such as voltage harmonics, voltage
unbalance, voltage flickers, and voltage sags and swells. APFs have the
capability of damping the harmonic resonance between an existing passive
filter and the supply impedance, but they suffer from high kVA ratings.
The boost converter constituting the shunt active filter requires a high
dc-link voltage in order to compensate effectively higher order harmonics.
On the other hand, a series active filter needs a transformer capable
to withstand full load current in order to compensate for voltage distortion.

Combining the advantages of both passive and active filters, hybrid
filter topologies are appealing.

They have been developed achieving the desired damping performance with
a significant reduction of KVA effort required by the power active filter.
They are cost-effective solutions to controlling voltage variations and
distortions as well as suppressing harmonics. Passive filters are also
used in this topology to carry the fundamental current component in a
series active filter and the fundamental voltage component in a shunt
active filter. UPQCs are the most effective devices to improve power
quality. Its configuration consists of a series and a shunt active filter
that usually share the energy source.

The series active filter cancels voltage harmonics and the shunt active
filter cancels current harmonics.

Active filter systems have been also developed for dc/dc converters.
These configurations of active filters are used for two purposes. The
first purpose is to remove high-frequency electromagnetic interference
(EMI) from input current of converter. The second reason is to remove
the voltage ripple from the output voltage of the converter. Among all
the configurations of active filters, UPQC is known as the best tool
for power quality improvement. The latter is used to cancel both current-based
and voltage-based distortions.

This chapter describes harmonic-producing loads, effects of harmonics
on utility line, and harmonic mitigation methods, especially, passive
active and also hybrid filters. Different topologies of active filters,
their applications, configurations, control methods, modeling and analysis,
and stability issues are detailed; moreover, simulation results are given
to show the performance of every topology studied.

2. Harmonic Production and Characteristics

Harmonics are periodic voltages and currents signals having frequencies
that are integral multiples of the fundamental frequency. In single-phase,
60 Hz power systems, odd harmonics such as 3rd, 5th, 7th, … are present
on the ac side with the third harmonic being dominant, whereas even harmonics
are found on the dc side. In three-phase three-wire, 60 Hz power systems,
only non-triplen odd harmonics such as 5th, 7th, 11th, 13th, … are present.
Harmonic distortion of supply voltage is caused because of the supply
impedance and the presence of rich harmonic currents drawn by residential,
commercial, and industrial loads such as switch-mode power converters,
adjustable speed drives, elevators, electronic ballasts, air conditioners,
arc welders, battery chargers, copy machines/printers, personal or mainframe
computers, UPSs, silicon-controlled rectifier (SCR) drives, and x-ray
equipment. The term total harmonic distortion (THD) gives the measure
of harmonics content in a signal and is generally used to denote the
level of harmonics present in the voltage or current signals. The quality
of the energy became a major concern because of the recommended international
standards such as IEEE-519, "IEEE Recommended Practices and Requirements
for Harmonic Control in Electrical power Systems" and IEC-6002-3.
The IEEE standard 519-1992 establishes the recommended guidelines for
harmonic currents and voltage control in utility distribution systems.
The standard specifies harmonics current and voltage limits at the PCC.
The European harmonic standard, IEC-555, proposes absolute harmonic limits
for individual equipment loads.

3. Characterization of the Disturbances

Several parameters are used in power systems to characterize distortion
and harmonic content of a waveform and their effects: the PF, the THD,
the distortion factor (DF), and the crest factor (CF).

The nonlinear load current iL is generally expressed by ....

Where:

ϕ1 is the phase angle of the fundamental load current

θs = ωt, ω is the frequency of the network

IL1 is the rms amplitude of the fundamental load current

ILh is the rms amplitude of the hth harmonic load current

ϕh is the phase angle of the hth harmonic load current

3.1 Power Factor

The apparent power, (volt-amperes), of a power system is given as ...

3.2 T otal Harmonic Distortion

The THD makes it possible to evaluate the difference between the real
waveform and the sinusoidal waveform for the current or the voltage.
It is used to quantify the levels of the current flowing in the distribution
system or the voltage level at the PCC where the utility can supply other
customers. It is defined as the ratio of the rms amplitude of harmonics
to the rms amplitude of the fundamental component of the voltage or current
as given in the following equation:

3.3 Distortion Factor

The DF is defined as the ratio between the rms value of the fundamental
current and the rms value of the same current:

When the current is perfectly sinusoidal DF = 1, the latter decreases
when the current is distorted.

3.4 Crest Factor

Another important quantity that characterizes the quality of the source
current is the CF. The CF is the ratio between peak values to the total
rms value of the same current:

For a sinusoidal waveform, CF is equal to 1.41. The peak factor can
reach values higher than 4 and 5 for much distorted waves, especially
for diode rectifiers feeding capacitive loads.

4. Types of Harmonic Sources

Nonlinear loads can generally be classified into two types, namely,
voltage-source type of nonlinear loads (or voltage-fed type of harmonic-producing
load) and current-source type nonlinear loads (or current-fed type of
harmonic-producing load). These two types of harmonic sources have completely
distinctive dual properties and characteristics. A voltage-source type
nonlinear load may consist of loads like a diode or thyristor rectifier
with a large smoothing capacitor at the load end. It is used in electronic
equipments, household appliances, ac drives, and in power converters
such as switch-mode power converters, UPSs, variable frequency drives
(VFD), etc. Harmonics generated by these loads have become a major issue.
The recommended types of compensating filters for this type of harmonic
source are series passive, active, and hybrid filters (a combination
of passive and active filters).

A current-source type of nonlinear load may consist of a diode or thyristor
rectifier with sufficient inductance on the dc side such that it produces
a constant direct current. It is used in applications such as dc drives,
battery chargers, etc. The current at the input of the rectifier contains
a large amount of harmonics due to the switching operation of the rectifier.
It is recommended in [1] that for optimum harmonics compensation with
these types of loads, parallel passive filters, active filters, and hybrid
filters (a combination of the shunt or series passive and the shunt or
series active filter) should be used due to the high dc-side impedance
of the load that will force the compensating current to flow into the
source side instead of the load side. Figures 1 and 2 show typical single-phase
and three-phase current-source nonlinear loads. These bridge rectifiers
feeding on the dc side an inductor:

LL = 10 mH in series with a resistor RL = 12 Ω.

The single-phase supply voltage (vs), the load current (iL), and its
spectrum analysis are shown in FIG. 3. The measured THD of the load current
is 11.31%.

The simulation results of the three-phase current-source nonlinear load
are presented in FIG. 4. The supply voltage (vs1), the load current (iL1),
and the spectrum of the load current in phase 1 are depicted in the same
figure. The measured THD of the current generated by the nonlinear load
is approximately 27.06%. The results show that the current contains a
large number of odd harmonics. This distorted current causes a distorted
voltage drop on the supply conductors, leads to voltage distortion in
the supply systems, and results in poor power quality.

FIG. 1 Single-phase current-source type nonlinear load.

FIG. 2 Three-phase current-source nonlinear load.

Figures 5 and 6 show, respectively, single-phase and three-phase voltage-source
type of nonlinear loads. These rectifiers are feeding a dc load constituted
by a dc capacitor CL = 1000 μF connected in parallel with a resistor
RL = 12 Ω. FIG. 7 illustrates the supply voltages (vs), the load current
(iL), and the spectral analysis of the supply voltage and the load current.
The THD of the supply voltage and load current are 9.45% and 115.98%,
respectively. It is interesting to note that the load current contains
a large amount of odd harmonics, with the third harmonic being dominant.

In FIG. 8, the supply voltage (vs1), the load current (iL1), and the
spectrum of the supply voltage and load current in phase 1 are presented.
One can notice that the THD of the supply voltage and the load current
in phase 1 are 7.74% and 72.37%, respectively. It can be seen that the
current and voltage waveforms of voltage-source nonlinear loads are much
more distorted than those of the current-source nonlinear loads. This
important distortion of the voltage is created due to discontinuity in
the supply current.

FIG. 3 Steady-state response of single-phase current-source nonlinear
load: (a) voltage and current waveforms and (b) spectrum of load current.

FIG. 4 Steady-state response of three-phase current-source nonlinear
load: (a) voltage and current waveforms in phase 1 and (b) spectrum of
load current.

FIG. 7 Steady-state response of single-phase voltage-source nonlinear
load: (a) voltage and current waveforms, (b) spectrum of load current,
and (c) spectrum of source voltage.

FIG. 8 Steady-state waveforms of three-phase voltage-source nonlinear
load: (a) supply voltage and current in phase 1, (b) load-current spectrum,
and (c) source-voltage spectrum.

5. Filters Used to Enhance Power Quality

Harmonic reduction is becoming more and more relevant due to the limitations
required by interactional standards such as the IEC 1000-3-2 or EN61000-3-2
and IEEE-519. Several mitigation methods are available that permit substantial
reduction of harmonics components. Different power filters have been
installed in power systems to keep the harmonic distortion within acceptable
limits. Conventionally, passive filters alone have been broadly used
for harmonic mitigation; these devices have the advantages of being simple
to design, not expensive to install, reliable, and require low maintenance
efforts.

However, they have several drawbacks, such as large size, possible parallel
and/or series resonance that could be created with both load and utility
impedances, and filtering characteristics strongly affected by source
and load impedances. To overcome the disadvantages of the passive
filters, various types of APF have been developed to improve power performance.
But, APF topologies suffer from high cost due to high KVA rating of the
converter, and are less reliable. Hybrid active filters (HAFs) provide
improved performance and have become a cost-effective solution to harmonic
elimination, particularly for high-power nonlinear loads. The other alternative
is the use of a UPQC to compensate voltage and current problem simultaneously.
However, the use of UPQC is an expensive solution.

5.1 Passive Filters

Passive filters are combinations of inductors, capacitors, and damping
resistors connected in series or in parallel to present the appropriate
high or low impedance to the current or voltage harmonics. Various topologies
of passive filter are available and have different compensation characteristics
and applications.

They are generally used as shunt passive filters or series passive filters.

5.1.1 Shunt Passive Filters

The shunt passive filter is a series-tuned resonant circuit having low
impedance at the tuned frequencies.

It can also provide limited reactive power compensation and voltage
regulation. Single-tuned, first-order, second-order, and third-order
high-pass passive filters are commonly used configurations.

Generally, one or more passive filter branches are designed for low-order
harmonics and then one highpass filter is designed for the rest of the
higher order harmonics. The shunt passive filter is very effective for
compensating current-source nonlinear loads type of generated harmonics.
These filters though quite useful pose various practical problems. The
filter may create a series or parallel resonance with the source impedance
resulting in the amplification of the harmonics with negative consequences.
The frequency variation of the power system and tolerances in filter
components affect its compensation characteristics. As a result, the
size of the components in each tuned branch becomes impractical if the
frequency variation is large. Overload occurs when the load harmonics
level increases and consequently high current and voltages circulate
in the passive branches; therefore, protection circuits are generally
added to prevent such cases. Moreover, the supply impedance strongly
influences the performance of the shunt passive filter, and since the
source impedance may not be easily determined, the performance of the
shunt passive filter becomes difficult to predict. Figures 9 through
12 show the most used passive filter configurations.

FIG. 9 Series-tuned second-order resonant branch (inductance, capacitance,
and resistance in series).

FIG. 10 First-order high-pass passive filter.

FIG. 11 Second-order high-pass passive filter.

FIG. 12 Third-order high-pass passive filter.

FIG. 13 Equivalent harmonic diagram seen as of points M and N.

The equivalent dynamic circuit of a series-tuned, second-order resonant
branch at harmonic scale as seen between points M and N is shown in FIG.
13.

The source impedance is given by the following equation:

The quality factor (Q) of the passive filter, which is defined as the
ratio of capacitive reactance (Xc) or inductive reactance (XL) to the
resistance (rh) at tuned frequency, becomes infinite. Therefore, Q can
be expressed as....

Where:

fh is the tuned frequency

C is the filter capacitance

L is the filter inductance On the other hand, the parallel impedance,
also known as anti-resonance impedance, which can involve amplification
and overvoltage at the frequency frh, can be expressed as in Equation
38.18a. The amplification factor depends on the quality factor of the
filter:

Indeed, the decrease of the quality factor of the filter inductance
reduces the overvoltage at the resonance frequency. Also, at the resonance
frequency, the impedance is not null and the specific hth harmonic is
not completely deviated. The ratio of the impedance after filtering to
the impedance before filtering as a function of frequency when the resonant
filter is tuned to fifth harmonic is given in FIG. 14.

FIG. 14 Compensation characteristic of series-tuned resonant circuit.

To eliminate several harmonics, the idea consists in placing a tuned
filter by harmonic. The elimination of k harmonics requires the parallel
connection of k-tuned filters. In practice, each passive filter element
employs three tuned filters, the first two being for the lowest dominant
harmonics followed by high-pass filter elements. Figures 15 and 16 show
the single-phase and three-phase tuned shunt passive filters. The ratio
of the impedance after filtering with the impedance before filtering
as a function of frequency when the resonant shunt passive filter is
tuned at the fifth and seventh harmonic is given by FIG. 17.

5.1.2 Series Passive Filters

Series passive filters are constituted of parallel resonant branches
connected in series with the nonlinear loads. They provide high impedance
to the harmonic currents and prevent them from flowing into the power
system. These filters also help reduce the current ripple on the dc side
of the rectifier circuit.

They are of low cost, simple to implement, and have been used to limit
harmonics caused by large loads. The series passive filters suffer heavily
from lagging PF operation for a whole range of operation.

On the other hand, a finite small voltage drop across the finite inductive
reactance and resistance of the coil occurs at fundamental frequencies
due to the difficulty in designing sharply tuned filters, and large drop
occurs at harmonic frequencies due to current at harmonic frequencies
escaping from the block that has been created by these filters. The series
passive filter has been found suitable for voltage-fed type of harmonic-producing
loads. Generally, each series passive filter element employs three tuned
filters, the first two being for the lowest dominant harmonics followed
by high-pass filter elements. In each series passive filter element,
two lossless LC components are connected in parallel for creating a harmonic
dam to block harmonic currents. All the three components of the series
passive filter are connected in series. Figures 18 and 19 show the general
schemes of single-phase and three-phase series passive filters.

FIG. 17 Compensation characteristic of series-tuned shunt passive filter
that is tuned at the fifth and seventh harmonic.

FIG. 18 Single-phase series passive filter.

FIG. 19 Three-phase three-wire series passive filter.

5.2 Active Power Filter

To overcome the limitation of passive filters, APFs were developed to
provide better dynamic control of current harmonics and voltage distortion
control. This is achievable thanks to the developments in solid-state
switching devices and control technology in recent years. APFs can be
classified based by a number of elements in topology, supply system,
and the types of converter used in their circuits. They are single-phase
(two-wire), three-phase three-wire, and three-phase four-wire voltage-
or current-source inverters used to generate the compensating voltage
or current that is injected into the line. Current source active filters
employ an inductor as the dc energy storage device. In voltage-source
active filters, a capacitor acts as the energy storage element. Voltage
source active filters are cheaper, lighter, and easier to control compared
to current-source active filters. Several APF design topologies as illustrated
in the block diagram shown in FIG. 20 have been proposed. They can be
classified as follows: shunt active power filter (SAPF), series APF,
hybrid shunt active filter, hybrid series active filter, and UPQC. They
use PWM-controlled current-fed or voltage-fed converters with inductive
and capacitive energy storage elements, respectively.

5.2.1 Shunt Active Power Filter

The shunt active filter operates by injecting harmonic current into
the utility system with the same magnitudes as the harmonic currents
generated by a given nonlinear load, but with opposite phases to maintain
a sinusoidal current at the PCC. The major aim of the shunt active filter
is to compensate harmonic currents yielding an improvement of the PF.
It can also be used as a static var compensator in power system networks
for compensating for other disturbances such as voltage flicker and imbalance.
The SAPF offers some advantages such as the following: source-side inductance
does not affect the harmonic compensation capability of the SAPF system,
cost-effective for low to medium KVA industrial loads, can damp harmonic
propagation in a distribution feeder, do not create displacement PF problems,
and utility loadings. On the other hand, the APF topologies suffer from
high KVA rating of the power electronic inverter for high-power industrial
loads. This is due to the fact that the converter must withstand the
line frequency, utility voltage, and supply harmonic current. In addition,
it does not compensate for the harmonic in the load voltage. Figures
21 and 22 show a single-phase and a three-phase voltage-fed shunt active
filter. A large capacitor connected to the dc bus of the converter behaves
as a voltage source. A single-phase and a three-phase current-fed shunt
active filter are shown in Figures 23 and 24. They use an inductive element
for energy storage.

======

Active power filter Shunt active filter Current source inverter Voltage
source inverter Series active filter Unified power Hybrid active filter
quality conditioner Hybrid shunt active filter Shunt active filter in
parallel with shunt passive filter Shunt active filter in series with
shunt passive filter Hybrid series active filter Shunt active filter
with series passive filter Series active filter in parallel with shunt
passive filter Series active filter with series passive filter

FIG. 20 Configurations of APF for power quality improvement.

======

FIG. 21 Single-phase voltage-fed shunt active filter.

FIG. 22 Three-phase voltage-fed shunt active filter.

FIG. 23 Single-phase current-fed shunt active filter.

FIG. 24 Three-phase current-fed shunt active filter.

The inductor behaves as a controllable nonsinusoidal current source
to compensate for the harmonic current requirement of nonlinear loads.
A diode is used in series with the self-commutating device for reverse
voltage blocking.

A four-pole switch type, a capacitor midpoint type, and a three single-phase
bridge configuration of a four-wire SAPF are shown in Figures 25 through
27:

FIG. 25 Four-pole four-wire shunt active filter.

FIG. 26 Capacitor midpoint four-wire shunt active filter.

FIG. 27 Three single-phase bridge four-wire shunt active filter.

5.2.2 Series Active Power Filter

The series APF is connected in series with the utility system through
a matching transformer so that it prevents harmonic currents from reaching
the supply system or compensates the distortion in the load voltage.
It is controlled in such a way that it can present zero impedance, at
the PCC, to the fundamental frequency and high impedance to harmonic
frequencies to prevent harmonic currents from flowing into the system.
It injects the necessary voltage needed for compensation of voltage harmonics,
voltage sags, and swells in dynamic voltage restoration, voltage flicker,
and other voltage disturbances that distort the desired sinusoidal waveform
at the PCC. It is also used to damp out harmonic propagation caused by
resonance with line impedance and shunt passive filters. The series active
filter is effective for compensating such voltage-source nonlinear loads.
The function of the series active filter is not to directly compensate
for the current harmonics of the load, but to isolate the current harmonics
between the load and the source. A drawback to the series active compensator
is its inability to directly compensate for current harmonics, balance
the load current, suppress neutral currents, and compensate the reactive
power. In addition, it carries full load current and must withstand large
power ratings. In the event of a failure of the filter's transformer,
the load will lose the power supply. Series active filters are designed
either as controllable voltage sources (voltage-fed converter type) or
as controllable current sources (current-fed converter type). A single-phase
and a three-phase voltage-fed series active filter are shown in Figures
28 and 29. Figures 30 and 31 show a single-phase and a three-phase current-fed
series active filter. FIG. 32 shows a three-phase four-wire voltage-fed
series active filter.

FIG. 28 Single-phase voltage-fed series active filter.

FIG. 29 Three-phase voltage-fed series active filter

5.2.3 Unified Power Quality Conditioner

The UPQC is a combination of series and SAPF, which are connected back
to back and sharing a common self-supporting dc link. The series filter
is controlled as a voltage source; hence, it is used for voltage compensation
while the shunt filter compensates for harmonic currents. Hence, UPQC
has the advantages of both the series and shunt filter, simultaneously.
Although its main drawback is its high cost and complexity of control,
interest in UPQC is growing due to its superior performance.

It can compensate significant power quality issues, such as, voltage
harmonics, voltage sag, voltage swell, voltage unbalance, voltage flicker,
current harmonics, load reactive power, current unbalance, and neutral
current. A single-phase voltage-fed and current-fed UPQC are shown in
Figures 33 and 34. Figures 35 and 36 show the schematic of a three-phase
three-wire and a three-phase four-wire UPQC.

FIG. 30 Single-phase current-fed series active filter.

FIG. 31 Three-phase current-fed series active filter.

FIG. 32 Three-phase four-wire voltage-fed series active filter.

FIG. 33 Single-phase voltage-fed UPQC.

FIG. 34 Single-phase current-fed UPQC.

FIG. 35 Circuit configuration of a three-phase three-wire voltage-fed
UPQC.

5.2.4 Hybrid Filters

APFs have the capability of damping harmonic resonance between an existing
passive filter and the supply impedance, but they require a large current
rating with low efficiency and harmful disturbance to neighborhood appliances.
HAF topologies that combine the advantages of both active and passive
filters are more appealing in terms of cost and performance. They are
cost-effective by reducing the KVA rating of the active filter as much
as possible while offering harmonic isolation and voltage regulation.
Two types of HAFs have been developed: a shunt HAF and a series HAF.
The shunt hybrid filters consisting of shunt active filter and shunt
or series passive filters connected in series or in parallel with each
other combine the advantages of both filters. This is an attempt to reduce
the high KVA rating of the shunt active filter without compromising its
functions. A single-phase and a three-phase shunt HAF are shown in Figures
37 through 42. The reduced switch hybrid filters along with load arrangement
are shown in Figures 38 and 40. Since one leg of the active filter is
eliminated by a center tap capacitor, the gate drive circuit requirement
and associated electronic circuit including the number of current sensors
are eliminated. Thus, these topologies reduce the overall cost of the
system. Figures 43 and 44 show the schematic of a three-phase series
hybrid filter. The series hybrid filter is a series or parallel combination
of a series active filter and series or shunt passive filter. The series
active filter acts as a harmonic "isolator." It is to improve
the filtering characteristics and to solve the problems of the passive
filter. Hence, the rating of the series active filter is much smaller
than that of a conventional parallel active filter.

FIG. 36 Circuit configuration of a three-phase four-wire voltage-fed
UPQC.

FIG. 42 Three single-phase bridge three-wire voltage-fed shunt hybrid
filter.

FIG. 43 Three-phase three-wire voltage-fed series hybrid filter.

6. Control of Active Filters

The quality and performance of the APF depends partly on the modulation
and control method used to implement the compensation scheme. Several
control strategies can be used to regulate the current produced by the
filter: variable switching frequency, such as hysteresis and sliding-mode
controls allow direct control of the current, but make the design of
the output filter quite difficult as well as the reduction of the noise
level. PWM control eliminates these problems, but the dynamic
response of the current feedback loop reduces the ability of the filter
to compensate for fast current transitions. Many algorithms in time and
frequency domains are proposed to extract or estimate compensating harmonic
references for controlling the APFs. The most popular are the time-domain
methods such as the notch filter, the instantaneous reactive power theory
(IRPT), the synchronous reference frame (SRF) theory, high-pass filter
method, low-pass filter method, unity PF method, sliding-mode of control,
passivity based control, proportional integral (PI) controller, flux-based
controller, and sine multiplication method. The main advantage of these
time-domain control methods compared to the frequency-domain methods
based on the fast Fourier transformation (FFT) is the fast response obtained.
On the other side, frequency domain methods provide accurate individual
and multiple harmonic load-current detection.

The discrete Fourier transform, the Kalman filter, and the artificial
neural networks are the most harmonic estimation techniques. There are
two control techniques used to generate switching signals for APFs, namely,
the direct and the indirect control techniques. The direct method detects
harmonics in the loads and injects current through an active filter to
cancel the harmonics. On the other hand, the indirect method senses the
harmonics in an ac network, and injects harmonic currents using feedback
control to reduce the harmonics. It has been demonstrated that the direct
method is not robust enough when the time delay in the control circuits
is considered, and the indirect method is more reliable. Indeed, in the
literature, it was reported that the current-type nonlinear load exhibits
a step wave shape, and there is an instantaneous change from one step
to another. This requires instantaneous compensation, but the inherent
delay in the compensation using direct current control scheme results
in switching ripples in the supply current. It is also essential to find
why the direct control algorithm of APF suffers from this problem of
switching ripples. The reference APF current are fast varying nonsinusoidal
signals and the direct current control algorithm works on the principle
of feed-forward control, where, the reference current of the APF is compared
with its sensed current. Therefore, at a point in the ac cycle, the direct
current controller does not have accurate information about the shape
of the actual (sensed) supply current. Therefore, even if there are switching
ripples in the supply current, the direct current controller does not
compensate the ripples due to lack of exact information.

Section 7 addresses the analysis of the single-phase shunt active power
filtering scheme with two control techniques. Direct and indirect current
control techniques are presented with the use of a unipolar PWM (U-PWM)
and a bipolar PWM (B-PWM) applied to the single-phase shunt active power
filter (SPSAPF) to compensate the current harmonics and the reactive
power. It is demonstrated that by using the averaging technique,
the direct consequence of using the U-PWM is that the transfer function
of the SPSAPF becomes a pure gain, which simplifies the tuning of the
regulator parameters. Also, the U-PWM pushes back the first significant
harmonic rays toward twice the switching frequency 2fsw. Furthermore,
it eliminates the rays groups that are centered on the odd multiples
of the switching frequency. In addition to the current compensation loop,
a voltage loop is also designed in order to regulate the dc bus voltage
and to stabilize it at a designed value. The current and voltage regulators
are designed by applying the linear control theory on a small-signal
frequency domain model of the filter. This mathematical model is derived
by using the state-space average modeling technique and, then, applying
the small-signal linearization process.

The SPSAPF is analyzed based on effective THD levels and response to
changing dynamics. The results concerning the two PWM control techniques
are verified by simulation, and experimental results on a 1 kVA prototype
obtained using the direct and indirect current control strategies confirm
the predicted performance and the superiority of the indirect current
control technique with the U-PWM.