Phase Angle Balance Control for Harmonic Filtering of A Three Phase Shunt Active Filter System

Transcription

1 Phae Angle Balance Control for Harmonic Filtering of A Three Phae Shunt Active Filter Sytem Souvik Chattopadhyay, V. Ramanarayanan Power Electronic Group Department of Electrical Engineering, Indian Intitute of Science, Bangalore 56, India ouvik@ee.iic.ernet.in,vram@ee.iic.ernet.in Abtract-Thi paper propoe a new trategy for harmonic filtering of a three-phae hunt active filter ytem. The hunt harmonic filter control objective i defined a: balance the phae angle of the input current with the phae angle of the line frequency component of the load current. Thi objective i achieved in dicreet implementation without ening the input voltage. The controller ue a phae hifting method on the ened input current and then applie the reitor emulator type input current haping trategy on the phae-hifted current. In implementation Texa Intrument DSP baed unit TMS3F4 EVM i ued a the dital hardware platform. The control algorithm i computationally imple yet the harmonic filtering performance i hh. The analyi, imulation and experimental reult of a three-phae hunt active filter prototype on a 5A non-linear load are preented. I. INTRODUCTION Shunt active filter (SAF) i a current controlled voltage ource converter (VSC) of Boot topology,a can be een in F.. We can identify two major function in the control of hunt active filter. Firt : it mut generate a current reference containing only thoe harmonic component that are preent in the load current and econd : a very hh bandwidth current controller need to be implemented. Thi current controller hould be able to extract the ame actual current wavehape from the SAF a dictated by the reference. The unique feature of active filter control i the method of harmonic extraction. The method hould be uch that each harmonic component in the reference i exactly equal in magnitude and phae to the correponding harmonic component in the load current. But it i difficult to den a harmonic filter that would eparate out the fundamental from the lower order harmonic without any phae hift. So conventional filtering technique can not be applied in a trahtforward manner. Moreover there can be unbalanced current in the phae which may alo have to be filtered out from the input. The harmonic extraction theorie that have addreed thee iue and provided implementable olution are the Intantaneou Active and Reactive Power Method [],the Synchronou Reference Frame Method [3], the Intantaneou Active and Receptive Current Component Method[4]. All thee method involve computation uing the ened input voltage. But the input voltage waveform of a real life ytem can have ditortion. So to filter out the ditortion in the input voltage a phae locked loop (PLL) circuit i a mandatory requirement for thee method. Den of a hh performance PLL i not eay and i an additional computational burden on the dital controller. Thi paper propoe a new trategy, denoted here a the Phae Angle Balance (PAB) control technique, for harmonic filtering of a three-phae hunt active filter ytem. The control objective of a hunt active filter i defined here a: balance the phae angle of the input current with the phae angle of the line frequency component of the load current. Thi objective i achieved in dicreet implementation without ening the input voltage. The controller ue a phae hifting method on the ened input current and then applie the reitor emulator type input current haping trategy on the phae-hifted current. Phae Angle Balance control ha the flexibility to compenate only for the harmonic of the load current o input voltage a-b-c v g Input current OR Filter current Current Senor 3-ph Source ih Current Senor Shunt Harmonic Filter Sw Sw3 Sw5 L L L Sw Sw4 Sw6 Sw Sw Sw3 Sw4 Sw5 Sw6 C il 3-Ph Non-linear Load Current Senor Voltage Senor Dital implementation (TMS3F4) of phae angle balance (PAB) controller for harmonic filtering F. Schematic diagram of a three phae hunt active filter ytem with the propoed controller 87

2 F. Control objective of a three phae hunt active filter that the current rating requirement of the converter i kept lower. However if required thi method can compenate for the harmonic a well a the reactive content of the load and thereby hape the input current like input voltage. Any other reitor emulator type active filtering trategy [5] i not available with thi flexibility. In ection II, we have explained the baic principle of operation of the propoed Phae Angle Balance controller. Two method for the determination of the phae hift of the input current are alo provided. The imulation reult of the propoed control cheme are preented in ection III. In ection IV the experimental reult of the prototype hunt active filter ytem are preented. II. PHASE ANGLE BALANCE CONTROL Phae Angle Balance Method i an extenion of the current haping method that ha been decribed in []. For hh power factor rectifier the input current i haped to follow the input voltage in each witching period. The difference for the hunt active filter (SAF) i that the phae hifted input current i made proportional to the input voltage. The control objective for a three phae hunt active filter, defined in F., i to make the input current devoid of hher order harmonic that i otherwie preent in load current i l. In order to keep the rating of the active filter low, current in the hunt filter i h hould contain only the harmonic current but not the reactive current of the load. The reactive component of the load current ha to be upplied from the input ide. Mathematically the function of the hunt active filter can be expreed a = vg R e e j v g axi vg Control Objective: and are input current and voltage vector repectively. can be poitive or negative depending on whether the load i inductive or capacitive repectively. R e i the emulated reitance correponding to active power tranfer in the poitive direction. The required phae hift depend on the load power factor. In implementation either the input current can be ened directly or obtained by ummation of the load and the filter current. The baic current haping e j ϕ ϕ α axi vg Re () technique decribed in [] i applied on the input current. For determination of the phae hift load current need to be ened. Let u aume that the input voltage (v g ) i inuoidal and balanced. v g = v g + jv g = V g co( f t) + jv g in( f t) f i the angular frequency of the input voltage.. (, ) i a ytem of orthogonal and tationary axe a hown in F.. The variable with uffix or repreent component along that particular axi. A line current haping controller, imilar to the one that ha been decribed in [] for hh power factor operation of three-phae boot rectifier, can be ued here to make ( ) proportional to (v g ). It ha been hown in F. that from the input current ( ), ( ) can be obtained by phae hifting by an angle. Therefore, = + j () = I g e j( ft) = I g co( f t) + ji g in( f t) = e j = co( ) in( ) + j( co( ) + in( )) Where, = + j = I g e j( ft ) It can be noted that if we can make the phae hift equal to the phae angle of the poitive equence component of the fundamental frequency load current then the input current will conit only of active and reactive current component of line frequency. Thi general principle of filtering the harmonic current i denoted here a the Phae Angle Balance (PAB) control technique. The block diagram of the PAB controller i given in F.3. We can ue either Method I, hown in F.4(a) or Method II, hown in F.4(b), to get the deired phae angle. Method I: For a three-phae three-wire ytem (i.e no zero equence current), the non-linear load current (i l ) can be expreed a a ummation of harmonic of poitive equence and negative equence current component. It ha been aumed that there i no dc component in the load current. i l = k= (i lk_p + i lk_n ), where k i the harmonic number In the uffix of a variable p and n indicate poitive and negative equence component repectively. The pace phaor in the above equation are decompoed into (, ) axi component a (6) and (7). i l = k= i lk = k= i lk _p + k= i lk _n = k= (I lk_p co(k f t + k_p ) + I lk_n co( k f t + k_n )) (3) (4) (5) (6) 88

3 F.3 Block diagram of the Phae Angle Balance (PAB) control technique. inϕ coϕ α 3-ph to -ph converion input current ening i l = k= i lk = k= i lk _p + k= i lk _n = k= ih Shunt Active Filter Phae Shifting α Dc Voltage Sening Vo load current ening 3-ph to -ph converion il α (I lk_p in(k f t + k_p ) + I lk_n in( k f t + k_n )) il Dital Two Axi Reitor Emulator Type Current Mode Controller And Space Vector Implementation Sw Sw Sw3 Sw4 Sw5 Sw6 (7) F.4(a) Determination of in( ) and co( ) by Method I il α α Magnitude Calculation il α D-Q Tranformation of Load Current F.4(b) Determination of in( ) and co( ) by Method II q_p co( θ) in( θ) Low Pa Filter il Magnitude Calculation ild ilq Low Pa Filter Computation of in( θ) and co( θ) PI Control ler Phae Information il_pd coϕ Computation of coϕ and inϕ il_pq inϕ α coϕ inϕ m =leading -=lagging The angular frequency of the fundamental component i f. Therefore the magnitude of can be written a (i l ) x i l x= i l + i l = k= (I lk_p + I lk_n ) + f p All the frequency dependent term are combined together a f p. We can eliminate the hh frequency component by a LPF, then x i l x fil = k= (I lk_p + I lk_n ) The magnitude of the input current i made equal to the magnitude of the load current by the outer loop PI controller. Therefore the PI controller enure that I g =x i l x fil = k= (I lk_p + I lk_n ) (8) (9) () F.4(c) Load current pace phaor (i l ), (i l_p ) in tationary (, ) and ynchronou (d_p, q_p) reference frame I g co( ) = I l_p co( _p ) d_p Ilp_d Ilα θ Ig α Il φ Igα Il_p(fundamental comp) Il -Ilp_q (fundamental+harmonic) (3) _pi the phae angle of the poitive equence component of the fundamental frequency load current with repect to the input voltage. Therefore Ig ω f The output of the PI controller give u the magnitude of in( ),or in( ). By monitoring and i l we can determine whether the load current i leading or lagging the input voltage. Thi information i binary in nature becaue it ha only two value : m = + for lagging current and m = for leading current. in( ) = m in( ) co( ) = in ( ) () () Thi method of calculation of phae hift i computationally imple but not very exact. The reaon for that i given below. From the power balance condition = co ( I l_p k= (I lk_p + I lk_n ) co( _p)) = co (K co( _p )) K can be expreed a K = +T D Where the total ditortion coefficient T D = I l_n + k=(ilk_p +I lk_n ) I l_p T D i defined a (4) (5) (6) 89

4 (a) 5(b) (a) (b) Iha Ila Iha Ila (c) 5(d) 6(c) 6(d) Co(Phi) Ild Ilq Co(Phi) Sin(Phi) Ild Sin(Phi) Ilq (e) (f) (e) (f) co(theta).5 in(theta).5 co(theta).5 in(theta) (g) 5(h) 6(g) 6(h) F.5. Simulation reult of the phae angle balance controller with load coniting of A(peak) diode rectifier and A (peak) reactive current (a) input phae current a (b) a (c) filter current i ha (d) load current i la (e) in( )(f) i l_pd, i l_pd (g) co( ) (h) in( ) Ideally we would like the controller to make the input current phae hifted by an amount equal to the phae angle of the poitive equence component of the load current, = _p. Then the line frequency, poitive equence current will to be upplied from the input at the optimum power factor. However it can be een from (4) that thi principle i not exactly followed in thi implementation. Intead a phae angle error i introduced that depend on the total ditortion coefficient T D of the load current. Thi would mean that the input current though free from F.6. Simulation reult of the phae angle balance controller with A(peak) rectifier load (a) input phae current a (b) a (c) filter current i ha (d) load current i la (e) in( )(f) i l_pd, i l_pd (g) co( ) (h) in( ) harmonic will not deliver power at the optimum power factor. However in mot cae thi error in magnitude of the input current i innificant. For example if T D. i 8% a would be in cae of a diode bridge rectifier then the input current magnitude will be 3.8% more than what i poible to achieve through perfect compenation. So if we approximate k= (I lk_p + I lk_n ) a I l_p then I g = I l_p (7) = _p (8) 9

5 Method II : It i poible to implement perfect compenation for harmonic with ome extra computational requirement on the controller. The poitive equence component of the fundamental current i l_p can be extracted from the load current in the ynchronouly rotating (d _p, q _p ) reference frame. For that, it i not even neceary to ene the input voltage v g a would be required in other method. Thi i becaue the internal control variable i proportional to v g. So, a hown in F.4(c), co( ) = i/ g ( / ) / +( ) (9) better dynamic can be extracted by the current controller. However in thi cae the choice i dictated by the limitation of the general purpoe TMS3F4 dital controller that ha been ued for the hardware implementation of the control algorithm. (MHz CPU, 6.6 Sec ADC ) The imulation reult for the method II of Phae Angle Balance control are preented in F.5 and F.6. The reult obtained from method I are practically the ame. In imulation two cae are conidered : () the load i diode rectifier type non-linear and alo ha line frequency reactive current component of A peak - i hown in F.5. and () the load ha no line frequency reactive component but conit only of A peak non-linear current - i hown in F.6. in( ) = / / ( ) / +( ) () IV. EXPERIMENTAL VERIFICATION Therefore, i l_pd = i l co( ) + i l in( ) i l_pq = i l in( ) + i l co( ) i l_pd i l_pq () () Subequently we pa and through low pa filter to eliminate the ac content in the waveform and get the dc quantitie (i l_pd, i l_pq ) correponding to the poitive equence fundamental component of the load current. From (i l_pd, i l_pq ) the in( ) and co( ) can be computed a co( ) = in( ) = i l_pd (i l_pd ) +(i l_pq ) i l_pq (i l_pd ) +(i l_pq ) (3) (4) Alternatively the magnitude of i l_p can be obtained from (5) and the phae angle can be determined by the cloed loop PI controller a in Method I. i l_p = i l_pd + i l_pq III. SIMULATION RESULTS (5) The propoed controller that generate the witching pule for the power converter of the hunt active filter i imulated in the MATLAB-SIMULINK (verion 5.3) imulation package. The non-linear load i of A peak. The hape of the current and it rie and fall time are o choen that it approximately model a three-phae diode bridge rectifier. The per phae inductance of the dened hunt active filter i.75mh. The output dc voltage of SAF i regulated by an outer loop PI regulator. The witching frequency of the converter i choen to be KHz. It i deirable that the witching frequency of the SAF i made a hh a poible. At hher witching frequency the filter inductance can be made lower and The propoed Phae Angle Balance harmonic filtering algorithm i implemented on the general purpoe dital hardware platform of Texa Intrument DSP TMS3F4 (MHz CPU, 6.6 Sec ADC ). It ha three 6-bit regiter [6] -,CMP, CMP and CMP3 to control the individual duty cycle of the witche. We ene two phae current of the harmonic filter (intead of that the input current can alo be ened),two phae current of the non-linear load and the output dc voltage of the hunt harmonic filter. The phae hifted input current i haped to follow the input voltage uing the reitor emulator type dital current mode control algorithm that ha been i decribed in detail in []. The power hardware of the prototype PWM converter i built on IPM (Intellent Power Module) witched at KHz that correpond to the control loop time of T = Sec. The per phae filter inductance i.75mh. The filter and load current are meaured but input voltage are not ened. The output of the hunt harmonic filter i regulated at 375V. A non-linear load of.7a(rm) rating i contructed uing a three-phae diode bridge rectifier connected to the reitive load. For verifying the performance of the phae hift control algorithm, decribed in ection II of thi paper, a predominantly reactive load of 4.7A(rm), i contructed by adding an induction motor under no load to the already available nonlinear load of the diode bridge rectifier. The meaured value of THD of the rectifier load current i 5.7% and the THD of the input current with Phae Angle Balance control i 6.5%. In thi cae the input voltage i not an ideal ine wave but itelf ha.8% THD. There i no ubtantial difference in the input current THD reult between the two method of APB control that are propoed in thi paper. The experimental wave form are hown in F.7 and F.8. The harmonic performance can be further improved by uing a lower value of per phae inductance. However in order to ue a lower value of inductance the witching frequency of the converter ha to be increaed. For that a better controller than TMS3F4 need to be elected becaue it ADC converion time of TMS3F4 i too hh for hher frequency operation. 9

6 7(a) 7(b) Diode rectifier - ch: input current, ch: load current, ch3:active filter current ch4: in( ) (from DAC)(d) Diode rectifier plu IM(no load) - ch: input current, ch: load current, ch3:active filter current ch4: in( ) (e) Diode rectifier - ch: input current, ch: load current, ch3: input current magnitude (from DAC)ch4: filtered load current magnitude i l (f) Diode plu IM (no load) - ch: input fil current, ch: load current, ch3: input current magnitude ch4: filtered load current magnitude (g) Diode plu IM (no load) - without PAB control - for hh power factor operation- ch: input current, ch: load current, ch3: active filter current ch4: in( ) (h) Diode plu IM (no load) - without PAB control - for hh power factor operation- ch: active filter current, ch: in( ), ch3: input current magnitude ch4: filtered load current magnitude i l fil Scale:: a, i la - 5A/div, i ha - A/div, v ga - 7V/div, in - /div,, -.5A/div i l fil 7(c) 7(d) 8(a) 8(b) 7(e) 7(f) 8(c) 8(d) 7(g) 7(h) F. 7. Experimental reult of a Shunt Active Filter ytem - Phae Angle Balance Control Method I (a) Diode rectifier - ch: input voltage (v ga ), ch: input current (a ), ch3: load current (i la ), ch4: active filter current (i ha ) (b) Diode rectifier plu IM (no load) - ch: input voltage, ch: input current, ch3: load current, ch4:active filter current (c) 8(e) 8(f) 9

7 control i a very imple but effective technique for achieving the harmonic filtering objective for load that upport unidirectional power flow from input to load. REFERENCES [] Souvik Chattopadhyay and V. Ramanarayanan, Dital implementation of a line current haping algorithm for three phae hh power factor Boot rectifier without input voltage ening, in APEC, pp (g) 8(h) 8. Experimental reult of a Shunt Active Filter ytem - Phae Angle Balance Control Method II (a) Diode rectifier - ch: input voltage (v ga ), ch: input current (a ), ch3: load current (i la ), ch4: active filter current (i ha ) (b) Diode and IM(no load) - ch: input voltage, ch: input current, ch3: load current, ch4:active filter current (c) Diode rectifier - ch: input current, ch: load current, ch3:active filter current ch4: in( ) (from DAC) (d) Diode plu IM(no load) - ch: input current, ch: load current, ch3:active filter current ch4: in( )(e) Diode rectifier - ch: input current, ch: load current, ch3: d-axi load current i l_pd (from DAC) ch4: q-axi load current (f) Diode plu IM (no load) - ch: input current, ch: load current, ch3: d-axi load current ch4: q-axi load current (g) Diode rectifier - ch: input current, ch: load current, ch3 : co( ) (from DAC), ch4 : : in( ) (h) Diode plu IM (no load) - ch: input voltage, ch:input current, ch3: phae hifted input current a (from DAC),ch4: co( ) Scale: : a, i la - 5A/div, i ha - A/div, v ga - 7V/div, in, co( ), in( ) - /div, i l_pd, i l_pq, a -.5A/div [] H.Akagi, Y. Kanazawa and A. Nabae, Intantaneou reactive power compenator compriing witching device without energy torage component, IEEE Tran. on Indutry Application, Vol. IA-,No. 3, May/June 984. [3] S. Bhattacharya, D. M. Divan and B. Banerjee, Synchronou reference harmonic iolator uing active erie filter, EPE Conf. Record, 99, Florence, Italy. [4] V. Soare, P. Verdelho and G.D. Marque, An intantaneou active and reactive current component method for active filter, IEEE Tran. on Power Electronic, Vol. 5 Iue 4, July, pp [5] K.M. Smedley, L. Zhou, and C. Qjao Unified contant -frequency integration control of active power filter - teady tate and dynamic, IEEE Tran. on Power Electronic, Vol. 6, No. 3, May, pp [6] TMS3C4x DSP Controller Peripheral Library and Specific Device - Reference Set - Volume, Literature Number : SPRU6B December 997. VI. CONCLUSION In thi paper phae angle balance (PAB) control for filtering the harmonic component of the non-linear load i propoed for hunt active filter. The harmonic filtering objective i defined a the tak of balancing the phae angle of the input current with the phae angle of the line frequency component of the load current. To achieve thi objective the input current i ened, phae hifted by a pecific amount and then made proportional to the input voltage.two method are decribed in thi paper for determination of the required phae hift. The current mode control algorithm i input voltage enorle, without PLL and uitable for dital implementation with current being ampled only once in a witching period. Phae Angle Balance control ha the flexibility to compenate only for the harmonic of the load current o that the current rating requirement of the converter can be made lower than the converter with both reactive and harmonic compenation. However if required thi method can compenate for the harmonic a well a the reactive content of the load and thereby hape the input current like input voltage. In concluion it can be aid that phae angle balance (PAB) 93