Three-Phase Series-Buck Rectifier with Split DC- Bus Based on the Scott Transformer

Transcription

1 Three-Phase Series-Buck Rectifier with Slit DC- Bus Based on the Scott Transformer Alceu André Badin and Io Barbi Federal Uniersity of Santa Catarina/Deartment of Electrical Engineering/Power Electronics Institute P. O. BOX Florianóolis SC - Brazil alceu@ine.ufsc.br Abstract In this aer, a new unity ower factor isolated three-hase buck rectifier is resented. Based on the Scott transformer, this rectifier is simle and it has the caability to obtain low outut oltage. Besides, it rotects against short circuit and it needs no auxiliary circuit for inrush current. Using only two actie switches, it is able to generate symmetrical currents in the line and outut oltage regulated. The modulation is used conentional SPWM. The control has only one oltage control loo. Theoretical analysis, design rocedure, comlete simulation results with closed loo oeration are gien, as well as results of an exerimental erification. I. INTRODUCTION ooking for imroe the energy quality in the distribution networks and efficiency, it were created strict current harmonic limitation imosed on ower sulies [1]. Therefore, rectifier with ower factor correction are researched and used for the deeloments of ower sulies in recent years. In this aer, the unity ower factor three-hase rectifier buck with a simlified control loo technique, based on the Scott transformer is resented. The single-hase PWM buck re-regulator in Fig. 1 has some imortant characteristics such as the absence of inrush current, low DC outut oltage, rotection against short circuit, among others. outut inductor current rile distorts the inut current, with a significant third harmonic comonent. In [] rooses a control technique to eliminate the distortion on the inut current een when the outut inductor current resented large rile. In reference [3] is also resented control techniques more simle to eliminate the distortion on the inut current. In [4]-[5] the unity ower factor three-hase rectifier with a slit DC-bus based on the Scott transformer is resented. The roosed toology is show in the Fig.. In this alication use two single-hase buck rectifiers in continuous conduction mode. There is a outut oltage with a slit DC-bus and the outut inductors are couled. It is reduced de size and cost of rectifier. The C inut current filter of high frequency make use the leakage inductances of transformers and thus there are not necessary inut inductors. Figure. Unity ower factor isolated three-hase rectifier buck series. Figure 1. Single-hase buck re-regulator. In continuous conduction mode, with a low frequency outut inductor designed in suck a way that if behaes as a constant current source. Therefore the size and weight of the outut inductor, in this case, is much bigger. In order to otimizer the size and weight, the inductance o may be decreased, so it no longer behaes as a constant current source. Although, increasing the II. SCOTT TRANSFORMER The Scott connection is realized with two single hase transformers, T M and T T. The rimary windings are fed by two different oltages, V AO (t) e V CB (t), that are generated from a symmetrical three hase system V A (t) V B (t) e V C (t). The connection is reresented at Fig. 3. Each secondary winding is simly a single hase winding, and the oltage across it and the current in it do not differ from what would be exected in an ordinary single hase transformer. In the case of the three hase side, howeer, it is interest to consider the actual oltages and currents, which are as follows: /08/$ IEEE 516 Authorized licensed use limited to: UNIVERSIDADE FEDERA DE SANTA CATARINA. Downloaded on Noember 13, 009 at 06:16 from IEEE Xlore. Restrictions aly.

2 3 V = V AO CB (1) I I AO By multilying the oltage across each transformer by the current in it, the equialent size of each transformer is obtained. In the case of main transformer, this is equal to times the grou outut; and in the case of the teaser transformer, 0.5 times the grou outut. Therefore, in a Scott connected grou, the two hase windings are equialent to the windings of two ordinary single-hase of the same outut, but on the three hase side the winding of the main transformer is increased by 15.5% aboe what would be required in a single hase transformer of the same outut. Assuming that the rimary and secondary windings of an ordinary single hase transformer each occuies the same sace, then, in a Scott connected grou it is necessary a transformer of 7.75% greater caacity in the main transformer than a single hase transformer. sect (t) and secm (t) reresent a two hase oltage system, with a hase angle 90 o between then. The hasor diagram is reresented at Fig. 4. CB t V sin( w t) (3) int t V cos( w t) (4) inm The urose of using a buck PFC is to correct the ower factor of the structure by forcing the inut current to follow the shae of the rectified secondary oltage. For that, instantaneous aerage duty cycles of the switches are: d t K sin( w t) (5) T d t K cos( w t) (6) Where K is the modulation index. M A T T V sect (t) B T M O C V secm (t) Figure 3. Scott connected transformers. Figure 5. Three-hase rectifier equialent circuit. The buck diode oltages are, therefore, multilication between inut oltages rectifiers and aerage duty cycles of the switches: V t V K w t DT sin( ) (7) Figure 4. III. Phasor diagram of Scott Transformer. THEORETICA ANAISIS In the unity ower factor isolated three-hase rectifier theoretical study, only the secondary circuitry will be taken into account. Therefore, the secondary windings of the Scott transformer are considered to be ideal AC ower sources. The full-bridge diode rectifiers were substituted by ower sources that reresent the rectified secondary oltage int (t) and V inm (t). The toology of Fig. can be reduced to the circuit of Fig. 5. The secondary oltages of the Scott transformer are sine and cosine waeforms [5]. Therefore, the rectified oltages at the inuts of the buck conerters are: V t V K w t DM cos( ) (8) The equialent circuit of Fig. 5 can be reduced to the circuit of Fig Authorized licensed use limited to: UNIVERSIDADE FEDERA DE SANTA CATARINA. Downloaded on Noember 13, 009 at 06:16 from IEEE Xlore. Restrictions aly.

3 i T (t) R o + + DT(t) C (t) - m dt k 1 k (11) m dm k 1 k (1) DM(t) c i M (t) C R o + (t) - o (t) Where k is the magnetic couling coefficient; Considering model couled inductor show in Fig. 8(c), equialent circuit can be reduced to Fig. 9. M V T(t) - Figure 6. Equialent circuit of the outut filter. dm m dt Considering V (t) and V (t) constant the equialent circuit can be reduced to Fig. 7. V M(t) M1 Where: Figure 9. Equialent circuit of the outut filter. V Mi cos( wt ) I' M ( t) jw dm ( I' ( t) I' ( t)) jw 0 M T m (13) Figure 7. Equialent circuit of the outut filter. Where: V t V M wt V t (9) M i cos( ) V t V M wt V t (10) T i sin( ) V Mi cos( wt ) I' T ( t) jw dt ( I' ( t) I' ( t)) jw 0 T M m (14) Soling equations (13) and (14), obtains the inductor current riles (only AC signals): V Micos( wt) I' T ( t) (15) 4 j w (1 k) The equialent circuit of the couled inductor can be show to Fig. 8. VMicos( wt) I' M ( t) 4 j w (1 k) (16) The inductor current riles of the each buck PFC is show to Fig. 10, with arametric alues. Figure 8. Equialent circuit of the couled inductor. Where: 518 Authorized licensed use limited to: UNIVERSIDADE FEDERA DE SANTA CATARINA. Downloaded on Noember 13, 009 at 06:16 from IEEE Xlore. Restrictions aly.

4 Figure 10. Inductor current riles. In a buck conerter the outut current is bigger than the inut current. The limit for the outut inductor current rile is the one that guarantees that the outut inductor current equals the inut current in one oint only. IV. CONTRO STRATEGY Each buck PFC resents its own oltage control loo (Fig. 11). The outut oltage is sensor and comared to a reference oltage. The resulting error is injected in an aroriate oltage controller. The outut of the oltage controller is multilied by a sensor of the rectified inut oltage and diided by a sensor of the current in the outut inductor. The resulting modulation signal is comared with the saw-tooth signal, generating the drie signal to the switch. The feedforward strategy the modulation signal resents a distortion that eliminates the inut current distortion due to the outut inductor current rile. Both transfers functions of the lant oltages loo were obtained from model of Fig. 6 and can be seen in (17) and (18). The equialent series resistance (R esr ) of the outut caacitor was taken into account. V (1 ) s V sresr C DT s R esr s C 1 s C Resr 1 R R (17) Figure 11. Control loo block diagram. V. SIMUATION RESUTS The results of two simulations are resented to check the alidity of the study until this oint. The first simulation aims to erify the erformance of the current loo. The design secifications of the rototye can be seen in Table 1. TABE I. DESIGN SPECIFICATIONS. Parameters Value ine frequency (f r ) 60 Hz RMS line oltage (V in ) 380 V Secondary oltage (V sect ) 0 V Rated ower (P o ) 6 kw Minimum rated ower (P omin ) 3 kw Outut oltage (V o ) 00 V Switching frequency (f s ) 0 khz Efficiency 90% Outut oltage rile (V e V ) % V (1 ) s V sresr C DM s R esr s C 1 s C Resr 1 R R (18) In Fig. 1 shows the inut current I T (t) and I M (t). The total harmonics distortions (THD) of the inut currents for full load oeration are: THD IM =.15% and THD IM =.10%. 519 Authorized licensed use limited to: UNIVERSIDADE FEDERA DE SANTA CATARINA. Downloaded on Noember 13, 009 at 06:16 from IEEE Xlore. Restrictions aly.

5 time (s) Figure 1. Currents I T (t) and I M (t) at each buck PFC Figure 15. Outut oltage (t), (t) and o (t)/. Fig. 13 shows the secondary oltage V sect (t) and secondary current I T (t). Detail of crossoer shows to Fig. 14. The second simulation aims to erify the erformance of the control loos when the rectifier suffers load ariations, from 50% of the rated load to 100%. The results for the inut currents can be seen in Fig. 16. Figure 13. Inut Current i M (t) and inut oltage sect (t). Figure 16. ine currents during a 50% increase in the load In Fig. 17 shows the outut oltage, V (t), and inductor current I M (t), when the rectifier suffers a load ariations, from 50% of the rated load to 100% time (s) Figure 14. Detail crossoer of the inut Current i M (t) and inut oltage sect (t). In Fig. 15 shows the outut oltage of the each buck PFC, (t) and (t), and outut oltage o (t). Figure 17. Voltage (t) and current i T (t) during a 50% increase in the load. 50 Authorized licensed use limited to: UNIVERSIDADE FEDERA DE SANTA CATARINA. Downloaded on Noember 13, 009 at 06:16 from IEEE Xlore. Restrictions aly.

6 VI. EXPERIMENTA RESUTS A laboratory rototye of the isolated three-hase buck rectifier based on the Scott transformer with neutral oint was imlemented to roe the theoretical studies. Both of the PFC modules are controlled by Unitrode UC3854B [6]. The design secifications of the rototye can be seen in Table 1. Fig. 18 is show a hotograh of the laboratory rototye. Figure 0. Phase oltage T (t) 100V/diision and 5ms/diision) and current i T (t) (10A/diision and 5ms/diision). Fig. 1 resents the inut current i T (t) and inductor current i T (t). The inductor current is on the limit. The outut inductor current rile is equals the inut current in one oint only. Figure 18. Photograh of the laboratory rototye. The exerimental results ware obtained with minimum rated ower (3 kw). The reference currents are obtained by scaling the measured rectified secondary oltages of the Scott transformer. These are the references of the current controller, which determines the shae of the boost inductor currents. The inductor currents are measured by Hall sensors. Fig. 19 shows inut currents i T (t) and i M (t) of the buck PFCs, which are 90 o hase-shifted from each other with equal amlitudes. They are nearly sinusoidal in shae. Both currents do not resent distortion at zero crossing, which is characteristic of single-hase boost PFC conerters. Figure 1. Currents i T (t) and i T (t) (10A/diision and 5ms/diision). Fig. resents the rile in outut oltages (t) and (t). Note that the 10 Hz rile in oltages (t) and (t) are out of hase which results in ractically no 10 Hz rile in outut oltage o (t). Figure 19. Currents i T (t) and i M (t) of each boost PFC (0A/diision and 5ms/diision). Figure. Outut oltage rile(v (t) and V (t)) (V/diision and ms/diision). In Fig. 0 shows the secondary oltage V sect (t) and secondary current I T (t). 51 Authorized licensed use limited to: UNIVERSIDADE FEDERA DE SANTA CATARINA. Downloaded on Noember 13, 009 at 06:16 from IEEE Xlore. Restrictions aly.

7 VII. CONCUSIONS In this aer it is resented and studied a control strategy to a simlified isolated three-hase Buck based unity ower factor single-hase buck rectifier and Scott transformer, oerating in continuous conduction mode. It resents only two switches and a balanced slit DC-bus. A 6 kw laboratory rototye was imlemented. The exerimental results demonstrate the erformance of the roosed system. The resulting inut line currents are nearly sinusoidal in shae, een rocessing only 3 kw. The low-ass C inut filters are obtaining with leakage inductances of Scott Transformer. Is not necessary add inductors, only caacitors. The ower factor is indeendent on the relation between the outut oltage aerage alue and the inut oltage eak alue. The outut inductors are couled. It is reducing de size and cost of rectifier. REFERENCES [1] IEEE Recommended Pratictices and Requirements for harmonics Control in Eletric Power Systems, IEEE Std. 519, 199. [] K. Hirachi, T. Iwada, K. Shibayama, A secific control Imlementation on buck-tye Actie Power Filtering Conerters. INTEEC Proceedings, 1995, , [3] F. Pöttker de Souza, I. Barbi, A Unity Power Factor Buck Pré- Regulator with Feedforward of the Outut Inductor Current. Alied Power Electronics Conference and Exosition, [4] A. A. Badin, I. Barbi, Simlified control technique for threehase rectifier PFC based on the Scott transformer. IEEE International Symosium on Industrial Electronics, Vol., Montreal, July 006. [5] A. A. Badin, I. Barbi, Unity ower factor isolated three-hase rectifier with neutral oint based on the Scott transformer. Alied Power Electronics Conference and Exosition, , Dallas, TX, 006. [6] P. C. Todd, UC3854 controlled ower factor correction circuit design, Unitrode Cor., Unitrode Alication Note U-134, Merrimack, NH, Authorized licensed use limited to: UNIVERSIDADE FEDERA DE SANTA CATARINA. Downloaded on Noember 13, 009 at 06:16 from IEEE Xlore. Restrictions aly.