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Power quality improvement by suppressing harmonics using power active filters.


This Paper deals with the harmonic elimination in Power System by adapting various methods. Due to the development of Power Electronics technology, more Power Electronics appliances are used, which leads the serious harmonic pollutions.

Using Shunt Active Filters we can eliminate these kinds of harmonics. The development of new Shunt Active Filter is presented. The concept of proposed Shunt Active Filter and its operating Principle, Control Theory is also discussed. The filtering scheme provides harmonic suppression at the source so that the source will supply high quality power to linear load [6].

The 'Power Quality' has become the 'buzzword' in the last one decade due to increase in quality-sensitive load, like computers, non-linear switched devices, which are the sources of disturbance to create poor power quality & awareness of implications of power quality.

Quality means customer satisfaction, which cannot be defined absolutely. It is defined with reference to consumer expectations. The quality of a product is thus measured by using yardstick of consumer satisfaction. Electric power quality is satisfaction of its customer; a consumer is satisfied if he is able to use power through his equipment and devices to serve his purpose. This is possible only if his equipment and devices have Electromagnetic Compatibility (EMC) with the supply quality; Thus, EMC is the measure of power quality [1].

The SIMULINK/MATLAB is a highly developed graphical user interface simulation tool. It has proved instrumental in implementing the graphical based controller. The Simulation tool has been used to perform the modeling and simulation of the customer power controller for a wide range of operating conditions. The Simulation results of the proposed filter are discussed.

The purpose of this paper is to review the results obtained during the present work before proceeding with the conclusion of the work done. The primary objective of this paper is to model and develop the proposed new Shunt Active Power Filter for the current harmonics suppression using SIMULINK/MATLAB, for power quality improvement.


Filters are used to restrict the flow of harmonic currents in the Power Systems. It is a LC circuit, which passes all frequencies in its pass bands and stops all frequencies in its stop bands. There are two basic types of filters. The simplest method of harmonic filtering is with passive filters. It uses the reactive storage components, namely capacitors and inductors. It has two types. Shunt passive filter is the Combination of L and C elements, which are connected in parallel with the line. It will restrict the flow of harmonics through the line. Fig-1 shows the configuration of Shunt passive Filter.



Series passive filter is the combination of L & C in parallel, which are connected in series with the supply as in fig-2, which has the ability to eliminate harmonic amplification of shunt passive filter. The series active filter needs a much smaller KVA rating than a conventional shunt active filter, and as a result, the combined system has good filtering characteristics and high efficiency. This paper presents an optimum design of the shunt passive filter that makes possible a great reduction in the required KVA rating of the series active filter. It can minimize the peak voltage across the series active filter and reduce the required KVA rating of the filter to 60 percent. A computer simulation geared to practical applications of large three-phase. Thyristor rectifiers are used to compare the compensation characteristics of the optimized system with those of a combined system that uses a conventional shunt passive filter. Active Filters are newly emerging devices for harmonic filtering, which will use Controllable Sources to cancel the harmonics in the Power Systems. The basic principle of operation of an Active Filter is to inject a suitable non-sinusoidal voltage and currents in to the system in order to achieve a clean voltage and current waveforms at the point of filtering. [10].



Shunt Active Filters are connected in parallel to the load. The system configuration is shown in fig-3. It consists of a Voltage Source Inverter and a filter inductor is connected in series. It performs a harmonic current suppression to the line. Where as series active filter shown in fig-4, is connected in series with the load. The major advantages of the Series Active Filter are, it maintains the output voltage waveform as sinusoidal and balances the three-phase voltage.

Harmonic Measurement

Importance of monitoring PQ

In a case study where the end-user equipment knocked off-line 30 times in 9 months but there were only five operations on the utility substation breaker. There were so many events, which will result in end-user problems that never show up in the utility statistics. One example is capacitor switching, which is quite common and normal on the utility system, but can cause transient over-voltage that disrupt manufacturing of machinery. Another is a momentary fault any where in the system that causes voltage to sag briefly at the location of the customer, which might cause an adjustable-speed drive or a distributed generator to trip off, but the utility will have no indication that anything will miss on the feeder unless it has power quality monitor installed [11].

Harmonic analysis

If a case study is conducted at any textile mill, which has 2 transformers in which transformer 1 is supplying non-linear load. There are 4 feeders on transformer1 (2MVA), which were supplying the non-linear loads. The harmonic distortion can be observed from the below diagrams.





It can be observed from the above waveforms and tables that the transformer contains current harmonics of around 16% and voltage harmonics of 4%. Major harmonics in Current waveform are 5th and 7th harmonics. Hence it is strongly recommended to mitigate the 5th and 7th harmonics in current waveform [11].

Power Quality Evaluation:

Transform rating: 2 MVA [9]

Electrical System = 415V,50 Hz, 3 Ph 3 Wires (With capacitor bank switched ON)

Apparent Power, S = 624.499 kVA

Real Power, P = 700 kW

Reactive Power, Q = 500 kVAR

Power Factor, PF = 0.97(assuming lagging) (1)

Source Voltage, V

(Phase-A), [V.sub.A] = 243 Vrms [, A] = 4.1%

(Phase-B), [V.sub.B] = 243 Vrms [, B] = 4.4%

(Phase-C), [V.sub.C] = 243 Vrms [, C] = 4.2% (2)

Load Current, [I.sub.L]

(Phase-A), [I.sub.L, A] = 1500 A [, A] = 15.2%

(Phase-B), [I.sub.L, B] = 1260 A [, B] = 16.0%

(Phase-C), [I.sub.L, C] = 1560 A [, C] = 13.2%

(Neutral), [I.sub.L, N] = 120 A [, N] = 20.6% (3)

Power & Current are calculated by using CT Ratio 400:1

The total harmonic current for each phase is calculated as follows:

[I.sub.H] = [] x ([i.sub.L]/[square root of ((1+[])))]

[I.sub.H, A] = 0.152 x (1500/[square root of ((1+[0.152.sup.2])))] = 225.41A

[I.sub.H, B] = 0.16 x (1260/[square root of ((1+[0.16.sup.2])))] = 199.06A

[I.sub.H, C] = 0.132 x (1560/[square root of ((1+[0.132.sup.2])))] = 204.14A

[I.sub.H, N] = 0.206 x (120/[square root of ((1+[0.206.sup.2])))] = 24.21 A (4)

Hence from the above calculation it is observed that the transformer contains a harmonic current of 200A per phase. Hence it is strongly recommended to reduce harmonic currents.

Harmonic analysis:

In a case study conducted in the textile mill there were total 3 transformers in which transformer no3 (1.5MVA) is supplying 6 ring frames, which were of variable frequency drives of 50kw each. It is observed from the diagrams of figure-5 that the transformer is supplying a highly non-linear load current of 25% THD. In which major harmonics are 5th and 7th.

Power Quality Evaluation:

Transformer rating:(1.5MVA) [9]

Electrical System = 415 V, 50 Hz, 3 Phase 3 Wires

Apparent Power, S = 834 kVA

Real Power, P = 815 kW

Reactive Power, Q =180 kVAR

Power Factor, PF = 0.98(assuming lagging) (5)

Source Voltage, V

(Phase-A), V = 399.2 Vrms [, A] = 4.5%

(Phase-B), [V.sub.B] = 397.5 Vrms [, B] = 4.2%

(Phase-C), [V.sub.C] = 397.1 Vrms [, C] = 4.3% (6)

Load Current, [I.sub.L]

(Phase-A), [I.sub.L, A] = 1260 A [, A] = 25.2%

(Phase-B), [I.sub.L, B] = 1260 A [, B] = 24.5%

(Phase-C), [I.sub.L, C] = 1140 A [, C] = 25.0% (7)

Power & Current are calculated by using CT Ratio 600:1

The total harmonic current for each phase is calculated as follows:

[I.sub.H] = THDIfund x ([i.sub.L]/[square root of ((1+[]))]

[I.sub.H, A] = 0.252 x (1260/[square root of ((1+[0.252.sup.2])))] = 307.89A

[I.sub.H, B] = 0.245 x (1260/[square root of ((1+[0.245.sup.2])))] = 299.83A

[I.sub.H, C] = 0.25 x (1140/[square root of ((1+[0.25.sup.2])))] = 276.49A

Hence from the above calculation it is observed that the transformer contains a harmonic current of 300A per phase. Hence it is strongly recommended to reduce harmonic currents

Installation of Active Harmonic Filter

A 100 Amp Active Harmonic filter may be installed in PDB2 of Plant III supplied by TX1.The Plant III has got 30 Ring frames, each ring frame having VFD (Variable Frequency Drive) of 50 KW rating. 30 Ring frames have been grouped into four groups. A Power Distribution Board (PDB) supports each group. PDB2 supports 8 numbers of ring frames. Single line diagram of the same installation is given below in figure-6.

There were facing a Problem of High current harmonics more than 25% in PDB2.

Tripping of the Circuit breaker and Higher Temperature of cable and transformer (Tx1). The 100 Amp APF Unit may be installed across the load of PDB2.


Transformer: 4 MVA, 33 KV/440 V, [DELTA]/Y, [6]

PFC: power factor correction capacitors (50 KVAR cap Bank)

Solid-state harmonic filter rating: 415 V, 50 Hz, 3-Ph-3 wire, 100 Amp AHF.

VFD Rating: Each VFD is of 50 KW, 440 V, 50 Hz (9)

Slight Distortion in current wave is, because of higher harmonic current present in the system than the rating of the AHF installed at the time of measurement. Due to variation in load sometimes-Harmonic current exceeds rating of AHF & work in full correction mode voltage waveform.

Comparative Study:

Installation & Commissioning

1. Easy Installation.

2. Installation without affecting the Production.

3. Time required to install the AHF is less than 45 Min.

4. User Friendly control Panel.

5. Maintenance can be done easily without disturbing load efforts.

6. Current transformers are to be connected at load side for Current Sensing.

7. R, Y, & B terminals of AHF to be connected In shunt with the Load point

Calculation of THD from the tables:

1. Current THD:

Before AHF Installed Current THD ([I.sub.THD]) was: 30-31%

After AHF Installed Current THD ([I.sub.THD]) brought down to: 5-6%

2. Voltage THD:

Before AHF Installed Current THD ([I.sub.THD]) was: 6-7%

After AHF Installed Current THD ([I.sub.THD]) brought down to: 4-5%

3. Improved power factor (up to Unity) without power factor correction capacitors.



Description of System configuration

The system configuration of the proposed shunt active filter is shown in the figure-7. It consists of a three-phase source, which is connected to a three-phase non-linear diode bridge rectifier circuit. The Shunt Active Power Filter is connected in shunt with the load. It consists of the voltage source inverter in series with the inductor and capacitor. The triggering for the inverter circuit is given through the control circuit. The inverter can be implemented by IGBTs operating in hard-switched Pulse-Width Modulation (PWM) mode to provide sufficient bandwidth for the filtering function.

Operating Principle:

Three-phase bridge rectifier with RL loads (non-linear loads) is connected to the three phase three wire distribution system as shown in fig. Due to the nature of the nonlinear loads, harmonics are injected in to the system. Shunt Active Power Filter is connected in shunt with the load to suppress the harmonics. The Voltage Source Inverter (VSI) generates a compensating harmonics current in to the phase conductors through the inductor and capacitor sets connected in series with it. The generated harmonic currents cancel each other with out affecting the fundamental part of the source current.

Control Strategy

There are three stages in the active filtering technology. In the first stage the essentials voltage and current signals are sensed using power transformers and current transformers to gather accurate system information. In the second stage, compensating commands in terms of current or voltage levels are derived based on control methods and AF configuration. In the third stage of control, the gating signals for the solid-state devices of the AF are generated using PWM techniques. In this we have used the instantaneous p-q theory for deriving the compensating signal.

Control Algorithm

The generalized theory of the instantaneous reactive power in three phase circuits is also known as instantaneous power theory, or P-Q theory. It is based on instantaneous values in three-phase power systems with or without neutral wire, and is valid for steady state or transitory operations, as well as for generic voltage and current waveforms. The p-q theory consists of an algebraic transformation (Clarke transformation) of the three-phase voltages and currents in the a-b-c co-ordinates to the [alpha]-[beta]-0 coordinates, followed by the calculation of the p-q theory instantaneous power components. As explained above the first step in p-q theory is to transfer the a-b-c frame of voltage and currents into [alpha]-[beta]-0 coordinates.



The instantaneous powers "p" and "q" are calculated using the equations given below

[p.sub.0] = [v.sub.0]*[i.sub.0] instantaneous zero sequence power.

p = [v.sub.[alpha]]*[i.sub.[alpha]]+[v.sub.[beta]]*[i.sub.[beta]] instantaneous real power.

q = [v.sub.[alpha]]*[i.sub.[beta]]-[v.sub.[beta]]* [i.sub.[alpha]] instantaneous imaginary power


Where p = [bar.p] + p ~

q = [bar.q] + q ~

[p.sub.0] = [[bar.p].sub.0] + [p~.sub.0]

[[bar.p].sub.0] = Mean value of the instantaneous zero sequence power--corresponds to the energy per time unity which is transferred from the power supply to the load through the zero sequence components of voltage and current.

[p~.sub.0] = Alternated value of the instantaneous zero sequence power--it means the energy per time unity that is exchanged between the power supply and the load through the zero sequence components. The zero-sequence power only exists in threephase system with neutral wire. Furthermore, the system must have unbalanced voltages and currents and/or 3rd harmonics in both voltage and current of at least one phase.

[bar.p] = Mean value of the instantaneous real power-corresponds to the energy per time unity which is transferred from the power supply to the load, through the a-b-c coordinates, in a balanced way (it is the desired power component).

p ~ = Alternated value of the instantaneous real power-it is the energy per time unity that is exchanged between the power supply and the load, through the a-b-c coordinates.

q = Instantaneous imaginary power- corresponds to the power that is exchanged between the phases of the load. This component does not imply any transference or exchange of energy between the power supply and the load,but is responsible for the existence of undesirable currents, which circulate between the system phases. In the case of a balanced sinusoidal voltage supply and a balanced with or without harmonics, q(the mean value of the instantaneous imaginary power) is equal to the conventional reactive power (q=3.V.[I.sub.1.sin][PHI])


As seen in above figure-9, [bar.p] is usually, the only desirable p-q theory power component. The other quantities can be compensated using a shunt active filter. [[bar.p].sub.0] Can be compensated without the need of any power supply in the shunt active filter. This quantity is delivered from the power supply to the load through the active filter. This means that the energy previously transferred from the source to the load through the zero sequence components of voltage and current, is now delivered in a balanced way from the source phases. It is also concluded from figure-9 that the active filter

DC Bus is only necessary to compensate [??] and [[??].sub.0] , since these quantities must be stored in this component at one moment to be later delivered to the load. The instantaneous imaginary power (q), which includes the conventional reactive power, is compensated without the contribution of the DC Bus. This means that, the size of the DC Bus does not dependent on the amount of reactive power to be compensated. The compensation reference currents in [alpha]-[beta]-0 components can be calculated by using the below mentioned equations.


Since zero sequence current must be compensated, the reference compensation current in the 0 coordinate is [i.sub.0] itself. [i.sub.c0.sup.*] = [i.sub.0], In order to get the compensating currents in a-b-c reference frame the inverse transformation of [alpha]-[beta]-0 to a-b-c is applied.



Features of pq theory:

The above figure-10 synthesizes the reference current calculations of instantaneous p-q theory.

1. It is inherently a 3-phase system theory.

2. It is based on instantaneous values, allowing excellent dynamic response.

3. Its calculations are relatively simple (it only includes algebraic expressions that can be Implemented using standard processors).

4. It can be applied to any three-phase system (balanced or unbalanced, with or without Harmonics in both voltages and currents).

Simulation Results



















The Proposed scheme of Shunt Active Filter is shown in Figure-9. The Circuit diagram for both with & without filter is shown in figure-12. In this active filter is connected in shunt with the 3-phase supply line.

The load taken for the simulation is Diode and Thyristor bridge rectifier with RL load. The table shows the values of R and L for both diode and Thyristor rectifiers.

As explained in the control algorithm there are basically three main steps in instantaneous PQ theory.1st step is converting the voltage and currents in ABC reference (stationary) frame into alpha beta zero sequence (which is also stationary) frame as shown in fig7.2.

2nd step is to calculate the instantaneous real and reactive powers and extracting reference compensating currents for compensating the harmonic currents and reactive power at the source side of the system as shown in fig 6.3

3rd step is to generate the reference compensating current by using an IGBT based voltage source inverter as shown in fig 6.1.

The below shown results will depict the effectiveness of the active filtering algorithm. It can be seen from the figure-13 that the source current has become a pure sinusoidal wave and it has been observed form POWER GUI tool that source current THD has been reduced from 24% to 2%. It can be seen from the figure-13 (d) that load power oscillations are reduced with active filtering.

The same active filter is connected to Thyristor based bridge rectifier which is containing more harmonics in current compared to the diode bridge rectifier as mentioned in 7.2.The current THD is reduced from around 44% to 3% hence the designed active filter effective with high percentage of THD also. The designed active filter is capable reducing around 50 amps of harmonic current in load. The simulation diagram for Active harmonic filter with Thyristor based bridge rectifier load is shown in fig 7.6

From fig 6.4(a) it can be seen that the load current waveform has been improved and the source see a sinusoidal current waveform. It can also be observed from figure that the power variation at the load side is reduced at the source side in addition to this the active harmonic filter is capable of supplying the reactive power to the load and the power factor at the source side is also increased.


1) The model of the proposed new Shunt Active Power Filter is realized. The developed new Shunt Active Power Filtering technology is implemented for a system feeding a non-linear load. It is simulated using the highly developed graphic tool SIMULINK available in MATLAB. The results reveal that the proposed new Shunt Active Filtering technology is simple and effective and is suitable for practical applications for the power quality improvement.

Harmonic Distortion of Load Voltage

Dead        Vab_50Hz   THD    H5     H7     8 kHz
Time (us)   V (rms)    (%)    (%)    (%)    (%)

0             380      1.27   0.21   0.13   0.74
5             354.6    2.13   1.43   0.79   0.82

3) From (1) & (2) it is absorbed that by the suppression of voltage and current harmonics by the use of new active filtering technology, the power quality can be improved as discussed in the various chapters.

Future Scope: The hardware implementation of the proposed filter can be done by using IGBT based Inverter Bridge and DSP/MICROCONTROLLER based controller.


[1] FACTS for power quality improvement in grids feeding high speed rail traction Grunbaum, R.Electric Machines & Drives Conference, 2007. IEMDC apos;07. IEEE International Volume 1, Issue, 3-5 May 2007 Page(s): 618 - 623 Digital Object Identifier 10.1109/IEMDC.2007.382738

[2] A Novel Power Quality Enhancement Scheme in Low Voltage Distribution System Using Modulated Power Filter Compensator Sharaf, A.M.; Mahasneh, H.A.; Biletskiy, Y. Clean Electrical Power, 2007. ICCEP apos;07. International Conference on Volume, Issue, 21-23 May 2007 Page(s):171 - 174 Digital Object Identifier 10.1109/ICCEP.2007.384206

[3] Ronny Strernberger, Student member, IEEE, and Dragan Jovcic, Senior Member, IEEE. " Theoretical Frame work for Minimizing Converter Losses & Harmonics in a Multilevel STATCOM", IEEE Transactions on power delivery vol.23. N0.4 Oct 2008

[4] J.Jesus Chavez, Student Member, IEEE, and Abner Ramirez, Senior Member, IEEE. "Dynamic harmonic domain modeling of transients in 3-phase transmission lines" IEEE Transactions on power delivery vol.23. N0.4 Oct 2008

[5] Woei-Luen Chen, Member, IEEE, Yung-Hsiang Lin, Hrong-Sheng Gau, and Chia-Hung Yu. " STATCOM Controls for a Self--Excited Induction Generator Feeding Random Loads" IEEE Transactions on power delivery vol.23. N0.4 Oct 2008

[6] Hedeaki Fujita, Member, IEEE, and Hirofumi Akagi, Fellow, IEEE " The Unified Power Quality Conditioner: The Integration of Series--Shunt--Active Filters" IEEE Transactions on power electronics vol.13. N0.2 March 1998

[7] Hirofumi Akagi, Fellow, IEEE. "New Trends in Active Filters for Power Conditioning" IEEE Transactions on Industry applications vol. 32. N0.6 Nov/Dec 1996

[8] D.Deniel Sabin & Ashok Sundaram. "Quality Enhances". IEEE Spectrum Feb-1996

[9] Mukul Rastogi, Rajendra Naik, and Ned Mohan, Senior Member, IEEE. "A Comparative Evaluation of Harmonic Reduction Techniques in Three-Phase Utility Interface of Power Electronic Loads" IEEE Transactions on Industry applications vol. 30. N0.5 Sep/Oct 1994

[10] "Understanding FACTS" by Narain G. Hingorani & Laszlo Gyugy Harmonics reduction using continuously reactive power compensation in HVDC links, Karim Shaarbafi, Ph D. student of power electronic Eng.Seyyed Hossein

[11] Hosseini, Ali Aghagholzadeh Electrical Engineering Dept., Faculty of Engineering, University of Tabri.

Sardar Ali (1), P. Sujatha (2) and Dr. Anjaneyulu (3)

(1) E-mail:

(2) E-mail:

(3) E-mail:
Table 1: Current Harmonic

Amp       A      B      C      N

THD%f   15.2   16.0   13.2   20.6
H3%f     0.8    1.4    0.7    5.0
H5%f    14.5   15.1   12.6    4.6
H7%f     4.1    4.8    3.5    4.3
H8%f     0.8    0.4    0.5   17.8
H11%f    0.6    0.9    0.8    2.9
H13%f    0.9    0.8    0.9    2.7
H15%f    0.1    0.1    0.1    1.9

Table 2: Voltage Harmonic

Amp       A     B     C      N

THD%f   4.1   4.4   4.2   67.6
H3%f    0.2   0.2   0.2    6.0
H5%f    4.0   4.2   4.0   59.5
H7%f    0.8   0.9   0.8   20.0
H9%f    0.3   0.2   0.3   21.6
H11%f   0.3   0.3   0.2    8.2
H13%f   0.2   0.3   0.3    4.4
H15%f   0.1   0.0   0.1    1.3

Power & Energy

             A           B           C         Total

kW          0.53        0.43        0.56        1.53
kVA         0.55        0.45        0.58        1.59
kVAA   [??] 0.15   [??] 0.13   [??] 0.15   [??] 0.46
PFs         0.96        0.96        0.97        0.96
Cos         0.97        0.97        0.98
Arms        2.2         1.9         2.4

             A           B           C

Vrms       244.1       243.9       243.9

Table 4

Parameter   Before AHF installation   After AHF installation
               (Cap Bank ON)              (Cap Bank OFF)

Avg Curr           307.88                      294.68
Avg KVA            221.57                      212.28
Max KVA               278                         258
KW                  211.6                       210.8
PF                  0.955                       0.994
KVAR                67.11                       19.81
ITHD                24.5%                        6.5%
VTHD                 6.4%                        5.3%
Voltage             415.5                       415.5

Table 5: Transformer Primary side

Parameter      Before AHF     After AHF
              Installation   Installation

[I.sub.THD]       19.6          16.3%

[V.sub.THD]       2.7%           2.4%

Voltage           33KV           33KV
PF at TX          .976           .986
Max KVA           1590           1540

Table 6: Transformer secondary side

Parameter      Before AHF      After AHF
              Installation   Installation

[I.sub.THD]       20.5%           16%
[V.sub.THD]        4.7%          4.2%

Table 7: Parameters for Diode Bridge load:

Load parameters       Active filter parameters

Resistance = 2 ohms   IGBT Inverter DC Bus = 700V
Inductance = 1.5mH    Inverter Coupling Inductance/Phase = 1.5mH
                      Ripple Filter Parameters
                      Resistance/phase = 4 ohms
                      Capacitance/phase =36 [micro]f
                      Capacitance/phase =72 [micro]f

Table 8: Parameters for Thyristor Bridge rectifier load:

Load parameters       Active filter parameters

Resistance = 2 ohms   IGBT Inverter DC Bus = 700V
Inductance = 1.5mH    Inverter Coupling Inductance/Phase = 1.5mH
                      Ripple Filter Parameters Resistance/phase
                      = 4 ohms
                      Capacitance/phase = 36 [micro]f
                      Capacitance/phase = 72 [micro]f
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Author:Ali, Sardar; Sujatha, P.; Anjaneyulu
Publication:International Journal of Applied Engineering Research
Article Type:Report
Date:Nov 1, 2009
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