International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.

55 (2015)
© Research India Publications; httpwww.ripublication.comijaer.htm

A Novel Digital Control Algorithm for

Three Phase Parallel Active Filter (PAF)
P.Pugazhendiran 1, J.Baskaran2 ,V.Vijayakumar3
1
IFET College of Engineering, Villupuram, Anna University, India.
2
Adhiparasakthi Engineering College, Melmaruvathu, AnnaUniversity, India
1
IFET College of Engineering, Villupuram, Anna University, India.
pugazh.ceg@gmail.com 1,baski11@gmail.com2, jpsvijayakumar@gmail.com3

1
Abstract— A new control algorithm for generating case of the generalized pq theory [l, 2]. In Mahesh. K
reference filter currents of a three phase parallel active Mishra, Avinash Joshi, Arindam Ghosh extended the theory
filters is projected in this paper. The generalized pq theory and proposed a wide vector equation used for filter current.
is used for controlling the output. Many algorithms have been The instantaneous active and reactive power has been
developed based on this theory. The unique feature in the defined in terms of instantaneous balanced
proposed algorithm is that, it provides harmonic
components (Symmetrical Components) [3]. All the
compensation, reactive compensation and holds unbalance
in source as well as load. Moreover it will work even if the above algorithms fail miserably if the source voltage is
source voltage contains harmonics content. This paper unbalanced and contains harmonics. This paper suggests
suggests a modification in the method depends on generalized an alteration in the technique based on generalized pq
pq theory. The proposed method takes care of the voltage theory, the proposed method takes care of the voltage
unbalancing and harmonics in source voltage itself using unbalancing and harmonics in source voltage itself [3, 6].
parallel active filters. The Voltage source inverters are acted as
an active filter to mitigating the power quality issues. The II. VOLTAGE SOURCE INVERTER ACTING AS PAF
proposed algorithm has been simulated and validated with real
time parameters using Matlab/SIMULINK.
The basic structure of a parallel active power filter
Index Terms— Parallel Active Filter (PAF), Total Harmonic is given in Fig. 1. This VSI based dynamic filter generates
Distortion (THD), Voltage Source Inverter (VSI), Pulse Width currents for mitigating the unwanted current components in
Modulation (PWM), Moving Average Filter (MAF) . the load current [9, 13, 18].

I. INTRODUCTION
The performance of a parallel active power filter is
subjected to several elements, out of which the generation of
reference current is the utmost important one among the
others. The technique to produce the reference current
pattern is accountable for the reference currents. Since,
it must be tracked by an inverter to generate the
preferred mitigating currents that will compensates the
harmonic currents produced by non-linear loads [20].
Many approaches for producing the reference
template were projected in the literature [1- 3], out of
which, one can be highlighted: the method suggested
by Akagi et.al [1]. They suggested (i.e. instantaneous
reactive power theory) p-q theory for deploying the
reference currents needed to shoot up into the system
or grid at the associated current issues at nonlinear load.
But it is based on the assumption that the Supply
voltage is well-adjusted without any null (zero) sequence
components. Since, the p-q theory has encouraged a lot of
works dealing with active power filter compensation Fig.1. Typical parallel active filter configuration
strategies. In Fang Zheng Peng, and Jih-Sheng Lai,
proposed generalized theory of reactive power in the The instantaneous load current is consists of three
value of instantaneous at all three phase power systems components as given by equation (1)
which provide a comprehensive description of rapid
(instantaneous) reactive power, which is considerably il = i p + iq + ih (1)
effective to sinusoidal or non-sinusoidal stable (balanced)
or unstable (unbalanced), three-phase power systems Where, ip,iq and ih are the fundamental active
with or without zero order (sequence) voltages and/or element, reactive element, harmonic element of the load
currents. They proved, the pq theory defined as a special currents [6].

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 International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.55 (2015)
© Research India Publications; httpwww.ripublication.comijaer.htm

The supply is supposed to feeds only the basic active The equivalent control is introduced to the designed
element of load current. The filter supplies the reactive arrangement as well as the steadiness analysis must be
as well as harmonic Component [5]. The fundamental finished for this situation. The control strategy for closed
active component is called the source reference current loop operation is carried over in this paper with the help of
while the sum of reactive and harmonic component are Matlab/SIMULINK modeling to compensate the current
called the filter reference current which is supposed to problems in a system.
supply by the filter. The PAF is consists of a three-phase
VSI with six regulated switches. The alternating current
IV. MODELING OF INVERTER
(AC) side of the inverter is coupled across with the non-
linear load. The dc side is coupled to a capacitor which gets In PAF, Voltage Source Inverter (VSI) is acted as a
charged through the anti-parallel diodes of the lattice to filtering out the harmonics. Before investigating
inverter.Fig.2 shows a typical configuration of a parallel the closed loop system, modeling has to be done for the
active power filter connected to a three phase system. In Fig inverter. The Inverter is modeled as a three phase converter
2 the reference current calculator corresponds to the control with control signals of ua, ub and uc. The state variables
algorithm for extracting the filter reference current and the chosen are filter current and capacitor voltage [2, 3, 8, 9, 15,
sliding mode current controller (SMC) corresponds to the 20].
switching control strategy for triggering VSI [7, 13].

Fig 3.Three phase Parallel (shunt) active filter

The Fig.3 illustrates the inverter used as a parallel

active filter. The State equations are obtained by applying
Kirchoff’s Laws at Point of Common Coupling (PCC) is
di fa
Vs a L fa i fa rfa uaVc 0 (5)
Fig 2. Nonlinear load compensated by ideal PAF dt
When ua=1 for +Vdc and
III. SLIDING MODE CONTROLLER
ua=-1 for -Vdc
The control method adopted for Parallel Active Filter
(PAF) is Sliding Mode Control (SMC). It is a strong control (Where, Vc1=Vc2=VC)
arrangement, which depends on the idea of modifying the
arrangement of the regulator or controller in react the The remaining two phases b and c can also be obtained in
fluctuating level of the system in order to get a favored similar way. For obtaining the derivative of the capacitor
result. A fast activating switch control action is provided to voltage first consider the position of the the switches as
switch between the several assemblies in addition to the ua=1, ub=1 and uc=1, the current is obtained as [8],
track of the structure is forced to move along a preferred
switching manifold at state space [4, 8]. The behavior of the
closed loop structure is a consequence resolute by the 1 dVc1 1 dVc 2
I fa I fb I fc 0 and 0 (6)
sliding surface [9, 10, 19]. C1 dt C 2 dt
.
Then seeing the remaining switch positions as ua= ub=u c=-1,
1 s( y, y ) 0 then capacitor current is given by,
u (t ) (2)
1 1 dVc1
s( y, y ) 0 0 (7a)
C1 dt
Where the switching function is defined by,
1 dVc 2
s ( y, y ) ky y (3) I fa I fb I fc 0 (7b)
C 2 dt
Where, k = positive scalar. For easy analysis, consider C1=C2=C; and Lfa = Lfb = Lfc = L,
For obtaining stability, the corresponding control is put while seeing the inductive resistance (rlf) the expressions are
as follows
on when S=0, therefore S 0 .The above said condition is
entails the entire system is in sliding surface; the equivalent vsa i fa .rfa Vc .ua
I fa vsa 0 (7c)
control is obtained from [9, 19] S 0 L fa L L
Finally the state matrix representation [9,15] of the PAF
Ueq =-(S1B)-1( S1 AX S1C S1 X r ) (4)
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 International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.55 (2015)
© Research India Publications; httpwww.ripublication.comijaer.htm
T /2
x Ax B( x)u C 2 2
is (8) v average
v as2 vbs2 vcs2 dt (15)
Where, T 0

rlf 1 1 The modified equations for the reference filter currents

1 1
0 are given by equation (16):
I fa L 2L 2L 2L 2L va
X= A= C ,B= C ,C= ;
vc1 0 0 0 0 0 p.vas
2 2 iaf ila (16)
vc 2 C C 0 2
0 0 0 0 v
2 2 average

U=ua VII. REFERENCE CURRENT GENERATION FLOW

CHART
V. REFERENCE CURRENT GENERATION The Fig 3.a shows the reference current generation for
The source current representation for with and mitigating the current harmonics due to the switching power
without filter is electronic devices. Based on the flow chart steps switching
pulses are generated and the generated pulses are used to
Without PAF i s il filtering the harmonics by using three phase inverter [7, 10,
(9)
With PAF is i f il 12].
(10)
The instantaneous load current is composed of
il ip iq ih
(11)
The generalized pq theory defines active component of the
load current for one phase is specified by equation (12)
p.vas
iap 2
(12)
v vbs2 vcs2 as
In the above equations p is the average value of the
instantaneous power, vas,vbs,vcs are the source voltages of
three phases[16-18]. Similarly iap is the active fundamental
element of the load current as like source reference current.
The filter reference current is deduced by subtracting the
actual current with source reference current [10, 12, 18].
The filter reference current for any one of the 3 phases is
written in equation (13)
p.vas
iaf ila 2
(13)
v as vbs2 vcs2

In the equation (13) iaf* ,ibf*, icf* are the reference filter
currents, and it is nothing but the addition of harmonic and Fig.3.a Flow Chart for reference current generation
reactive current of the resultant load [6, 7, 16, 17].
VIII. SIMULATION RESULTS AND DISCUSSION
VI. PROPOSED ALGORITHM The nonlinear load considered for simulation is three
In equation (13) the denominator part will be a constant, phase bridge rectifier fed resistive load. The simulation is
if all three voltages are pure sine and balanced, which is not done using MATLAB/Simulink. The load parameters
mostly the case. In the numerator the average power is a considered are as follows. Input source voltages: va=230V,
constant parameter. Hence a constant multiplied by a sine vb=230V,vc=230V, load resistance: R=50ohms. The
and divided by a time varying quantity will not give a sine nonlinear load and the parallel active filter with voltage
wave. If the denominator part is constant then the source source inverter have been modeled based on its dynamic
reference current will follow the shape of the source voltage. equations in MATLAB /SIMULINK.
This can be achieved by extracting the average component
of the denominator part. This is done by using a moving The following observations are made from the
average filter in the same way as that the power is extracted simulation results. The control algorithm provides reactive
from actual instantaneous power based on average and harmonic power compensation. The THD in source
component. The square of the amplitude of the voltage in current has been reduced to 3.71% from 27.92 % to after
vector form is given by equation (14): compensation. The source current has been balanced after
compensation. The rms current of fundamental active
2
v 2
v as vbs2 vcs2 (14) component and the actual source current after
compensation are found to be nearly same and are equal to
6.82 amps and 6.879 amps respectively for all the phases.

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 International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.55 (2015)
© Research India Publications; httpwww.ripublication.comijaer.htm

Fig 6. Fundamental active Component of Source Current with the proposed

algorithm

Fig 3.b PAF over all simulation setup

The supply current is coinciding with the supply voltage

after compensation. A result is presented in Figs 4 to 11 and
Harmonic spectrum before and after compensation for
proposed PAF is shown in table 1.

The supply current before the reimbursement is shown

in Fig 5 and the Fundamental active Component of Source
current with the proposed algorithm is shown in Fig 6. Filter
reference current obtained in open loop for the algorithm
proposed is shown in Fig 7.
Fig7. Filter Reference current
The generated reference current for compensation using
controller is exactly follows the reference filter current
shown in Fig 7. This is called actual filter current and is
presented in Fig 8. Compensated source current in all three
phases by using proposed algorithm is shown in Fig 9 and it
is used for compensating the current issues due to the
nonlinear loading operation.

Fig 8. Actual filter current

Fig 4. Source voltage

Fig 9. Compensated source current

Fig 10 shows the Supply current after reimbursement is

exactly coincides with the Source voltage.

Fig 5. Source Current before Compensation

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 International Journal of Applied Engineering Research, ISSN 0973-4562 Vol. 10 No.55 (2015)
© Research India Publications; httpwww.ripublication.comijaer.htm
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been triggered using a hysteresis controller and it has been
verified only in simulation verified in simulation.

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