Advanced Control of Active Filters. in a Battery Charger Application. Martin Bojrup


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1 Advanced Contol of Active Filtes in a Battey Chage Application Matin Bojup Lund 999
2 ii Cove pictue Measuement on the dynamic esponse of the MRI hamonic filte contolle: load cuent (top), esulting line cuent (middle), and injected active filte cuent (bottom), see futhe page. Depatment of Industial Electical Engineeing and Automation (IEA) Lund Institute of Technology (LTH) P.O. Box 8 S 22 LUND SWEDEN ISBN CODEN:LUTEDX/(TEIE2)/24/(999) Copyight 999 Matin Bojup Pinted in Sweden by Univesitetstyckeiet, Lund Univesity Lund 999
3 Abstact In this thesis, a high pefomance battey chage fo electic vehicles (EVs) is investigated. By including active powe line conditioning capabilities in the battey chage, a viable concept fo a fast chaging infastuctue is obtained, beneficial both to the EV uses and the powe distibutos. The thesis contains discussions on modelling aspects and design consideations fo the poposed battey chage, based on a caie wave modulated self commutated 2level voltage souce convete topology. Futhemoe, model based contolle synthesis is employed, and thoough analysis of the contolle chaacteistics is given. Emphasis is put on the active filteing pefomance of the battey chage. The weaknesses of the model based contol system in active filte applications ae evealed, whee especially the inheent phase deviation of the contol system and the sensitivity to system paametes deteioates the pefomance of the active filte. In ode to ovecome the deteioating popeties of the model based contolle, a contolle stuctue fo active filtes based on seveal integatos in multiple efeence fames is poposed. It is shown, both theoetically and expeimentally, that the poposed contolle exhibits low sensitivity to system paametes and povides fo complete compensation of the inheent phase deviation. The esult is excellent conditioning pefomance of the poposed active filte contolle in steady state.
4 iv Abstact
5 Acknowledgements Fist of all, I would like to expess my sincee gatitude to Pe Kalsson fo invaluable discussions duing this poject and a lot of laughs. He has poven to be the pefect fiend, colleague and tavelling companion. I would also like to thank my supeviso Pofesso Mats Alaküla fo his encouagement and suppot duing this wok. I am also gateful to Pofesso Las Getma fo initiating the eseach poject and fo fuitful discussions and comments along the way. The suppot fom the extenal membes of the efeence goup, Andes Lasson (Volvo Technological Development) and Ulf Thoén (Sydkaft AB), has been geatly appeciated. I wish to thank all my colleagues and the staff at IEA fo poviding a ceative and stimulating atmosphee, and in paticula to the lunch paty, to D. Ulf Jeppsson fo maintaining the UNIX system, to Getachew Dage fo maintaining the wokshop and to Bengt Simonsson fo his helping hands in the laboatoy. I would like to thank my immediate family fo thei suppot duing this time. I am also indebted to Kain fo he love and suppot. This wok has been financially suppoted by Elfosk, within the Elekta eseach pogam, which is gatefully acknowledged. Matin Bojup
6 vi Acknowledgements
7 Contents Intoduction. Backgound....2 Chaging Fast chaging Influence on the powe gid Desied capabilities of the chage infastuctue Thesis outline Gid conditioning 7 2. Reactive powe compensation Voltage contol Load balancing Hamonic eduction in distibution netwoks Design and modelling of the battey chage 9 3. System desciption Convetes DClink Line filte Battey filte Taget system specification Contol Oveall contol Battey cuent contol Line cuent contol DClink voltage contol... 57
8 viii Contents 5 Active filte contol 6 5. Identification based on the load cuent Oiginal active filte contolle Diect phase compensation of the active filte contolle Rotating integatos in active filte contol Expeiments 9 6. Expeimental setup Cuent contol veification Step esponse Chaging of battey equivalent Load balancing of a twophase load Hamonic filteing of a diode ectifie Dynamics of the MRI contolle... 7 Conclusions 3 Refeences 5 A Vecto notation A. Thee phase to two phase tansfomation... A.2 Vecto tansfomation...2 A.3 Instantaneous powe...3 B Nomalisation 5 B. Pe unit definition...5
9 Intoduction This chapte povides an intoduction to the thesis and points out the main objectives with the wok caied out leading to this thesis.. Backgound The Electic Vehicle (EV) has lately, duing the past yeas, been focus fo thoough eseach whee the majo concen is to find a eplacement fo the Intenal Combustion Engine (ICE) diven vehicle. Envionmental aspects such as the ai pollution due to the emissions of the ICE ae the key diving foces fo the eseach in altenatives to the ICE. Anothe fact is the limited oil esevois, which in the futue inevitable will be depleted. The temendous development in the aea of powe electonics in tems of size, ating and fequency chaacteistics togethe with adapted electical motos make high pefomance EV dives possible. The pefomance chaacteistics, i.e. top speed and acceleation capabilities, of the high pefomance EV dives ae simila o supeio to the ones obtained by an ICE [6]. The majo shot coming of the pesent EV oiginates fom the limited enegy stoage onboad the vehicle, which educes the obtainable diving ange compaed to an ICE. High powe/enegy density stoage of electic enegy is had to achieve and expensive. But impovement in battey technology fom leadacid to NiCd o NiMH has consideable inceased the stoage capacity. The diving ange of pesent EVs is appoximately 7 km pe one full chage, though dependent on the dive. In ode to incease the diving ange of the EV, onboad chaging systems such as ICE diven geneatos o fuel cells can be included. This gives highe feedom in the opeation of the ICE. The ICE can be opeated optimally with espect to emissions, efficiency o fuel consumption leading to impoved oveall pefomance [3]. Acceleation and peak powe ae then povided by the electical supply. Fuel cells, on the othe hand, povides electical powe though electochemical
10 2. Intoduction eaction. Fuel cells have high powe/enegy density and the electochemical eaction is almost emission fee, but they ae expensive and still lot of development has to be done [28]. To futhe povide a wide takeup of EVs, a public chaging infastuctue is needed [3]. Such a infastuctue would incease the mobility of the EV dive at least if fast chaging, i.e. chaging with high powe, is an altenative. The diving ange in between chaging instants would be the same, but fast chaging educes the chaging time consideably. Theefoe, an EV dive may accomplish a 23 times longe daily diving ange. The topic fo this thesis is to descibe how such a fast chaging station could be implemented in ode to obtain the best use of the investment..2 Chaging Fast chaging Chaging of a battey is accomplished by the supply of a DC cuent to the battey. The DC cuent povides electical chages that ae stoed in the battey though an electochemical pocess. Since cuent is defined as the tanspot of electical chages pe time, the enegy deliveed duing the chaging pocess is detemined by the amount of DC cuent supplied and the elapsed time. Electical enegy is tansfeed fom geneato to consume in AC quantities. These AC quantities have to be conveted into DC quantities in a contollable manne suited to the voltage/cuent levels of the battey. This is the task fo the battey chage. Battey chages ae gouped in onboad and offboad chages. An onboad chage is individual to the EV and located intenally in the vehicle, while the offboad chage is of extenal and stand alone type. Offboad chages ae typically located at a chaging station o at paking aeas. The pinciple of opeation is the same but the onboad chage pefoms the AC to DC convesion inside the vehicle, thus the vehicle is chaged fom an AC supply. The offboad chage, on the contay, acts as a DC supply whee the AC to DC convesion is made elsewhee fom the vehicle point of view. Both onboad and offboad chages can opeate fom single o thee phase AC supplies. Onboad chages ae mainly of single phase type, since weight and size ae cucial in vehicle applications. Howeve, a theephase chage can povide thee times the chaging powe of a single phase chage, since all thee phases can be loaded as much as the single phase chage loads one phase. Theefoe, the chaging time is educed by
11 . Intoduction 3 a thid. Table. lists pefomance chaacteistics of diffeent types of chages, when chaging an EV coesponding to a diving distance of km. The enegy consumption of the EV has been estimated to 2 kwh pe km in the compaison. The coesponding efuelling time fo ICE vehicle is also given in Table.. Table. Chaging powe and time coesponding to a diving distance of km. Fuse / Phases Powe Time Type 6 A 3 kw 7 h Onboad 6 A 3 kw 2 h Offboad 2 A 3 75 kw 5 min Fast chaging station /2 min Gas station Offboad chages opeate eithe in a conductive o inductive manne. Conductive chages supply the electical enegy though conductos all the way fom the DC output of the chage to the teminals of the battey. Inductive chages, on the othe hand, have a galvanic isolated intemediate stage, i.e. a high fequency tansfome, between the chage DC output and the battey. The pimay winding of the high fequency tansfome is pefeably located in the connecto handle wheeas the seconday winding is located inside the EV [9][29], thus poviding galvanic isolation thoughout the chaging pocess. Howeve, the losses in the high fequency tansfome can educe the enegy efficiency of the chaging pocess. A majo poblem associated with chaging of EV batteies is the lack of standadisation, since almost evey ca manufactue has thei own idea of how it should be done. Due to this, both types of chages ae focus fo development and eseach [5][2]. Heeafte, only thee phase conductive chages ae investigated..3 Influence on the powe gid An intoduction of EVs will have an impact on the powe gid and its opeation. Typically, onboad chages will be used duing night time when the ca is paked at a esidence. Usually, at night time the load on the powe gid is low esulting in high voltage, which causes the powe gid opeatos to disconnect capacito banks and even inset eactos in ode to maintain the voltage magnitude. In this sense, EV chaging at night can even impove opeation of the powe gid, since the load is smoothened out ove the day. Fast chaging and maintenance chaging at ca paks, on the contay, ae used mainly duing day time in ode to
12 4. Intoduction incease the opeating ange of the vehicle. At day time the load on the powe gid is high and an incease due to fast chaging can cause oveload situations [2][3]. EV chages will contibute to the incease of powe electonic loads on the powe gid, since powe electonics is necessay in the AC to DC convesion of electical enegy. Powe electonic loads cause, due to thei nonlinea chaacteistics, distoted o nonsinusoidal cuents on the powe gid. The distoted cuents cause voltage distosion due to self impedance in cables and tansfomes. Voltage distosion leads to ageing and even to damage and malfunction of sensitive electical equipment, i.e. computes. Othe hamonic cuent geneating electical equipment ae switched mode powe supplies (SMPS) in computes and television sets, adjustable speed dives (ASD) and fluoescent lights. Cuent and voltage distotion ae consideed as a poblem in the opeation of the powe gid, despite difficulties in detemining the eal consequences of thei pesence. Anyway, the poblem with hamonics have been ecognised and standads fo electical equipment, in tems of hamonic emission and immunity, ae being established. As a consequence heeby, fast chages based on line commutated convetes, i.e. diode o thyisto bidge ectifies, though cheap and elatively simple should not be used, since they do not fulfil these standads on hamonic emission into the powe gid. This is concluded fom Figue., whee a typical nonsinusoidal line cuent fom a line commutated chage togethe with its hamonic content ae given Figue. t [ms] h h I h /I [%] IEC 34 I h /I [%] Left) Line cuent fom a thyisto based chage and its hamonic content. Right) Compaison of the cuent hamonics against the IEC 34 egulation on cuent hamonic emissions.
13 . Intoduction 5.4 Desied capabilities of the chage infastuctue A fast chage should be capable of chaging EVs with a gidfiendly cuent, i.e. sinusoidal line cuent with unity powe facto. It should also be flexible enough to handle vaious battey types and sizes, with thei individual voltage levels and cuent ating. In ode to incease the usage of a fast chage infastuctue, additional capabilities in tems of powe gid conditioning can be included. This means that instead of having a negative impact on the powe gid, a chage infastuctue may povide a tool fo powe gid opeatos to maintain, at least locally, a high powe quality. The nonideal, i.e. eactive, hamonic and asymmetic pats of the load cuent should be geneated locally by the gid conditioning chage. The powe gid supply would then only have to povide the active powe of the load cuent. Figue.2 descibes schematically the opeation pinciple of such a gid conditioning chage. In this way, the load on distibution lines and tansfomes will be loweed and a moe efficient supply of electical enegy obtained. Electical supply P supply P load S load Electical loads P chaging S conditioning Battey chage + Gid conditione Figue.2 Schematic diagam of chage with gid conditioning capabilities. Anothe, moe contovesial capability that can be included in a fast chage infastuctue deals with peak powe geneation, i.e. active powe suppot at high load. EV batteies attached to fast chages may be consideed as a potential enegy eseve. Duing shot peiods of time, enegy could be boowed fom the batteies and supplied to the powe gid. This means that the fast chage opeates as an electonic gas tubine. This last capability has to be applied with geat cae, since the pimay function of a fast chage has to be to chage EV batteies and not the opposite.
14 6. Intoduction Summaising this discussion leads to a numbe of capabilities desied fo a fast chage infastuctue: Chaging of EVs with sinusoidal line cuent and unity powe facto. Gid conditioning capabilities such as eactive powe compensation, hamonic filteing and load balancing. Possibility to act as an electonic gas tubine, i.e. povide active powe suppot duing shot peiods of time by boowing enegy fom the EV batteies..5 Thesis outline Chapte povides a shot intoduction of the aim of the poject leading to this thesis. In Chapte 2, basic gid conditioning concepts ae investigated and tools to accomplish gid conditioning ae pesented. Chapte 3 descibes implementation, design and modelling consideations fo the poposed fast chage. In Chapte 4, the basic contol stuctues and model based contolles fo a fast chage ae given. In Chapte 5 specific active filte contol stuctues ae thooughly investigated. In Chapte 6 expeimental esults of a chage pototype ae pesented, veifying the poposed concept fo fast chages. Finally in Chapte 7 conclusion emaks ae given. Note that this thesis is one of two, evolved fom the battey chage poject. The accompanying thesis [8], teats hadwae elated aspects of the battey chage, while this thesis focuses on contol and functional aspects.
15 2 Gid conditioning This chapte descibes the intention of powe gid conditioning, its oigin and how it may be done. The powe gid connects the poduces of electical enegy with the consumes, making it possible to tansfe powe fom the centalised geneatos to the distibuted consumes. Theefoe, it is desied that the voltage magnitude and fequency at the consume end ae close to thei nominal values and that the amount of voltage distotion is low. The powe gid consists of two pats, the tansmission netwok and the distibution netwok. The tansmission netwok consists of a meshed gid connecting diffeent egions, it has high voltage and is theefoe capable of high powe tansfe with low losses. The distibution netwok, on the contay, consists of adial feedes inside a egion whee the powe flows downsteam fom substations to consumes. The adial configuation in combination with the self impedance of the conductos causes load cuent dependent voltage dops and at the end of a feede all upsteam load contibutions to the voltage distosion is seen. Anothe poblem is that esonance between conductos and capacito banks o loads can be enegised by distoted cuents which also causes voltage distotion. Electic enegy tansfe though the powe gid is instantaneous, i.e. thee is no stoage of electical enegy. Theefoe, fequency contol is obtained by matching the active powe poduction in the geneatos with the powe used at the consume end, including the losses in the distibution and tansmission netwoks. If this is not achieved, the excess o the deficit of electical powe poduced will acceleate o deceleate the geneatos esulting in a fequency vaiation. 2. Reactive powe compensation Voltage contol Reactive powe compensation is equivalent to voltage contol, that is by contolling the eactive powe flow in a netwok substantial contol of the voltage magnitude is achieved.
16 8 2. Gid conditioning Reactive powe Q in contast to the active powe P can not be conveted into mechanical wok, i.e. kinetic enegy o heat etc. Instead, the eactive powe epesents an enegy oscillation between load and geneato. Still eactive powe is a necessay quantity in electical engineeing, since it is used in electical motos to build up the magnetic flux used to convet electical enegy into mechanical wok. Reactive powe is a steadystate popety and defined as the imaginay pat of the complex valued appaent powe S * S = U I = P+ jq (2.) Poblem Losses and lage distance tansfe Conside a loss less single distibution line with eactance X l connecting a stiff (efeence) bus to a load bus as in Figue 2.. E P X l V δ Q Figue 2. Powe flow in a single line with eactance X l. The tansfe of active and eactive powe, fom the stiff bus, ove the line ae stongly coupled and given by P E = V sin δ X l Q = E X E V cosδ l ( ) (2.2) whee E is the voltage at the stiff bus, V is the voltage at the load bus and δ is the load angle, i.e. the angle diffeence between the voltages at the stiff and load bus []. Fom (2.2) it is found that lage amount of eactive powe tansfe esults in a significant voltage dop ove the line. This is due to the dependency between the load angle δ and the active powe tansfe. It is also seen that a weak line, i.e. lage X l, educes the powe tansfe capability. The eactive cuent contibutes to the msvalue of the supply cuent, thus esulting in a highe supply cuent than needed in ode to delive the active powe to the load. This affects the efficiency of the powe gid since in eality lines ae not loss less, thee exist a small esistance in addition to the eactance fo evey conducto. The losses in the conductos ae popotional to the squae of the cuent, theefoe it is
17 2. Gid conditioning 9 obvious that also active as well as eactive losses incease with a lage amount of eactive powe tansfe. Solution Local poduction Local poduction of eactive powe at a load bus deceases the amount of eactive powe tansfe ove the line. Reducing the msvalue of the supply cuent to only coespond to the active powe demand at the load bus. Theefoe, the losses and the voltage dop acoss the line ae deceased and a moe efficient powe gid is obtained. As a consequence, the voltage at the load bus may be contolled close to the nominal value by contol of the eactive powe input. Shunt capacito banks povide a simple way of eactive powe compensation, see Figue 2.2. In a capacito excited by a AC voltage the cuent leads the voltage by 9 in phaso notation, this phase lead coesponds to a injection of eactive cuent and thus poduction of eactive powe. Shunt capacito banks ae common in distibution netwoks, whee they ae connected and disconnected accoding to the load fluctuation ove the day in ode to maintain a voltage magnitude close to the nominal. X l Q X l Q X l X C Figue 2.2 Reactive powe compensation; shunt (left), TCR (middle) and seies (ight). A moe sophisticated solution includes a thyistocontolled eacto (TCR) in paallel with the shunt capacitos. Such a device may even absob eactive powe duing light load condition in addition to the poduction of eactive powe. This combination can be teated as a capacito bank with vaiable and even negative capacitance. Synchonous condenses, i.e. synchonous motos with no mechanical load, can also contol thei eactive powe output to povide eactive powe compensation. Anothe fom of eactive powe compensation involves seies capacitos, whee capacito banks ae installed in seies with the line. Thei effect is to educe the total seies impedance of the line seen fom the souce, theeby educing line voltage dops and inceasing the line loadability. Lately, even moe advanced compensato stuctues, whee powe electonics ae included, have been poposed. The unified powe flow
18 2. Gid conditioning contolle (UPFC) consists of two convetes connected back to back, one in seies with the line and the othe one in paallel [8]. The UPFC ae intended fo use on tansmission systems and is vey flexible. It can pefom shunt eactive compensation as well as voltage magnitude and load angle contol and line impedance compensation. 2.2 Load balancing A balanced powe gid is obtained if the thee phases ae equally loaded, i.e. each phase tansfes one thid of the total powe. This gives the best utilisation of the powe gid, since duing balanced conditions the phases ae affected in the same way. Electical loads can be eithe one, two o thee phase loads but since a line feeds seveal loads, the total load seen fom the souce is balanced if a lage numbe of loads ae distibuted to the diffeent phase conductos. Hence fo the tansmission netwok and the highe voltage distibution netwoks the balanced condition holds tue. But close to the consumes, the load may be imbalanced due to the low numbe of loads to be distibuted. Imbalanced loads o systems may be analysed in steady state by the use of symmetical components []. In accodance with symmetical components, any balanced o imbalanced thee phase quantity ae esolved into thee sequence components, that is positive, negative and zeosequence components. The positivesequence component consists of thee phasos with equal magnitude and 2 phase displacement in positive sequence, i.e. in abc sequence, efe to Equation (A.5). The negativesequence component also consists of thee phasos with equal magnitude and 2 phase displacement howeve in negative sequence, i.e. in acb sequence, while the zeosequence component have zeo phase displacement. Figue 2.3 shows an example of an abitay unsymmetical cuent sepaated into sequence components. I pc I pa I nb I na I za I zb I zc I b I c I a I nc I pb a) b) c) d) Figue 2.3 Sequences of an abitaily unsymmetical cuent, a) positivesequence, b) negativesequence, c) zeosequence and d) total cuent.
19 2. Gid conditioning In Appendix A., the definition of the thee phase to two phase vecto tansfomation (abc αβ) is given. A balanced system with positivesequence, abc sequence, and msvalue I can be epesented as a vecto in the stationay efeence αβfame as αβ jω t i = 3Ie (2.3) p while fo a balanced system with negativesequence, acb sequence, the vecto is accoding to αβ jω t i = 3Ie (2.4) n Positive and negativesequence components can thus be sepaated by the otation diection of the vecto in the stationay efeence αβfame, see Figue 2.4. Applying the same tansfomation on a balanced system with zeosequence gives iz αβ = (2.5) This means that zeosequence components ae not epesented in the stationay efeence αβfame. β s β s ω t s β β s ω t s α α s α α Figue 2. 4 Rotation diection of vectos in the stationay αβfame, positivesequence (left) and negativesequence (ight). In ode to obtain a vecto tansfomation that is valid also fo zeosequence components, the given tansfomation in Appendix A. can be modified accoding to [25] whee an abc αβ vecto tansfomation is used. Poblem Neutal cuents and voltage distosion Imbalanced loads cause a distoted cuent distibution among the phase conductos as well as cuent in the neutal conducto. This leads to highe losses in the phase conductos than fo a balanced load and may cause oveload of a phase conducto. Neutal cuents can esult in ciculating cuents in tansfomes and additional losses in the windings and in the neutal conducto.
20 2 2. Gid conditioning Anothe aspect of imbalance cuents and especially neutal cuents is the effect on the voltages at the load due to the self impedance in the conductos. The voltage dop in the neutal conducto affects the vitual zeo potential at the load, esulting in an imbalance of the thee phase voltages in tems of magnitude and phase displacement. This may cause ove and/o undevoltage in some of the phases, which in tun may cause malfunction o destuction of the electical equipment connected to these phases. Solution Rediection of load cuents The most obvious solution to load imbalances is to physically connect simila loads to diffeent phase conductos. Load balancing may also be obtained by ediection of load cuents by a symmetisation unit, see Figue 2.5. The aveage powe used in the load mainly coesponds to the positivesequence of the load cuent due to the balanced supply voltage of positivesequence. Theefoe, the negative and/o zeosequence components of the load cuent may be geneated locally and thus eliminating those components fom the supply cuent. The esulting supply cuent would only contain the positivesequence component of the load cuent and the unsymmetical load is seen as an equivalent symmetical load fom the supply point of view. A B C 2I I 6 I 6 3I N 3I I 6 I 6 I A B C I I 2 I 2 3I 3I N 2I 2 I 2 I 3I 3I Neg. seq. symmetisation Neg. and Zeo seq. symmetisation Figue 2.5 Load balancing pinciple, negativesequence compensation (left), and negative and zeosequence compensation (ight). In Figue 2.5, the basic opeation pinciples of such symmetisation units ae shown. In the left figue, the negativesequence cuent of the load is povided by the compensato, thus esulting in only zeo and positivesequence supply cuents. Total symmetisation is obtained in the ight figue by injection of the negative and zeosequence cuents, esulting in a balanced supply cuent of positivesequence and no neutal cuent.
21 2. Gid conditioning Hamonic eduction in distibution netwoks Electical equipment made of powe electonics connected to the gid causes, due to thei nonlinea chaacteistics, nonsinusoidal gid cuents. Accoding to the fouie analysis, evey peiodic wavefom can be teated as the sum of one o seveal sinusoidal wavefoms of diffeent fequencies, i.e. the fundamental fequency and multiples of the fundamental fequency. Since the gid voltage is nealy sinusoidal, the powe used by a hamonic geneating load coesponds to the fundamental fequency component of the load cuent. The cuent hamonics ae a nondesied bypoduct due to the switched opeation of line commutated ectifies, i.e. diode o thyisto contolled ectifies. Cuent hamonics ae in fact a steadystate poblem due to thei peiodic and epetitive behaviou. Howeve, cuent tansients oiginating fom inush cuents at stat up o at distinctive shut downs o disconnection of loads can also be teated, in accodance to the fouie analysis, as cuent hamonics despite thei lack of peiodicity. Still, tansients and thei coesponding cuent hamonics will disappea afte a sufficiently long time. A typical load cuent and the coesponding hamonic spectum of a line commutated thee phase ectifie can be seen in Figue.. The cuent contains the fundamental fequency and hamonics of ode 6k± whee k is any positive intege. The cuent hamonics can futhe be esolved into sequence components, i.e. positive and negativesequences, accoding to Table 2.. The absence of zeosequence hamonics is due to the balanced configuation of thee phase line commutated ectifies. Howeve, single phase line commutated ectifies have additional zeosequence hamonics of ode 6k+3 whee k = [,,2 ]. Table 2. Hamonic sequences of thee phase line commutated ectifies. Sequence Hamonics Positive, 7, 3, 9, Negative 5,, 7, 23, The load cuent of a line commutated ectifie can be epesented in the stationay efeence αβfame as αβ j( ωt+ φ) jh ( ωt+ φh) jh ( ωt+ φh) i = 3I e + 3I e + 3I e (2.6) Load h h= 6k h= 6k+ h
22 4 2. Gid conditioning whee I and I h ae the msvalues of the fundamental cuent component and the individual cuent hamonics, espectively. Equation (2.6) gives a tue epesentation of the cuent of thee phase line commutated ectifies, wheeas fo single phase ectifies the cuent hamonics of zeosequence ae absent. This is due to the vecto tansfomation used, efe to (2.5). Poblem Losses and voltage distotion Cuent hamonics esult in a distoted voltage, which may cause malfunction of sensitive electical equipment, i.e. computes. The voltage distosion is due to the voltage dop acoss the nonzeo impedance of cables and tansfomes. The cuent hamonics can also cause sevee voltage distosion due to esonance oscillations between the self inductance in the distibution netwok and the shunt capacito banks fo eactive powe compensation [3][4]. Cuent hamonics cause additional losses in the distibution netwok. Tansfomes ae exposed to themal stesses due to an incease in ion losses caused by the distoted voltages and cuents. This may lead to deating as well as pematue ageing of the tansfomes. Fo electical motos, the voltage distosion causes themal stesses and toque ipple. Solution Hamonic filteing Cuent hamonics ae often teated as a local poblem at least fo one feede in the distibution netwok. The impedance of the distibution netwok dampens the hamonic popagation [4]. Theefoe, hamonic filteing should be pefomed neaby the souce of the cuent hamonics fo the best esult. If this is done, othe equipment will be unaffected by the hamonic poducing load. Hamonic filteing o compensation is accomplished by the use of passive filtes, shunt active filtes (AF) o seies active filtes. Passive filtes Passive filtes fo hamonic eduction povide low impedance paths fo the cuent hamonics. The cuent hamonics flow into the shunt filtes instead of back to the supply. A typical shunt passive filte and the esulting equivalent impedance seen fom the load ae shown in Figue 2.6. The passive filte consists of seies LC filtes tuned fo specific hamonics in combination with a geneal high pass filte used to eliminate the est of the highe ode cuent hamonics.
23 2. Gid conditioning 5 Gid C 5 C 7 C hp Load L 5 R 5 L 7 R 7 L hp R hp 5th ham. filte 7th ham. filte High pass filte Impedance Z Figue Hamonic Typical passive filte fo eduction of cuent hamonics and the equivalent impedance seen fom the load. The pefomance of a passive filte is stongly dependent on the system impedance at the hamonic fequencies [27]. The system impedance depends on the distibution netwok configuation and the loads. Theefoe, design of passive filtes involves thoough system analysis in ode to obtain adequate filteing pefomance of the filte. Shunt active filtes A shunt active filte consists of a contollable voltage o cuent souce. The voltage souce convete (VSC) based shunt AF is by fa the most common type used today, due to its well known topology and staight fowad installation pocedue, see Figue 2.7. It consists of a DClink capacito C dc, powe electonic switches and filte inductos L f. Gid i Line i AF i Load Load Figue 2.7 L f C dc VSC Pinciple configuation of a VSC based shunt active filte. The opeation of shunt AFs is based on injection of cuent hamonics i AF in phase with the load cuent i Load hamonics, thus eliminating the hamonic content of the line (supply) cuent i Line. That is, suppose the load cuent can be witten as the sum of the fundamental cuent component and the cuent hamonics accoding to
24 6 2. Gid conditioning iload = iload, fund + iload, hamonics (2.7) then the injected cuent by the shunt AF should be i = i, (2.8) AF Load hamonics The esulting line cuent is iline = iload iaf = iload, fund (2.9) which only contains the fundamental component of the load cuent and thus fee fom hamonics. Figue 2.8 shows the ideal phase cuents when the shunt AF pefoms hamonic filteing of a diode ectifie. The injected shunt AF cuent completely cancels the cuent hamonics fom the load, esulting in a hamonic fee line cuent. i Load  i AF  i Line Figue 2.8 t [ms] Shunt AF opeation pinciple when pefoming hamonic filteing. Fom the load cuent point of view, the shunt AF can be egaded as an vaying shunt impedance. The impedance is zeo, o at least small, fo the hamonic fequencies and infinite in tems of the fundamental fequency. Seies active filtes Seies active filtes ae connected in seies with the line though a matching tansfome. VSCs ae appopiate as the contolled souce even fo seies AFs, thus the pinciple configuation of seies AFs ae simila
25 2. Gid conditioning 7 to shunt AFs. The filte inductos of shunt AFs ae eplaced with the seies tansfome, see Figue 2.9. Gid i Line v AF i Load Load Figue 2.9 VSC C dc Pinciple configuation of a seies AF system. The opeation pinciple of seies AFs is based on isolation of the hamonics in between the load and the supply [4]. This is obtained by the injection of hamonic voltages acoss the tansfome line side. The injected hamonic voltages affect the equivalent line impedance seen fom the load. The equivalent line impedance can be egaded as infinite o lage fo the hamonics wheeas it should ideally be zeo fo the fundamental fequency component. The opeation pinciple of a seies AF is illustated in Figue 2.. i S i Sh Z eq = i Lh Z S v S v AF i L i Sfund Z eq = i Lfund i AF Z AF Z Sfund i AFfund Z AF v Sfund Z Sh v Sh Z AF i AFh a) b) c) Figue 2. Seies AF opeation pinciple; a) single phase equivalent of seies AF, b) fundamental equivalent cicuit, and c) hamonic equivalent cicuit. Hamonic isolation is achieved by means of the infinite impedance fo the cuent hamonics in seies with the line. That is, no cuent hamonics can flow fom load to souce o fom souce to load. Cuent hamonics poduced elsewhee on the souce side of the seies AF can not intefee with loads on the load side, i.e. passive filtes and shunt capacitos. Hybid filtes Hybid filtes consists of combinations of shunt/seies AFs and shunt passive filtes. The main pupose of hybid filtes ae to educe the initial costs and to impove efficiency. The passive filtes educes the bulk hamonic content of the load cuent, wheeas the active filte handles the est of the hamonic content not filteed by the passive filtes. Theefoe, the ating of the active filte can be deceased compaed to a stand alone active filte and thus educing the initial cost. Figue 2. shows two
26 8 2. Gid conditioning examples of hybid filte configuations, additional configuations have been epoted in the liteatue [3]. Load Load Shunt AF Shunt Passive Filte Seies AF Shunt Passive Filte Figue 2. Hybid filtes; left) combination of shunt AF and shunt passive filte, and ight) combination of seies AF and shunt passive filte. The optimal solution to hamonic eduction in distibution netwoks in tems of pefomance chaacteistics consists of the combination of a shunt and a seies AF with a common DClink, see Figue 2.2. This combination is efeed to as the unified powe quality conditione (UPQC) and has the same configuation as the UPFC, efe to Section 2. page. Howeve, the UPQC fo distibution netwoks diffe fom the UPFC fo tansmission netwoks in tems of opeation, pupose and contol stategy [3]. The conditioning functions of the UPQC ae shaed by the seies and shunt AFs. The seies AF pefoms hamonic isolation between supply and load, voltage egulation and voltage flicke/imbalance compensation. The shunt AF pefoms hamonic cuent filteing and negative sequence balancing as well as egulation of the DClink voltage shaed by the AFs. The DClink voltage egulation is obtained by balancing the active powe fom both the VSCs including all losses in the passive as well as the active components. i Line v AF i AF i Load Load Seies AF Shunt AF Figue 2.2 Configuation of the unified powe quality conditione (UPQC).
27 3 Design and modelling of the battey chage In this chapte, the hadwae topology of the investigated chage is discussed. Design consideations and modelling aspects of the chage ae given. The topology selection fo the battey chage is based on the desied capabilities fo a fast chage infastuctue deived in Section.4: Conductive chaging of EVs with sinusoidal line cuent. Gid conditioning capabilities such as: Hamonic filteing Load balancing Reactive powe compensation Possibility to act as an electonic gas tubine. These equiements and estictions indicate the type of powe electonics to use. Line commutated convete topologies can not be used, due to thei nonsinusoidal line cuent. The second point hints the usage of an active filte topology as the line side inteface of the battey chage. Finally, the thid capability is accomplished if the battey chage is designed fo bidiectional powe flow, i.e. the powe can flow in eithe diection between the powe gid and the EV batteies. 3. System desciption The selected battey chage topology able to cay out the listed capabilities is shown in Figue 3., whee the point of common coupling (PCC) towads the powe gid and an EV battey attached to the chage ae also shown. The battey chage consists of a DClink capacito C dc, a thee phase VSC connected back to back to a half bidge convete, and passive filtes.
28 2 3. Design and modelling L 2 L V dc C dc L L 2 PCC C C Vbatt Figue 3. The selected battey chage topology. The passive filtes ae, due to the switched opeation of the VSCs, essential components of the battey chage topology. The output voltage pulses of the VSCs ae filteed by the passive filtes, educing the high fequency cuent components. Hence, sinusoidal line cuents as well as a low ipple battey cuent ae obtained. The passive filtes shown in Figue 3. consist of thid ode LCLfiltes instead of odinay fist ode Lfiltes. The easons ae discussed in Sections 3.4 and 3.5. At the moment, it is sufficient to say that the high fequency as well as the low fequency chaacteistics of the LCLfilte ae beneficial compaed to the Lfilte configuation, i.e. the equivalent impedance is high fo the high fequency cuent components and low fo the active filte cuents. The line side of the battey chage topology foms an equivalent shunt AF. The decoupling of the convetes povided by the DClink capacito ensues that the functionality of the shunt AF is sepaated fom the battey side opeation. Theefoe, hamonic filteing as well as load balancing and eactive powe compensation may be accomplished by the topology selected. Howeve, the lack of a neutal conducto makes balancing of zeosequence cuent components impossible. Thus, only negativesequence cuent balancing is achievable accoding to the discussion in Section 2.2. Finally, the half bidge DCDC convete enables chaging of EV batteies of diffeent voltage levels. Futhemoe, it allows bidiectional battey cuent and thus that the battey chage is able to function as an electonic gas tubine. 3.2 Convetes The VSCs of the battey chage consist of fou bidge legs as shown in Figue 3.2. Thee bidge legs fom the line side convete while the fouth bidge leg acts as the DCDC convete at the battey side. A bidge leg, o a half bidge convete, consists of two tansistos each
29 3. Design and modelling 2 connected to an antipaallel diode. The tansistos should pefeably be IGBTs due to thei supeio oveall pefomance chaacteistics, i.e. fowad voltage dop and switching times, in the powe ange consideed. Soft ecovey feewheeling diodes should be used in ode to get less snappy tunoff at evese ecovey of the diodes. Dive V dc C dc s Dive Figue 3. 2 The VSCs of the battey chage including dives fo the DC/DC half bidge convete. In Figue 3.2, additional dives needed to convet the logic contol signal s into gate voltages suitable to the IGBTs ae also shown. The logic invete block pio to the lowe IGBT dive ensues that the IGBTs ae not simultaneously conducting. Thus, shot cicuit of the DClink capacito is pevented. Ideally, thee ae no losses in the VSCs and cuent commutation is instantaneous. Howeve, this is not the case fo pactical VSCs. In pactical convetes the powe electonic devices ae not loss less and the switching times ae finite. When a powe electonic device is conducting cuent, a simultaneous voltage dop of a few volts acoss the device appeas. The finite switching times imply that the cuent and the voltage ae simultaneously high, leading to lage instantaneous powe dissipation duing the switching tansients. These losses, efeed to as conduction losses and switching losses, ae the limiting factos in the cuent loading capability and the switching fequency. Stay inductance futhe stesses the powe electonic devices at the switching instants [27]. Theefoe, snubbes to educe these effects ae often needed. The finite switching times futhe imply that duing commutation both IGBTs may conduct at the same time, since one IGBT is tuning on and the othe one is tuning off. This implies that the DClink capacito is shot cicuited at the switching instants. This is pevented by additional contol logic in the dives. The tun on of the IGBTs ae delayed in ode to guaantee the completion of the tun off pocess of the othe Insulated Gate Bipola Tansisto
30 22 3. Design and modelling IGBT. This is efeed to as the intelock o blanking time, since both IGBTs have tempoaily logic low contol signals. The enegy efficiency of the VSCs may be impoved by the use of softswitched topologies instead of the hadswitched shown in Figue 3.2 [7]. Extensive analysis of esonant convetes ae pefomed in the paallel thesis [8] and theefoe not consideed in this thesis. Instantaneous modelling In ode to model the VSCs, the simplified equivalent cicuit of Figue 3.3 is used. The equivalent cicuit is deived based on the ideal popeties of the VSCs. Hence, the uppe valve of the half bidge is conducting when the logic contol signal o the half bidge switching state s is high, wheeas the lowe valve conducts when s is low. High and low value of the switching state coesponds to one and zeo, espectively, in the following discussion. u a i a v a s a v link+ u b u c i b i c v b s b v c s c V dc v link s batt i batt u batt Figue 3.3 Simplified ideal model of the VSCs. The instantaneous unipola output voltage u batt of the half bidge DC/DC convete is given by u batt = sbattvdc (3.) whee s batt coesponds to the switching state of the DC/DC convete. When s is low, the output voltage is ideally zeo, since the output teminal is shoted by the lowe valve to the DClink negative supply ail. If, on the othe hand, the switching state s is high then the uppe valve is active. Hence, the output teminal is connected to the positive supply ail of the DClink and the output voltage ideally equals the value of the DClink voltage, i.e. V dc. A simila appoach fo the thee phase VSC gives the following expessions fo the output phase voltage potentials with espect to the DClink negative supply ail
31 3. Design and modelling 23 v v v = s V a a dc = s V b b dc = s V c c dc (3.2) whee s a, s b and s c ae the switching states of the bidge legs fo phases a, b and c, espectively. Fom (3.2) the output phase voltages seen fom the neutal point of the powe gid ae given by u = v + v = s V + v u = v + v = s V + v u = v + v = s V + v a a link a dc link b b link b dc link c c link c dc link (3.3) whee v link is the voltage potential of the DClink negative supply ail with espect to the powe gid neutal. The voltage potential of the DClink negative supply ail can be detemined fom the balanced condition of a thee phase system, which states that the phase quantities should sum up to zeo accoding to Equation (A.3). That is, since then v link is given by u + u + u = (3.4) a b c sa sb sc v link = + + V dc (3.5) 3 which indicates that the DClink supply ails of the VSCs ae floating with espect to powe gid neutal. The coesponding voltage vecto of the thee phase VSC in the stationay efeence αβfame, obtained by the space vecto tansfomation given in Appendix A., is defined as u u ae j j2π αβ 2 3 = 3 + ube + uce j4π 3 (3.6) Insetion of (3.3) into (3.6) gives the following expession fo the voltage vecto j2π j4π j j2π j4π αβ u = 3 savdc + sbvdce + scvdce + v link e + e + e (3.7) which can be simplified to =
32 24 3. Design and modelling j π αβ u 2 23 = V 3 dc sa + sbe + sce j4π 3 (3.8) since the zeosequence component v link vanishes in the tansfomation as indicated in (3.7). Fom (3.8) it can be concluded that, since the thee switches can be combined in 2 3 =8 ways, eight discete voltage vectos may be geneated by the thee phase VSC. The voltage vecto space fo the thee phase VSC is displayed in Figue 3.4. Hee, the switching states fo the coesponding voltage vectos ae indicated by the notation u. sss a b c u β u u u u u α u u Figue 3.4 Voltage vecto space fo the thee phase VSC. The length of the voltage vectos ae 2 3 V dc accoding to (3.8), except fo the zeo voltage vectos obtained when all switching states ae equal, i.e. the combinations and. Pulse Width Modulation Pulse width modulation (PWM) methods ae employed in ode to detemine the instantaneous switching states of the VSCs [5][34]. Usually, modulation of the VSCs ae obtained though cuent contol. In closed loop PWM techniques, the cuent contol is included in the switching state geneation, wheeas open loop PWM is based on efeence voltages obtained fom cuent contolles. In the latte case, the efeence voltages ae then used to find the instantaneous switching states of the VSCs eithe by geneal space vecto modulation o, in paticula by caie wave modulation. Closed loop modulation by hysteesis contol Closed loop contol of the VSCs is obtained by hysteesis o toleance band cuent contolles. The pinciple of hysteesis contolles is to keep
33 3. Design and modelling 25 the cuent within the hysteesis dead band. This is shown in Figue 3.5 togethe with the pinciple scheme of a hysteesis cuent contolle. i a V dc i batt i a * i a s a s b s c s batt i batt i batt * Figue 3.5 Top) Pinciple scheme of a hysteesis cuent contolle. Bottom) Basic cuent wavefom included the contolle dead band (dashed). The basic pinciples of hysteesis cuent contolles causes the cuent to be within the hysteesis dead band. This implies high dynamic pefomance, since the cuent is able to follow almost any efeence cuent tajectoy. Hence, hysteesis cuent contolles ae vey attactive in active filte applications. Howeve, thee ae some inheent dawbacks with this contol method [5] such as lack of intecommunication between the individual hysteesis contolles and theefoe no stategy to geneate zeo voltage vectos. a tendency to lock into limit cycles of high fequency switching. geneation of subhamonic cuent components. the cuent is not stictly limited by the hysteesis dead band as can be seen in Figue 3.5. This can happen wheneve the gid voltage vecto has an component that opposes the active switching state vecto. Anyway, the vaying switching fequency complicates the convete design. Futhemoe, the intended use of LCLfiltes make hysteesis cuent contol less pacticable.
34 26 3. Design and modelling Open loop modulation by tiangula caie PWM Open loop contol of VSCs is usually obtained by caie based PWM stategies. In Figue 3.6, the pinciple scheme and signal flow diagam fo tiangula caie PWM VSCs with additional symmetisation is shown. Hee, the instantaneous switching states ae geneated fom compaatos, which compae the voltage efeences with the caie signal. At the cossing instants of a voltage efeence and the caie signal, the coesponding switch state changes and commutation of the bidge leg is initiated. i a V dc i batt i a * i u a * s a a s b s c s batt PI u az * u PI batt * PI u z PI i batt * Figue 3.6 Signal flow diagam of caie based PWM. An impotant popety of caie based PWM techniques is detemined by the fequency modulation atio m f defined in [27] as m f = f caie f (3.9) whee f caie and f ae the caie signal fequency and the fundamental gid fequency, espectively. Accuate time aveage output voltages ae obtained, if the amplitude of the caie signal coesponds to the instantaneous DClink voltage of the VSCs. Hence, the peak value of the caie signal fo the thee phase VSC should be û caie = V 2 dc (3.) wheeas the caie signal used fo the unipola halfbidge convete should vay between zeo and V dc. Figue 3.7 shows in detail the logic contol signal geneation of tiangula caie PWM of a thee phase VSC. Since high pefomance is equivalent
35 3. Design and modelling 27 to a lage m f, which implies that the voltage efeences can be assumed constant duing the caie peiod as shown in the figue. V 2 dc V 2 dc s a u caie u az * u bz * u cz * s b s c Figue 3. 7 Detailed desciption of the logic contol signal geneation fo a thee phase VSC with tiangula caie PWM. Fom Figue 3.7, it can be concluded that the switching fequency f sw of the VSC is given by the caie signal fequency, i.e. f sw = f (3.) caie Symmetisation of the phase voltage efeences, as indicated in Figue 3.6, by the subtaction of a zeosequence component u z given by * * * * * * max ( ua, ub, uc)+ min ( ua, ub, uc ) uz = (3.2) 2 impoves the pefomance of the PWM method [5][34]. The utilisation of the DClink voltage inceases and the cuent ipple deceases. Futhemoe, the phase cuents ae equal to thei aveage values at the tiangula ca ie endpoints. Sampling of the phase cuents at these instants educes the need of aveage filtes fo contol puposes in digital contolles. Hence, the sampling ate f s and the coesponding sampling time T s ae given by f T s s = 2 fcaie (3.3) = f s As stated befoe, a lage m f means that the fequency of the caie signal is much highe than the gid fequency. Hence, only small changes in the voltage efeences occu between two peiods of the caie signal. This
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