HYBRD POWER FTER Prof. B.S. athika, Ms.Sreeevi.G. Dr.Raa yer Abstract This paper eals with a haronic filter to eliinate source current haronics in power syste. t is a cobination of shunt passive filter an series active filter. The ain section of the active power filter, a voltage source inverter, is connecte between ac source an nonlinear loa through a series transforer. Current haronic copensation is achieve by injecting equal funaental current instea of haronic current at the point of connection, thereby blocking the haronics an iproving the power quality of the connecte power syste. n this a hysterisis base current control schee is eploye to erive the gating signal of the AF. Reference current of the controller is the funaental coponent of the source current. The principle of operation of the propose filter along with current control schee an funaental etection ethos are iscusse in etail. The perforance of the filter is verifie by the siulation results of current an voltage type haronic proucing loas.. NTRODCTON The increase in use of power electronics relate appliances such as ajustable spee rives, static power supplies an asynchronous ac-c links in win an wave generating systes prouces serious haronics probles. These power converters behave as non-linear loas to ac ains an inject haronics an result in lower power factor an efficiency of power syste []. n orer to solve the proble of haronic pollution effectively, any haronic liitation stanars, such as EEE59-99, EC000-3-, an EC000-3-4, have been establishe. Conventionally, passive C filters have been use to eliinate line current haronics an to increase the loa power factor ue to its low cost. However in practical applications these passive filters present the following isavantages []. i) Sensitivity to the variation of power syste ipeance; ii) Sensitivity to frequency variation of the utility; iii) The risk of series an parallel resonance; iv) The filter frequency is fixe, an not easy to ajust. Shunt active power filters [3] have eerge as an alternative solution for power quality probles. A shunt active filter is controlle in such a way to actively shape the source current into sinusoi by injecting the copensating current [4]. The isavantage of the shunt active filters are the VA rating of the power electronic converter in active filters becoe very large because they ust withstan the linefrequency voltage an supply the haronic currents. This large VA rating results in high cost, high electroagnetic interference an high power losses. Due to these reasons, the use of active filters in power syste has been liite to low power applications [5]. Hybri filter is a cobination of series active filter an parallel passive filter. n this, the passive filter filters the oinant haronics, an the power converter is use to enhance the filter perforance an to protect the passive filter fro parallel resonance. Hence the capacity of power converter is saller than that of the parallel active power filter for the sae non-linear loa. Besies the voltage stress applie to the power electronic switches in the power converter is low. As a result, hybri filter is suitable to high power applications. n this paper, a hybri filter an its control are presente. The filter can be ipleente using a voltage source inverter an funaental etection unit. Avantage of the filter is in this filter only the funaental coponent nees to be etecte an tracke. The funaental is a single frequency an can be easily etecte through a funaental etector. The avantage of funaental etector is its siple an robust structure, which lens itself to easy ipleentations in software/harware environents an its high transient spee. n this filter a hysterisis current control schee is eploye.. SYSTEM CONFGRATON AND OPERATON PRNCPE The syste configuration of the filter is shown in Fig..t consists of voltage source, series transforer, funaental etection unit, active filter an a passive filter shunte with haronic proucing loa. The funaental coponent of the power utility current, which is a single frequency, can be etecte through a funaental etector. The whole current control an inverter circuit are use to prouce the funaental copensating current. A series transforer with an air gap is chosen an its priary wining is connecte in series between the source an haronic proucing loa. The funaental current instea of 008 Australasian niversities Power Engineering Conference (APEC08) Paper P-65 page
Fig.. Syste configuration of hybri filter the haronic currents of the transforer s priary wining is etecte an followe by applying a voltage source PWM inverter so as to prouce a copensating funaental current. The copensating funaental current is injecte to the seconary wining of the series transforer. When the injecte funaental current an power utility funaental current coponent satisfy the funaental agnetic flux copensation conition, the ain agnetic flux of the series transforer to funaental can be copensate to zero. Then the propose filter exhibits very low priary leakage ipeance at the funaental frequency an naturally agnetizing ipeance to haronics so that the haronic currents are force to flow in to the passive filter branches. As a result, the series transforer iproves the source ipeance to haronics rather than funaental an it really acts as haronic isolator.. THE COMPENSATON PRNCPE Fig. shows the series transforer circuit. N an N are the turns of the priary an seconary wining of the transforer; the turns ratio is represente by k N /N.The priary wining is connecte series between the source an haronic- proucing loa, the priary current consists of the funaental current an the haronic currents.i i +Σi (n).the funaental coponent i () is etecte fro the power utility current i an followe by applying a voltage source inverter so as to prouce a funaental current i (), which has the sae frequency as i (). i () is inversely in phase injecte to the seconary wining of Fig.. Series transforer circuit the transforer. For the funaental an haronics, there are two cases in ters of the law of superposition. ) For the funaental, fro the transforer circuit, the voltage equations are ( r + jω ) + jωm () ( r + jω ) + jωm () Where ω Funaental angular frequency. Self-inuctance of priary wining. Self-inuctance of seconary wining. M Mutual inuctance between priary an seconary wining. Equations () an () can be change into [ r + jω ( km )] + jωkm ( / k) (3) [ r + jω ( M / k)] + jωkm ( / k) (4) Where km S eakage inuctance of priary wining. M / k S eakage inuctance of seconary wining. ω s x Funaental leakage inuctive reactance of priary wining. ω s x Funaental leakage inuctive reactance of seconary wining. () j ω km Z Funaental agnetizing ipeance of the transforer. f the injecte funaental current i satisfies / k 0 i.e., N + N 0 (5) then (3) an (4) will siplify into [ r + jω ( km )] (6) [ r + jω ( M / k)] ( r + jx ) (7) Fro terinals AX, the equivalent ipeance of the transforer is () Z AX / r + jω( km ) r + jx Z () () Z eakage ipeance of the priary wining of the series transforer. (5) is the conition of the funaental agnetic flux copensation (FMFC). (8) 008 Australasian niversities Power Engineering Conference (APEC08) Paper P-65 page
3 Fro terinals AX, the equivalent ipeance of the transforer is () ( ) A. Moel Derivation Z AX / r + jω( km ) r + jx Z A continuous perioic signal u ( can be represente by (9) () Z eakage ipeance of the priary wining of the series transforer Fro terinal ax () Z ax / r + jx Z (0) The equivalent ipeance of the seconary wining referre to priary wining is () Z ax / r + jx Z () () () The priary an seconary ipeance (8) an (0) suggest that the equivalent ipeance of the series transforer to the funaental is the corresponing leakage ipeances. ) For the nth-orer haronic: Since only a funaental current is injecte to the seconary wining of the transforer, i oesn t inclue any orer haronic. i (n) 0. n this case (3) can be change into [ r + jnω ( km )] + jnωkm () ( r + jnx + jnz ) Fro the priary wining, the equivalent ipeance is () Z ( r + jnx ) + jnz nz AX () () / (3) ( V. FNDAMENTA DETECTON AGORTHM. n 0 [ A n sin( nω + B cos( nω] (4) The funaental coponent of the signal is ( A sin( ω + B cos( ω (5) Base on least square approach, the following Jis iniize to obtain an estiate of funaental coponent of (. J ( k, k ) ( u y( k, k )) (6) k, k - Estiate of A, B respectively y( k( sin( ω + k ( cos( ω (7) The paraeter vector (k, k ) can be estiate using the Graient Descent etho J ( k, k ) ( k, k ) t μ (8) t ( k, k ) μ is the algorith learning rate which eterines convergence rate of algorith. The estiate k an k then have to satisfy the following equations k( μ sin( ω 0( u( y( ) (9) t k ( μ cos( ω0( u( y( ) (0) t n Accoring to () an (3) the equivalent ipeance of the transforer to the nth-orer haronic is n ties the agnetizing ipeance. Fro the above analysis, in the hybri power filter with FMFC, if the injecte funaental current an the funaental coponent of overall current of priary wining satisfy the funaental agnetic flux copensation conition N +N 0, the funaental ain agnetic flux is copensate to zero. Then fro the priary wining, the series transforer exhibits very low ipeance (priary leakage ipeance Z () ) at the funaental frequency an siultaneously exhibits agnetizing ipeance to haronics () (nz for nth-orer haronic). Therefore, the series transforer in the hybri power filter with FMFC iproves greatly the source ipeance to haronics rather than funaental an really acts as a haronic isolator. Then the haronic currents are force to flow into the passive C filter branches. Fig. 3. Block iagra of funaental etection algorith. These equations (9) an (0) constitute a non-autonoous ynaical syste, which has a unique perioic solution in a neighborhoo of (A, B ). This ynaical syste can be interprete as a funaental etector whose input an output are ( an y( respectively. The structure of the filter is siple. Fig.3 is easily ipleente by a few arithetic an eleentary atheatical operations. t is suitable for real-tie ebee applications ipleente in the for of both software an harware. Another feature of the filter is its insensitivity to 008 Australasian niversities Power Engineering Conference (APEC08) Paper P-65 page 3
4 paraeters μ an μ. These paraeters change the transient spee of the algorith with out any consierable ipact on perforance. V. CRRENT CONTROER The current control schee of the ultiple voltage source PWM converters ust provie the following: i) Quick current controllability, ii) Suppression of the haronics cause by the switching operation, an iii) Equalization of the average switching frequency of each PWM converter. Generally there are two basic schees for PWM current control. The first, carrier signal in which PWM switching sequence is eterine by eans of coparing the current error signal aplifie by gain with a triangular carrier signal. The secon schee, hysterisis current control schee, consists of iposing a ea ban or hysterisis ban aroun the reference current. Whenever the actual current tries to leave the ban, the appropriate switch is on (off), forcing the current to return to the ban. This enables quick current controllability. n this filter hysterisis current control schee is applie. Fig. 4 shows the block iagra of the hysterisis current PWM controller. n this i * is the funaental reference current an i be the feeback current. The change process of current is as follows: when i-i * h, the output of the controller is at low level, an the appropriate switching evices are turne on, an then i begin to ecrease, when i-i * -h, the output of the controller changes into high level, an the other appropriate switching evices are switche on, an then i begin to increase. Therefore, i is always fluctuating between i * +h an i * -h. n every perio, the current change process consists of two stages, which are ecrease fro i * + h to i * -h an the increase fro i * -h to i * +h, respectively.t an t be the rise tie an fall tie in one perio. T s ( t + t is the switching cycle. Suppose represents the voltage of the c sie of the inverter, enotes the su of filter inuctance an the leakage inuctance s of the seconary wining of the transforer. R is the su of resistance coponent of an the leakage inuctance of seconary wining. The following expressions can be achieve. * i h t t t () R i Fig. 4. Hysterisis current PWM controller. * i h + t t t () Ri f the funaental reference i* sin (ωt+θ) here θ is the initial phase angle. When R is neglecte, equations (0) an () are siplifie as follows: h t ω cos( ωt (3) h t ω cos( ωt (4) Switching cycle T s ( can be calculate as 4h T ( t + t (5) s ω cos ( ωt n this filter, is very sall, an can be easily chose to be uch ore than ω. The switching frequency f s ( of the controller is esigne to be uch ore than the frequency of power haronics an can be easily filtere. V. SMATON RESTS To eonstrate the filter a siulation circuit was evelope on the basis of the syste configuration Fig.. The current type haronic- proucing loa consist of ioe full brige with resistive an inuctive loa is use for siulation Current type haronic- proucing loa (R). For this type of loa, the turns ratio of the transforer is :,the rs value of source voltage an current are 30V an.3a, respectively. The inverter voltage is 0V. The passive filter branches consists of thir, fifth an seventh tune C filters. The siulate wavefors are shown in Fig. 5. 008 Australasian niversities Power Engineering Conference (APEC08) Paper P-65 page 4
5 Fig. 5. Source current wavefor with current type haronic proucing loa in three cases: (a) no filter is ae; (b) only passive filters are ae; an (c) the hybri filter is ae. Fig.5 (a)-(c), respectively, shows the source current wavefors with current type haronic proucing loa in three cases: without any filter, with passive filters are ae, an with the hybri filter is ae. For coparison, the total haronic istortion (THD) of all the three cases is calculate. THD of the source current with no filter ae is 35.4 with passive filter ae is 7.3% an with hybri filter THD is 4.8%. Therefore by aopting hybri filter approach, the perforance is better. aplification probles cause by single-tune filters, EEE Trans.on power Del., pp. 800-806, 998. [3] W.M. Gray, M.J. Saotyj, an A.H. Noyola, Survey of active power line conitioning ethoologies, EEE Trans.on power Del., vol.5, July 990, pp. 536-54. [4] M. Ares, J. Hafner, an K. Heuann, Three phase four wire shunt active filter control strategies, EEE Trans.on power Electron., vol., July 997, pp. 3-38. [5] H.Akagi, A. Nabae an S. Atoh, Control strategy of active power filters using ultiple voltage source PWM converters, EEE Trans.on n.application., vol.6, Nov-Dec 990, pp. 983-990. [6] H.Karii, M. K. Gharteani, M. R. ravani, An aaptive filter for synchronous extraction of haronics an istortions, EEE Trans.on power Del., vol.8, Oct 003, pp. 350-356. [7] i, Q. Ch, Z. Jia, J. Ke A novel active power filter with funaental agnetic flux copensationters, EEE Trans.on power Del., vol.9, April-004, pp. 799-805. Conclusion This paper presents a hybri filter in ters of the characteristics of the transforer an the law of super position. n this filter, when the injecte funaental current an the funaental coponent of the power utility satisfy the funaental agnetic flux copensation conition, the series hybri power filter with FMFC exhibits priary leakage ipeance at the funaental frequency an agnetizing ipeance to haronics. This is verifie using two types of haronic proucing loas. n this filter, only the funaental current coponent nees to be etecte an tracke. The funaental can be easily etecte through a funaental etector, therefore the etecting an tracking with rapi response is ore easily realize than haronic etecting an tracking. The hysterisis current control schee, which is robust an siple, is incorporate so that the whole syste has excellent ynaic perforance. REFERENCES [] B.Singh, K. Al-Haa, A. Chanra, A new control approach to three-phase active filter for haronics an reactive power copensation, EEE Trans.on power systes,. Vol.3, Aug. 998, pp. 33-38. [] C.J.Wu, J.C. Chiang, S.S. Yen, C.J. iao, J.S. Yang, an T.Y.Guo, nvestigation an itigation of haronic 008 Australasian niversities Power Engineering Conference (APEC08) Paper P-65 page 5