Single phase STATCOM with 3kV IGCT based Step Down PWM AC Choppers
Description
Niveau: Supérieur, Doctorat, Bac+8
128 4 Chapter 4 Single-phase STATCOM with 3.3kV IGCT based Step-Down PWM AC Choppers 4.1 Introduction The use of direct AC/AC conversion PWM AC Choppers for single-phase reactive power compensation (capacitive operation) can provide an interesting alternative to the classical STATCOM solutions based on Voltage Source Inverters. Higher reactive power compensation capabilities at given semiconductor ratings, lower semiconductor power losses and smaller compensation capacitor are the main arguments offered by the PWM AC Choppers. However, the AC Choppers' control and switching pattern generation (PWM) is directly related to the AC input signal, which requires specific solutions dealing with the non-linear behaviour of these topologies. This chapter is arranged as follows. First, the PWM AC Chopper topologies are briefly introduced to show their general characteristics. Then, the conversion structure for a 3MVAR single-phase STATCOM for the 25kV/50Hz SNCF substations based on step-down PWM AC Choppers equipped with 3.3kV IGCTs is proposed. The operation and design of a basic 1MVAR STATCOM module based on 3.3kV IGCTs and PWM AC Choppers is detailed in terms of components dimensioning, control strategy, etc. 4.2 Direct AC/AC Conversion with PWM AC Choppers Multiple converter topologies can be applied to provide AC/AC conversion. Considering the number of conversion stages, they can be classified as either Direct or Indirect AC/AC converters, [BHO-93].
128 4 Chapter 4 Single-phase STATCOM with 3.3kV IGCT based Step-Down PWM AC Choppers 4.1 Introduction The use of direct AC/AC conversion PWM AC Choppers for single-phase reactive power compensation (capacitive operation) can provide an interesting alternative to the classical STATCOM solutions based on Voltage Source Inverters. Higher reactive power compensation capabilities at given semiconductor ratings, lower semiconductor power losses and smaller compensation capacitor are the main arguments offered by the PWM AC Choppers. However, the AC Choppers' control and switching pattern generation (PWM) is directly related to the AC input signal, which requires specific solutions dealing with the non-linear behaviour of these topologies. This chapter is arranged as follows. First, the PWM AC Chopper topologies are briefly introduced to show their general characteristics. Then, the conversion structure for a 3MVAR single-phase STATCOM for the 25kV/50Hz SNCF substations based on step-down PWM AC Choppers equipped with 3.3kV IGCTs is proposed. The operation and design of a basic 1MVAR STATCOM module based on 3.3kV IGCTs and PWM AC Choppers is detailed in terms of components dimensioning, control strategy, etc. 4.2 Direct AC/AC Conversion with PWM AC Choppers Multiple converter topologies can be applied to provide AC/AC conversion. Considering the number of conversion stages, they can be classified as either Direct or Indirect AC/AC converters, [BHO-93].
- ac choppers
- direct ac
- single-phase statcom
- step-down pwm
- power levels
- input voltage
- could lead
- indirect exchange
- pwm ac
Informations
| Publié par | profil-zyak-2012 |
| Nombre de lectures | 30 |
| Langue | English |
| Poids de l'ouvrage | 4 Mo |
Extrait
4 Chapter 4 Single-phase STATCOM with 3.3kV IGCT based Step-Down PWM AC Choppers 4. 1 Introduction The use of direct AC/AC conversion P WCMh oApCpers for single-phase reactive power compensation (capacitive ope rcaatino np)rovide an interesting alternative to the classical STATCOM solutions based on Voltage Sovuerrct e rIsn. Higher revaec tpiower compensation capabilities at given semiconductor lroawtienrg s ,emiconductor power losses and smaller compensation capacitor are the main sa rogfufemr e dn tby the PWM AC Choppers. However, the AC Choppers’ col natrnod switching pattern generationi s( PdiWreMc) tly related to the AC input signal, which requires specific sdoleuatl i onngs with the non- libn e haraviour of these topologies. This chapter is arranged as follows. FPirWst,M tAhCe Chopper topologies are briefly introduced to show their general characteristicsh. eT hceo n ,v etrsion structure for a 3MVAR single-phase STATCOM for the 25kV/50Hz SNCF substations based on step-down PWM AC Choppers equipped with 3.3kV IGCTso ipso sperd. The operation andg nd oefs ia basic 1MVAR STATCOM module based on 3.3kV IGCTs and PWM pApC erCsh ois detailed in terms of components dimensioning, control strategy, etc. 4. 2 Direct AC/AC Conversion with PWM AC Choppers Multiple converter topologies can be ap p rloievidd et oAC/AC conversion. Considering the number of conversion stages, they can fbi e d cl a s sieither Direct or Indirect AC/AC converters[,B HO-93]. Indirect converters use ina t e r lminekdsi (DC or AC) using at least two different conversion stages, which geenqeuriarlely trhe use of asgtoer elements. Direct converters provide a direct link betwseoeunr cteh ea nd the load without additional storage elements. Nevertheless, pafsilstievres are always requir efdil tteor out the high frequency harmonics introduced at the input and ou t hpruotu sgihd etshe converter switching operation. Among the AC/AC direct converters, the vCeyrctleorcsoannd Matrix converters are distinguished by their ability to adjust the output frequenacgye afrnod mv oal tspecific inApCut voltage source. They also provide bi-directional power tranlsifteier sc, aapllaobwiing the use of active loads (e.g. motors in regeneration mode). On the other hand, the hAoCp pCer topologies, analogtoou stlhye well known DC choppers, provide direct AC/AC conversion only betow eAeCn stowurces characterised by the same fundamental frequeFnicgyu,r e 4-1. They can be trea t e qd uaivsalent transformers where the 128
Chapter 4 Single-phase STATCOM wiVt hI G3C.3T kbased Step-Down PWM AC Choppers
turns ratio can be electronically m oNdeifvi e drt.heless, although they can provide instantaneously bi-directiona l trpaonwsefrer, they allow the ptroawnesrfer flow in one direction only according to the loadu’rs e.n aTthe AC Choppers arem nalolry designed to guarantee the power transfer between a fixveodl tAaCg e source (e.g. they ugtriilidt) and a passive AC load. The load voltage (i.e. its RMS value) can bde tao dcjuosntterol the power demand, but the power exchange (either active or reactive) isn oendl yb yd etfhie nature of tshsei vpea load (resistive, capacitive or inductive). Forpl e x, awmith an AC Chopper, anc t i nvde uload will enre vbe able to operate as a capacitive load hfer oimn ptut side point of view i. s Tdhiuse to the fact that the phase of the AC volt a pgpel ied to the load cannot bee dm. oItd iifsi only defined by the input voltage and the transfer ratio of the AC Chopper.
Figure 4-1.- Principle of AC Chopper direct AC/AC conversion
4.2. 1 Overview of PWM AC Choppers Topologies The concept of the AC Chopper is n.otLi n e wfrequency thyristor based direct AC/AC converters are widely used in AC powera pcpolnictraotli ons such as industrial heating, lighting control, soft starting and speed controllertsi ofon r minodtuorcs, power conditioning, etc. In these applications, the load voltage is controller d abtiyo nr eotfa the thyrrsi’s tfoiring angle, which generates lagging power factosir gnainfidc ant current harmontihces satu pply side, as well as relevant current and voltamgeo nhicars at the output side. The use of the PWM pattern generation wdaesr ecdo an s ia solution tpor oivme the input power factor and to eliminate s pceucrirfiecnt harmonics of thyristor based direct AC/AC Converters, which led to the introduct iostne po-fdown PWM AC Chopp[eCrsH, O-89][, CHO-95]. This topology is derived from the clasespi-cdalo wstn PWM DC choppehre, rew the two segment semiconductors (unidirectvi o lnt a lg e and current) have beene dr ebpyl afcour segment turn-off semiconductors (voltage and current reFviegrusribel e4)-,2 .
Figure 4-2.- Single-phase step-down PWM AC-chopper derived from the step-down DC-chopper The output voltage is simply adjustedn abtyi nagl ttehre two available semiconductors states inherent to this topology. During the act itvhe e pihnapsuet, voltagea pips lied to the load, and during the freewheeling phase a shorpta-ctihr ciusi tp rovided to the load cFuirgreurnet, 4-3. However, the practical application of thii s sliolmui t ieodn by relevant technical drawbacks. Most significant problems are due to the need sonfu b buelkr y circuits to provide safe switching conditions to the reversible semiconductors.
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Chapter 4
ial rentiffet fo erucurts csha pe-glin shegiru e-4uter.sF the basi6 shows ht sid eoivaa ru saluctrerfftienneg emasheb lareerff o],e thg inme,systs-Y20[ EM pol andase y-phrof tob is helgncas ben pp aedliohppres rtcuuterrential PWM AC Cfeif-Don Nedllca-os eht ,sesac h suc. Inointal puerttun i pnt ehteectod y tlnncomrepnenatsum eb he load int f ttuar lop ,ht een
Figure 4-7.- 3-level multi-cell differential PWM AC Chopper (step-down)
Figure 4-8.- 3-level multi-cell non-differential PWM AC Chopper (step-down) Classical applications of PAWC MC hoppers have begehnt iling control, soft starters for induction motors, industrial heating, etc. During twh ey ep a rsst, fneew applica tic o n s idering PWM AC Choppers in the field of poweitri ocnoi n dg such as power line icoonnerdsi,t phase shift control in transmission lines, power electronic transformmoneircs ,f ihltaerrs, series compensators, Flicker
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Chapter 4 Single-phase STATCOM wiVt hI G3C.3T kbased Step-Down PWM AC Choppers Figure 4-10.- Active energy exchange phase Figure 4-11.- Freewheeling phase Figure 4-12.- Indirect energy exchange phases During the indirect exchange phases g ebnye ratth e d switching cell dead times (all semiconductors off), the decoupling capa c ihtao r gs eadr eb y the output current source through the freewheeling dioEdqe.s ,4 -1,E q. 4-2. Inversion of the ocuutrpruetn t polarity implies that the conducting diodcehsa nge from 1- D 2C (I OUT <0) to 2 -DD 1C (I OUT <0) and vice versa. As a result, the decoupling capacitors chargienngt cduur r ing the dead timebse craenp resented by the absolute value of the output source current (mreocdteif)i.e rT his means t h ea tv toltage of the decoupling capacitor will be incerde absy a certain amo Δ uVn C t D 1 = Δ V CD2 on every dead time sequEeqn.c e, 4-3. This continuously charging procehses doefc otupling capacitors can not be allowed, therefore, adaptatiotnh eo f switching pattern to avoei d etffh e csts and to guarantee the safe operation conditions osf etmhiec onductors is required. I CD1 = C D1 ⋅ I Eq. 4-1 C D1 + C D2OUT D2 I CD2 = CC + C ⋅ I OUT Eq. 4-2 D1 D2 t + T D Δ V CD1 = Δ V CD2 = C D1 + 1C D2 ⋅ ∫ I OUT (t) ⋅ dt Eq. 4-3 t Providing that only the Aacntidv eFreewheeling phases can be applied, the transition from one sequence to another must be assured ahveo isdiimnugl ttaneous condu(citniopnu t voltage short-circuit) or blocking (indirect exchange ofp hthase e)f our controlled semiconductors. Such operation requires additional informationi cahb osuwti tcw h ing pattern has to be provided to guarantee the proper operation for each speticoifni.c Ecitohnedri detecti otnh eo finput voltage or the output current sign can be [LuEsFe-d0, 1]. However, since the output current ripple, introduced by the switching o np, eirsa tuisually relativelye rh itghhan the input voltage ripple, accurate detection of the input voltage tpeolcahrniticy ailsly more feasible than detection of the output current polarity. The switching strategy, adohpeti inngp tut voltage polarity detceocntisoisn,t s in short-circuiting one of the switching cells according to tvhoel tiangpe utpolarity, wt h ilee other switching cell operates in PWM mode witahd dtieme generation. For eex,a imf ptlhe pinut voltage I N Vis 133
Figure 4-13. I -N >V0 ⇒ SC 2 short-circuited, _ SC _1 PWM operated
Chapter 4 Single-phase STATCOM wiVt hI G3.C3Tk based Step-Down PWM AC Choppers positive, the switching c _ e 2 l li sS sChort-circuited tahned switching cel _ l 1 SoCperates in PWM, Figure 4-13. Ohnetother hand, wh I e N ni s Vnegative, _ S 1 Cis short-circuited an _ d 2 oSpCerates in PWM, Figure 4-14. This switching strategy rtehsep uencitdsi rectional voltage characteristic of the switching cells, limiting the voltage of tplhien gd eccaopuacitors to ctohrer esponding half cycle voltage of the input source. During t h ael f octyhcelre , the switching cell voltage is zero.
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Chapter 4 Single-phase STATCOM wiVt hI G3.C3Tk based Step-Down PWM AC Choppers
the output fundamentqaul efnrecy and the DC link voltacgoen sitsa nt, the duty cycle evolution has the same sinusoidal evolaust itohne desirepdh ase voltagFei,g ure 4-16. In the step-down PWM AC chopper, the same principle cani ebde. aAsp ptlhe input voltage already has the desired output voltage wavefotrhme a nvde rage output voltage is proportional to the duty cycle, the duty cycle is changed exclusively ttoh em RodMifSy amplitud et hoef output voltaFgige,u re 4-17.
v IN = V DC ;VVn ( ) 〈 v I v NOU = T 〉 V I = N ⋅ Vs O in UT ϖ⋅⋅ sti ) n; ϖ ⋅ t ; 〈 v OUT 〉 = D 2 C + OUT × si ϖ × t ; ( ) 〈 〉 ct ; v OUT = 1 + VV OUT × sin (ϖ × t ) = α (t); 〈〈 vv OINUT 〉〉= VV OINUT = α (t) = e 〈 v IN 〉 2 DC Figure 4-16.- Steady-state duty cycle for a FVigSuI re 4-17.- Steady-state duty cycle for a single-phase leg phase step-down PWM AC Chopper From a dynamic control point of viecwa,l lcilnaessair averaging modelling techniques for PWM DC/DC converters have been successfullyt oa pspeliv e dral PWM AC Chopper topologies for different applicati[oVnEs,N -97b[],F ED-01]. In these applica t ihoen sA, C Choppers are designed to deal with low power values at hignhg sfwrietqcuhiencies, aimincgo tpoe with the modelling inaccuracies introduced by the assumtphtieosnes toefc hniques w h e ny tare applied to PWM AC Chopper[s,K OR-01]. After averaging the model, a small-signiasl ombotadienl ed by linearisation of the averaged system equations around a stable operation spomianltl.- sTighne al model is valid only for small deviations around the operating point. HoweAvCe rC, hPoWppM ers are excited with large levels of AC quantities and the effectiveness o df etllhiinsg mtechnique is reduced significantly unless the sinusoidal voltage change is taken into account. Another parameter that hsaisg naif icant influence on tlhidei tyv aof the averaged model techniques, even for DC/DC convertersc, oins vtehrtee r switching frequency. If the switching frequency is relatively low acnudr rtehnet and voltage switcphipnlegs rion the converter passive components are relatively high, thien gs wfritecqhuency terms can not be neglected and have influence on the average models. Severalg sm o lduteiloli n s have been developed dealing with the influence of the switching freaqnude ntchye switching ripple ef[fKeRctEs-,9 0][, LEH-96a], [LEH-96b],[ VER-99]. Most of the proposedt iosnos luuse complicated mathematical representations of the s,y sptreomviding a better approxni mofa titohe converter behaviour reconstruction. However, their applicantieoanr fc o rnltirollers’ implemioe n tisa tvery restricted and they are mainly used to develop acmc u lraatitoe ns imodels with reduced computation time. The non-linear characteroispteirca tion of the AC Choppeurs (osiidnal input voltage), together with the additional non-lineair obuer h a sv with any other pcoownevrerter (dead times, adapted switching patterns, etc), complicate thieo na popfl ilciantear contsrtorla tegies. Instantaneous compensation of the AC Ch’os ppneorn-linearity or use oonf- li n ear control techniques (hysteresis controllers) can provide trheed rdeyqnuaimic performaonf cteh e AC Choppers.
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Figure 4-18.- Hybrid reactive power compensation system, ( p -a sascitvive e6 3MMVVAARR) The STATCOM solution presented her e di s obna s ingle-phase step-down PWM AC Choppers using 3.3kV IGCTs. The required 3MVAR rpeoawcteirv ecompensation cilaitpya ibs obtained by arranging three 1MVAR modules in parallFeilg (unre= 34)-,1 9. The modules are connected to a single input LC filt F e-rC F ()L on the secondary side of ea -spihnaglse step-down transformer. The secondary nominal volta gthe e otfransformer N i E s T =V1.06k R V MS (m C ≈ 23.6), which imposes a maximum nominal voltage value of 1.5kVt oa tdhaep tIeGdC T ratings. The output load of each PWM AC Chopper consiosft sa nother LC circu S i-tC( V )L with capacitive behaviour at the network frequency to provide the desired react icvoe mppoewnesration function. The smoothing inductor L S is required to provide a current source bfeorh athvieo luorad and to limit the switching current ripple. To guarantee the baloapnecreadt ion of the 3 modulehs , meoacdule has its own control loop and receives the same compensation reference.
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