Compensated Single-Phase Rectifier

Transcription

1 Copensated Single-Phase Rectifier Jānis DoniĦš Riga Technical university Abstract- Paper describes ethods of rectified DC pulsation reduction adding a ensation node to a single phase rectifier. I INTRODCTION In any cases there is a need to rectify single-phase AC. In addition, for exaple, in soe technical applications (single-phase AC electrified transport and other) power has any hundreds of kilowatts. High quality rectified voltage of ost of the rectifier circuits has rectifier bridge circuit, but also in the rectified voltage has high variable voltage onent that does not provide that high-quality load voltage. For the voltage shape iproveent there is widely used the capacitive filter [,3], which is charging at the oent when AC voltage reaches its peak and discharges when the output voltage decreases shifting stored energy to the load thus equalizing the load voltage. While rectifier with capacitance filter iproves the rectified voltage for, it has the disadvantage that its AC current has very bad shape with large agnitude because charging takes place in very short tie interval. When connecting capacitor in parallel to the bridge rectifier output, load voltage shape significantly iproves. However, the capacitor capacity is large, especially if the load current is large. Filter capacity can be approxiately defined as Iload (0,5T + arcsin( )) C = ω, () where voltage reduction of the aplitude value, half period 0,5T=0,5/f while, but ω=πf ( is and ratio). If, for exaple, the load current is 00A, =0V, =3V, f=50hz, then necessary capacity of capacitor is 4450µF. Such a capacitor while half-period is charging in a very short tie with a current aplitude of ore than.5 ka and the current THD =.66, i.e., with very bad shape of the current. For of network current can be significantly iproved involving an inductance into AC power side []. This inductance can be approxiately calculate as 0,63..(0,5T arcsin( )) L= ω, (),7. I load and in case of this exaple, its inductance ust be 9H having a RMS current of about 0A and saturation current near 00A. Although such coils and filter capacitor diensions and weights are large, this significantly degrades the syste s efficiency. These deficiencies can be prevented adding to a siple rectifier an elevated voltage ensation junction, circuits can be created a nuber of ways, soe of which are observed in this study, and the solution is called the singlephase ensated rectifier. This rectifier output curve ared the rectifier, which has only capacitive filter, is less dropping within incident of load current rise and within a certain range of load current growth, load voltage reains constant. II DEVELOPMENT OF SINGLE-PHASE COMPENSATED RECTIFIER Copensated rectifier is developed providing of additional direct voltage source, a voltage just above the (- ) value, and connecting it to the load through the transistor VT that is operating in eitter follower ode with the eitter base steering voltage (- ) (Fig..). When the load voltage instantaneous value is slightly below the (- ), Fig.. Schee of ensated rectifier and diagras transistor opens and the ensation voltage will be connected to the load by aintaining the load voltage instantaneous value at the reference voltage (- ) level. sing the ensated rectifier, part of the load power is provided fro the unregulated bridge rectifier and part fro the ensation unit. 48

2 Operation tie of each of the sources is depending on the level of : if it is higher, longer operates source of ensation: t = arcsin( ), ω (3) ( = + 0,63. t R sl ( + ) t ) (5) but operation tie of ain rectifier is t = 0, 5T. t Curves of t = t f = f ( ) and t = f ( ) at frequency f=50hz are shown in Fig.,a. As it can be seen both operation ties are equal at =0.9. In order to reduce power losses, it is rationally to adopt only slightly (by -5 V) greater than reference voltage (- ). Introducing base capacity /R load, transistor relative loss expression will be PTRa = ( + 0,63.. t ) (6) ( + ). t, where it would be appropriate to accept =.0(- ). Basing on this assuption, at f=50hz calculated curve P Tra =f( ), displayed in Figure 3. As can be seen, at increase relative loss decreases. At =0.3 the relative loss is , or at =3V, R load =Ω, it will be 68W. Noticing that the power transistor ust be rated for at least about 50A and loading capacity will be alost 3kW, dissipated power will be sall. Fig.. Dependence of conductivity ties of both part of rectifier (a) and averaged voltage of load (b) on relative voltage drop Load voltage s average value can be deterined as sl sl = = + 0,63.. t. (4). This curve, depending on the at f = 50Hz is depicted in Fig.. b. As it can be seen as goes higher, load goes lesser and at = 0.3 it is At the sae tie it should be noted that the ain rectifier bridge rectified DC voltage s ratio to is , i.e., lesser than in ensation syste at = 0.3. Transistor VT of ensation circuit operates in eitter follower ode, i.e., it has large power losses. When the transistor is conducting, the collector-eitter voltage is = +, VT where is a ensation source DC voltage, which ust be greater than (- ), i.e., reference voltage. Current, conducted by the transistor is I. So the average transistor dissipated power is P = I ( + ) t = TRa Fig. 3. Dependence of relative losses of transistor on relative voltage drop Main rectifier ust be calculated on the average load current + 0,63.. t I average = (7) Rsl or I vid. Rsl I average = = + 0,63.. t. While the ensation source average current (8) I average = ( + 0,63.. t ). t, but this source power will be P. Rload P = = I.,0.( ) average (9) This way, at = 0.3, the ensation source should be calculate on 0.8 unit of relative power, or, for exaple, at = 3V, R load = Ω ensation power capacity ust be 3.7 kw. 49

3 III DEVELOPMENT OF COMPENSATION SORCE Such source can be created as a bridge rectifier with connected capacitor-reactor filter at its output, the ensation source will be able to feed fro the sae AC as ain rectifier (Fig. 4). Required coil inductance and capacitor capacitance have to provide that instantaneous eanings of capacitor s C F voltage will be ore as (- ). RMS value of reactor s current accepting as current through reactor is passing, ties longer as it s ain rectifier operation interval can be found as: I = I,4t f. (5) Lef ca. Fig.4. Possible schee of realization of ensation source Calculation of such filter approxiately [] can be realized at approach that eanings of capacitor voltage are decreasing linearly in way of consuing fro junction ensation current (Fig.5): I t u sl. cf = c in + cf, (0) C where cin is inial value of voltage, cf is voltage difference between axiu and iniu values, but tie t is between 0 un t kop. In siilar way capacitor s voltage is rising in way of operation of ain rectifier: Ica. t uc = c in +, () C where I ca is an averaged value of capacitor current in way of operation of the ain rectifier when t is between 0 to t. Accepting necessary cf, fro (0) can be found capacitance Isl. tkop C=, () cf but fro () - obtain necessary averaged current of capacitor charging cf. C Ica =. (3) t As it can be seen fro Fig.5 this current is slightly saller as agnitude of reactor s current I L which is about,3 ties bigger. Inductance of coil can be found fro siplified equation for capacitor s charging circuit I L. LF = 0,63( c in ) 0, 5 cf t Here fro [0,63( L = F c in ) 0,5 cf ]. t,3i sl. tkop. (4) Fig.5. Voltage and current diagras of filter on Fig.4 If for instance =0,3, R sl = Ω, I sl =5A, f=50hz, cin =30V, cf =35V, then L=,05H, C= 700µF, but RMS current of reactor is about 90A. If ensation schee is not used in the case of rectifier with filter ust be L=,45H with RMS current 40 A. Because ass of reactor is proportional to LI ef it should be possible state that reactor of filter for ensated schee will be three ties lighter. But capacitance of filter capacitor for ensated schee have to be C=700 µf when in case of ordinary rectifier only 9656 µf. Rising of capacity is need because providing condition cin > (- ). There is possible apply another schee with capacitor type voltage doubler (Fig. 6). With the nubers are indicated the following eleents: - transforer secondary winding of which has a center tap; - ain rectifier bridge; 3 - active inductive load; 4 - diode-capacitor voltage doubler; 5 - ensation transistor; 6 reference voltage unit, which is presented as diode and capacitor; 7 Mltisi virtual instruents to collect data for load characteristics of syste. 50

4 7 8 is siilar to the rectifier in the previous case, but the difference is that the power rectifier with capacitor 6 perfors only reference voltage function. Outer curve o this circuit is shown as 3 rd in Fig. 7 and Fig. 6. Copensated single-phase rectifier with voltage doubler.load IV INVESTIGATION OF LOAD CHARACTERISTICS For above entioned schees have been using siulation progras obtained their load characteristics, i.e. dependence of load voltage on load current. Was applied as uter progra PSIM as also MultiSIM. Characteristics are presented on Fig. 9 and 0. Load characteristics of syste with voltage doubler is shown as nd in Fig 7 and 8. Characteristics for circuit with additional elevated voltage are shown as st in Fig. 9 and 0. Load characteristics for schee 8 are presented as 3 rd. Characteristics 4 are for ordinary rectifier with capacitor at output. 7 I.load Fig. 9. Load characteristics of rectifiers (PSIM) Fig. 7 Copensated single-phase rectifier with the additional elevated voltage rectifier and capacitance.load 7 I.load Fig. 8. Copensated single-phase rectifier with the additional elevated voltage rectifier and capacitance On Fig. 7 is presented another version of ensation voltage source when it s possible apply transforer with elevated value of voltage of secondary winding s part. In addition, in this case, the transforer secondary winding has the id-point output, but the ensation rectifier is connected to the transforer output voltage of about 40% higher than the output at which is connected to a ain rectifier. The action differs only in that the load is ensated by the lower voltage level and sipler ensation accuulator than the case with voltage doubler. Single-phase ensation rectifier shown in Fig Fig. 0. Load characteristics of rectifiers (Multisi) To are efficiency of rectifier ensation option to the siple rectifier with capacitance filter load characteristics of four rectifier circuits are shown in Fig. 9 and 0. The characteristics are taken by changing the rectifier load gradually fro 0 to large values and collecting data of average current and voltage easureents. All observed circuits had being easured at each load value but the load current and voltage average value reading is perfored and data are suarized in the Fig. 9 and 0. Copensated rectifier circuits were siulated according to the pictures illustrated in Fig. 6, 7, 8, all ensated rectifier circuits are without ain rectifier capacitor. A siple rectifier circuit was siulated according to the Fig.. All of the ensated rectifier circuit s ensation accuulator capacitances are chosen equal to siple rectifier capacitance, this way the load curves shown in Fig 9 and 0 are arable. 5

5 accuulator, which appears in the Fig. 7, so the easureents will be shown only for this option. I load I I rect Fig.. Siple rectifier with capacitance filter Estiating load curves displayed in Fig. 9 and 0 can be seen that the characteristics and has a critical value of the load to which they provide a very stable voltage - st curve to ~ 5 A, but nd curve ~ A. It follows that the ensated rectifier with additional elevated voltage rectifier and capacitance shown in Fig. 7 gives the highest efficiency. Circuit s shown in Fig. 6 - single-phase rectifier with voltage doubler - efficiency is lower, the load curve provides a stable voltage to the load current value of A, and ore increasing current value of the load voltage drop becoes fastest of all the reference rectifier options, and circuit negative characteristic is its lex construction. Copensation rectifier s, circuit Fig. 8, the load curve 3 rd in Fig. 9 and 0 is siilar to that of a siple rectifier with capacitance filter circuit in Fig., and load curve 3 rd in Fig 9 and 0 the output average voltage value decreases in all areas where the output current average value is rising. V COMPENSATED SINGLE-PHASE RECTIFIER CRRENT AND VOLTAGE SHAPE MEASREMENTS Current and voltage shapes were obtained siulating a rectifier circuit using NI Multisi virtual instruents. In ; ref; load I rect; I ; I load I rect I view of the data collection that is shown in the Fig. 9 and 0, it can be concluded that the widest possible use can obtain the ensated single-phase rectifier circuit with the addition of elevated-voltage rectifier bridge and I load Fig. Current and voltage easureent points (Multisi) CE Fig. 3 Load current shapes (Multisi) CE ref Copensated rectifier with the additional elevated-voltage rectifier bridge and accuulator capacitance is shown in Fig. with subsequent easureent points. Here I rect - current share when load is receiving fro the ain rectifier; I current share when a load is receiving fro elevatedvoltage rectifier with accuulator capacitance of ensation; I load total load, these three currents at any point of tie is linked by the first Kirchhoff's law i load (t) = i rect (t)+i (t), the current easured fors are shown in Fig. 3. Measured voltages are load - load voltage; - ensating source voltage, ref - the reference voltage is the level that transistor keeps the voltage on the load unless the ensation accuulator has sufficient energy, CE - a potential difference the ensation transistor collector and eitter, the voltage points the transistor operation ode. Exaining the nature of voltage CE Fig. 4, curve shows the rectifier load is less than the critical ( > load ), the transistor operates in aplifier ode, i.e. CE voltage is bigger than the value of "0". In case = load, following CE =0 then these intervals, the transistor is operating in switch ode, the intervals corresponding to ties when the energy accuulator ensation has expired and has not been able to keep the load voltage for the reference voltage level ref. CONCLSIONS Fig. 4 Voltage shapes (Multisi) load. A siple rectifier with filter capacitance load curve is decreasing at any load current rise in the range. 5

6 . Any ensated rectifier circuit option load curves are less dropping aring to siple rectifier s with capacitive filter load curve. Copensated rectifier with the additional elevated-voltage rectifier bridge and storing capacitance in a certain range of load current rise-tie voltage value reains constant. 3. Most effective ensated rectifier option is the circuit with elevated-voltage rectifier bridge and accuulator capacitance, its load curve shows the best characteristics of the voltage source aong observed circuits, as well as technical construction is siple. 4. Through the rectifier load characteristic studies with NI Multisi and Powersi PSIM software, there are distinctions, it can be explained by the fact that the NI Multisi circuit eleents are assigned with a nuber of real properties, but PSIM uses to idealized eleents, but calculations of these progra reliability is not unequivocal. REFERENCES [].И. М. Чиженко, В. С. Руденко, В. И. Сенько, Основы преобразовательной техники, Высшая школа,974-46стр. []. Rodionova I. Rankis I. Operational Regies of Input Filters// Electronics and Electrical Engineering.- Kaunas:Technologija, 006 N 4(60) p [3] Mohan N., ndeland T., Robbins W. Power Electronics: Converters. Application and Design. NY: John Wiley and sons, p 53