An Overview of High Performance Current Controllers for Active Power Filters

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1 An Overview of High Performance Current Controller for Active Power Filter P. Shriva 1, J. Tamarakar PG Student, Department of ECE, Sri Satya Sai Intitute of Science and Technology, Sehore, M.P. India 1 Aitant Profeor, Department of ECE, Sri Satya Sai Intitute of Science and Technology, Sehore, M.P. India ABSTRACT: Power quality problem caue advere effect on utilitie and cutomer. Among the variou power quality problem the harmonic have been the mot dicued becaue of their undeirable effect on power ytem. The ue of active filter for power quality improvement i widely adopted thee day. But the performance of active power filter mainly depend on the control technique. With the availability of variou control cheme it become confuing to chooe the bet one. Thi paper preent a comparative tudy of variou control cheme and dicue the uperiority of high performance controller for active power filter. KEYWORDS: hyterei controller, fuzzy controller, high performance controller. I. INTRODUCTION Traditionally the load ued by conumer are linear and paive in nature which have le effect on power ytem but now a day mot of the load are non linear in nature. Thee nonlinear load are the main caue of harmonic in power ytem. A a reult, the voltage at different bue of power ytem network i getting ditorted and the utilitie connected to thee bue are not operated a deigned. The harmonic current pollute the power ytem cauing problem uch a tranformer overheating, voltage quality degradation, rotary machine vibration, detruction of electric power component and malfunctioning of medical facilitie etc. In order to face the problem of harmonic, many olution have been propoed. Thee olution included modification on the load itelf for le harmonic emiion like the cae of pecial tructure ingle phae and three phae rectifier, and PWM rectifier. Or the connection on the polluted power grid of other traditional or modern compenation ytem. Mot of traditional harmonic reduction olution include the ue of harmonic trapping paive filter baed on RLC element. However, paive filter have the demerit of fixed compenation, large ize, and reonance. During the lat three decade, reearcher were encouraged by the development of power electronic indutry, the revolution in digital ignal proceing production and the increaing demand for efficient olution of power quality problem including harmonic problem. They were encouraged to develop modern, flexible, and more efficient olution for power quality problem. Thee modern olution have been given the name of active compenator or active power filter. The objective of thee active power filter abbreviated motly APF i to compenate harmonic current and voltage in addition to elective reactive power compenation. Although there are different type of APF, the Shunt APF i till the mot famou and ued type APF. The main function of Shunt Active Power Filter i to cancel harmonic current occurring in power grid. The organization of thi paper i a follow. Thi paper initially dicue the variou current control cheme for hunt active filter. The PI controller i firt decribed followed by Hyterii controller, dead beat controller and fuzzy controller with their limitation. Finally the limitation are overcome by high performance current controller like multiple reonant controller, Proportional integral plu reonant controller, vector PI controller and PI plu VPI controller. Copyright to IJAREEIE

2 II. LITERATURE REVIEW The literary of APF i very rich and cover many apect including power topologie, control theorie, and harmonic extraction and reference generation method of APF. Since their baic compenation principle were propoed around 1970, much reearch ha been done on active filter and their practical application. The paper [1] ha dicued the difference in the dynamic performance of the three mot popular current control technique for active filter application. The tudy how dead beat controller give fatet performance but it performance depend upon the parameter of active power filter, the reult how a certain uperiority of the hyterei control but till it have reonance problem and chance of witching frequency loe. The need for high performance controller arie from thee limitation. A current control cheme for elective harmonic compenation with hunt APF ha been propoed in [,3]. The method employ an array of reonant current controller, one for the fundamental, and one for each harmonic, implemented in fundamental reference frame. Each pair of harmonic filtered by one controller thu reduce number of controller. The VPI control had uperior behavior in term of tability and wider BW in the experimental tet. It maintain table operation at high frequencie due to the lack of reonant peak in the frequency characteritic [3]. Overall tability improved by elective harmonic compenation but thi method i expenive in term of real time calculation. [4] Propoe a detailed nonlinear control technique for a three-phae SAPF. It ha two inner current loop and an outer dc bu voltage regulator loop. The main problem with thi method i the delay that occur when the control ytem i digitally implemented. To overcome thi drawback, a computational control delay compenation method i propoed, which delaylely and accurately generate the SAPF reference current. In [5] author demontrate the feaibility of four-witch three-phae inverter. The common mode voltage generated by the four-witch three-phae converter i evaluated and compared to that provided by the tandard ix-witch three-phae inverter. It ha been found that the reduced witch topology of the AF ytem operate well with the control algorithm [6]. Through extenive experimentation, it ha been found that it i poible to eliminate two voltage enor which may be needed for reliable operation with other control technique. It reduce cot and complexity of real time ytem and it alo raie the reliability and work well in preence of ditortion in the voltage at PCC. A. PI controller III. CURRENT CONTROLLER Fig 1 how the block diagram of the propoed PI control cheme for the active power filter. The DC ide capacitor voltage (Vdc) i ened and compared with a reference voltage (Vdc,ref). Thi error e =Vdc,ref Vdc at the nth ampling intant i ued a input for PI controller. The error ignal i paed through Butterworth deign baed Low Pa Filter (LPF). The LPF filter ha cutoff frequency at 50 Hz that can uppre the higher order component and allow only fundamental component. The PI controller i etimate the magnitude of peak reference current Imax and control the dc-ide capacitor voltage of voltage ource inverter. It tranfer function i repreented a in (1): (1) Where, [KP =0.7] i the proportional contant that determine the dynamic repone of the Dc ide voltage control and [KI =3] i the integration contant that determine it ettling time. The proportional integral controller i eliminating teady tate error in the DC-ide voltage. Copyright to IJAREEIE

3 The conventional verion of the linear current controller perform a ine-triangle PWM voltage modulation of the power converter uing a the modulating ignal the current error filtered by a proportional integral (PI) regulator. It i worth noting that we have here conidered the original analog implementation of the PWM technique, ince it enure to the ytem the fatet poible peed of repone. A udden change in the modulating ignal i indeed intantaneouly turned into a duty-cycle variation, without the unavoidable delay equal to one-half of the modulation period, in the cae of pace-vector modulation (SVM), or to a whole modulation period, in the cae of ampled PWM. The application of thee modulation technique can only reduce the ytem peed of repone. Neverthele, the linear current control technique with analog PWM, although very imply implementable by mean of analog circuitry, provide a rather unatifactory performance level a far a active filter application are concerned. Thi i mainly due to the limitation of the achievable regulator bandwidth which, a it i well known, i implied by the neceity of ufficiently filtering the ripple in the modulating ignal. Thi neceity compel one to keep the loop gain croover frequency well below the modulation frequency. Thi reflect in a poor rejection of the diturbance injected into the current control loop, mainly due to the ac line voltage at the fundamental frequency. To overcome thi limitation, recent verion of the linear current control exploit the rotating frame. It i noticeable that, for thi kind of application, to perform the tranformation, there i no need to know the intantaneou phae angle of the inuoidal waveform. The main advantage of uch a olution i that the fundamental harmonic component of voltage and current ignal appear contant to the current regulator. Therefore, the line voltage, which i almot inuoidal, i een by the current regulator a a contant quantity. A a conequence, the rejection of thi diturbance i much more effective. On the other hand, the bandwidth limitation of the PI regulator, which remain unchanged, till implie ignificant error in the tracking of the high-order harmonic component of the current reference. In active filter application, thee error uually reflect in a not completely atifactory quality of harmonic compenation. [8]. B. Hyterei current control - Hyterei current control i a nonlinear cloed-loop current control method and currently the mot widely ued in the APF control, which form a dead zone of a given current by hyterei comparator. The baic idea of hyterei current control i to keep the current inide the hyterei band by changing the witching tate of the converter each time the current reache the boundary. A hyterei current controller i ued to generate witching plue required for VSI. In the hyterei controller the reference compenation current i compared with the actual current that i being injected by the compenation circuit. A poitive pule i produced if the actual current tend to decreae below the lower hyterei limit, while the negative pule i produced if the current exceed the upper hyterei limit. Thu in a hyterei current controller the actual compenation current i forced to tay within a particular hyterei band. [9] The witching action of the PWM converter i controlled through hyterei error of the feedback current and command current. Thi method ha the character of imple circuit, mall witching lo, fat dynamic repone, but the witching frequency of the ytem repone peed and current tracking accuracy i affected by the hyterei broadband. In addition, independent control of three phae reult in phae interference in none neutral line three-phae inverter. The baic implementation of the hyterei current controller derive the witching ignal from the comparion of the current error with a fixed hyterei band. Although imple and extremely robut, thi control technique exhibit everal unatifactory feature. The main one i that it produce a varying modulation frequency for the power converter. Thi i, in general, reponible for variou problem, from the difficulty in deigning the input filter to the generation of unwanted reonance on the utility grid. Another negative apect of the baic hyterei control i that it performance i negatively affected by the phae current interaction, which i typical of three-phae ytem with inulated neutral. Fig. how the implified cheme of the implementation of uch a controller. Copyright to IJAREEIE

4 Many improvement to the original control tructure have been uggeted by indutrial application. Firt of all, phae current decoupling technique have been devied. Secondly, fixed modulation frequency ha been achieved by a variable width of the hyterei band a function of the intantaneou output voltage. Thi i achieved either by mean of a phaelocked loop (PLL) control or by a feed forward action operating on the control threhold. A can be een, the controller modifie the hyterei band by umming two different ignal. The firt i the filtered output of a PLL phae comparator and the econd i the filtered output of a band etimation circuit The band etimator implement a feed forward action that help the PLL-baed circuit to keep the witching frequency contant; in thi way, the output of the PLL circuit only provide the mall amount of the modulation of the hyterei band which i needed to guarantee the phae lock of the witching pule with repect to an external clock ignal. Thi alo enure the control of the mutual phae of the modulation pule. All of thee proviion have allowed a ubtantial improvement in the performance of the hyterei current controller, a i dicued in [8]. It i worth adding that, in different application, uch a drive or PWM rectifier, uch control complexity may not be actually neceary, ince the required dynamic performance i normally lower, and conventional, non hyteretic, technique can be completely atifactory C. Dead-Beat Control (DBC) The method i actually a pre control, and the baic idea i to calculate the witch control to the next ampling period by the load current and compenation current in the previou ampling period. The advantage are fat dynamic repone, and eay oftware implementation, the diadvantage are the amount of calculation and dependence on the ytem parameter. In addition to the introduction of a variety of current tracking control trategy, there are many modern control method, uch a Sliding Mode Control (SMC), Space Vector Control (SVC), etc. Application of active power filter control algorithm hould be baed on the harmonic ituation, conidering the control accuracy, repone peed, and the ytem immunity factor. In the conventional implementation, the digital control calculate the phae voltage, o a to make the phae current reach it reference by the end of the following modulation period [10] [13]. The calculation are often performed in the frame, and the pace-vector modulation (SVM) trategy, which very well uit the digital implementation, i applied to the witching converter. Thi i eentially the ituation depicted in Fig. 3. Copyright to IJAREEIE

5 An important advantage of thi technique i that it may not require the line voltage meaurement in order to generate the current reference. Indeed, the deadbeat control algorithm implie an etimation of the line voltage intantaneou value, which can, therefore, alo be ued for the current reference generation. On the other hand, the inherent delay due to the calculation i indeed a eriou drawback for thi technique [13]. Due to the high required peed of repone, it become the main limitation in active filter application and may imply an unatifactory performance level. In the more recent verion [14] of the deadbeat controller, thi delay i reduced by ampling the control variable and executing the control routine twice in a modulation period. The turn-on and turnoff time of the power converter witche are, therefore, decided eparately in two ucceive control period. A a conequence, the aforementioned delay in current reference tracking can be reduced to a ingle modulation period. Thi can be further compenated by adopting a prediction technique for the current reference [14]. Accordingly, the control algorithm interpolate the reference value for the current modulation period from thoe calculated in the preceding one. Thu, by anticipating the current reference, the teady tate tracking error can be virtually eliminated. On the other hand, the implied derivative action caue increaed error and overhoot in the preence of udden reference change. In practice, however, it turn out that, on the whole, the reference prediction technique provide a performance improvement. Another key iue in thi kind of control technique i the effect of the input filter commonly ued to eliminate reidual high-frequency harmonic component in the line current, which are due to the inverter modulation. Thee filter are not normally accounted for in the control algorithm and therefore, undermine the tability of the current loop. To guarantee the control tability, certain over izing of the ytem reactive component may be neceary. D. Fuzzy Logic Controller Fuzzy logic control i deduced from fuzzy et theory in 1965; where tranition i between memberhip and non memberhip function. Therefore, limitation or boundarie of fuzzy et can be undefined and ambiguou; FLC are an excellent choice when precie mathematical formula calculation are impoible. Copyright to IJAREEIE

6 Fig 4 how block diagram of the fuzzy logic control cheme. In order to implement the control algorithm of a hunt active power filter in a cloed loop, the dc capacitor voltage V DC i ened and then compared with the deired reference value V DC,ref. The error ignal e =V DC,ref V DC i paed through Butterworth deign baed LPF with a cut off frequency of 50 Hz; that pa only the fundamental component. The error ignal e(n) and integration of error ignal i termed a ce(n) are ued a input for fuzzy proceing. The output of the fuzzy logic controller limit the magnitude of peak reference current Imax. Thi current take care of the active power demand of the non-linear load and loe in the ditribution ytem. The witching ignal for the PWM inverter are generated by comparing the actual ource current (ia,ib,ic) with the reference current (ia*,ib*,ic *) uing the HCC method. The main advantage of fuzzy control are it linguitic decription, independence of mathematical model, robutne, and it univeral approximation. A fuzzy logic controller i coniting of four tage: fuzzification, knowledge bae, inference mechanim and defuzzification. The knowledge bae i compoed of a data bae and rule bae and i deigned to obtain good dynamic repone under uncertainty in proce parameter and external diturbance.the data bae coniting of input and output memberhip function, provide information for the appropriate fuzzificztion operation, the inference mechanim and defuzzification. The inference mechanim ue a collection of linguitic rule to convert the input condition into a fuzzified output. Finally, defuzzification i ued to convert the fuzzy output into control ignal. In deigning of a fuzzy control ytem, the formulation of it rule et play a key role in improvement of the ytem performance. [15] IV. HIGH PERFORMANCE CURRENT CONTROLLER A. Reonant controller In order to effectively regulate the upply current to be inuoidal, the current controller mut have high gain at harmonic frequencie, which can be achieved by mean of multiple reonant controller tuned at harmonic frequencie. The tranfer function of reonant controller i given a (): K rh G PR K ph () ( h h 1, 5, 7,11,13... ) The main diadvantage i that each reonant controller ha a reponibility to regulate only one harmonic component. Thu, the controller complexity and computational burden are ignificantly increaed if large number of harmonic current are required to be compenated. B. PI plu Reonant controller- The diadvantage of reonant controller can be overcome by uing PI controller with the conventional reonant controller. The aim of the PI controller i to regulate the fundamental current wherea reonant controller are ued to control the harmonic current. However, the delay time caued by the effect of the APF and digital implementation mut be taken into account if the high-order harmonic are compenated. The tranfer function of thi controller i given in (3). But only conidering the delay time caued by digital implementation and neglect the effect of the APF: the tability margin are reduced, and undeired peak appear in the cloed-loop frequency repone when the order of the compenated harmonic increae. In addition, a PI controller i utilized to compenate a mall voltage drop reulting from the ytem impedance and the erie tranformer. It i alo ued to improve the dynamic repone of the erie APF. Conequently, the voltage control cheme for the erie APF conit of a PI controller and a reonant controller, and the combined tranfer function i given a follow: [16].(3) Copyright to IJAREEIE

7 C. Vector PI controller- In order to overcome thee problem, an alternative olution of the reonant controller, the VPI controller, ha been introduced [7]. The tranfer function of the VPI controller (4) in the -domain i.(4) The VPI controller i able to cancel the coupling term with the form1/(lf + RF ) by electing the reonant gain a Krh = KphRF /LF, where LF and RF are the inductance and the equivalent reitance of the LF inductor, repectively. Owing to thi advantage, the VPI controller i able to remove anomalou peak appearing in the cloed-loop repone without demand of delay compenation.[7] D. PI plu VPI controller- Adopting the uperiority of the VPI controller over reonant controller, the reonant controller given in G PI-R are ued to replaced by controller G VPI, [7] the tranfer function of the propoed PI-VPI current controller (5)in the -domain i given a follow: K i1h K ph K rh G PI VPI K p1 (5) ( h ) h V. SUPERIORITY OF PI PLUS VPI CONTROLLER In order to invetigate the uperior characteritic of the propoed PI-VPI controller over the traditional PI controller, Fig. 5 decribe open-loop Bode diagram for the traditional PI and the vector PI controller for the cae of h = 6n, n = , ω = π60 rad/, Kph = 1, and Krh = KphR F /L F. A illutrated in Fig. 5, at low frequency (le than 0 Hz), the gain of both the PI and propoed controller are high and comparable, but at high frequencie, i.e., elected reonant frequencie (6ω, 1ω, 18ω, 4ω, and 30ω), the gain of the PI controller i ignificantly reduced while the propoed controller produce very high gain for enuring zero teady-tate error in compenating the harmonic current. A hown in Fig. 6, the VPI controller provide unity gain and zero phae-hift for all elected reonant frequencie while there are undeired peak in the cloed loop of the PI-R controller. Thu, the ue of VPI controller ignificantly improve the tability margin and accuracy of the propoed current controller. Copyright to IJAREEIE

8 VI. CONCLUSION Thi paper ha dicued the different current control technique ued for harmonic compenation in three phae hunt active power filter. The PI controller, Hyterei controller, dead beat controller and fuzzy controller have been explained with their advantage and diadvantage. The Superiority of VPI high performance current controller i alo explained with variou bode diagram. REFERENCES 1. S. Buo, L. Maleani, and P. Mattavelli, Comparion of current control technique for active filter application, IEEE Tran. Ind. Electron, vol. 45, no. 5, pp. 7 79, Oct C. Lacu, L. Aiminoaei, I. Boldea, and F. Blaabjerg, High performance current controller for elective harmonic compenation in active power filter, IEEE Tran. Power Electron, vol., no. 5, pp , Sep C. Lacu, L. Aiminoaei, I. Boldea, and F. Blaabjerg, Frequency repone analyi of current controller for elective harmonic compenation in active power filter, IEEE Tran. Ind. Electron,, vol. 56, no., pp , Feb S. Rahmani, N. Mendalek, and K. Al-Haddad, Experimental deign of a nonlinear control technique for three-phae hunt active power filter, IEEE Tran. Ind. Electron, vol. 57, no. 10, pp , Oct M. B. de R. Corrêa, C. B. Jacobina, E. R. C. da Silva, and A. M. N. Lima, A general PWM trategy for four-witch three phae inverter, IEEE Tran. Ind. Electron., vol. 1, no. 6, pp , Nov B. N. Singh, B. Singh, A. Chandra, P. Ratgoufard, and K. Al-Haddad, An improved control algorithm for active filter, IEEE Tran. Ind. Electron, vol., no., pp , Apr Quoc-Nam Trinh and Hong-Hee Lee, An Advanced Current Control Strategy for Three-Phae Shunt Active Power Filter IEEE Tran. Ind. Electron, vol. 60, no. 1, Dec Simone Buo, Luigi Maleani and Paolo Mattavelli Comparion of Current Control Technique for Active Filter Application IEEE Tran. Ind. Electron, vol. 45, no. 5, Oct J. B. Dixit, Amit Yadav Electrical Power Quality Laxmi Publication, Ltd., Jan 1, K. P. Gokhale, A. Kawamura, and R. G. Hoft, Dead beat microproceor control of PWM inverter for inuoidal output waveform ynthei, IEEE Tran. Ind. Applicat., vol. IA-3, pp , Sept./Oct T. Kawabata, T. Miyahita, and Y. Yamamoto, Dead beat control of three phae PWM inverter, IEEE Tran. Power Electron., vol. 5, pp. 1 8, Jan T. Ohnuki, O. Miyahita, T. Haneyohi, and E. Ohtuji, High power factor PWM rectifier with an analog pulewidth prediction controller, IEEE Tran. Power Electron., vol. 11, pp , May P. Tenti, A. Zuccato, L. Roetto, and M. Bortolotto, Optimum digital control of PWM rectifier, in Proc. IEEE IECON 94, 1994, pp D. G. Holme and D. A. Martin, Implementation of direct digital predictive current controller for ingle and three phae voltage ource inverter, in Conf. Rec. IEEE-IAS Annu. Meeting, 1996, pp Uma Maheh.J, Muthak Ahmed Shaik, Fuzzy logic controller of a erie active power filter for Power Quality improvement International Journal of Modern Engineering Reearch (IJMER) Vol., Iue., Mar-Apr 01 pp Quoc-Nam Trinh and Hong-Hee Lee, Novel Control Strategy for a UPQC under Ditorted Source and Nonlinear Load Condition Journal of Power Electronic, Vol. 13, No. 1, January C. Lacu, L. Aiminoaei, I. Boldea, and F. Blaabjerg, High performance current controller for elective harmonic compenation in active power filter, IEEE TRANSACTIONS ON POWER ELECTRONICS, vol., no. 5, pp , Sep Copyright to IJAREEIE 110