bl

o

nKeywords:Active power filterPower qualityElectric arc furnace

power theory. In the second part, the DC capacitor voltage regulator is applied, producing a referencecurrent and a proper voltage regulator is developed. For the third part of the control system, we use a PIcontroller to provide some conditions that follow the reference current in a complete cycle, and generatefiring pulses by the hysteresis method. The proposed control system not only reduces the voltage andcurrent harmonics in power systems but can also improve the power quality indices. The above designwas implemented in the EAF systemof theMobarakeh steel complex (Isfahan, Iran). The simulation resultsshow the effectiveness of the APFs in improving the power quality indices.

Crown Copyright 2010 Published by Elsevier Ltd on behalf of ISA. All rights reserved.

1. Introduction

Due to the advancement of power electronic devices, regardingpower and speed performance, their application in power indus-tries is increasing. The diversity of these devices encourages theirapplication in making AC/DC converters, dimmers, electric motordrives, and other similar appliances and equipment. However,there exist some problems regarding the application of theseelements and the usage of this equipment because they can trans-fer power by cutting the voltage or current waveforms and bringabout disturbances in the network. Among other loads used inpower systems the EAF is very important and can cause most ofthe power quality disturbances in power systems. Themost impor-tant issue related to EAF modelling is the manner in which its arcis modelled. An electric arc is a nonlinear phenomenon for whichmany models have been presented so far [15]. In general, themodelling procedures of EAFs are suggested in both the time [15]and frequency [6] domains. Modelling in the time domain is aneffective method to analyse the EAF operations. The frequencydomain models are suitable for analysing and investigating theexternal network. Some of the time domain models are based

Corresponding address: Department of Elec. Eng., University of Isfahan, Hezar-Jerib St., P. Code: 8174673441, Isfahan, Iran. Tel.: +98 311 7934073; fax: +98 3117933071.

E-mail addresses: Hooshmand_r@eng.ui.ac.ir, Hooshmand_r@yahoo.com(R.A. Hooshmand), Torabian_mehdi@yahoo.com (M. Torabian Esfahani).

on nonlinear equivalent circuits [1,2], founded on the arc volt-agecurrent characteristic curve. Of course, there exist somemod-els that are based on random and stochastic process models [3,4]and harmonic and nonlinear models.

One piece of equipment that has shown good ability inharmonic elimination is the active power filter (APF) device. Theuse of such filters is a proper method to reduce the harmonicdisturbances of voltage and current, sudden voltage distortion,voltage notch and the transient disturbances. The APFs usuallywork by eliminating disturbances; they produce similar signals tothe system but in opposite directions. Up to now, many differentmethods have been presented in designing APF control systems[718]. Some of these methods are based on the instantaneouspower theory in order to produce the reference current. However,the drawback of these methods is that loads with heavy changescannot perform the compensation properly. In addition, it is notpossible to eliminate all harmonics and voltage imbalances. Ofcourse one of the advantages of these methods is their simplicityin implementation, and the ability to come up with differentdefinitions for computing the reference current [710]. Anotherway of computing the reference current is using the synchronousmethod [1114]. In this method, the computation of the referencecurrent is based on the solution the d-q equations using low-pass filters. Another criterion used is based on the active currentcomponents in which there is an access to three states of equalactive powers, currents and resistances [1517]. Therefore, thethree-phase source current shall be without any reactive currentgenerated by an active power filter.ISA Transactions 5

Contents lists availa

ISA Tran

journal homepage: www.e

A new combined method in active filter din power systemsRahmat Allah Hooshmand , Mahdi Torabian EsfahanElectrical Engineering Department, University of Isfahan, Isfahan, Iran

a r t i c l e i n f o

Article history:Received 8 June 2010Received in revised form2 December 2010Accepted 6 December 2010Available online 28 December 2010

a b s t r a c t

The voltage & current harmpower quality. In order to impaper, a new control system(EAFs) is presented. The sysregulation of DC capacitor vosystem is presented based o0019-0578/$ see front matter Crown Copyright 2010 Published by Elsevier Ltd ondoi:10.1016/j.isatra.2010.12.0010 (2011) 150158

le at ScienceDirect

sactions

sevier.com/locate/isatrans

esign for power quality improvement

i

nics produced by nonlinear loads in power systems cause a reduction inprove the power quality, active power filters (APFs) can be used. In thisfor designing active filters despite nonlinear loads of electric arc furnacestem is composed of three main parts: computation of reference currents,ltage, and production of firing pulses. In the first part, the active filter controlthe combination of the synchronous detection method and instantaneousbehalf of ISA. All rights reserved.

R.A. Hooshmand, M. Torabian Esfahani

Active Power Filter

Pow

er

Syst

em Electrical Arc Furnace

Load

Fig. 1. Diagram of an EAF connected to the APF and the rest of power system.

There are two drawbacks of using this method. First, it requiresmany converters; second, the filter response is also accompaniedwith a time delay. In addition, this method is not able to eliminatethe harmonics in amodewhere the heavy load changes are not in asinusoidalmode. Anothermethod to produce the reference currentis using the frequency domain method [1820]. There are twodrawbacks to this method: first, it needs many cycles in order toestimate the current; and second, it is not able to produce suitablereference currents in the transient mode with heavy load changes.Most of these methods are not able to optimise the elimination ofdisturbances in the system. They are also not suitable for nonlinearand time varying loads such as EAFs; and they cannot eliminate theproduced disturbances in a propermanner. However, by designinga suitable control system for the APF, the harmonics of the systemwill be eliminated, and the power quality indices can be improved.

In this paper, a new combined method is proposed. In thismethod, the APF control system comprises three fundamentalsections: the reference current computation, the DC capacitorvoltage regulator, and the firing pulses production. In the first part,the reference currents are determined based on the combinationof instantaneous power theory and the synchronous detectionmethods. In the second part, regarding the importance of stabilityof DC capacitor voltage, a method is introduced where in thisvoltage becomes effective in reference current production. In thethird part of the proposed control system, the actual current ofthe APF with the reference current produced passes through a PIcontroller in order to follow the actual reference current at alltimes. Then, the firing pulses of the APF are computed throughthe hysteresis method. Using this control system, not only arethe harmonics eliminated, but the power quality indices in thesystem are also improved. To evaluate the operational accuracyof the proposed control system, the APF is modelled in the worstconditions with a power system loaded with EAF. The intendedmodel for the furnace load is adopted from a real arc at theMobarakeh Steel Complex based on the statistical-probabilisticmodel and accurate-sampling process. The random flicker in arcvoltage is also considered in the arc model. The simulation resultsindicate the high accuracy and correctness of the APF operation toimprove the power quality indices.

2. Modelling the power system

In this section, the modelling of nonlinear loads such asEAFs and the corresponding supplying system are discussed. Theelectric diagram of a supplying source of an EAF with the APFis illustrated in Fig. 1. In this figure, bus 1 indicates the point of

common coupling (PCC), which is the supplying bus of the EAFtransformer. In order to change the input active power of the EAF,/ ISA Transactions 50 (2011) 150158 151

Fig. 2. The APF control system.

the transformer TF (MV/LV) is used. This transformer is equippedwith a tap changer located at the secondary winding to have theability of changing the voltage of the furnace. The arc furnace is alsoconnected to the PCC through the transformer TS , (HV/MV). In thisfigure, XC and RC are the reactance and resistance of the connectingcable line to the furnace electrodes, respectively. Also, XLsc is theshort circuit reactance at the bus PCC.

In Fig. 1, the TAPF transformer connects the APF to power system.The purpose of applying this transformer is to reduce the voltagelevel on the APF for implementation. The APF used is composedof six insulated-gate bipolar transistors (IGBT) switches and oneDC capacitor. Among many reasons in using IGBT, the high speedin switching and lack of necessity of commutation circuits areimportant.

2.1. The statistical-probabilistic model of electric arc

In this section, the arc modelling of the EAF is performed basedon the statistical-probabilistic model. This modelling, to a greatextent, incorporates the nature of random electric arc and has theleast error regarding the actual operating of the arc. Accordingly,one can expect that, in the system studies, there is a betterconfidence in the results compared with the other models.

This model is based on the system identification, that is, in aspecific EAF, its arc model is determined through sampling the arccurrent in the initial melting (scrap), mild melting (plating), andrefinement stages. After sampling the arc current in each stage ofthe furnace operation, the electric arc is modelled in the form of avoltage source dependent on the inlet current. Then, according tothe arc current, the arc voltage value based on previous obtainedsamples is obtained through interpolation. In other words, at anytime period in the simulation process, the arc voltage is obtainedwith respect to the value of arc current. As a result, the VIcharacteristic of the arc becomes completed. The advantage of thismodel compared with other models is the capability of describingthe EAFs behaviour in the time domain. Also, in order to make themodelling more precise and to provide different conditions of arcoperation, the random flicker of the voltage is considered for themodel with zero mean and variance 1 and with flicker limit from 4to 14 Hz.

3. Proposed control system for APF

The structure of the proposed control system for the EAF is

illustrated in Fig. 2. There are three constituent parts of this system.The first part includes the computation of reference currents, in

sii

= e ee epq . (6)

The currents i and i can be divided into the following twocomponents:[ii

]=[

e ee e

]1 [p0

]+[

e ee e

]1 [0q

]=[ipip

]+[iqiq

](7)

where,

ip = ee2 + e2p, iq = ee2 + e2

q

e e

source have any reactive current components. For this purpose,the three-phase load power is calculated first. Then, an attemptis made to let the three-phase source currents follow the actualpowers evenly without any reactive current. Therefore, the APFprovides the reactive and harmonic currents of the load [1517].For example, the basic current source for a single-phase system ispresented as follows:

is(t) = Im v(t)Vm =2pVm

v(t)Vm

=2pV 2m

v(t) (10)

where Im is the peak value of source current after compensationand p is the total active power of the three-phase system.

Balancing the phases is accomplished in three stages. In the first152 R.A. Hooshmand, M. Torabian Esfahani

Fig. 3. The block diagram of the in

which the instantaneous power theory method and synchronousmethod are combined. The second part includes the DC capacitorvoltage regulator system of APF, and the third part includes thefiring pulses for APF based on the hysteresis method, in a mannersuch that the actual current of the filter is able to follow thereference current at all times.

3.1. First part: reference current computation

3.1.1. Instantaneous power theory methodIn the first part of the EAF control system, the reference

currents are computed through the instantaneous power theory, asillustrated in Fig. 3. According to this figure, the converted vectorsof voltage and currents are used in an system as follows inorder to compute the instantaneous power:

[ee

]=23

1 12

12

0

32

32

eaebec

(1)

[ii

]=23

1 12

12

0

32

32

iaibic

. (2)

The instantaneous active and reactive powers in the power systemare calculated as follows:p = e i + e i = eaia + ebib + ec ic (3)q = e i + e i= 1

3[ea(ib ic)+ eb(ic ia)+ ec(ia ib)]. (4)

The above equations can be expressed in terms of a matrix form:[pq

]=[

e ee e

] [ii

]. (5)

Therefore,[ ] [ ]1 [ ]ip = e2 + e2p, iq = e2 + e2

q./ ISA Transactions 50 (2011) 150158

tantaneous power theory method.

It should be noted that the active power is transferred throughcomponents ip and ip, and components iq and iq donot have anyrole in this power transportation. Therefore, the reference currentsfor the reactive power compensation (the main component andharmonic components) are calculated as follows:[irir

]=[

e ee e

]1 [0q]. (8)

When the currents and voltages are sinusoidal and balanced, boththe p and q powers will be of constant values. However, withharmonics or imbalanced currents, the p and q powers are nolonger constants, and they can be expressed as follows:

p = p+ pq = q+ qp = p2 + phq = q2 + qh

(9)

where,

p, q: Active and reactive powers relevant to non-harmonic andbalanced currents

p, q: Active and reactive powers relevant to the harmonic andcurrent negative components

p2, q2: Active and reactive powers relevant to the negativecomponent of a three-phase currents

ph, qh: Active and reactive powers relevant to the harmoniccomponents of a three-phase currents.

In order to improve the power quality indices in power systems,different parameters can be defined to create reference currents,as shown in Table 1. Based on the available definitions in this table,the majority of the power quality indices in power systems can beset in allowable ranges.

3.1.2. Synchronous detection methodIn this section, in order to compensate for the load unbalancing

and the reactive power, the synchronous detection method isapplied. The goal here is not to let the three-phase currents of thestage, the three-phase active powers become equal to one another.In the second stage, the three-phase currents are equalised. In the

Fig. 5. Detection of synchronous signal in equal power mode.Fig. 4. The control system designed for the APF.

third stage, the observed resistance from the source will becomeequal to all three phases. These stages are designed according toFig. 4.First stage: access to equal active powers

In this stage, the line currents after compensation (e.g., iac_p) canbe calculated as:

iac_p = (p/3)(2van(t)/V 2am)ibc_p = (p/3)(2vbn(t)/V 2bm)icc_p = (p/3)(2vcn(t)/V 2cm)

(11)

where van(t) is the voltage of phase a andVam is its amplitude. Now,if we subtract the currents of Eq. (11) from the load currents, thecurrents that are to be produced by the APF can be found:

ican(t) = ian(t) iac_p(t)icbn(t) = ibn(t) ibc_p(t)iccn(t) = icn(t) icc_p(t)

(12)

where ian(t) is the load current of phase a and ican(t) is the APFcurrent. This method is summarised in Fig. 5.Second stage: access to equal currents

In this stage, the line currents after compensation (e.g., iac_c) areexpressed as:

iac_c(t) = (2pvan(t))/[Vam(Vam + Vbm + Vcm)]ibc_c(t) = (2pvbn(t))/[Vbm(Vam + Vbm + Vcm)]icc_c(t) = (2pvcn(t))/[Vcm(Vam + Vbm + Vcm)].

(13)Fig. 6. Detection of synchronou(e.g., ican) are as follows:

ican(t) = ian(t) iac_c(t)icbn(t) = ibn(t) ibc_c(t)iccn(t) = icn(t) icc_c(t).

(14)

This stage is summarised in Fig. 6:Third stage: access to equal resistances

In this stage, the line currents after compensation (e.g., iac_r ) canbe calculated as:

iac_r(t) = (2pvan(t))/(V 2am + V 2bm + V 2cm)ibc_r(t) = (2pvbn(t))/(V 2am + V 2bm + V 2cm)icc_r(t) = (2pvcn(t))/(V 2am + V 2bm + V 2cm).

(15)

Therefore, the APF currents are as follows:

ican(t) = ian(t) iac_r(t)icbn(t) = ibn(t) ibc_r(t)iccn(t) = icn(t) icc_r(t).

(16)

This mode is summarised in Fig. 7.To obtain the maximum value of each phase voltage (Vam), one

can calculate the following for all the three above stages:

V 2am = v2an(t)+ v2an(t T/4). (17)In this equation, van can be a sine or a cosine wave wherevan(t T/4) is obviously a cosine or a sine wave, respectively.In other words, for each state, equation V 2am = [Vam cos(t)]2 +[Vam sin(t)]2 is applicable. Also, T is equal to the period in time.

3.1.3. The two previous methods combinedAfter evaluating and analysing the instantaneous power theory

and the synchronous detection methods for the first part of theproposed APF control system, the first and second sections aremerged with each other.

At the beginning, the initial reference currents are computedbased on the instantaneous power theory. Then, based on differentdefinitions for reference current parameters (in accordance toTable 1), the harmonics of the load current and the flicker ofCompensating for the main component of the reactive power 0 iqCompensating for the unbalanced current and the main component of the reactive power ip2 iq + iq2Compensating for harmonics, unbalanced current and the reactive power ip iqCompensating for harmonics and flicker of the voltage ip iq + (r/x)ip

Hence, the currents those are to be produced by the active filterR.A. Hooshmand, M. Torabian Esfahani / ISA Transactions 50 (2011) 150158 153

Table 1The list of parameters applied in the APF control.

Compensating parameters The variables used in the control system

The objective of compensating ipc iqcCompensating for harmonics and balancing the current ip iqs signal in equal current mode.

3.2.1. PI controller calculationAs it was mentioned, with regard to Fig. 8, the DC capacitor

voltage (Vdca) generates the error signal of e after being comparedwith the reference capacitor voltage (VdcRef ) in the followingequation:

e(n) = VdcRef Vdca(n). (19)In this equation, e(n) is the resulting voltage error at the nth

operation of an active filter.Then, in order to produce suitable firing pulses, the hysteresis

method is applied. In this method, according to Fig. 9, an attemptis made tomake the output current of the APF follow the referencecurrent. In this regard, one hysteresis band is introduced. Now, theassumption is that the reference current is positive. When the APFcurrent is equal to the lower boundary, the switch Ig1 is closed,and the switch is opened when the higher boundary is reached.For the half cycle where the reference current is negative, thein such a way that the capacitor voltage remains stable. For thispurpose, Fig. 8 shows the DC capacitor voltage regulator system. InFig. 8, the capacitor voltage produces an error signal (e) after beingcomparedwith the reference voltage. This signal and its derivationproduce the signal (Icp) after passing through a PI controller inseries with a limiter. This signal represents the exchanged activecurrent peak between the APF and the system. This current is inphase with the corresponding voltage. It can be multiplied by theper-unit voltage andwill be changed into a sinusoidal current withan Icp peak that is in phase with the voltage. This signal is thenadded to the reference signal explained in Section 3.1 to producethe final reference signal.

proposed PI controller. The controller is presented below:

G(s) = kp + kis +n

m=1

ksmss2 + (m1)2 . (21)

The second term of this equation is considered in order to keepthe current error at zero in the steady state. The third term isfor the ability to follow the harmonics reference currents in orderto eliminate the load harmonics. In this equation, m shows theharmonic order. By designing this control system, it can be claimedthat, in addition to the improvement of power quality indicesto a desired extent, the actual current of the APF consistentlyfollows the reference current. This will, in fact, provide the desired154 R.A. Hooshmand, M. Torabian Esfahani

Fig. 7. Detection of synchronous

Fig. 8. DC voltage capacitor regu

the voltage can be eliminated. In order to eliminate the loadunbalancing and to improve the reactive power in power system,these currents are gathered with the reference currents computedthrough the synchronous detection method. Therefore, in theproposed method, the APF can improve the power quality indicesin the power system. In the next stage, these currents are combinedwith the results from the capacitor control system, and the finalreference currents are made.

3.2. Second part: DC capacitor voltage regulator system

According to Fig. 1, the DC capacitor voltage must be kept ata constant level for the proper operation of the EAF. ConsideringEq. (18), if the capacitor voltage is constant, the real power valueexchanged between the capacitor and the power system in stableconditions will be zero.

d(v2c )dt

= 2Cpe. (18)

However under transient conditions, the capacitors voltagechanges, and the active power is thus exchanged with the system.Therefore, this power should be included in the control systemsampling instant. Also, the PI controller output (o(n)) in the nthsampling instant is calculable by Eq. (20). This signal in fact/ ISA Transactions 50 (2011) 150158

signal in equal resistance mode.

lator system in the active filter.

represents the peak value of the active current (Icp) exchangedbetween the active filter and the power system.

o(n) = o(n 1)+ Kp{e(n) e(n 1)} + Ki e(n) = Icp (20)where Kp and Ki are proportional and integral gain constants of thevoltage PI controller. Also, o(n), o(n 1), e(n) and e(n 1) are thecontroller output and the voltage error at the nth and (n 1)thsampling instant, respectively.

Therefore, with regard to the solution of Eq. (20), the Kp and Kivalues of the PI controller can be calculated within the minimumerror. For this purpose, it is necessary that the amplitude of thereference supply current (Icp) must be estimated. For solving theproblem, many methods were presented which in this paper, thecoefficients of PI controller are calculated based on the variablestructure control [21,22].

3.3. The third part: producing firing pulses

In this section, in the purpose of the actual current of theactive filter is to follow the final reference current in all conditions,and thus the difference of these two currents passes through amode above is repeated for the switch Ig2. This operation causesthe actual current of the APF to follow the reference current. By

R.A. Hooshmand, M. Torabian Esfahani

Fig. 9. Hysteresis switching method.

Fig. 10. Waveforms of the arc voltage and current.

selecting a smaller hysteresis band, the APF current comes closerto the reference current; hence, fewer harmonics are obtained. Itshould be noted that with the shrinkage of the hysteresis band,the switching frequency of the semiconductorswill becomehigher,and this increase will in turn make them very costly.

4. Simulation results

4.1. The initial data and the EAF load model

The selected parameters for the circuit diagram presented inFig. 1 are based on actual values used in the Mobarakeh steelcomplex, which are summarised in Table 2.

Fig. 10 shows the waveforms of voltage and current of the ac-tual electric arc (computed based on the statistical-probabilisticmodel). Fig. 11 illustrates the electric arc voltagecurrent charac-teristic of the electric arc by considering the random flicker.By studying the above two figures, the designed model of theactual electric arc at the Mobarakeh steel complex can adopt the/ ISA Transactions 50 (2011) 150158 155

Fig. 11. Voltagecurrent characteristic of the arc.

Table 2The power system parameters in Mobarakeh steel complex.

Different parameters

Supplying system Ssc = 11 000 MVA, V = 400 kV, Xth = 14.5,f = 50 Hz

Arc furnace load

Values of the flexible cablesLa = 4.0459 H, XLa = j0.0013, Ra = 7.1 Values of electrodesLa = 2.37 H, XLa = j0.744 m, Ra = 129.7 Values of the electrode armsLa = 3.32 H, XLa = j0.0010, Ra = 3.55 Final valuesLeq = 4.0959H+3.32H+2.37H = 9.7859H,Xeq = j3.074 m

Table 3Different parameters of the designed APF.

Active power filter c = 1000 FPI controller (second part) Kp = 0.428, Ki = 0.0068PI controller (third part) Kp = 66.5, Ki = 382, Ksm = 132 for

m = 1, 3, . . . , 17Hysterias band T = 0.0001

types of power quality phenomena in power systems well, inaccordance to rapid changes in their characteristics. Moreover, thismodel is able to establish the worst conditions for the APF design.

4.2. The results with the APF

In this section, the results of the APF design for the nonlinearload of the EAFs are presented and compared with the state ofbeing without APF. The different parameters of the designed filterare shown in Table 3. As the value of hysteresis band is selected tobe equal to 0.0001, the switching frequency is therefore equal to10 kHz. This frequency correctly shows the APF capabilities.

Considering the nonlinear load of the EAF and the designedcontrol system, Fig. 12 shows the actual current and the computedreference current waveforms. In this figure, the actual currentfollows the reference current in a suitable manner. The resultingerror of these two currents is about 2%. This issue shows the desireddesign of the APF control system. Fig. 13 shows the waveform ofthe DC capacitor voltage of the APF with its reference. The DCcapacitor voltage follows the reference voltage and improves theAPF operation. In this figure, the reference voltage error arisingfrom the proposed method is about 0.9%, which justifies the

suitability of the control system. Hence, it can be argued that thecapacitor voltage is stabilised well; as a result, the steady state

156 R.A. Hooshmand, M. Torabian Esfahani

Fig. 12. Waveforms of the reference current and actual current of the APF.

Fig. 13. Waveforms of the reference voltage and actual voltage of the DC capacitor.

value of the exchanged active power between the capacitor and thepower system in permanentmodewill be zero. Thus, the operationof the APF control system is more desirable.

The waveforms of the three-phase voltages and currents at thePCC bus, without and with the APF are shown in Figs. 14 and 15,respectively. In Fig. 14, the voltage variations are significantlyreduced and the voltage profiles are improved. As shown inFig. 15, in the absence of the APF, the low-order harmonics ofthe currents are injected into the network. These harmonics andcurrent fluctuations in the PCC bus when using the APF aresignificantly reduced and therefore the currentwaveformbecomesclose to a sinusoidal form.

The current harmonic spectra in the first bus (BUS1) of thepower system (Fig. 1) with and without the APF are depicted inFig. 16(a) and (b), respectively. The harmonic spectrum presented,based on the percentage of the fundamental component, isexpressed for each harmonic order. In this figure, the harmonics ofthe main bus current are substantially reduced. For example, the5th harmonic without the APF was about 19% of the fundamentalcomponent, and the onewith the APF it is reduced to 2.6%. The THDis equal to 26.79%without the APF and is reduced to 7.59%with theAPF.

The APF creates the high-order harmonics in the current andthe voltage. It should be noted that the switching frequency in

the designed APF is 10 kHz. The harmonic spectrum shown inFig. 14 is up to 750 Hz. It is because the designed APF is capable/ ISA Transactions 50 (2011) 150158

a

b

Fig. 14. Waveforms of the three-phase voltages at the PCC bus; (a) without theAPF; (b) with the APF.

of deleting and improving the low-order harmonics, which are themost important issues in the power quality problem.

The reactive power in the BUS1 of the power system withand without the APF is shown in Fig. 17. This figure shows thatthe designed APF is capable of reducing flicker and improvingthe profile in the system main bus in addition to improving theharmonic and compensation as well as reactive power. It shouldbe noted that the reactive power of the system is decreased toabout the zero level. This fact helps to prevent the power loss thatis essential in the system.

Figs. 18 and 19 show the curves of the power factor and flickerof the voltage with and without APF in the main bus of the system(BUS1). In Fig. 18, it should be noted that the power factor withoutAPF is about 0.6, which fluctuates randomly. While using the APF,this value is constant at about 1. On the other hand, in Fig. 19, theflicker of the voltage in themain bus of system shows a substantialreduction.

4.3. The advantages of the APF design

The advantages of the APF design are:

1. With APF, the actual current of the APF follows the referencecurrent at all times, which is true even if the flicker of thevoltage fluctuates randomly. The error produced here is about2%.

2. By obtaining a constant DC capacitor voltage, the real powerexchange between the capacitor and the power system is zero

in all conditions. This situation leads to the desired operation ofthe filter.

R.A. Hooshmand, M. Torabian Esfahani

a

b

Fig. 15. Waveforms of the three-phase currents at the PCC bus; (a) without theAPF; (b) with the APF.

a

b

Fig. 16. The main bus current harmonic spectrums; (a) without the APF; (b) withthe APF.

3. The current and voltage harmonics in the main bus of the

power system are improved, and the THD value in this bus issignificantly reduced./ ISA Transactions 50 (2011) 150158 157

Fig. 17. Waveforms of the system reactive power of themain bus,with andwithoutthe APF.

Fig. 18. The Power factor curve in power system with and without the APF.

Fig. 19. The curve of flicker of the voltage with and without the APF.

4. The reactive power in the main bus of the system reachesalmost zero, and the power factor is naturally increased in thisbus.

5. The flicker of the voltage is reduced, and the voltage profile isimproved at the 1-p.u. level in the main bus of the system.

5. Conclusion

In this paper, a new control system is proposed for designingan active power filter. This system consists of three parts:the computation of reference currents, a DC capacitor voltageregulator, and the production of firing pulses. In each part, a

method is presented in order to improve the performance ofthe APF control system. In order to evaluate the operation of

158 R.A. Hooshmand, M. Torabian Esfahani

the proposed system in the worst conditions, the supply systemof the Mobarakeh steel complex is modelled here. The electricarc modelling is also performed based on statistical-probabilisticsampling from the voltage and current of an actual arc. Thesimulation results show that the system is able to improvethe reactive power, the power factor and the power qualityphenomenon and to eliminate the harmonics in the power system.Another important advantage of this method is the possibility ofits actual implementation for the investigation of power systemoperations.

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[2] Hooshmand R, Torabian Esfahani M. A new time domainmodel for electric arcfurnace. J Electr Eng, JEEEC 2008;59(4):195202.

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A new combined method in active filter design for power quality improvement in power systemsIntroductionModelling the power systemThe statistical-probabilistic model of electric arc

Proposed control system for APFFirst part: reference current computationInstantaneous power theory methodSynchronous detection methodThe two previous methods combined

Second part: DC capacitor voltage regulator systemPI controller calculation

The third part: producing firing pulses

Simulation resultsThe initial data and the EAF load modelThe results with the APFThe advantages of the APF design

ConclusionReferences