A proportional resonant current controller for selective harmonic compensat

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014 2055
A Proportional-Resonant Current Controller
for Selective Harmonic Compensation in a
Hybrid Active Power Filter
Leopold Herman, Student Member, IEEE, Igor Papic, Senior Member, IEEE, and Bostjan Blazic, Member, IEEE
Abstract—This paper deals with reactive power compensation
and harmonics elimination in medium-voltage industrial networks
using a hybrid active power filter. It proposes a hybrid filter as a
combination of a three-phase, two-level, voltage-source converter
connected in parallel with the inductor of a shunt, single-tuned,
passive filter. This topological structure greatly decreases the
voltage and current stress over the elements of the active filter.
Since the topology is composed of a single-tuned branch, the
control algorithm also has to ensure sufficient filtering at other
harmonic frequencies. We propose using a proportional-resonant,
multiloop controller. Since the controller is implemented in a
synchronous-reference frame, it allows us to use half the number
of resonators, compared with the solution using proportional-in-
tegral controllers in the harmonic-reference frame. Theoretical
analyses and simulation results obtained from an actual industrial
network model in PSCAD verify the viability and effectiveness
of the proposed hybrid filter. In addition, the simulation results
are validated by a comparison with the results obtained from a
real-time digital simulator.
Index Terms—Harmonic distortion, hybrid filters, proportional-
resonant controller, reactive power.
I. INTRODUCTION
NONLINEAR loads, which, these days, form a large por-
tion of the overall electrical load, are known to be a major
source of current harmonics in the electrical system. In addition,
most of these loads impose varying reactive-power demands
that have to be compensated in order to improve the power
factor (PF) and efficiently deliver the active power to the loads.
This results in harmonic distortion-related problems, reducing
the quality of the electrical power and the performance of the
power system. The operation of these devices may, therefore,
prove to be very problematic [1].
Traditional solutions to reduce the harmonic current flows
into the supply system and to improve the power factor at the
customer-utility point of common coupling (PCC) involve the
placement of resonant-tuned passive filters at the PCC of the
Manuscript received August 23, 2012; revised February 19, 2014; accepted
July 03, 2014. Date of publication August 19, 2014; date of current version
September 19, 2014. This work was supported by the Slovenian Research
Agency under the Electrical Power Systems Research Program no. P2-356.
Paper no. TPWRD-00887-2012.
The authors are with the Faculty of Electrical Engineering, University of
Ljubljana, Ljubljana 1000, Slovenia (e-mail: leopold.herman@fe.uni-lj.si; igor.
papic@fe.uni-lj.si; bostjan.blazic@fe.uni-lj.si).
Digital Object Identifier 10.1109/TPWRD.2014.2344770
load. These filters represent a well-established technology; how-
ever, in addition to their fundamental task of providing reactive-
power compensation and harmonics filtering, they may cause
unwanted resonance conditions. Their other limitation is an in-
ability to adapt to the changing conditions in the network and
their size [2]–[4].
With the development of power electronics, active filters are
becoming increasingly important, because they do not cause res-
onance with the system, and they also enable rapid dynamic re-
sponses to changing conditions. The central part of these devices
is the power converter, which is controlled in such a way in order
to improve selected power-quality (PQ) parameters. The main
disadvantage of active filters is their high investment and oper-
ating costs [5]–[7].
To overcome the aforementioned disadvantages, passive and
active filters can be combined into a single device. This enables
us to minimize the required power ratings of the active filter
(reducing the overall price) and to dampen the harmonic reso-
nances caused by the passive part. These devices are referred to
as hybrid active power filters (HAPFs) [8]. Various hybrid-filter
topologies have been proposed for reactive-power compensa-
tion and harmonic-current filtering. The most common variants
are when the active filter is added in series to the passive filter
(series HAPF) [9]–[12] and a parallel configuration of the pas-
sive and active parts [13]–[15]. In the first case, the required
voltage rating of the voltage-source converter (VSC) is small,
since most of the voltage drop occurs on the capacitor; however,
the active filter may conduct a fully rated fundamental current if
no coupling transformer is used. In the second case, the VSC has
a lower current rating, but it has to be designed for the nominal
voltage. An extensive overview of other topological structures
can be found in [16]–[19].
The hybrid filter investigated in this paper is composed of
a three-phase, two-level, VSC connected in parallel with a pas-
sive filter inductor (Fig. 1). This topology has not received much
attention since it was first reported in [20], where the authors re-
ported poor dynamical behavior of the active part of the filter.
This paper proposes an improved version of the topological
structure and using a proportional resonant current controller
implemented in the SRF. The main advantage of this structure is
that the voltage drop on the capacitor reduces the VSC voltage
ratings, while the inductor conducts the fundamental reactive
current. In this paper, the rating requirements of this topology
are analyzed and compared with the series HAPF topology and
the pure active filter. As will be shown, the required power
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2056 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 29, NO. 5, OCTOBER 2014
Fig. 1. Scheme of the proposed shunt-connected HAPF.
rating of the active part is very low, typically only 1%–2% of
the load rating.
Since the topology investigated in this paper is composed of
a single-tuned branch, the control structure also has to ensure
sufficient filtering of the current harmonics at other harmonic
frequencies. Therefore, designing the controller is an important
and challenging task, due to its impact on the performance and
stability of the overall system. The control principles previously
presented for different HAPF structures are mainly wideband,
where the proportional (P) control law is the most common and
is usually implemented in the SRF [6], [9], [14]. Due to the
high value of the proportional constant required for sufficient
filtering, the proportional control structure does not perform
well with the topology investigated herein. Namely, it shows
poor transient performance [22]. The poor dynamic behavior of
this topology in the case of a fluctuating load has also been re-
ported in [20], where a proportional-integral (PI) regulator im-
plemented in the HRF was used to control the active part of the
HAPF.
To enhance the transient performance of the HAPF, this
paper proposes a proportional-resonant current controller
implemented in the SRF. Resonant controllers have taken on
significant importance in recent years due to their high selec-
tivity and good performance [23]–[29]. They are equivalent to
the conventional PI controllers implemented in the HRF for the
positive- and negative-sequence reference frames. Thus, they
can achieve similar steady-state performance to PI controllers,
with the following advantages:
• only one regulator is needed for compensating both har-
monics at ;
• if implemented in the HRF, it allows half the number of res-
onators, compared with the solution using PI in the HRF;
• better selectivity and, thus, improved transient perfor-
mance.
Fig. 2. Simplified equivalent circuit of the HAPF.
This paper is organized as follows. The system configuration
and rating analysis are presented in Section II. In Section III, the
control system is developed. The simulated network is described
in Section IV and the proposed HAPF performance is evaluated
in Section V. Simulation results are validated in Section VI.
Finally, conclusions are drawn in Section VII.
II. SYSTEM CONFIGURATION
Fig. 1 shows the proposed circuit configuration. In this
system, a nonlinear load is supplied by a balanced voltage
source and compensated by the proposed HAPF. A ripple
filter is used to reduce the high-frequency harmonic currents
injected into the network.
In Fig. 2, a simplified, equivalent circuit of the proposed topo-
logical structure is shown. The symbols used are as follows:
supply voltage, and are the supply-system resistance and
inductance (short-circuit impedance and transformer impedance
connected in series), and are the load resistance and in-
ductance, is the passive filter capacitance, and are
the passive filter resistance and inductance, and , and
are the ripple filter inductances and capacitance. The ac-
tive part of the HAPF is presented with an ideal voltage source,
while the nonlinear load is considered to be a current source .
A. Rating Analysis
The active filter has to provide a small component of funda-
mental voltage at the PCC in order to divert the fundamental
reactive current to flow in the inductor. This voltage is equal to
the voltage drop on the inductor for a pure passive filter and it
depends on the tuned frequency of the passive part. It can be
expressed as
(1)
Here, is the fundamental frequency (i.e., 50 Hz) and is the
tuned frequency of the passive filter.
The harmonic voltage across the active part of the hybrid
filter consists of two components: the component due to the dis-
torted supply voltage and the component due to the har-
monic load current flowing in the passive impedance. This
voltage is given by
(2)
To obtain the worst case, no load impedance was considered
when deriving (2), and the coupling impedance of the active

HERMAN et al.: PROPORTIONAL-RESONANT CURRENT CONTROLLER FOR SELECTIVE HARMONIC COMPENSATION 2057
TABLE I
EXAMPLE CASE PARAMETERS
Fig. 3. Active filter rated power versus tuned frequency of the passive part.
filter was neglected and the harmonic current flowing in the
system was considered to be zero (ideal filtering).
The current flowing in the active part also consists of two
parts: the harmonic component due to the distorted supply
voltage and the distorted load current. It is given by
(3)
The power-rating requirement of the active part is finally given
by
(4)
To better illustrate the power-rating requirements of the in-
vestigated HAPF’s topological structure, let us consider the (hy-
pothetical) example case in Table I. For this case, the required
power of the active part is given by
(5)
As is clear from Fig. 3, the power rating strongly depends on
the tuned frequency of the passive filter. The hybrid active filter
of the proposed topology is thus the most suitable for tuned filter
branches, where the tuned frequency is as close as possible to
the filtered harmonic.
For the same load conditions, the rating of the active part for
the series HAPF is equal to 0.0251 p.u., while the required rating
of the pure active filter for this case is 0.25 p.u. (which does
not include the reactive power compensation). An extensive
overview of the power-rating requirements for most common
current-sink HAPF topologies can be found in [30].
III. CONTROL SYSTEM DESIGN
Proportional-resonant (PR) controllers are equivalent to con-
ventional PI controllers implemented in the -reference frame,
separately for the positive and negative sequences. Therefore,
the PR controller is capable of simultaneously tracking the ref-
erence for the positive and negative sequence with zero steady-
state error. For example, a sixth harmonic PR compensator is
effective for the fifth and seventh harmonics of both sequences;
hence, four harmonics are filtered with one PR filter imple-
mented in the SRF [23].
A. PR Controller Transfer Function
The relationship between the -components and the
-components is given by an anticlockwise rotating Park’s
vector
(6)
(7)
Thus, the influence of the Park transformation can be expressed
as the frequency shift of all the frequencies in the frequency
domain. The equivalent transfer function of the PR controller is
in the SRF. can be derived from a PI controller imple-
mented in positive- and negative-sequence HRFs, taking into
account (6) and (7)
(8)
(9)
For the nonideal integrators of , the PR
controller transfer function takes the form
(10)
where is the cutoff frequency, representing the limits of
the integrator. In this paper, several HPR controllers are added
in a cascade to control several harmonics simultaneously. The
current controller takes its final form
(11)
Equation (9) describes an ideal PR controller with infinite gain
at the tuned frequency and no phase shift and gain at the other
frequencies. The disadvantage of such a controller in practical
applications is the possible stability problem associated with in-
finite gain, which can be avoided by using nonideal integrators:
(10). Another feature is the very narrow bandwidth of the ideal
PR controllers, which makes them highly sensitive toward slight
frequency variation in a typical power system [23].
B. Control Algorithm
Fig. 4 shows the proposed control scheme, which includes
harmonic detection (Fig. 5), the PR-current regulation (11), dc

Fig. 4. Control block diagram of the HAPF.
Fig. 5. Block diagram of the harmonic current detection.
Fig. 6. DC bus voltage control.
voltage control (Fig. 6), and the fundamental current diversion
controller (Fig. 7). The PR controller is closed with the mea-
sured and filtered values from the actual system. Since the objec-
tive is to eliminate the harmonics from the supply current, this
current represents the PR controller input. At first, the funda-
mental-frequency component has to be extracted from the mea-
sured current. This is done by using first-order, high-pass filters
in the fundamental frequency and synchronous reference frame
with a cutoff frequency of 20 Hz. Fundamental angular fre-
quency is obtained using a conventional qPLL system [31]. Due
to the fact that the supply current may also contain, in the case of
unsymmetrical line conditions, some fundamental negative-se-
quence component, this component is suppressed by the second
part of the harmonic detection unit, where Park’s transformation
is performed with a clockwise synchronous rotation. As a result,
the output of the harmonic detection unit is the supply-current
harmonics which, in the next step, are compared to the reference
values. The difference is then applied to a PR controller tuned
on the resonance frequencies of , , and . The
output of the controller is the variable , which is added
to the voltage reference .
Fig. 7. Feedforward control for the fundamental current diversion.
C. DC Bus Voltage Control
Proper control of the dc bus voltage is essential for the op-
eration of this HAPF. The principle of controlling the dc bus
voltage is based on active power control, that is, charging the
dc capacitor with active power will increase the voltage while
releasing a certain amount of active power, will decrease it. Ac-
cording to -theory, a dc component in the -coordinates cor-
responds to the active power and, thus, dc bus voltage control is
implemented in the SRF.
As can be seen from Fig. 6, the difference between the ref-
erence value and the measured and filtered actual value is ap-
plied to a PI controller, which adjusts the direct axis current (the
quadrature axis is set to zero). A low-pass filter (LPF) with a
cutoff frequency of 15 Hz eliminates the harmonics from the
measured dc bus voltage. The resulting control signal is added
to the voltage reference .
D. Fundamental Current Diversion
In order to achieve the minimum current rating of the ac-
tive part, the fundamental-frequency filter current needs to be
diverted into a parallel inductance. This is done with a simple
feedforward controller, represented in Fig. 7. It calculates the
voltage appearing across the passive filter inductor , which
would occur in the absence of the active filter, using (1). As a
result, only a small fundamental frequency current is flowing
through the active elements, which is required for charging the
dc capacitor.
E. Current Control Transfer Function
Fig. 8 shows the main current control block diagram of the
hybrid filter. The harmonic content of the system current is fil-

Fig. 8. Main current-control block diagram.
tered from the measured system current. The transfer function
for the harmonic-detecting circuit can be expressed as
(12)
where is the cutoff angular frequency of the HPFs that ex-
tract the dc component in the –coordinates, and is the fun-
damental angular frequency. The detected harmonic current
is compared with the current reference , and the difference
represents the input to the PR controller. This results in the
production of the reference voltage to be generated by
the inverter.
In the real control circuit [implemented on a digital signal
processor (DSP)], the output signal is inherently delayed with
respect to the input signal. The time delay is represented as
(13)
where 100 s is the sampling period. The plant transfer
function is defined as (see Fig. 2)
(14)
Finally, the open-loop transfer function of the hybrid filter con-
troller is given by
(15)
F. Filtering Characteristic
The filtering characteristic of the HAPF depends primarily
on the control algorithm; however, all of the algorithms have
something in common. It is well known that power-electronic
converters in harmonic filtering applications may be controlled
to behave in a similar fashion to a passive element [21], [32]. As
will be shown, the PR-controlled HAPF proposed herein mimics
several parallel resonant circuits added in parallel and tuned to
the characteristic harmonic frequencies.
The filtering characteristic can be obtained by calculating the
equivalent model of the network (Fig. 2). Applying Kirchhoff’s
circuit laws yields
(16)
Fig. 9. Equivalent circuit for the filtering characteristic analysis.
Taking into account (11) and (14) and neglecting the resis-
tance of the passive filter inductor, we obtain
(17)
Letting:
(18)
we obtain
(19)
Equation (19) defines the filtering characteristic of the HAPF,
which depends on the passive filter inductor and capacitor
equivalent impedances and , the system impedance
, and the active power filter transfer function given by (11).
As can be seen from Fig. 9, representing a single-phase equiv-
alent circuit of the system with a connected HAPF, the active
filter behaves as a pure resistor , with several parallel
circuits added in series.
IV. NETWORK UNDER STUDY
To illustrate some practical implications of the proposed
PR-controlled HAPF and to evaluate its filtering performance,
the operation is demonstrated on a real industrial network
model. A simplified scheme of the modelled system is shown in
Fig. 10. This system was chosen because it represents a typical
example of an industrial network with a poorly designed pas-
sive compensator, producing unwanted resonant amplifications
of the current harmonics. Since the proposed HAPF topology
enables retrofitting applications, the existing reactive power
filter is upgraded with the active part. As will be shown, the
active part damps the resonances and ensures sufficient filtering
of the characteristic harmonics.

Fig. 10. Simplified single-line diagram of a real industrial network.
A. System Data
The network parameters are given in Table II. A 35-kV dis-
tribution network is powered through a 110-kV transmission
system, which is represented by a stiff voltage source with a
short-circuit power rating of 3750 MVA. The passive filter rated
at 5.4 MVAr is connected on the secondary side of the trans-
former TR I (35-kV voltage level). Two large, adjustable-speed,
thyristor-controlled, motor drives—DCM I and DCM II (rated
at 2.5 MW and 2.15 MW)—are also connected to the network.
These two motor drives are the main source of the harmonic
distortion in the network. The linear load is modeled with the
impedance .
B. Active Filter Parameters
The parameters of the active part are given in Table III. The
rated power of the active filter is 75 kVA, which is approxi-
mately 1.4% of the passive part’s rating. The ripple filter block
in Fig. 1 is used to reduce the high-frequency harmonic currents
(generated by the PWM inverter) injected into the network. Its
resonant frequency is approximately 2600 Hz.
The dc side of the VSC is only built with a capacitor. To keep
the voltage ripple as low as possible, the capacitance is set to
1200 F. The dc bus voltage is controlled to a value of 4 kV by
a PI controller with 2 and 0.5 that adjusts the
reference active current. The firing pulses for driving the semi-
conductor switches [insulated-gate bipolar transistors (IGBTs)]
are generated using PWM, with a 10-kHz carrier signal.
C. Tuning of the Main Current Control Loop
The parameters of the PR controller are most often tuned by
means of Bode diagrams [27]–[29]. Fig. 11 shows the Bode
diagrams of the open-loop transfer function (15) for positive
and negative frequencies on a linear scale. The diagrams are
obtained with controllers tuned at harmonics
, with 0, 100, 250, and 450
TABLE II
INDUSTRIAL NETWORK PARAMETERS
TABLE III
ACTIVE FILTER PARAMETERS
and 0.5, 2, and 15, respectively. Since the controller is
implemented in the SRF, only one regulator is needed for com-
pensating both harmonics at 1, 2, 3. To observe
the effect of the harmonic detection transfer function, the har-
monic detection transfer function is included in the calculation
of the open-loop transfer function.
As can be seen, the proportional constant defines the
crossover frequency at which the magnitude is 0 dB. In
other words, it defines the bandwidth of the filter. To ensure
the stability, has to be high enough so that all of the fil-
tered harmonics are lower than the . When the resonant terms
are added, the overall frequency response is modified only in
the vicinity of each tuned harmonic frequency and, thus, their
impact on the stability can be neglected. The Bode character-
istic depicted in black, is obtained with 15 and
450, which ensures stable operation of the overall system with
a phase margin of 76 and provides an adequate tradeoff be-
tween the bandwidth, the transient performance, the selectivity,
and the stability.

Fig. 11. Open-loop Bode diagram of the main current control loop.
Fig. 12. Frequency characteristics of harmonic propagation/damping.
V. HYBRID FILTER PERFORMANCE EVALUATION
The HAPF filtering characteristic is analyzed using (18). The
steady- and transient-state performances are demonstrated in
PSCAD software.
A. Filtering Performance Analysis
Fig. 12 shows the relationship between the and for dif-
ferent values of and . When 0, the HAPF
behaves as a pure passive filter tuned to 314 Hz. It creates a
parallel resonance point very close to the 5th harmonic com-
ponent with the ratio reaching high values of more than
5 dB. This may result in overheating and a shorter lifespan for
certain equipment (transformers, cables, filters), the occurrence
of noise and vibrations (motors, generators), the incorrect op-
eration of certain devices (computers, printers), and equipment
outages or destruction. In the past few years, several cases were
reported by this particular customer, related to the problem of
harmonic resonance.
To overcome this problem, the active part of the HAPF needs
to dampen the parallel resonance. As can be seen from Fig. 12
Fig. 13. Simulated waveforms in the steady state—passive filter.
and follows from (19), increasing the proportional control part
causes the active filter to act as an additional fictitious re-
sistance added in series to the system impedance that
increases the damping performance of the filter. The parallel res-
onance gets completely damped for values of . It should
also be noted that is null at the fundamental frequency and,
thus, no additional losses occur at this frequency due to the op-
eration of the active filter.
On the other hand, by increasing the integral control part
, high equivalent system impedances at the selected har-
monic frequencies are created. If is high enough, the system
impedance is much higher than the filter impedance, which di-
verts almost all of the harmonic currents injected by the non-
linear load into the filter branch. This can be seen in Fig. 12 as a
low (negative) gain of the ratio at , 1, 2,
3. Thus, it can be assumed that the HAPF will show very good
filtering performance for these harmonic currents produced by
the load. The filtering performance will be further evaluated.
B. Steady-State Performance Evaluation
In this subsection, the results of computer simulations using
the PSCAD software package are shown. The HAPF results are
compared to the results obtained with a pure passive filter. Due
to the transparency, only the waveforms for one phase (L1) are
shown.
Fig. 13 shows simulated waveforms of the passive filter
(voltage at the filter PCC , supply current and load
current ) in the steady state. The harmonic content of the
and in terms of the percentage of the fundamental
component is given in Table IV. It is clear from the waveforms
that the 5th harmonic is particularly problematic. It reaches
a value of more than 15% of the filter’s fundamental current,
while the 7th harmonic reaches only 0.79%. Consequently, the
voltage at the filter PCC is also highly distorted with
the 5th harmonic. It reaches a value of 4.5%. These results were
expected, as the frequency-response characteristics showed that
there is a parallel resonance point close to the 5th harmonic.
Fig. 14 shows the simulated waveforms of the HAPF under
the same conditions as Fig. 13. After starting the active filter,
the voltage and current distortions decrease significantly. Both
waveforms are nearly sinusoidal. The supply current has the
THD reduced to 1.47%. The 5th and 7th harmonic components
are very small, that is, 1.39% and 0.35%. The PCC voltage THD

TABLE IV
HARMONIC CONTENT OF VOLTAGE AT THE PCC AND SUPPLY CURRENT IN
PERCENT OF THE FUNDAMENTAL COMPONENT
Fig. 14. Simulated waveforms in the steady-state—HAPF.
is now only 0.52%. It is also important to note that there is a
very low fundamental frequency component in the active filter
current in the steady state.
C. Transient State Performance Evaluation
Fig. 15 shows how the hybrid filter behaves during startup. At
first, only the passive filter is operating, and after 50 ms of sim-
ulation, the active filter starts. The system needs approximately
150 ms to reach the steady state. After that, the supply current
and the PCC voltage waveforms become almost sinusoidal.
Fig. 16 shows the HAPF active and reactive power outputs. It
is clear that the active filter does not affect the generation of the
reactive power. It even slightly increases after putting the active
part in the operation.
Fig. 17 shows simulated waveforms of the hybrid filter for a
step load decrease/increase of 50%. The supply current is dis-
torted for approximately half a cycle after the occurrence of the
load change and becomes almost a sinusoidal waveform with
a THD of 1.47%. The load change does not produce any other
unwanted effect (e.g., unstable operation) and, thus, it can be
Fig. 15. Simulated waveforms in the transient state—HAPF startup.
Fig. 16. Simulated waveforms in the transient state—active filter and reactive
power output during HAPF startup.
Fig. 17. Simulated waveforms in the transient state–50% step load decrease/
increase.
concluded that the current control loop as well as the dc-side
control loop work properly and stably during load variations.
VI. RTDS RESULTS
In this section, the PSCAD simulation results are validated
by comparing them with the results obtained from the real-time

Fig. 18. Schematic overview of the testing setup.
Fig. 19. RTDS waveforms in the steady state—HAPF.
digital simulator (RTDS), which is one of the simulators that
makes real-time calculation of power-system electromagnetic
phenomena possible. The achieved calculation time steps
are about 50 s for the modelling of power-electronics ele-
ments such as IGBT converters, even as low as 1.5 s [33].
Special hardware also makes it possible for the importing and
exporting of signals to external devices, which is a basis for
the closed-loop testing of external equipment (e.g., DSP) with a
power system model. In this way, the RTDS user has the possi-
bility to analyze the external device itself as well as its impact
on the rest of the modelled system. Therefore, we can consider
the model within the RTDS simulator to be a replacement of a
real system [34].
A schematic overview of the testing setup is presented in
Fig. 18. Within the RTDS simulator, a power system model
with the proposed HAPF (Fig. 1) has been created, where
the output signals correspond to the following: three-phase
voltages at the filter PCC , three single-phase system
currents , and the voltage at the dc side of the inverter
TABLE V
HARMONIC CONTENT OF VOLTAGE AT THE PCC AND SUPPLY CURRENT IN
PERCENT OF THE FUNDAMENTAL COMPONENT
Fig. 20. RTDS waveforms in the transient state—50% step load increase.
. By determining the analog output card (GTAO) ratio, the
output from the RTDS simulator is in the form of seven voltage
signals . These signals are fed into the
Texas Instruments TMS320F28335 hardware platform (32-b
floating-point, 150 MHz) as the control system that converts
[analog-to-digital converter (ADC)] and amplifies these signals
and produces voltages and currents that correspond to those
in the model. The proposed control algorithm (Fig. 4) is im-
plemented in the C language, using the TMS320C2000 Code
Compose Studio as a development environment and a pre-
warped bilinear (Tustin) transform as a digitization technique.
The hardware produces six firing pulses that are led back to the
RTDS simulator via the digital input card (GTDI).
A. Steady-State Performance Evaluation
Fig. 19 shows the RTDS results of the HAPF operation under
the same conditions as Fig. 14. As can be seen, the system cur-
rent and the PCC voltage waveforms are nearly sinusoidal. The
supply current has the THD reduced to 2.01%, while the PCC
voltage THD is only 1%. The harmonic content of the and

in terms of the percentage of the fundamental component is
given in Table V.
B. Transient State Performance Evaluation
Fig. 20 shows simulated waveforms of the hybrid filter for a
step load change from 50% to 100%. The supply current is
distorted for less than three fundamental cycles and after that,
the benefits of the HAPF are clearly seen. The load change does
not produce any other unwanted effect.
A comparison of the results in Figs. 14–17 and Figs. 19 and
20 shows very good matching, which validates the simulation
results obtained with PSCAD software.
VII. CONCLUSION
In this paper, a hybrid active power filter for reactive power
compensation and harmonics filtering has been presented. It is
composed of a small-rating VSC connected in parallel with the
inductor of a shunt single-tuned passive filter. Since the rated
power of the active filter is relatively low, the HAPF represents
a viable solution for reactive power compensation and harmonic
filtering.
A PR current control scheme for selective harmonics com-
pensation with the HAPF has been proposed. As shown, each
controller acts as a resonant filter tuned to a certain harmonic
frequency. The proper selection of the parameters ensures high
selectivity and improves the transient performance of the HAPF.
Another key feature is that each pair of harmonics ,
is filtered by one controller and, thus, important
savings in terms of computational burden are achieved.
Theoretical analysis, along with the simulation results, ob-
tained from a real industrial network model, verifies the effec-
tiveness of the proposed hybrid filter, which represents an ex-
cellent solution for reactive power compensation and harmonic
filtering.
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Leopold Herman (S’06) was born in Trbovlje, Slovenia, on April 16, 1984. He
graduated from the University of Ljubljana, Faculty of Electrical Engineering
in 2008.
Currently, he is a Researcher at the Faculty of Electrical Engineering, Uni-
versity of Ljubljana, Ljubljana, Slovenia. His research interests include power-
quality and power system simulations.
Igor Papic (S’97–M’00–SM’06) received the B.Sc., M.Sc., and Ph.D. de-
grees in electrical engineering from the Faculty of Electrical Engineering of
the University of Ljubljana, Ljubljana, Slovenia, in 1992, 1995, and 1998,
respectively.
Currently, he is a Professor at the University of Ljubljana. From 1994 to
1996, he was with Siemens Power Transmission and Distribution Group,
Erlangen, Germany. In 2001, he was a Visiting Professor at the University
of Manitoba, Winnipeg, MB, Canada. His research interests include power
conditioners, ﬂexible ac transmission systems devices, power quality, and
active distribution networks.
Bostjan Blazic (S’02–M’06) received the B.Sc, M.Sc., and Ph.D. degrees in
electrical engineering, from the University of Ljubljana, Slovenia, in 2000, 2003
and 2005, respectively.
Currently, he is an Assistant Professor with the Faculty of Electrical Engi-
neering, University of Ljubljana. His research interests include power quality,
smart grids, mathematical analysis, and the control of power converters.

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