A New 12kW Three-phase 18-Pulse High Power Factor AC-DC Converter with Regulated Output Voltage for Rectifier Units

Transcription

1 A New 12kW Three-phase 18-Pulse High Power Factor AC-DC Converter with Regulated Output Voltage for Rectifier Units Falcondes JosC Mendes de Seixas (*) and vo Barbi Federal University of Santa Catarina - Department of Electrical Engineering Power Electronics nstitute P.O. Box Florian6polis - SC - Brazil Phone: fax: falcon@inep.ufsc.br (*) s with the UNESP - SBo Paul0 State University - Abstract - This work presents a new high power factor three-phase rectifier based on a Y-connected differential autotransformer with reduced kva and 18-pulse input current followed by three DC-DC boost converters. The topology provides a regulated output voltage and natural three-phase input power factor correction. The lowest input current harmonic components are the 17'h and the 19*. Three boost converters, with constant input currents and regulated parallel connected output voltages are used to process 4kW each one. Analytical results from Fourier analyses of winding currents and the vector diagram of winding voltages are presented. Simulation results to verify the proposed concept and experimental results are shown in the paper.. NTRODUCTON n telecommunication systems, conventional input stage of power supplies is widely realized as voltage DC-link converters. n many cases, the AC supply voltage is connected to a DC link voltage by a diode rectifier with output filter capacitor. A high-frequency DC-DC converter, with regulated output voltage, is then connected in series with the DC-link voltage. Motivated by high efficiency, high power density, low cost and robustness, an uncontrolled threephase bridge directly connected to the AC supply has been used. However, this concept shows, as disadvantage, high low-frequency current harmonics, due to the output capacitor, which leads to a distortion of the AC supply voltage. Various international standards like EEE-5 19 and EC-1OOO-3-2, were proposed to maintain the THD (Total Harmonic Distortion) and the Power Factor (PF) at acceptable levels. This is especially important for guaranteeing universal applicability of telecommunication rectifiers. Furthermore, the conventional power supplies are being replaced more and more by three-phase modular rectifier systems, due to the advantages of the latter concerning operational behavior, system technology, and costs [Ol]. n the same way that the great number of works developed for power factor correction in single-phase systems, the techniques for three phase systems to operate with power factor correction are growing, either through converters with one or more associated switches, or by using specially connected transformers. n general, the techniques that use transformers or mixed systems, with transformers and static converters, operate well in higher power systems [02,03]. A parallel connection of three six-pulse bridge rectifier is used to obtain 18-pulse uncontrolled converter. Usually, an nterphase Transformer (PT) connected on the DC sides of the three bridge rectifiers is required to absorb the instantaneous voltage differences between the bridges. When a non-isolated transformer is used, two PT are needed: one connected to the positive outputs, and another connected to the negative outputs. The size, the loss and the cost of the PT are, therefore, increased. Figure 1 shows the basic diagram for a conventional non-isolated 18-pulse converter using two PT to improve parallel connection on the DC sides. SOURCE n i, Vb Non-isolated transformer 18-pulse Bridge 3 i - Bridge 3. Figure 1 : 18-Pulse Converter with Conventional PT. This work proposes an 18-pulse converter using a Y-connected differential auto-transformer. The autotransformer is designed to feed three six-pulse bridge rectifiers displaced in phase by 20'. The autotransformer power rate is only 22% of the output kva and the lowest order harmonics are the lp and the 19. n order to provide parallel-connected and regulated output voltages, without an PT, three boost converters with an additional diode [04] were proposed. Six small size high-frequency boost inductors, as shown in figure 2, replace the large interphase transformers. The continuous conduction mode and output regulated voltage are guaranteed by basic and simple control strategy of the boost converter. The inductor currents of each converter are kept constant with a little high frequency ripple /99/$ EEE

2 .L;, SOURCE 0 Vector diagram of the autotransformer winding voltages The three secondary voltage systems are obtained by the combination of ratios between primary and secondary windings. The vector diagram used to obtain all voltage system is shown in figure 4. AUTOTRANSFORMER Figure 2 Proposed Topology of the 18-Pulse Regulated Boost Converter Eiach boost converter is studied separately and a simple.passive lossless snubber circuit [05] is applied to reduce the over-current through the switch and the losses, during the diode reverse recovery. This snubber circuit provides a1 good turn-on transition for the switch. Eixperimental results with relevant waveforms to verify the power factor and details of switch commutation are presented.. ANALYSES OF THE PROPOSED 18- PULSE Y -CONNECTEDFFERENTAL AUTOTRANSFORMER. To simplify the analyses of the autotransformer, the boost converters are eliminated and, furthermore, three independent inductive loads, at the same values, are connected on the DC sides of the bridge rectifiers, as shown in figure 3. The main equations and relationships of winding voltages and currents are studied. Figure 4: Vector Diagram The primary windings of the autotransformer are formed by La, Lb and Lc, Y-connected and linked to the line voltages Va, Vb and Vc. n this connection, a virtual neutral point N is generated. The secondary windings are designed, in such way that, the turns ratio and the connection between them and the primary winding generate three different threephase systems with 20' phase-shift from each other. These voltages feed the rectifiers. All the windings of La, Lan, La1 and La2 are coupled together in the same limb core, the resulting voltages Va, Van, Val and Va2 are in phase. The same applies to phases "b" and "c", as shown in figure 4. The magnitude of the voltages across the secondary windings, is obtained as follows. sin( 20') VLbl = VLc2 = Vu. = V~ (01) sin( 100') VLcl = VLa2 = Vb. sin(20') = Vb (02) sin( 100') t Figure 3: A Simplified Circuit of Proposed Topology for Analyses The autotransformer is supplied by a three-phase balanced voltage system. The output voltages are composed by three rectifier systems of three-phase voltages, also balanced. One of these systems is placed in the same phase of the supply voltage and the others are placed of t20" and -20, with regard to the supply system. The 18-pulse converter is obtained when each output voltage system is connected to a six-pulse diode rectifier. Three identical loads with current source characteristic are used. sin( 20') = Vc VLal = VLb2 = Vc. sin(100') (03) The winding turns-ratio (n2) that ensure a displacement of 20" is given by (04). n2=va= =2.88 (04) Vbl Va This result shows that the secondary turns are 2.88 times lower than the primary turns. The magnitude voltages between each secondary terminal with respect to the virtual neutral point N are obtained as follows. sin( 60') Vbl = Vc2 = Vu. = Vu (05) sin( loo') sin( 60') Vcl = Vu2 = Vb. = Vb (06) sin( loo')

3 sin( 60') We can observed that this winding (Lan) Val = Vb2 = VC. = Vc (07) conducts the current 1013 during 120" (2n/3), starting from sin( 100') 30" (n/6). Thus, the expression of the current results: We can observe that the voltage magnitudes of each three-phase system are about 88% reduced in an (t) = :.?.? d.cos(k.b)sin(k.cn) ( 15) comparison input phase voltages. The third secondary three-phase voltage system Were, k=l, 3,5,... is in phase with the primary one. ts voltages however, The waveforms of all currents through the must be in agreement with the others secondaries. So, the secondary windings are the same, only with a 20" phasefollowing equations must be fulfilled shift among each three-phase system. The other currents VLan =VU -0,8794.V~ = V~ (08) are represented by the same equation of an as shown below. Therefore, all Dhases are adiusted. VLbn = Vb - 0,8794.Vb = Vb (09) VLcn = Vc - 0,8794.V~ = V~ (10) bn(t) = n~.e.~i.co{k.~)s~n~.[m.t+~)] 3 (16) Were, VLan, VLbn and VLcn are the magnitude of the voltages across the secondary windings Lan, Lbn and Lcn, respectively. cn(t) = 7x3 ~.~.~i.co{k.~)sin~.(at-$)] (17) The winding turns-ratio to ensure 88% from primary voltage (nl), without displacement is given by. n the other secondary three-phase system, the currents are expressed by: nl=-=---- Va Vb - Vc 1 --=8.29 (11) Van Vbn Vcn 0,1206 al(t) =:.F.F+.co@.$)i{k.[ at-%)] (18) This result shows that this secondary turns is 8.29 times lower than the primary turns. The magnitude voltages for these secondary terminals, with respect to the virtual neutral point N, are obtained as follows. Van = V~ (12) Vbn = Vb (13) Vcn = V~ (14) Analysis of winding currents The technique to eliminate harmonic of current in the multiple pulse converters requires current-mode operation to the load. n this case, the 18-pulse converter, formed by three-bridge rectifier, each converter conducts 1/3 of the load current (10/3). The currents in the secondary windings are the same to the input currents of each bridge. The waveform of the input Va and the current through one secondary winding (Lan) are shown in figure 5. These waveforms are adopted as angular reference to represent the other ones. ), + cl(t) = ~.~.Fi.co{k.~)in~.~ 140.n +%) bl =~.~.~i.co@.$)nk.( 113 (Ut (19) K3 (20) To the last system, the currents are expressed by: a2(t) =!.~.F~.co~.~)si{k.~t 1OO.K (21) a bz(t) =t..".z'.co n 3 k k + -)] c2(t) =!.E.~~.co{k.$).in[k~-~)] n 3 k k (23) The primary winding currents (a, b and C) can be obtained by considering the currents of the three secondary winding coupled to the same limb core and by turns-ratio (n1=8.29 and n2=2.88). As mentioned boton, winding with the same index (a, b or c) are coupled in the same limb. a(t) = nl 1 Figure 5: Primary Reference Voltage and Secondary Current to Phase indexed by "a" The waveform of an can be decomposed in Fourier series by the conventional way. By the way, when discontinuous function is considered, the series terms can be obtained by inspection. We can observe that this waveform presents alternate symmetry, the negative half cycle is an inverted reproduction of the positive half cycle. Thus, the even harmonic are zero and there are not terms in cosines. The average value is also zero. c(t) = Figure 6 shows the primary current Qa). t is obtained by composition of equation (24) from (04), (1 l), (15), (18) and (21). This solution is easily found and plotted through MathCad program s Figure 6: Primary Current Waveform

4 0 Anrdysis of input phase currents The input phase currents ia, ib and ic are obtained by summing all currents through windings at same node as shown in figure 3. So, the follow equations can be written. ia( t) = a(t) + an(t) + bl(t) + c2(t) (27) ib(t) = b(t)+ bn(t) + cl(t) + a2(t) (28) ic(t) = c(t) + cn(t) + al(t) + b2(t) (29) Figure 7 shows the phase currents (Ea, ib and ic). They are obtained by composition of equation (27), (28) and (29) from (15) to (26) ~... ~ - ' T l T. - 2 The Power Factor (PF) is calculated by (33). The displacement between the phase current and the phase voltage is negligible. 1 PF = da = (33) Figure 9 shows the frequency spectrum of the input phase current ia(t). We can observe that 18-pulse converter presents only harmonic orders n.(18fl), for n=1,2,3,... The lowest harmonic orders are the 17'h and the 19". 6 i %ia 5; ;... i. : in nn i Harmonic orders Figure 9: Frequency Spectrum for nput Phase Currents Figure 7: Three-phase nput Currents Figure 8 shows a detail of the current and voltage wavefonn of one phase. 4 r T... """... TT^ Figure 8: Current and Voltage Waveforms of one nput Phase Harmonic analysis for input phase current Total Harmonic Distortion (THD) is calculated by relationship between rms values of the fundamental components and all phase current harmonics. n these calculations MathCad program has considered the first lo00 harmonic components (k=1,3, ). The total rms value for all harmonics of the phase current ia and the rms current for fundamental component ial (for k=l), are calculated through (30) and (31): ia =,/-!-.rnia(t)' 2.n 0.dot Where, ia(t) is solved by (27) for k=1,3,5,... and ial(t) is solved by (27) for k=l. Now, from (30) and (31), the THD is easily obtained by (32). 0 Normalised winding currents The shape of all secondary winding currents are equal, but each one is shifted in phase to generate the three-phase systems. Anyway, all rms value of secondary currents are the same. Then, they can be calculated through (34). 1 an =o. -. ~an(t)'.doc - an= (34) 2.n o The primary winding currents have the same m value and can be expressed by (35)..dot a = o (35) a = o.,/l.rny2 2.n 0 Where, an(t) and a(t) are the secondary and the primary winding currents, respectively. Other currents are calculated with the same way. We can observe that the secondary current rate is 27.2% while the primary current rate is only 7.8% of the load current o. 0 Power rate of the autotransformer The voltage across secondary windings with the same phase than primary ones, h, Lbn and Lcn, are reduced by turns ratio nl. Therefore, the relationship between these voltages is expressed for (08), (09) and (lo), and replaced by (36).,EL VLan = Va (36) nl With VLan = VLbn = VLcn and Va = Vb = Vc The voltage across secondary windings with phase shift of 20' in relation to primary ones, Lal, La2, Lbl, Lb2, Lcl and Lc2, are reduced by turns-ratio n2. Therefore, the relationship between these voltages are expressed for (Ol), (02) and (03), and replaced by (37). Va VLal=- VLal = Va (37) n2 With VLal = VLa2 = VLbl = VLb2 = VLcl = VLc2 Expression (38) and (39) give the average voltage from output of the rectifier.

5 Vo =-. 6 2n (fi.& va.sin(ot))ht 2.a 0 Vo = Va (39) The secondary power rate VA is obtained by summing of the product of voltage and current of each winding, as in (40), from (34), (36) and (37). S2 = PO (40) The primary power rate VA is obtained from equations (35) and (39) and is expressed by (41) S1 = 0.114P0 (41) Where Po = vo.10 (42) Thus, the autotransformer power rate is given by average value between S1 and S2. Then, S = 0.218P0 (43) 14-2 Non-regulated inductor currents To verify the relevance of the boost current control for the proposed strategy, some simulation analyses have been investigated. Figure 11 shows the simulation result waveforms for current through the inductors when three nonregulated boost converters (at constant frequency) are employed. We observe that these currents oscillate in the same frequency as the rectified voltages. n addition, the boost inductance and the switching frequency define the ripple PROPOSED BOOST CONVERTER STRATEGY The boost converter is chosen because it provides a simple regulated output voltage and regulated inductor current. Furthermore, it is very easy to obtain three parallel connected boost converters. +zoo Figure'l 1: nductor Currentsfor Non-Regulated Boost Converter The amplitudes of these oscillation currents are very large in comparison with the average current values. Furthermore, the boost converter can be operate in DCM for reduced load, as shown in figure 12. These problems make the non-regulated boost converter impracticable.,ll l_l. - - ^ x m Figure 10: Parallel Connected Boost Converters 0 Parallel connected boost scheme Figure 10 shows a basic scheme to allow parallel connection of the boost converters [04]. The conventional boost converter can not be used because when two or more switches are closed, the current flowing through the first boost returns through the second one. The diodes D12, D22 and D32 added in the conventional boost can solve this problem. Another problem occurs when all switches are simultaneously open. The current flows through the first boost, through the load and returns through the second one. This problem is solved by division of each boost inductor in two series connected inductors, as shown in figure 10. After the switch turn-off, the lower inductor forces the current from upper one to comes back through it. n this way, the lower inductors L12, L22 and L32 are necessary. This connection is very important to the operating of the circuit. t improves an independent operation mode between the boost converters. Figure 12: nductor Currents for Non-Regulated Boost Converter (Reduced Load) The oscillations of the inductor currents are reflected to input phase current. To reduce the low frequency component of the inductor currents, a bigger boost inductor have used. Then, we could eliminate the boost converters and replace them by PT's. To solve this problem, the boost converters have to furnish regulated currents for all boost inductors. 0 Command circuit for regulated boost converter To improve regulated current in boost inductor and regulated output voltage, various command circuits such as constant hysteresis and average current-mode control were investigated. The simplest strategy to

6 ----- "l""_ 14-2 promote both regulated voltage and regulated current is shown in block diagram in figure 13. Current error amplifier Figure 13: Command Strategy for one Boost Converter constant frequency. Anyway, all boost converters operate independently and decoupled. Therefore, the synchronism between them is not necessary. n order to connect all boost converters in parallel, only one voltage regulator is necessary. Fortunately, the command circuit becomes simpler, as shown in figure 14. The simulation results for regulated boost inductor currents are shown in figure 15. We observed that...- ", j L, i Y lr 1) ~~ Figure 15: nductor Currents for Regulated Boost Converter V. EXPERMENTAL RESULTS Figure 14: Command Strategy for All Boost Converters This is a conventional scheme to control each boost converter. The output voltage error amplifier is used as reference by the current regulator and the output current error amplifier is connected to PWM controller at A prototype rated at 12kW, input line voltage equal to 380V and DC output voltage equal to 600V has been built and tested in the laboratory. The implemented circuit is shown in figure 16. i / BOOST 1. ;BOOST2 /...., l ~ l l h ~ : :..._.._.E :BOOST3!. +. Figure 16: Complete Circuit for one Boost Converter "33P i

7 The more relevance components and the main specifications are reported as follows. 0 Autotransformer and diode rectifiers The autotransformer circuit and the diode rectifiers are shown in figure Primary windings N(La, Lb, Lc) = 330 turns with 20AWG wire 9 Secondary windings N(Lan, Lbn, Lcn) = 40 turns with 15AWG wire 9 Secondary windings N(La1, Lbl, Lcl) = 114 turns with 15AWG wire 9 Secondary windings N(Lan, Lbn, Lcn) = 114 turns with 15AWG wire 9 Central area of the EE three-limb core = 27cm2 9 Three-phase bridge = SKD 30/08 A1 (Semikron) 0 Boost converters To reduce the stress current due the recovery diode and to reduce the turn-on losses on GBT, a regenerative snubber circuit (Ls, Dsl, Ds2 and CS) [05] is used. The more relevant components for boost converters are presented as follows. 9 Active switch S1= GBT (RG4PC3OW) > Boost inductors L11, L12 = 1mH (60 turns on core E65-26) Boost diodes D11, D12 = HFA15TB60 Snubber diodes Dsl, Ds2 = HFA15TB60 Snubber inductor Ls = 2pH (4 turns on core E30-7) Snubber capacitor Cs = 47pF Output capacitors CO = 470 pf 9 Current sensor = LA-25PN 9 PWM circuit = LM Operational amplifier = LM Optocoupler = HP 2211 Figure 17 shows a detail of the main switch tumon transition with the snubber circuit installed. TQK Stop. 250MW.S, and the THD of the input current measured are equal to 0.99 and 8.8%, respectively. TOK Run: 25.0WlS H KeS 22 Mar 1 QQQ Ref4 ioov 2.0Oms 16:11:41 Figure 18: nput Current and Voltage for Complete Circuit (2ms/Div, loov/div,.5a/div) The input phase voltage (Va) and the current through the input bridge (or through the secondary winding - an) are shown in figure 19. Rerl 10ov 2.00m.K 22 Mar !13!46 Figure 19: Bridge nput Current and Phase Voltage (2msDiv, OOVDiv, 2ADiv) Figure 20 shows the three currents displaced by 20 in phase, each one other. With this figure we can visualise one phase of each three-phase systems (al, an and a2). TOL Run. 25.0Wls HKS.. A All 1OOV 4 21 Marl Figure 17: Detail of the Main Switch Turn-on Transition (2OOnsDiv, 100VDiv, 2A/Div) Figure 18 shows the most important result of this work. t shows the waveforms for input current and input voltage in the same phase. We can observed the shape of input current between experimental result (fig. 19) and mathematical results (fig. 8) are the same. The input PF RP 22 Mar 1 QQ9 Ref3 10Omv Zooms 11:15:04 Figure 20: Bridge nput Currents for Each Three-phase System (2ms/Div, 2A/Div)

8 A detail of the boost inductor regulated current and the rectified voltage is shown in figure 21. Figure 21 : Boost nductor Current and Rectified Voltage (2Op/Div, 25OV/Div, 2ADiv) v. summary n this paper we presented a three-phase high input power factor rectifier, intended to be used in the design of (3 12kW rectifier unity for telecommunications. The converter is composed of a 18-pulse rectifier based on a differential Y-connected autotransformer and three six-pulse diode bridge, and three boost DC-DC converters;. From the studies reported in this paper, we draw the conclusion as follows: J The proposed circuit works according to the predicted models; J The input power factor and current THD are equal to 0.99 and 8.8%, respectively; J The low frequency 18-pulse autotransformer is rated at 22% of the output power. Therefore, the weight and volume are compatible with the telecclmmunication power supply specifications. J t employs three active switches that make this rectifier simpler and more reliable than the rectifier with six active switches. J t is controlled by very simple PWM dedicated integrated circuit, not requiring multipliers. 1 is the author s opinion that the proposed rectifier is a good candidate for 12 kw power supply design for telecommunications, with many advantages over the topologies presently being used in these applications. REFERENCES [Ol] J. W. Kolar, F. C. Zach A Novel Three-phase Utility nterface Minimizing Line Current Harmonics of High-Power Telecommunications Rectifier Modules, EEE Trans. on ndustrial Electronics, Vol. 44, pp Aug [02] Paice, Derek A. Power Electronic Converter Harmonic Multipulse Methods for Clean Power, N.Y., EEE Press, [03] S. Choi, P. N. Enjeti,. J. Pitel Polyphase Transformer Arrangements with Reduced kva Capacities for Harmonic Current Reduction in Rectifier-Type Utility nterface, EEE Trans. on Power Electronics, Vol. 11, pp , Set [04] G. Spiazzi, F. C. Lee mplementation of Single- Phase Boost Power-Factor-Correction Circuits in Three-phase Applications EEE Trans. on ndustrial Electronics, Vol. 44, pp Jun [05] A. Pietkiewicz and D. Tollik Snubber circuit and Mosfet Paralleling Considerations for High Power Boost-Based Power-Factor Correctors Proceedings of NTELEC 95, pp , ACKNOWLEDGEMENTS The authors gratefully acknowledge the valuable advice of ]:van Eidt Colling.