Vector Control Implementation for a Wound-Excited Synchronous Generator Considering the Damping Effect

Transcription

1 The 18 th Natonal Conference on Electrcal Drves CNAE 2016 Vector Control Implementaton for a Wound-Excted Synchronous Generator Consderng the Dampng Effect Csaba Szabo 1, Enkő Szőke 1, Mara Imecs 1, Ioan Iov Incze 1, Dan Claudu Rus 1 1 Faculty of Electrcal Engneerng, Techncal Unversty of Cluj-Napoca, Romana Abstract - The paper deals wth the mplementaton of a stator-feld-orentaton based vector control strategy for an electrcallyexcted salent-pole synchronous generator wth damper wndngs, supplyng an solated three-phase load. The generator s drven by an nducton motor whch operates n torque control mode. In order to mprove the behavor n transent operaton and to reduce oscllatons, the damper effect s taken nto account n the control structure at the computaton of the exctng current reference value on the reactve control loop. Keywords Electrcally Excted Synchronous Generator, Vector Control, Feld-Orentaton, Damper wndng effect, Dgtal mplementaton. 1. INTRODUCTION The Electrcally Excted Synchronous Machne (EE-SyM) s most sutable to be used n hgh-power applcaton ether as drve machne or as generator due to ts hgh power/weght and power/sze rato. The EE- SyG can operate wth arbtrary power factor, because the exctaton can be controlled ndependently. It can be used also for grd balance as t can produce reactve power f t s overexcted. For the above mentoned reasons the Electrcally Excted Synchronous Generator (EE-SyG) s stll the man suppler for Natonal AC Grd. If the generator s coupled to the man electrc grd as n case of wnd turbnes, the generator s forced to run at a constant speed, regardless of the varable wnd speed. The classcal control prncple of the EE-SyG s based on the control of the frequency and ampltude of the stator (armature) voltage. The ampltude s controlled by adjustng the reactve power by means of the exctng current, whle the frequency s determned by the actve power whch s controlled by means of the drvng turbne energy. For a good transent behavor a vector control system based on stator-feld orentaton (SFO) s appled that was ntroduced n [1] and frst publshed n [2]. The dynamc model of the SyM and also of the vector control system s based on the Space Phasor (SPh) Theory [3]. The EE-SyG used n expermentatons s provded wth complete (connected) damper wndngs havng an mportant role on the system stablty. The dampng has an mportant role on reducng speed oscllatons. Oscllatons of the SyG may affect also the power grd frequency where dsturbances or large transents can be sustaned or ncreased [4]. The damper wndng generates asynchronous torque n transent operaton whch helps at startng and also when load varatons occur (both electrcal and mechancal), provdng addtonal torque to synchronze the machne. Usually the damper crcut tme constant s much smaller than the armature tme constant or the exctaton one, and t s the frst that respond to perturbatons. Damper wndngs are also used to reduce the huntng phenomena of the SyM. Other advantage of usng damper wndngs refers to ts ablty to balance the termnal voltage on each phase of the SyG durng unbalanced loadng [4]. The modelng of the damper crcut s of a great mportance, and requres an accurate determnaton of the damper crcut parameters. The proposed vector control structure based on the SFO takes nto account the effect of the damper wndng on the reactve control loop where the exctng current reference s computed. 2. IDENTIFICATION OF THE DAMPER CURRENTS OF THE EE-SYG In a vector control system for EE-SyG f the machne has damper wndngs, the dynamc performances may be further mproved f the dampng effect s taken nto account nsde the feed-back loops n the computaton procedure of the control varables. In order to do ths, the dampng currents have to be dentfed based on the mathematcal model (MaMo). The man drawback of ths method s that t s

2 434 The 18 th Natonal Conference on Electrcal Drves CNAE 2016 nfluenced by some of the machne parameters, lke leakage nductances that are dffcult to dentfy. Based on the MaMo wrtten n rotor coordnates, there are two methods: one based on the flux model, and another based on current model (whch s sutable f the machne saturaton s also taken nto account). In ths paper the flux model s used. The computaton of the dampng current vector A = Ad + j s performed usng the general equatons of the SyM wrtten n rotor coordnates dθ θθ. The damper flux lnkage equatons for the drect and quadrature rotor axs are [3]: Ad = = t 0 t 0 R R Ad Ad dt dt (9) (10) Ad = L AdAd + Lmd ( sdθ + e ); (1) = L + L, (2) Aθ Aθ Aθ mθ sθθ where Ad, are the resultant fluxes, and Ad, the currents of the longtudnal and quadrature damper crcut, sdθ, sθθ the rotor orented longtudnal and quadrature components of the stator (armature) current and e s the exctng one, L md, Lmq are the longtudnal, respectvely the quadrature three-phase useful nductances of the salent-pole SyM, whle L Ad, L are the total nductances of the longtudnal and quadrature damper crcut whch ncludes also the leakage nductances: L Ad = L σ Ad + L (3) L = L σ + L (4) md ; mq. Fg. 1 Block dagram of the ar-gap flux compensaton based on the rotor-orented model [3], [7]. Based on (1) - (4) the followng expresson for the damper currents are obtaned [3]: Ad Aθ σad =, where σad = Ad mdθ ; LσAd (11) σaθ =, where σaθ = Aθ mθθ. L (12) σaθ The block dagram for the dentfcaton of the damper currents based on equatons (7) and (8) derved from the flux model s presented n fgure 2. Ad Aθ The resultant damper fluxes may be wrtten as: = = + σad mdθ + σaθ mθθ, Ad AdAd ; σ = L σ (5),, σ = L σ (6) where σad, σ are the longtudnal and quadrature rotor-orented components of the damper leakage flux [3]. The stator flux s obtaned by ntegraton of the stator-voltage equatons [8]. The useful ar-gap flux obtaned by compensaton from the dentfed stator flux based on the MaMo [3], [7]: mdθ = Lσσdσdθ (7) σdθ = L (8) mθθ σσθ σdθ σθθ The correspondng block dagram s shown n fgure 1. The resultant damper fluxes, Ad are dentfed by ntegraton of the damper crcut voltage equaton wrtten n rotor orented coordnates: Fg. 2 Block dagram of the dampng current computaton based on the damper crcut usng the flux model [3]. The damper crcut effect s taken nto account n the proposed vector control scheme used n generaton of the exctaton current reference value, where only the longtudnal dampng component Ad s used (computaton block Ad C ). 3. THE VECTOR CONTROL STRATEGY FOR THE EE-SYG The space-phasor dagram of the EE-SyG for transent operaton s presented n fgure 3:

3 The 18 th Natonal Conference on Electrcal Drves CNAE Fg. 3 Space-phasor dagram of the EE-SyG for transent operaton by takng nto account the damper crcut [3]. In generator operaton mode the energy transfer s reversed n comparson wth the motorng operaton mode f a motor MaMo s used. The machne torque wll be negatve for generator operaton, consequently the feld-orented quadrature component of the armature-current λ s also negatve, as t may be seen sq s n the fgure 3 [3]. The rotor-orented components of the machne currents there are also represented, beng used to compute n rotor-coordnates the exctng current. The rotor and the orentaton feld angular speeds ( ω, ω λs ) are noted as dfferent because of the transent operaton. They are equal only n steady-state operaton (synchronous runnng). The nduced e.m.f. d s due to the stator-flux varaton s also dt represented and t s perpendcular to the rotatonal ω. e.m.f. voltage ( ) λs s 3.1. The vector control structure The proposed vector control structure presented n fgure 4 s based on SFO, where on the two loops the stator voltage ampltude s controlled by adjustng the stator flux, whle the frequency s determned by the turbne drvng torque [3]. The orentaton stator flux two-phase natural components sd, sq are dentfed based on the voltage model wrtten n stator-fxed coordnates by ntegraton of the e.m.f. n the s C (Stator Flux Computaton) block [8]. Usng a Vector Analyzer (VA) the orentaton flux ampltude s s dentfed, beng used as feed-back sgnal n order to perform the flux control. The block also computes the λ s poston of the orentaton flux ( sn λs, cosλ s ) that s used for coordnate transformaton. In order to perform the computaton of the damper current, n rotor coordnates a coordnate transformaton s performed (CooT [D(θ)]) n order to obtan the sdθ, sθθ rotor-orented components of the orentaton flux. After that the ar-gap flux compensaton s performed n block mco based on equatons (9), and (10) as t s presented n fgure 1. The drect component of the dampng current s computed based on the structure presented n fgure 2 usng equatons (7), (9) and (11) (I Ad C block). On the actve loop the frequency control s performed. On the feed-back loop the rotor speed s computed based on the encoder sgnals, and based on ths the actual frequency f s s computed, whch s proportonal to the rotor speed. The frequency controller generates Ref sqλs - the SFO-ed torque producng (actve) component of the armature current. On the same loop the actve current controller wll generate the reference torque me Ref value for the drvng turbne. On the other hand, the reactve loop wll perform the control of the stator voltage ampltude. The voltage controller generates the stator flux reference value. At the output of the flux controller wll result ms Ref, the stator-flux based magnetzng current reference value.

4 436 The 18 th Natonal Conference on Electrcal Drves CNAE 2016 Fg. 4 Vector control system of the salent-pole EE-SyG based on stator-feld orentaton by takng nto account the dampng effect. Fgure 5 shows the block dagram of the exctaton current computaton takng nto account the vectorcharacter of the exctng current. Fg. 5 Block dagram of the exctaton current computaton based on the rotor-orented model consderng the dampng effect [3]. The exctaton current computaton s performed based the rotor-orented MaMo [3]: ( ms ) ( + s ) sd Ad e = dθ θ 1 (13) In the above expresson the ( ms ) dθ s the drect component of the stator flux based magnetzng current n dθ θθ coordnates. Ths operaton s performed by a smplfed coordnate transformaton block whch wll have only one nput, and only one multpler n comparson wth the classcal CooT wth and uses a transformaton matrx wth four elements [3]: Ref ( ) ( θ λ ) ms dθ = cos (14) ms s The ( θ λ s ) represents the load angle δ obtaned by an angle subtracton block, and represents the dfference between the rotor and stator (orentaton) feld poston. The exctaton current s generated by a controllable DC current source. It can be sad that n SFO-ed coordnates the e exctng current may also be expressed vectorally ( e = edλ s + jeqλs ), by means of ts edλ s reactve and actve components whch has nfluence on both of eqλs the control loops of the EE-SyG [3], [5]. The expermental set-up also ncludes a squrrelcage Inducton Machne (IM) fed by a Voltage-Source Inverter (VSI) that provdes the drvng torque for the SyG generated on the actve control loop. At the EE- SyG output a three-phase passve load s connected. 4. THE EXPERIMENTAL SET-UP The expermental set-up presented n fgure 6 conssts of a 4 pole 800W salent-pole EE-SyG drven by a VSI-fed 1.5 kw squrrel-cage IM workng n torque control mode. The generator supples a threephase star-connected R-L passve solated load.

5 The 18 th Natonal Conference on Electrcal Drves CNAE The EE-SyG exctaton s ensured by a controllable DC current source. The control varables for the IM (the drvng torque) and for the DC current source (the exctaton current) are generated by the control structure presented n fgure 4. Fg. 8 The r.m.s. value of the reference and actual armature voltage generated by the EE-SyG. Fg. 6 The expermental set-up [5], [7]. The mplementaton s realzed usng a DSP-based research and development system wth the dspace DS1104 controller Implementaton results Valdaton of the control structure presented n fgure 4 was performed n dfferent condtons, for both steady-state and transent operaton. As perturbatons were ntroduced varatons of the reference voltage and frequency. The test was performed for the startng of the SyG at rated voltage and frequency (50 Hz, 220V machne), followed by a varaton of the mposed reference values to 40Hz and 180 V, respectvely. Durng operaton the R-L load remans unchanged. Fg.9 Phase-to-phase armature voltages at startng of the EE-SyG Fg.10 Phase-to-phase armature voltages of the EE-SyG durng transent operaton due to varaton of mposed reference values. Fg. 7 The reference and the actual frequency of the SyG. Fg.11 The three-phase armature.e. loadcurrents at startng of the EE-SyG.

6 438 The 18 th Natonal Conference on Electrcal Drves CNAE 2016 taken nto consderaton n the control structure. The proposed control structure s realzed by takng nto account the rotor-orented drect component of the damper current (dentfed by usng the flux model) n the computaton process of the exctng current reference value on the reactve control loop of the EE-SyG. If the e exctng current s computed n SFO-ed coordnates, ts components ( edλs and ) eqλ s nfluences both the actve and reactve control loops, whch confers a vectoral character to the control structure. Fg.12 Phase-to-phase armature voltages of the EE-SyG durng transent operaton due to varaton of mposed reference values. Fg. 13 The space-phasor trajectory of the three-phase armature.e. load current. The dynamc response meets the expectatons. Hgher oscllatons occur durng transent operaton that follows the change of the mposed reference values of the frequency and armature voltage. The actual value of the f s stator frequency s computed actual (measured) rotor speed CONCLUSIONS By applyng a feld-orentaton to the armature current of the EE-SyG for the computaton of the control varables on the actve and reactve control loops, based on the general mathematcal model of the machne, better dynamc response s acheved also n transent operaton occur by varaton of the reference quanttes. If the machne s equpped wth damper wndngs, further mprovements of the machne behavor are acheved f the dampng effect s also REFERENCES 1. Árpád Kelemen, Mara Imecs, Metodă ş sstem de reglare automată vectorală a puter actve ş reactve a generatoarelor sncrone (n Romanan), Inventon Patent, RO, , Árpád Kelemen, Mara Imecs, Vector control system for frequency and voltage of synchronous generators, Proceedngs of Internatonal Conference on Electrcal Machnes, ICEM 90, Cambrdge, Massachusetts, USA, Árpád Kelemen, Mara Imecs, Vector Control of AC Drves, Volume 2: Vector Control of Synchronous Machne Drves, Ecrture Publsher, Budapest, 1993, ISBN E. Kyrakdes,R.G. Farmer: Modelng of Dampng for Power System Stablty Analyss, Electrc Power Components and Systems Journal, Volume 32, Issue 8, 2004, Prnt ISSN: , Onlne ISSN: pp Rus D. C., Incze I. I., Imecs Mára, Szabó Cs.: Vector Control Implementaton for a Wound-Excted Synchronous Generator Neglectng the Dampng Effect, ELECTROMOTION Journal, Vol. 20, Nr. 1-4, Jan.-Dec (4 ssues per vol.), Specal Issue: of the 10 th Jublee Symposum ELECTROMOTION 2013, Cluj-Napoca, Medamra Scence Publsher, ISSN X, pp Imecs Mára, Vector control of the current-excted synchronous generators, 2 nd Internatonal Conf. n Recent Achevements n Mechatroncs, Automaton, Computer Scence and Robotcs MACRo2010, Tg. Mures, Scenta Publshng House, Cluj-Napoca, 2010, ISBN ` , pp Rus D. C., Practcal Implementaton of Vector Control Strategy for an Electrcally Excted Synchronous Generator, (n Romanan.), PhD Thess, PhD supervsor prof. Imecs Mára,, Incze I. I., Szabó Cs., Imecs Mara, Flux dentfcaton for vector control of the synchronous motor drves, IEEE-TTTC Internatonal Conference on Automaton, Qualty and Testng, Robotcs, AQTR 2008 (THETA 16), May 2008, Cluj- Napoca, Romana, Volume II, pp , CD-ROM, IEEE Catalog Number: CFP08AQT. 9. Olmpu Stocuta ;Teodor Pana, Asymptotc stablty study of nducton motor sensorless vector control systems wth MRAS observer, Publshed n: Automaton, Qualty and Testng, Robotcs (AQTR), 2016 IEEE Internatonal Conference on, May 2016, Cluj-Napoca, INSPEC Accesson Number: , DOI: /AQTR