ABSTRACT This paper describes the implementation of Distributed Active Filters (DAFs). To share the workload of harmonic filtering, several Active Filter Units (AFUs) are placed on different locations which can perform the harmonic filtering without a direct communication. Different configurations of DAFs are described and are implemented using MATLAB /Simpower system Toolbox environment to observe their performances and comparison. KEYWORDS Distributed Active Filters (DAFs), Power Quality, Total Harmonic Distortion

1. Electrical and Electronics Engineering: An International Journal (ELELIJ) Vol 2, No 1, February 2013
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NOVEL CONTROL STRATEGIES FOR
DISTRIBUTED ACTIVE FILTER
Sakshi Bangia1
,P.R.Sharma3
and Maneesha Garg2
1,2
Department of Electrical Engineering, YMCA University of Science and Technology,
Faridabad, Haryana
sakshibangia@gmail.com1, prsharma1966@gmail.com2
3
Department of Humanities of Applied Science, YMCA University of Science and
Technology
garg_maneesha@yahoo.com3
ABSTRACT
This paper describes the implementation of Distributed Active Filters (DAFs). To share the workload of
harmonic filtering, several Active Filter Units (AFUs) are placed on different locations which can perform
the harmonic filtering without a direct communication. Different configurations of DAFs are described and
are implemented using MATLAB /Simpower system Toolbox environment to observe their performances
and comparison.
KEYWORDS
Distributed Active Filters (DAFs), Power Quality, Total Harmonic Distortion
1. INTRODUCTION
Most of the pollution issues created in power systems are due to the non-linear characteristics and
fast switching of power electronic equipment. Distribution system locates the end of power
system and is connected to the customer directly, so the power quality mainly depends on
distribution system. The reason behind this is that electrical distribution networks failures account
for major portion, just due to customer interruption. Conventionally these problems are solved by
passive L-C filters. But these L-C filters introduce tuning, aging, resonance problems and
these filters are large in size and are suited for fixed harmonic compensation. . Moreover,
the passive filter acts as a sink to the harmonics of the source voltage, because it presents
impedance at specific harmonic frequencies lower than that of the load impedance. Finally, the
compensation characteristics of passive filters are influenced by the source impedance, which is
not usually known accurately and depends on the instantaneous configuration of the power
network.
Recently active power line conditioners (APLC) or active power filters (APF) have been
developed to solve these problems basically designed for compensating the current-harmonics
and reactive power simultaneously to improve power quality [1]. Most of the active filters
developed are based on sensing harmonics and reactive Volt-Ampere requirements of the
non-linear load [2-3].and require complex control. Among which shunt active power filter is used
to eliminate load current harmonics and reactive power compensation. The shunt-connected
active power filter, with a self-controlled dc bus, has a topology similar to that of a static
compensator (STATCOM) used for reactive power compensation in power transmission

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systems. Shunt active power filters compensate load current harmonics by injecting equal-but
opposite harmonic compensating current.
In this case the shunt active power filter operates as a current source injecting the harmonic
components generated by the load but phase-shifted by 180°. Many harmonic extraction
techniques are available, and their responses have been explored. Proposed techniques include
traditional d−q [4] and p−q theory [5] based approaches and application of adaptive filters [6],
Wavelet [7], Genetic Algorithm (GA), Artificial Neural Network (ANN), etc., for quick
estimation of the compensating current [8].
A Distributed Active Filter system (DAFS) has been proposed to suppress harmonic distortion in
the power system as shown in figure 1 [9].The paper describes the analysis of parallel three phase
active power filters in three main different configurations to solve the problems of capacity
enlargement and load unbalance compensation encountered by the shunt APF.
Fig1: Distributed Active Filter System
2. PRINCIPLE OF OPERATION
The Increasing numbers of harmonic or sensitive loads are leading to more Active Power Filter
(APF) application. Similarly, parallel operation of APFs and their control strategies are
being given more emphasis for compensating high levels of harmonic and reactive current
in the distribution network.[10]
Shunt active power filter is controlled to draw/supply a compensating current from / to the utility,
so that it cancels current harmonics on the AC side, and makes the source current in phase with
the source voltage. The load current waveform, the desired mains current and compensating
current injected by the active filter containing all the harmonics, to make mains current
sinusoidal [11]. The synchronous reference frame theory or d-q theory is based on time-domain is
used for reference signal estimation. The objective of controlling methods and connection
topologies for Distributed Active Filter (DAFs) is divided in to following [12].
2.1. Central Controller Mode
In this mode compensation of distorted source current is done through multiple shunt Active
Filter Units (AFUs), connected parallel to different loads causing distortion. As shown in figure 2
all the AFUs are controlled by a central controller which will track the average inverter current
required. This mode exhibits the advantage of low switching losses. The main disadvantage is the
malfunction of any inverter will cause erroneous compensation due to the generation of a current
command which is not independent of other inverters. Addition of another load in the line needs

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again and again tuning of the controller to compensate for change in current harmonics and
reactive power compensation.
.
Fig 2 Central Controller Mode.
2.2. Common Dc-Link control
Another configuration of the connection topology of Distributed Active Filter System (DAFS) is
the Common DC Link Control. In this configuration Dc link capacitor of the all the Active Filter
Units are common. The advantage of this topology is the reduction in the cost of the
capacitors/system but design complexity of hardware increases. Common DC- Link Control is
shown in figure 3.
Fig 3 Common DC- Link Control.
2.3. Capacity Limitation Control
The In this mode multiple AFUs can operate independently and compensate the load harmonic
current according to its own capacity-limitation. Also an individual control circuit is used for each
inverter. Capacity Limitation Control is shown in figure 4. The output current of each APF is
optimized in such a way that the APF with large capacity compensates more current and the one
with small capacity compensates less current. Each APF only has to compensate the harmonic
component left by the previous APF on its load side. The main advantage is its easy maintenance
and installation and high flexibility, reliability due to no control interconnection and reduced

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power capacity demand of APFs. The main disadvantage is that the APF modules are not
identical and therefore replacement requires a similar one.
Fig 4 Capacity Limitation Control
3. SIMULATION RESULTS
Simulations are performed for implementing Distributed Active Filters configurations using
MATLAB SimPower system Toolbox. The parameters of the simulated system are shown in
Table 1.
The Synchronous Reference Frame or d-q control theory is used to calculate the reference source
currents. PCC voltages, load currents and the dc bus voltage of Active Filter are sensed and
Hysteresis control is used to generate gating signals. The performances of the Distributed Active
Filter Systems are analysed for all the three configurations.
.
3.1. Performance of DAFs using Central Controller Mode
Initially load1 is connected to the three phase source having THD of phase ‘a’ current waveform
of load1 is 4.50% as shown in fig 5.
Fig 5 THD of phase’a’ current waveform of load1 without filtering.

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Then at t=0.4s load 2 is also connected which is a non linear load Hence THD of phase ‘a’ current
waveform becomes 13.70% shown in figure 6. Performances of source voltage, source
current,load1 current,load2 current, filter1 current and filter2 current without filtering are shown
in figure 7.
Initially THD of current waveform of load 1 is less than 5% which is within the limits of IEEE
std-519.As soon as it crosses this limits, the proposed strategy can be integrated. With the THD
sensor, used to check the harmonics distortion of the line current controls the switching of the
Active Filters. The distributed active Filters are an effective solution by deploying multiple shunt
active filter units (AFUs) in the system to enhance its filtering performance. Based on the
measured current harmonic distortion at source current point, each AFU can dynamically adjust
its own filtering to maintain the filters and thus filter performance is appreciated settling the
source current harmonics to 3.18%.
Fig 6: THD of phase ‘a’ current waveform of load2 without filtering
Fig 7 Source Voltage, Source Current, Load1 Current, Load2 Current, Filter1 Current and Filter2 Current
without filtering

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.
Fig 8 Source Voltage, Source Current, Load1 Current, Load2 Current, Filter1 Current and Filter2 Current
with filtering.
Fig. 8 shows the performances of source voltage, source current,load1 current,load2 current, filter
1 current and filter 2 current with filtering. PI controllers in d-q frame control strategy, used in
implementing the central controllers can also be tuned to compensate for reactive power
compensation.
Reactive power is compensated after t=0.4 as figure 9 shows the reactive power compensation
between load and source. Figure 10 THD of source current phase ‘a’ with filtering using central
controller mode.
Fig 9 Reactive Power Compensation between load(blue) and source(green)

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3.2. Performance of DAFs Using Common Dc link Control mode
The performance of common Dc link can be analyzed by making the Dc link capacitor common
to of all the active filters . It can be seen that THD of source current phase ‘a’ are reduced to
4.42% as compared to load current as shown in figure 11.The figure 12 showing Source Voltage,
Source Current, Load1 Current, Load2 Current, Filter 1 Current and Filter 2 Current using
Common DC- Link Control.
Figure: 10 THD of source current phase ‘a’ with filtering using Central Controller Mode
Figure: 11 THD of source current phase ‘a’ with filtering using Common DC- Link Control

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Figure: 12 Source Voltage, Source Current, Load1 Current, Load2 Current, Filter 1 Current and Filter 2
Current using Common DC- Link Control
3.3 Performance of DAFs using Capacity Limitation Control Mode
The performance of DAFs in Capacity Limitation Control mode is same as analyzing the each
load and its associated active filter unit as an separate distribution system with active filter
connected in shunt.
4. CONCLUSION
In this work Distributed Active Filters configurations are implemented and analyzed that DAFs
are good for load sharing between the filters. Central Controller Mode proves to more effective
than Common Dc link Control. As it needs to tune the PI controller of only its Active Filter Unit
for the newly added load and do not disturb the tuning and the functioning of other AFUs. But in
Common Dc-link Controller mode tuning of Active Filter Unit are needed whenever there is
addition of load making the filtering process more cumbersome. Thus it is concluded that the
proposed strategies can effectively controls the working of active filters whenever needed.
Moreover they can be implemented such that filtering does not depend on grid condition or the
voltage harmonic distortion. Hence good stability and reliability, ease maintenance and
Installation and reduction in the hardware design complexity are its more advantageous features.
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Authors
Sakshi Bangia received the B.Tech degree in Instrumentation and Control from MDU
Rohtak and M.Tech degree in Electrical Engg. from YMCA UST in 2004 and 2006,
respectively. . She is pursuing Ph.D in Power Quality from Maharishi Dayanand
University, India. Presently she is working as Assistant Professor in Electrical
Engineering Department at YMCA UST Faridabad.
Dr. P.R.Sharma was born in 1966 in India. He is currently working as Head of the
Department in Electrical Engg. in YMCA University of Science and Technology,
Faridabad. He received his B.E Electrical Engineering in 1988 from Punjab University
Chandigarh, M.Tech in Electrical Engineering (Power System) from Regional
Engineering College Kurukshetra in 1990 and Ph.D from M.D.University Rohtak in
2005. He started his carrier from industry. He has vast experience in the industry and
teaching. His area of interest is Optimal location and coordinated control of FACTS
devices ,power system stability and control.
Dr.Maneesha Garg did her Ph.D(Physics) in 2002 from Kurukshetra University, India.
After that she worked as Research Associate granted by CSIR, in NIT Kurukshetra for 4
years. She has 18 publications in national, international journals and more than 30 papers
in conferences to her credit. Presently she is working as Assistant Professor in Humanities
and Applied Science Department, YMCA UST, Faridabad and guiding 4 scholars for their
research work.