Analog-Digital versus DSP Implementation of Park s Current Calculators for AC Mains Self-Powered Systems

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1 Anaog-Digita versus DSP Impementation of Park s Current Cacuators for AC Mains Sef-Powere Systems VASCO SOARES, PEDRO VERDELHO DEEA - Departamento e Engenharia Eectrotécnica e Automação,, CAUTL, CIC ISEL - Instituto Superior e Engenharia e Lisboa, IST - Instituto Superior Técnico Rua Conseheiro Emíio Navarro, ; Lisboa PORTUGAL Abstract: - This paper presents a comparison between an anaog-igita eicate circuit that evauates the Park s current components on three-phase systems an its counterpart impementation with a DSP. Both circuits operate with the instantaneous active an reactive current component i -i metho. These currents are obtaine with a synchronous reference frame which erives from the mains votage vector. The cacuators presente are suitabe to perform the irect an inverse current transformations in many sef-powere systems connecte to the ac mains even at noniea mains votage conitions. Laboratoria impementation aspects of the propose current cacuators are shown. Experimenta resuts showing their steay-state performance operation appie to a shunt active fiter appication are aso presente. Key-Wors: Park s transformation, anaog-igita systems, igita signa processing, current measurement, current cacuator, active fiter, harmonics. Introuction The use of Park s transformations is very usefu for the stuy of eectrica machines an power eectronic converters. The Park s current cacuators presente in this paper are base on the instantaneous active an reactive current component i -i metho []. This metho is speciay suite to the contro of many sef-powere systems connecte to the ac mains, ike active fiters (AF s), unifie power fow controers (UPFC s), avance static VAr compensators (ASVC s), PWM rectifiers, uninterruptibe power suppies (UPS s) an neutra current compensators (NCC s). In active fitering it as been prove that better performances are achieve uner unbaance an nonsinusoia votage conitions compare with other contro methos []-[3]. The aopte contro metho is base on a synchronous rotating frame erive from the mains votages without the use of a phase-ocke oop (PLL) [4]-[6]. Using the i -i contro metho many synchronisation probems are avoie an truy freuency-inepenent systems can be achieve. Contro Metho In the instantaneous active an reactive current component i -i metho []-[3] the conversion between the noninear oa currents i i an their corresponing components, i an i, are obtaine from a irect (), (3) an inverse (), () Park s transformations. A nu vaue for the zero votage seuence is consiere. The zero current seuence is aso nu since there is absence of neutra connection. The oa current components are erive from a synchronous reference frame where represents the instantaneous votage vector ange. cos( ) sin( ) π π T ( ) = cos( ) sin( ) () π 4π cos( ) sin( ) 3 3 i T = T ( ) i () i3 i = T ( ) (3) i i3 In Fig. the instantaneous votage an current vectors in the stationary an rotating frames are shown. The transformation ange is sensibe to votage

2 harmonics an unbaance votage sources therefore /t may not be constant. 0 β uβ i i uαβ, u Fig.. Instantaneous votage an current vectors. Instea of using a PLL to obtain the ange, thus avoiing improper current etermination, the rea instantaneous votage vector ange can be obtaine by, u β = tan, (4) uα where u α an u β are αβ components of the mains suppy votage, (5). u α = u (5) uβ 3 3 u3 0 Since the zero current seuence is consiere nu, as state before, the Park s transformations can be obtaine by the foowing expressions. π sin( ) sin( ) 3 i i = = π cos( ) 3 cos( ) π 3 cos( ) 3 ϕ iαβ uα α i cos( ) sin( ) π sin( ) 3 i 3 Anaog-Digita Impementation (6) () The cacuator circuit propose can perform the irect an the inverse Park s transformations []. The bock iagrams are presente in Figs. an 3. To perform the inverse Park s current cacuation the phase-to-phase votages u an u 3 have to be measure. After the measurement operation the votage signas pass through an anti-aiasing fiter an a votage imiter before the igita conversion. Using two -bit semi-fash anaogue-to-igita converters (ADC s) a high conversion rate can be achieve (samping time or freuency of t s < 5 µs, f s > 00 khz). An aress bus is estabishe from the most significant bits (MSB) of each ADC. The oss of bit in the conversion is irreevant since it represents the ADC error. The bus wi aress two 3 kb erasabe programmabe rea-ony memories ( s) where the sinusoia functions presente in (6) are store, with the ange etermine by (4). The outputs of the s are converte by two ua -bit 4-uarant mutipying igita-to-anaogue converters (DAC s). u u3 D.. A0..A6 D.. A..A3 f Gu Gu ADC ADC fs D.. A0..A6 D.. A..A3 Reógio i i f DAC A DAC B DAC A DAC B DAC A / DAC B Fig.. Inverse Park s transformation. 3 4 ic r ic r DAC 3A DAC 3B DAC 4A DAC 4B DAC A / DAC B Gi Gi cos() -sin() Fig. 3. Direct Park s transformation. sin(π/3) -cos(π/3) ic r ic r sin(π/3) G sin() cos() G cos(π/3) The measure current vaues i an i are imite an mutipie by the DAC s. The sum of these signas gives the currents i an i. The operation of the irect Park s current cacuation is simiar to the ast circuit. However the sinusoia functions store in the s are base in () an (4), so the resut wi be the reference currents i cr an i cr. The synchronisation circuit signas are obtaine with a master cock buit with a votage-controe osciator (VCO) an a D type fip-fop (FF). These signas enabe us to choose the sinusoia functions in the s since they are G G i i

3 mutipexe, an the proper seection of the rea (RD) an write (WR) operations in both ADC s an DAC s. In Fig. 4 is presente the aboratoria impementation of Park s current cacuator base in the circuits shown in Figs. an 3. Fig. 4. Anaog-igita impementation of the Park s current cacuator. In the experimenta prototype it is reuire the fastest possibe program execution so the SRAM is the mainy use memory. This processing system it is programmabe by a eicate interface an runs in rea-time inepenent of the computer an its workoa. There is a seectabe group of ADC s with 6 bipoar mutipexe anaog inputs ( bit resoution with 0. µs conversion time pus µs mutipexer setting time, or 6 bit resoution an 0 µs conversion time pus 4 µs mutipexer setting time). Anaog outputs are obtaine with two non mutipexe 6 bit DAC s (3 to 0 µs of setting time epening on signa range). For igita ata there are 6 bit TTL igita inputs/outputs. 4 DSP Impementation The igita impementation of the Park s Current cacuator is base on a DSP system (ADwin-Go), Fig. 5. Fig. 6. DSP interna structure of the Park s current cacuator. Fig. 5. DSP impementation of the Park s current cacuator. This stan-aone an compact moe is we suite for prototyping an it is euippe with the IEEE 3 bit foating point 40 MHz Super Harvar Architecture (SHARC) DSP ADSP 06 from Anaog Devices, Fig. 6. It presents a singe-cyce instruction execution with 5 ns instruction rate. The performance vaues are 40 miions of instructions per secon (MIPS), 0 miions of foating point operations per secon (MFLOPS) of peak performance an 0 MFLOPS of sustaine performance. The DSP as an interna static ranom access memory (SRAM) of 56 kb (5 ns). The externa ynamic ranom access memory (DRAM) is 4 MB (5 ns). Both memories can be use for program an ata. 5 Experimenta Resuts of the Park s Current Cacuators Appying the propose Park s cacuators to a shunt AF for harmonic current compensation, the inverse an irect current cacuations can be easiy performe, Fig.. The AF contro circuits are escribe in [] an []. The variabes i an i are the noninear oa currents in components that have to be fitere in orer to eiminate the DC components, i.e., to obtain the harmonics to be injecte in the main suppy, i cr an i cr. Therefore the variabes i cr an i cr represent the reference currents of the three-eg votage source converter (VSC) an so the harmonic currents that must be eiminate from the mains.

4 An AF experimenta prototype was buit with a kva three-eg IGBT VSC. Ha-effect current an votage sensors an simpe igita an anaog interface circuits were use. The ac current contro an c votage reguation were performe by anaog-igita controers/reguators or by the DSP system. With the anaog-igita Park s current cacuator a great ecrease in the tota harmonic istortion (THD) is obtaine, Figs. an 9. AC mains is i Noninear oa L i ic u, u3 ic αβ 3-eg VSC g3 αβ Current controer C ec Fig.. AF performance with the anaog-igita Park s current cacuator. () Mains votage u (00 V/iv). () Compensating current i c (5 A/iv). (3) Converter current i (5 A/iv), α = 0, THD i =,0 %. (4) Mains current i s (5 A/iv), THD is = 4,0 %. 5 ms/iv. 3 icαβ - icαβ r Harmonic current fiters αβ ic r ic r i nh - i nh - i h DC votage reguator δec - ec i h = 0 Fig.. Shunt AF contro circuits. The variabes measure in the prototype were: currents i c3, votages u, u 3 an e c. Switching signas an IGBT enabing signas g 3 an E 3, respectivey, were use to contro the three-eg VSC. The goba DSP execution time which correspons to the samping time of a variabes measure was 35 µs (samping freuency of.6 khz). The foowing resuts show the operation of the Park s current cacuators appie to an AF uner mains baance an sinusoia votage conitions, f = 50 Hz. The harmonic current compensation is performe over a noninear oa mae with a three-phase fu converter (firing anges α = 0 an α = 60 ). ec r Fig. 9. AF performance with the anaog-igita Park s current cacuator. () Mains votage u (00 V/iv). () Compensating current i c (5 A/iv). (3) Converter current i (5 A/iv), α = 60, THD i = 56,0 %. (4) Mains current i s (5 A/iv), THD is = 3,0 %. 5 ms/iv. Simiar performance vaues are obtaine with the DSP Park s current cacuator, Figs. 0 an. From the THD vaues obtaine in a experimenta resuts it can be concue that the greater resoution provie by the DSP system partiay compensates its ower samping freuency ( bit,.6 khz) compare to the anaog-igita system ( bit, 00 khz). It aso shou be pointe out that the DSP executing time (acuisition pus processing time) is a critica parameter with respect to current contro in the AF.

5 The effectiveness of the harmonic current compensation is very epenabe from it. Fig. 0. AF performance with the DSP Park s current cacuator. () Mains votage u (00 V/iv). () Mains current i s (0 A/iv), THD is = 3, %. (3) Loa current i (0 A/iv), α = 0, THD i = 5,9 %. (4) Compensation current i c (0 A/iv). 0 ms/iv. Fig.. AF performance with the DSP Park s current cacuator. () Mains votage u (00 V/iv). () Mains current i s (0 A/iv), THD is =, %. (3) Loa current i (0 A/iv), THD i = 60, %, α = 60. (4) Compensation current i c (0 A/iv). 0 ms/iv. 6 Concusions The contro metho use in the Park s current transformations is base upon the instantaneous active an reactive current component i -i metho. This metho enabes to obtain current cacuators for AF that may operate in variabe freuency conitions without any ajustments. Therefore, the harmonic compensation systems can work propery in a arge range of freuencies covering both 50 Hz an 60 Hz istribution systems. The anaog-igita Park s current cacuator is buit with inexpensive eectronic circuits. The DSP version of this current cacuator is much more expensive. However, since it s not an harware eicate circuit it presents the avantage of being programmabe with great gains on signa processing capabiity an fexibiity. Both systems propose exhibits a very fast ynamics an present a goo accuracy in the currents cacuation. In an AF appication the noninear oa current harmonics are eepy ecrease. There are no significant ifferences in the performance vaues between them. For Park s current cacuation there is not a uniue soution between anaog-igita an DSP impementations, in spite of being uite simiar with respect to performance vaues. The choice between them as to o with cost effective issues an epens in what type of appication the cacuator is neee. References: [] V. Soares, P. Vereho, an G. Marues, An instantaneous active an reactive current component metho for active fiters, IEEE Trans. on Power Eectronics, vo. 5, no. 4, Juy, 000, pp [] V. Soares, P. Vereho, an G. Marues, Active power fiter contro circuit base on the instantaneous active an reactive current i -i metho, Proc. PESC 9 Conf., ISBN X (CD-ROM), ISSN , vo., 99, pp [3] V. Soares an P. Vereho, Anaysis of active power fiters in freuency omain using the fast Fourier transform, Proc. EPE 9 Conf., ISBN , vo. 4, 99, pp [4] P. Vereho an G. D. Marues, An active power fiter an unbaance current compensator, IEEE Trans. Inustria Eectronics, vo. 44, no. 3, 99, pp [5] J. Tepper, J. Dixon, G. Venegas, an L. Morán, A simpe freuency-inepenent metho for cacuating the reactive an harmonic current in a noninear oa, IEEE Trans. Inustria Eectronics, vo. 43, no. 6, 996, pp [6] H. Fujita an H. Akagi, The unifie power uaity conitioner: the integration of series active fiters an shunt active fiters, Proc. PESC 96 Conf., 996, pp [] V. Soares an P. Vereho, Instantaneous Active an Reactive Current i -i Cacuator suitabe to Active Power Fiters, Proc. EPE-PEMC 9 Conf., ISBN (CD-ROM), vo., 99, pp. -4.