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
MODELLING OF HYBRID FILTER FOR HARMONIC
COMPENSATION IN POWER SYSTEMS
R. Ramesh, V.Ramachandran, P.Chandrasekar, K.Nithiyananthan, J.R.Maglin
Department of Electrical and Electronics Engineering
Anna University, Chennai -24
India
Abstract - The need for effective utilization of electrical power has increased the area of application of power
electronics. Due to extensive use of power control devices, an increasing deviation of the system voltage and current
waveforms is observed. The presence of harmonics in the power lines results in greater power losses and excessive
heating of electrical equipment in the power distribution system. An attempt is being made in this paper to model a
hybrid active filter to compensate for harmonics in power systems. The passive filter removes load produced
harmonics just as a conventional one does. On the other hand, the active filter plays a role in improving the filtering
characteristics of the passive filter. The hybrid active filter which is a combination of both active and passive filters is
modeled using P-Q theory and simulation studies are performed using PSIM-SIMCAD software. Experimental
results obtained from a prototype model are shown to verify the theory developed in this paper.
Key – Words: - Hybrid filter, Resonance, Instantaneous reactive power theory
1
passive filters are bulky and they are susceptible to
Introduction
The need for effective utilization of electrical
power has increased the area of application of power
electronics. But non-linear loads draw non-sinusoidal
currents,
injecting
current
harmonics
into
the
distribution system. The voltage drop across the
impedance due to these current harmonics gives rise
to voltage harmonics. Harmonics have damping
effects on electrical equipment such as transformers,
rotating machines, switchgear, capacitor banks, fuses
and protective relays. The effects are in the form of
increased losses and excessive heating that ultimately
lead to the equipment’s failure.
Shunt passive filters have been initially used
to compensate the current harmonics injected by large
industrial non-linear loads. The most common types
of shunt passive filters are the single tuned filter and
the high pass filter as they are the simplest to design
and the least expensive to implement [1]. But the
 MAIN AUTHOR AFFILIATION INFORMATION GOES HERE.
series and parallel resonance with the supply and the
load. Also the source impedance influences their
filtering characteristics. The large ratio of the source
impedance to the filter impedance will lead to better
filtering of load current harmonics. A single active
filter can be used for compensating multiple
harmonics as well as for damping resonance in a
power distribution system [1]. An active filter can be a
VSI or CSI operating on PWM and requires a high
bandwidth because of the non-sinusoidal nature of its
reference. The VSI is preferred over the CSI, due to
its lower initial costs and higher efficiency. Active
filters are currently expensive for larger ratings as
compared to passive filters due to the higher prices of
the switching devices, but it is admissible if several
other features are added in them to improve the power
quality of a distribution system.
2
State of the art
the efficiency. The advantages of both the active filter
and the passive filter are combined in a hybrid active
The control of the hybrid active filter requires
extraction of the harmonic component of the source
current. However the filter current has also been used
for the control of the hybrid active filter [1]. The
traditional method of extracting the harmonic
component in time domain is a notch filter at the
fundamental frequency. But this method gives rise to
errors due to changes in the system frequency.
Dr.Akagi et al [3] proposed the IRP theory that has
been widely used for harmonic extraction.
The hybrid active filter shown in fig.1.1 (b) was
proposed by Peng et al [4]. The extraction of harmonics is
filter. Hybrid active filters are shown provide a
practical and cost effective harmonic filtering solution
for non-linear loads in the range of 1MVA to 30
MVA.
A single phase equivalent circuit of HAFS is
shown is fig.1. The source voltage (Vs) and the source
impedance (Zs) are approximated, as the Thevenin’s
equivalent as seen from the PCC and the load as a
current source (IL). The hybrid active filter consists of
an active filter (Vaf) in series with a shunt passive
filter (Zf) in order to compensate the harmonics in the
distribution system.
based on the IRP theory. The active filter is a voltage
controlled VSI that inserts a resistance (K) at harmonic
frequencies. This ensures damping of resonance along with
the filtering of harmonics in the system. The compensation
characteristics and the stability of this hybrid active filter
system were discussed in [4]. The active filter acts as a
harmonic isolator by forcing the load current harmonics to
flow into the passive filter.
The hybrid active filter shown in fig.1.1. (c)
Fig 1: single phase equivalent circuit of the HAFS.
was proposed by Dr.Akagi et al [3] with a control
scheme based on the IRP theory. The IRP theory has
The control strategy of a hybrid active filter
been modified by the use of fundamental component
system depends on the harmonic detection method
of the voltage at the PCC in place of its instantaneous
used in determining the reference to the
value. The active filter is controlled as a voltage
active filter. The harmonic detection method used here
source to insert a resistance (K) at harmonic
for control of a hybrid active filter is Supply current
frequency. It acts as a harmonic compensator and
detection method ,In this method, Is is detected and its
improves the filtering characteristics of the passive
harmonic component Ish is extracted. The active filter
filter. In [5,6] illustrates the methods of simulation
is controlled to satisfy Vaf = Kc.Ish.
and the guidance.
This thesis deals with the hybrid active filter shown
3
in fig.1in which the active filter consisting of a VSI
Hybrid active filter
based on PWM is connected in series with the shunt
It contains both an active filter and a shunt
passive filter to reduce the initial costs and improve
passive filter. The supply voltage is assumed to be
sinusoidal and the non-linear load considered for
compensation is a 3-phase, 6-pulse diode bridge
It is observed that the lead characteristics
rectifier. The shunt passive filter consists of a single
extend the filtering capacity of the hybrid active filter
tuned filter for 5th harmonic, which is the dominant
from that of a single tuned shunt passive filter to that
harmonic in the rectifier load current. For the control
of a wide band filter .But the phenomenon of parallel
of active filter, the harmonic component of the source
resonance
current is extracted by the IRP
disproportionately. The resonant frequency ( Wp)
theory [3].
shifts towards lower side for higher values of T1. The
appears
if
T1
is
increased
magnitude of the resonant peak at W=Wp can be
4
Filtering characteristics
The filtering characteristics of the hybrid
reduced for a particular values of T1 and T2 by
increasing the value of K.
In contrast, the lag characteristics reduce the
active filter system compensating harmonics
generated by a non linear load are analyzed. The
harmonic equivalent circuit of hybrid active filter in
the lag-lead characteristics has been shown in Figure 2
and the filtering characteristics are shown in Figure 3.
filtering characteristics of the hybrid active filter from
that of a single tuned shunt passive filter to that of a
narrow band filter. Increasing the value of K can
increase the width of the pass band for a particular T 1
and T2.
5
control scheme
The computation of the reference to the active
filter involves the extraction of the harmonic
component of a signal. This extraction can be done
Fig 2 .Harmonic equivalent circuit of HAFS
using frequency-domain or time-domain technique
[7]. Hence time-domain techniques are preferred due
The above circuit represents a generalized
to
their
ease
of
implementation
and
little
case that includes lead characteristics and lag
computational burden. The harmonic extractions
characteristics. The lead characteristics are obtained at
based on the IRP theory have advantages over the
either T2=0 or T1>T2(when T2≠0).The active filter
notch
acts as a series RL circuit with a resistance of K ohm
performance from the system frequency variations.
and an inductance of KT1 henry for the lead
Hence, this is presently the most popular method of
characteristics when T2=0.
harmonic extraction. The P-Q theory is one of the
filter
in
the
independence
of
filtering
Similarly lag characteristics are obtained at
several methods that can be used in the control of
either T1=0 or T1<T2 (when T1≠0). The active filter
active filter [1]. The instantaneous voltages and
acts as a parallel RC circuit with a resistance of K
currents in three-phase circuits can be mathematically,
ohm and a capacitance of T2 / K farad for the lag
expressed as the instantaneous space vectors [3]. For
characteristics when T1=0.
simplicity, the three-phase voltages and currents
excluding zero-phase sequence components are
current harmonics produces voltage harmonics at the
considered.
PCC. A hybrid active filter is used to filter these
6 Modeling of hybrid filer
harmonics. It consists of an active filter connected in
The system consists of a power circuit and a
control circuit. The power circuit is composed of the
series with a shunt passive filter through a coupling
transformer.
various components of the hybrid active filter system.
The control circuit deals with the interfacing of a
micro-controller in order to generate the switching
signals for the active filter. The simplified schematic
of the hybrid active filter system is shown in a singlephase representation in Fig. 3 The system is
categorized into three broad sections. The source
comprises of an ideal voltage source in series with an
Fig.3 : Simplified schematic of the HAFS
inductive reactance. The non-linear load considered is
a 3-phase, 2.5 kW, 6-pulse diode bridge rectifier.This
lLoad injects currents harmonics into the system. The
7
Results
The modeling of HAFS is done by using C++
programming .The results are tabulated in Table 1
voltage drop across the line impedance due to the
Table 1: Parameters of HAFS
Fundamental frequency
f
= 50
Supply voltage
Vs
= 415
Load Power
P
= 2500
1 p.u.
= 68.89
Source inductance
Ls
= 0.0091
Source inductance
Ls
= 0.0183
Source resistance
Rs
= 0.3588
Load Current
Idc
= 4.56
Load Voltage
Vdc
= 547.7505
Load resistance
Rdc
= 120.1207
Load inductance
Ldc
= 0.0300
Passive filter inductance
Lf5
= 0.0260
Passive filter capacitance
Cf5
= 1.6586
Passive filter resistance
Rf5
= 11.3068
Switching ripple inductance
Lr
= 0.0116
Switching ripple capacitance
Cr
= 1.3875
Switching ripple resistance
Rr
= 0.3484
Switching frequency
fs
= 5600
Corner frequency
fc
= 3138.7488
* Switching harmonics are attenuated.
Leakage inductance
L1
= 0.0030
Leakage resistance
R1
= 0.0785
* Pulse number = 6
Order of the harmonics
5
7
11
13
17
19 23
25
Hz
Volts
Watts
Ohm
Henry for SCR = 24
Henry for SCR = 12
Ohm
Amps
Volts
Ohms
Henry
Henry
e-05 Farad
Ohm
Henry
e-06 farad
Ohm
Hz
Hz
Henry
Henry
A detailed simulation is made for the diode rectifier
8
Protection scheme for the HAFS
A
protection
scheme
is
analyzed
and
suggested for the hybrid active filter system to

Isolate the active filter from the remaining system
when a fault occurs in the PWM inverter.

Provide a starting sequence for its operation.
When a fault condition occurs in the MOS
gate driver, a fault latch is set and all its six output
drive signals are shunt down. It is necessary to isolate
the inverter to attend to the fault condition without
load with an RL circuit on its DC side. The effect of
commutating inductance is also considered. The
supply voltage is assumed to be sinusoidal in nature
and the harmonics are injected into the system by the
rectifier load. The coupling transformer is modeled as
an ideal transformer with a turns ratio of N1:N2 in
series with a leakage inductance along with its
resistive component.
The results were compared for different source
impedance .
disturbing the remaining part of the hybrid active filter
system. This is done by connecting the switches NO1
and NC1 to the coupling transformer as shown in fig.4

HAFS Without any Filter:

HAFS without the Active Filter:

HAFS With the Active Filter:
Table 2: Attenuation in the source current due to
the shunt passive filter for Ls = 9mH.
5
7
11
13
17 19
H
21.54
15.80
32.57
18.76
0.6
6
4.3
3
Ishp
(%)
12.19
9.82
22.81
13.36
0.4
8
3.2
9
Ahp
0.57
0.62
0.70
0.71
0.7
2
0.7
3
ILh
(%)
Fig 4. Circuit for isolating PWM inverter under
fault / starting conditions
The overall protection scheme for the hybrid
active system also considered for discussed.
9
Modeling and simulation result
The single-phase equivalent circuit of the
hybrid active filter system adopted as the simulation
model. The digital simulation is performed in
MATLAB – SIMULINK software using differential
equations to describe the working of the power circuit
Table 3 give the attenuation factor for Ls =
18mH. It is observed that attenuation factor for Ls =
18mH is less as compared to attenuation factor for Ls
= 9mH.
Table 3 Attenuation in the source current due to
shunt passive
filter for Ls= 18mH.
H
5
7
11
13
17
19
ILh
(%)
21.54
15.80
32.57
18.76
0.6
6
4.5
3
Ishp(
%)
7.46
7.08
17.53
10.37
0.3
8
2.5
9
Ahp
0.35
0.45
0.54
0.55
0.5
9
0.5
7
Table 4 gives the comparison of the current harmonics
between Ls= 9mH and Ls = 18mH. It is observed that
the harmonics are decreased due to change in the
source inductance.
Table 4: Comparison of 5th order harmonics due to
the shunt passive filter
Ls = 9 mH
Source current 1.1
harmonics
(Amps)
10
0.35

The DC bus voltage control of active filter is
to be incorporated into the control scheme.
The simulation of three-phase equivalent of
HAFS is to be performed using MATLAB
Simulink software.
The modeled HAFS can be fabricated and
experimentally tested. The results can be
compared with the results obtained through
the simulation.
References
[1]
[2]
[4]
Z.Peng,
H.Akagi,
A.Nabaz,
“
COMPENSATION CHARACTERISTIC OF
THE COMBINED SYSTEM OF SHUNT
PASSIVE
AND
SERIES
ACTIVE
FILTERS”, IEEE trans. on industry
applications, 1993, vol.29, no.1, pp. 144-152.
[5]
Dr.P.S.Bimbhra,
“
POWER
ELECTRONICS”, Khanne publishers, 1998.
[6]
GUIDELINES TO MATLAB-SIMULINK
SOFTWARE PACKAGE.
[7]
Z.Peng, H.Akagi, A.Nabze, “A NEW
APPROACH
TO
HARMONIC
COMPENSATION IN POWER SYSTEMS.
A COMBINED SYSTEM OF SHUNT
PASSIVE
AND
SERIES
ACTIVE
FILTERS”, IEEE Trans. on applications,
1990, vol.26,
pp 983-990.
Conclusions and future research
The following are some of the problems,
which need further investigation:

H.Akagi, Y. Kanazowa, A.. Naba,
“INSTANTANEOUS REACTIVE POWER
COMPENSATORS
COMPRISING
SWITCHING
DEVICES
WITHOUT
ENERGY STORAGE COMPONENTS”,
IEEE Trans on industry application, 1984,
vol. IA- 20, pp.625-630.
Ls = 18 mH
The modeling of HAFS had been done. The
protection scheme for isolating the active filter from
the remaining system under fault/starting conditions is
described. The control schemes are described for the
extraction of harmonic component of the source
current using IRP theory. The system was simulated
using MATLAB software package. The simulation
results for the control of the hybrid active filter shows
that the attenuation in the 5th order harmonic is the
maximum due to the single tuned shunt passive filter.

[3]
J.Afonso, C.Couto, J.Martins, “ACTIVE
FILTERS WITH CONTROL BASED ON
THE P-Q THEORY”, Conf at Universidade
do Minho, Portugal, Jan 2000.
H.Peng, H.Akagi, “NEW TRENDS IN
ACTIVE
FILTERS
FOR
POWER
CONDITIONING”, IEEE trans. on industry
applications, 1996,vol .32, no.6, pp. 13121320.