Unity Power Factor Control for Three Phase Three Level Rectifiers without Current Sensors Bingsen Wang Giri Venkataramanan Ashish Bendre Dept. of Electrical and Computer Engineering University of Wisconsin – Madison Madison, WI 53706 bingsen@cae.wisc.edu; giri@engr.wisc.edu Advanced Development Group DRS Power and Control Technologies Milwaukee WI, USA ashishrbendre@drs-pct.com Abstract— Three level rectifiers with reduced number of switches (such as the Vienna Rectifier) have been receiving wide interest in the past years to improve the input power quality of rectifier systems. In this paper, a new carrier-based pulse width modulation (PWM) control algorithm is proposed for such converters to eliminate the low frequency harmonics in the line current while achieving unity power factor at the rectifier input terminals. The operating constraints of the Vienna Rectifier with the carrier-based modulation strategy are examined carefully and the proposed control algorithm ensures that appropriate voltage/current directional constraints are met. A promising cost reduction opportunity can be seen with elimination of input current sensing to operate the Vienna Rectifier. The control algorithm is verified via Saber simulation and experimental results. Keywords - Unity power factor; Vienna Rectifier; Phase angle control; Carrier based PWM; I. INTRODUCTION New generations of adjustable speed drive and power supply producers are increasingly incorporating input power factor and waveform control to comply with various regulatory standards [1]. From time to time, specific applications such as aerospace power require careful regulation of the power converter front-end line current harmonics to minimize undesired interaction among the equipment connected to the same utility grid. A slew of new topologies including the ones based on three level power conversion have been proposed to realize high quality input waveforms [2-6]. Among these, the topology proposed by Kolar in 1994, called the Vienna Rectifier has the additional benefits of reduced controlledswitch count in addition to the general the benefits of threelevel converter. Thus, due to the opportunities of competitive cost reduction, Vienna Rectifier is generally considered attractive [5, 6]. Several control approaches have been proposed for the Vienna Rectifier. The initial application proposed the use of a hysteresis current controller. The switching signals are generated by comparison of reference current template (sinusoidal) and the measured mains currents [4]. Although this approach is easy to implement, the switching frequency is not constant, which is not desired for some applications because of inter-harmonics. The ramp comparison current control IAS 2005 presented in [6] derives the duty cycle by comparison of current error and the fixed-frequency carrier signal. The ripple current in the input inductor makes the current error noisy even though synchronization is carefully considered. The space vector modulation was analyzed in [3] with recognition of the constraints imposed by the unidirectional nature of the rectifier. For all the approaches mentioned above, the measurement of input currents is necessary. In this paper a new PWM control algorithm without input current sensing is proposed. Reactive component design considerations of three level converters are presented in Section II. In Section III, the operating limitations of the Vienna Rectifier are examined under the reduced switch realization. In Section IV, the state space averaged model for the Vienna Rectifier is developed. Then the proposed control algorithm and the controller design suitable for the Vienna Rectifier are presented in Section V. Computer simulations results verifying the proposed approached are presented in Section VI, backed up by experimental results in Section VII. The concluding section presents a summary of the paper. II. REACTIVE ELEMENT DESIGN CONSIDERATIONS A general three-phase three-level rectifier using ideal single pole triple throw (SPTT) switches is illustrated in Figure 1. The realization of the SPTT switch will be discussed in the subsequent section. Appropriate sizing of the reactive elements is crucial to fulfill the purpose of this active front-end instead of passive diode bridge, which is typically to meet the line current harmonic requirement prescribed by the regulation standards or certain applications. The sizing of the input inductors for three-level converters is very similar to the twolevel converter case. With proper modulation, the harmonics of pole voltages are pushed close to switching frequency and higher. For the same converter rating and harmonic specifications, the three-level converter input inductor is smaller since the peak-to-peak ripple voltage seen by the inductor is only half of the dc bus voltage. However, the dc-link capacitor sizing is quite different due to the third harmonic current flowing to the dc bus neutral point. For typical modulation schemes, the switch in each phase leg will be modulated between the top throw and the mid point for half cycle while in the other half cycle the switch will 1677 0-7803-9208-6/05/$20.00 © 2005 IEEE be toggled between the bottom throw and the neutral point. Thus, during any interval of π/3, the charging current to the top dc capacitor will be from either one phase-leg or two phaselegs. The dc link neutral current contains low frequency third harmonics due the inherent feature of three-level converters, which will usually increase the necessary dc link capacitor sizing unless special modulation function is applied [2]. The typical neutral current and its spectrum from sinusoidal PWM modulation are shown in Figure 2. The resulting size of the capacitor is generally bigger than the critical minimum value considered from dynamic interaction between the dc link capacitor and the input inductors [7]. This is in contrast with two-level converters, in which case, all three phase-legs supply the dc link current during each switching period. Thus, the dc bus sees no low frequency harmonics. La ia Lb + van vbn ib Lc + Cdc ic A La Qb RL Qc Db2 Lb ib ia + vbn + ic + vcn - 2 Da1 Cdc + vcn N RL Qa Cdc Da2 Da1 + Vdc - 2 +V Cdc A Qa ia - 2 0.006 0.008 0.01 0.012 0.014 0.016 Cdc t (s) 10 A 5 ia Cdc Da2 0 20 40 60 80 SWITCH REALIZATION CONSTRAINTS The ideal SPTT switch can be realized using different combinations of controlled switches and diodes. One of the realizations is the unidirectional topology with reduced count of controlled switches is the Vienna Rectifier as shown in Figure 3. In each phase-leg, only one controlled switch is used. With assumption of continuous conduction mode (CCM), the IAS 2005 +V dc - 2 RL +V Cdc A Cdc ia Vdc - 2 N RL Qa dc - 2 + Da2 +V dc - 2 (d) Figure 4. Conduction paths for phase-leg A when (a) the line current is positive and the controlled switch is off; (b) the line current is positive and the controlled switch is on; (c) the line current is negative and the controlled switch is off; (d) the line current is negative and the controlled switch is on; Figure 2. Waveform of the neutral point current and its spectrum for sinusoidal modulation. III. (c) 100 f (×60 Hz) Cdc Da1 + Vdc - 2 N Qa Da2 RL (b) Da1 10 + Vdc - 2 N dc (a) 0.004 2 Figure 3. Schematic of the power circuit of the Vienna Rectifier + Vdc 0 0.002 Vdc - Dc2 Lc 2 n RL ia 0 Vdc N C Da2 10 FFT(iN) (A) + Cdc A iN (A) B Qa Figure 1. Schematic of the three-phase three-level rectifier using ideal SPTT. 0 Dc1 - van N Db1 Cdc + + V dc - 2 Cdc Da1 rectifier pole voltages (vAN, vBN, vCN) have a definite state determined by on/off states of controlled switches and the polarity of line currents at any instant of operation. For instance, if the line current ia is positive and the controlled switch Qa is off, the voltage between the converter pole A and dc bus midpoint N vAN is Vdc/2. The conduction path for this case is illustrated in Figure 4. (a). If the line current ia is positive and the controlled switch Qa is on, the voltage vAN is 0, 1678 0-7803-9208-6/05/$20.00 © 2005 IEEE in which case the conduction path is illustrated in Figure 4. (b). Similarly, if the line current ia is negative, the voltage vAN can be either -Vdc/2 if the switch Qa is off or 0 if the switch Qa is on as illustrated in Figure 4. (c) and (d), respectively. This operating principle also applies to phase legs B and C. To avoid low frequency (lower than the switching frequency) harmonics in line currents, the rectifier phase voltages must be free of low frequency harmonics except for triplen harmonics, which may present on the modulation signals to increase the fundamental component without invoking over-modulation. Under CCM an important operating constraint can be recognized. If continuous sinusoidal PWM is used, the polarity of the line currents and the polarity of the imposed line to neutral voltage from the switching devices have to be identical. In the past this has been referred to as the pulse polarity consistency rule (PPCR) [8]. Thus on an averaged basis, the line currents have to be in phase with corresponding pole to neutral voltages. Otherwise, low frequency harmonic distortion will occur in both line currents and pole voltages. This requirement is equivalent to the unity power factor at the rectifier poles (NOT at the source voltages). On the other hand, under space vector modulation mode the input power factor angle at the rectifier input terminals may lie between (-π/6, π/6) [9]. Although this appears to be a drawback, realistic value of input inductors lead to a power factor at the line terminals to be greater than 0.98 for typical cases. IV. STATE SPACE AVERAGED MODEL With CCM, the SPTT in each phase leg can be replaced by the averaged model and the resulting equivalent circuit is shown in Figure 5. In this model, the harmonics close to switching frequency are neglected and the pole voltages only contain the fundamental component and possible third harmonics coming from the modulation signals. However, the third harmonics do not have effect on the line currents for three-wire configuration. So, in the subsequent analysis, only the fundamental component of the pole voltages is considered. 1 Vdc a a θ M cos ( M θ − I Lθ ) Vs cos ( − I L ) − L 2 I La d θ 1 a V I L = a Vs sin ( − I Lθ ) − dc M a sin ( M θ − I Lθ ) − ωe dt LI 2 Vdc L 1 3 2 V I La M a cos ( M θ − I Lθ ) − dc C2 RL where ILa and ILθ are amplitude and phase angle input current vector (also called dynamic phasor), respectively [10]. Similarly, Ma and Mθ are amplitude and phase angle of the modulation index space vector, respectively. Here, the phase of the three-phase ac source voltage is chosen as reference. This dynamic model has been shown to be convenient for regulator design based on phase angle coordinates. Under steady state, the dynamic model (2) reduces to a set of phasor relations visualized in Figure 6. Due to the operating constraints imposed by particular switch realization, the line currents are in phase with rectifier pole voltages. The phase angle between the source voltages and pole voltages is determined by the phase angle of modulation functions. It may be noticed that if the pole voltage is modulated such that it stays on the trajectory indicated by the dotted half circle, then the line current is forced to be in phase with the pole voltage. Furthermore, the phase angle between the source and pole voltages determines the active power flow, as is classically formulated in ac power systems theory. For balanced operation, the power flow to the rectifier is given by The state space model for the averaged system can be described by state equation (1). dx = Ax + Bu dt where x = [ia 0 0 A= 0 ma1 C dc ma 2 C dc ib ic 0 0 − ma1 La 0 0 − mb1 Lb 0 0 m − c1 Lc mb1 Cdc mc1 Cdc −1 RLCdc mb 2 Cdc mc 2 Cdc 1 RLCdc (1) ma 2 La m − b2 Lb mc 2 − Lc 1 RLCdc −1 RLCdc , vbn B= 1 La 0 0 1 Lb 0 0 0 0 0 0 0 0 1 Lc 0 0 van - La + a vbn n + + ib b ma1vc1+ma2vc2 ic c +- ma1ia+mb1ib+mc1ic Cdc mb1vc1+mb2vc2 B Lc (3) 2ωe L A Lb vcn - ia 3VsaVdc M a sin ( M θ ) +- Cdc mc1vc1+mc2vc2 C N ma2ia+mb2ib+mc2ic + vc1 - RL vc2 + +- Van , Μθ j*IaωL Ia VAN vcn ] . T With assumptions of La = Lb = Lc = L, and vc1 = vc2 = Vdc/2, transforming three phase abc quantities to synchronous reference frame in polar coordinates results in IAS 2005 P=− Figure 5. State space average model of the Vienna rectifier. − T vc1 vc 2 ] , u = [ van (2) 1679 Figure 6. Phasor diagram of unity-power-factor constraint at the rectifier input. 0-7803-9208-6/05/$20.00 © 2005 IEEE Notice the maximum power transfer occurs when Mθ = π/4. Pmax = − 3 (V ) a 2 s van - 2ωe L ma1ia+mb1ib+mc1ic vbn n Lb + Lc + - mc1vc1+mc2vc2 +- mα van vbn vcn I LAωe 0 0 M − 2L , 0 2 − RLC 0 A l = − Vdc M B 2 I LA L 0 polar Ma a rect. Vs vβ Cdc ma2ia+mb2ib+mc2ic vc1 RL vc2 + abc mβ rect. Μθ+Vsθ Vsθ polar + - Μθ * + PI regulator Vdc Figure 7. Block diagram of the proposed controller. 40 Magnitude (dB) 20 0 20 40 60 80 100 3 1 .10 4 3 1 .10 4 1 10 100 1 .10 1 10 100 1 .10 180 Phase (degree) (5) 90 0 90 180 i , x = i , vdc + cos Vdc To design the PI regulator, the transfer function between the phase angle Mθ and Vdc is derived from the state space averaged model formulated in dynamic phasors. The model described in (2) is nonlinear with respect to the control input Mθ. By linearizing the system at the nominal operating point, small signal model is obtained. A vα αβ A new control algorithm is proposed based on the phase angle control as shown in Figure 7. Only a voltage loop is used to regulate the dc bus voltage. No current sensing/estimation is needed in this algorithm. The error between the dc voltage reference and the dc voltage feedback is processed by the PI regulator. The output of the PI regulator becomes the phase angle difference between the source voltage and pole voltage. The phase angle difference is also used to calculate the modulation index space vector. Typically, the phase angle of the modulation index space vector is small. The amplitude of the modulation index vector varies only by a small amount during normal operation. So the power flow is controlled mainly by the phase angle Mθ, although the proper amplitude Ma is critical to ensure the unity power factor at the rectifier poles to achieve good quality of the input current waveforms. d x l l = Ax + Bu dt N αβ abc Cdc mb1vc1+mb2vc2 +- vcn The common practice of controlling the three phase boost rectifier, either two level or three level topology, resorts to multi-loop control [7, 11, 12]. Typically, the dc link voltage reference is compared against the feedback voltage and the error is fed to a PI regulator. The output of the PI regulator becomes the current reference. The inner current loop uses a proportional gain to calculate the required pole voltage reference. Due to the bandwidth limitation of the inner current loop, the controller is preferred to be implemented in the synchronous reference frame, which is complicated. Particularly to the Vienna Rectifier, the controller is even more complicated due to switch realization. The space vector modulation is often the choice to stay within the constraints. IAS 2005 +- PROPOSED CONTROLLER V. u = m Θ . ma1vc1+ma2vc2 (4) The maximum power point typically corresponds to unpractical large input current, which would not be suitable for practical applications. 0 where lA = − ωe A IL 3 M A 2 C La + A L Θ L Frequency (Hz) Figure 8. Bode plot of loop gain transfer function with a PI regulator. The small signal transfer function between mΘ and vdc may be determined to be G ( s) = 1680 vdc = G0 ⋅ mΘ 1 s s2 1 s + 2) (1 + )(1 + ωr Qc ωc ωc (6) 0-7803-9208-6/05/$20.00 © 2005 IEEE (V) : t(s) , 50.0 8ω 22 L V_ab 0.0 −50.0 2 1 ωc , and Go = − 3Vdc M A ⋅ R 2 ωr ωc 8ωe L 2 −1 ωr −100.0 (A) : t(s) 4.0 50.0 (V) It is interesting that the control transfer function of a three phase boost derived Vienna rectifier features a third order system with a real pole and a pair of complex poles under angle control. This is in stark contrast to more classical control approaches exhibiting a right half plane zero in their dynamics. 0.0 i_a 2.0 0.0 (V) : t(s) V_AN −2.0 −50.0 −4.0 0.25 A proportional and integral (PI) regulator is designed to stabilize the system by shaping the loop gain transfer function with adequate phase margin and gain margin. The compensated loop gain transfer function is plotted in Figure 8. The loop gain gives about 60 degrees phase margin and 10 dB gain margin. The bandwidth of the closed loop system is about 20 Hz. 0.26 0.27 0.28 0.29 t(s) Figure 9. Steady state simulation results of salient quantities with the proposed control algorithm: top two traces are sinusoidal line-to-line source voltage Vab and modulated pole-to-pole voltage VAB; bottom two traces are sinusoidal line current ia and modulated pole-to-neutral voltage VAN. (V) : t(s) 2.0 SIMULATION RESULTS A detailed Saber model has been built to verify the control algorithm. The salient parameters used in the small scale simulation model are listed in TABLE I. V_AN 50.0 (A) VI. V_AN 100.0 (V) RC + 2 2 , ω = ω2 + 3 M A c e 4 LC (A) Qc = ωr = 1 3M A2 R 0.0 (V) where (A) : t(s) 0.0 i_a −50.0 SALIENT PARAMETERS USED IN SIMULATION −2.0 2.0 35 Vrms 100 V 60 Hz 5 kHz 10 mH 0.2 Ω 180 µF 96 Ω V_AN 50.0 0.0 (V) Source voltages Van = Vbn = Vcn Dc link voltage Source Frequency Switching frequency Input inductances La = Lb = Lc Input resistances Ra = Rb = Rc DC capacitance Cdc Nominal load resistance R (A) TABLE I. (A) : t(s) 0.0 i_a −50.0 −2.0 0.265 0.27 0.275 0.28 0.285 t(s) The transient response of the pole-to-pole voltage vAB, input current ia and dc bus voltage Vdc to a step-up and step-down change of the output load is shown in Figure 11. a and Figure 10. Simulation of line current ia and pole voltage VAN for case (a) the magnitude of the modulation index is NOT controlled properly (upper panel) and case (b) with proper control of modualtion index (lower panel). V_AB 100.0 (V) 50.0 0.0 −50.0 −100.0 (A) 0.0 i_a 2.0 100.0 50.0 (A) : t(s) 4.0 150.0 (V) Under steady state operation, sinusoidal line-to-line source voltage Vab and modulated pole-to-pole voltage VAB are shown in the top panel of Figure 9. It is observed that the pole voltage at the rectifier slightly lags the source voltage. The small phase angle difference between the source voltage and the pole voltage results in appropriate power flow to regulate the dc bus voltage. The sinusoidal line current ia and modulated pole-toneutral voltage VAN are exactly in phase as shown in the lower panel of Figure 9. It is critical to satisfy the phasor relation defined by the diagram in Figure 6. Otherwise, the reduced switch realization of the converter results in low frequency distortion. To illustrate this point, distorted line current and pole voltage due to violation of the necessary phasor relation are shown in the top panel of Figure 10. compared to the proper line current and pole voltage shown in the lower panel of the same figure. In addition, the dc link voltage has more ripple voltage not shown in the figure. 0.0 (V) : t(s) Vdc −2.0 −4.0 0.24 0.26 0.28 0.3 0.32 0.34 0.36 0.38 0.4 0.42 t(s) (a) IAS 2005 1681 0-7803-9208-6/05/$20.00 © 2005 IEEE v AB 50 V/div V_AB 100.0 (V) 50.0 0.0 −50.0 −100.0 i_a 2.0 100.0 (A) (V) v ab 25 V/div (A) : t(s) 4.0 150.0 50.0 0.0 0.0 v AN 25 V/div (V) : t(s) ia 1 A/div Vdc −2.0 −4.0 0.24 0.26 0.28 0.3 0.32 0.34 0.36 0.38 0.4 0.42 t(s) (b) 5 ms/div Figure 11. Simulation of the dyanmic response of the pole-to-pole voltage vAB (upper panel), ac line current ia and dc bus voltage Vdc (lower panel) to a load step change: (a) load step-up; (b) load step-down. Figure 12. Steady state experimental results of salient quantities with the proposed control algorithm: top two traces are sinusoidal line-to-line source voltage Vab and modulated pole-to-pole voltage VAB; bottom two traces are sinusoidal line current ia and modulated pole-to-neutral voltage VAN. Figure 11. b, respectively. During the transients, both the poleto-pole voltage and the line current off-tune, but return to normal waveforms after the dc link voltage transient dies out and the rectifier settles to a new operating point. Line current ia 0.5 A/div VII. EXPERIMENTAL RESULTS In order to verify the proposed controller and the simulations, experiments have been conducted on a laboratory scale prototype. The steady state measurements are recorded in Figure 12. , which shows the line-to-line voltage vab and poleto-pole voltage vAB in the upper panel and line current ia and pole-to-neutral voltage vAN in the lower panel. Similar to the simulation results, leading phase angle of the source voltage with respect to the pole voltage and in phase between the line current and the pole voltage have been observed. In the experiment, the identified operation constraints are deliberately violated and the ac line current distortion is obvious as shown in the upper panel of Figure 13. Even the pole-to-neutral voltage is distorted due to the distorted zero-crossing of the line current. The transient response of the controller has been tested against the step changes in load. The pole-to-pole voltage vAB, input current ia and dc bus voltage Vdc to a step-up and stepdown change of the output load are shown in Figure 14. a and Figure 14. b, respectively. During the transient, both the poleto-pole voltage and the line current are off-tune, but return to normal waveforms after the dc link voltage transient dies out and the rectifier settles to a new operating point. The strong agreement between the experimental results and the simulation results are readily evident from the figures. VIII. CONCLUSIONS This paper has presented design oriented analytical solutions addressing the application of three level rectifiers, with particular focus on the Vienna rectifier. The harmonic loading on the dc bus capacitor due to circulating neutral currents have been identified as an issue to be considered in si- IAS 2005 Pole voltage v AN 25 V/div Line current ia 0.5 A/div Pole voltage vAN 25 V/div 2 ms/div Figure 13. Experimental measurement of line current ia and pole voltage VAN for case (a) the magnitude of the modulation index is NOT controlled properly (upper panel) and case (b) with proper control of modualtion index (lower panel). zing the dc bus capacitor. The operating constraint introduced by the reduced switch realization, namely unity power factor at the rectifier poles, as opposed to source terminals, is identified under typical carrier-based PWM mode control. Although the phase angle difference between the source voltage and pole voltage is small (less than a few degrees) for typical inductor values, unity power factor at pole voltages is critical in ensuring optimal ac line currents quality. For a particular design, the phase angle difference may be so small that the current distortion may not be obvious with unity power factor at ac sources. A new PWM control algorithm based on phase angle control is proposed to ensure operating constraints of the 1682 0-7803-9208-6/05/$20.00 © 2005 IEEE reduced switch realization are met. The system is modeled using dynamic phasor and the controller is designed based on that model. The proposed control algorithm is verified by Saber simulation and laboratory experiment. The match between the simulation and experimental results validates the modeling and the controller design. This new control algorithm is relatively simple compared to inner-current-outer-voltage multi-loop controller. Constant switching frequency provides better line current spectrum compared to the hysteresis current controller. Furthermore, this controller offers promising cost reduction opportunity by eliminating the line current sensors. v AB 50 V/div ia 1 A/div v dc 50 V/div 20 ms/div (a) vAB 5 0 V /d iv ia 1 A /d iv v dc 5 0 V /d iv 2 0 m s /d iv (b) ACKNOWLEDGMENT The authors would like to acknowledge support from The Boeing Company and the sponsors of the Wisconsin Electric Machine and Power Electronics Consortium (WEMPEC) at the University of Wisconsin-Madison. The work made use of ERC shared facilities supported by the National Science Foundation (NSF) under AWARD EEC-9731677. REFERENCES [1] T. S. Key and J. S. Lai, "Comparison of standards and power supply design options for limiting harmonic distortion in power systems," Industry Applications, IEEE Transactions on, vol. 29, pp. 688, 1993. [2] A. Bendre and G. Venkataramanan, "Modeling and design of a neutral point regulator for a three level diode clamped rectifier," in Conference Record of the 2003 IEEE Industry Applications Conference (Cat. No.03CH37459), Salt Lake City, UT, USA, 2003, pp. 1758. [3] J. W. Kolar and U. Drofenik, "A new switching loss reduced discontinuous PWM scheme for a unidirectional threephase/switch/level boost-type PWM (VIENNA) rectifier," in 21st International Telecommunications Energy Conference. INTELEC '99 (Cat. No.99CH37007), Copenhagen, Denmark, 1999, pp. 10 pp. [4] J. W. Kolar and F. C. Zach, "A novel three-phase utility interface minimizing line current harmonics of high-power telecommunications INTELEC. Sixteenth International rectifier modules," in Telecommunications Energy Conference (Cat. No.94CH3469-4), 1994, pp. 367. [5] N. B. H. Youssef, F. Fnaiech, and K. Al-Haddad, "Small signal modeling and control design of a three-phase AC/DC Vienna converter," in IECON'03. 29th Annual Conference of the IEEE Industrial Electronics Society (IEEE Cat. No.03CH37468), Roanoke, VA, USA, 2003, pp. 656. [6] U. Drofenik and J. W. Kolar, "Comparison of not synchronized sawtooth carrier and synchronized triangular carrier phase current control for the VIENNA rectifier I," in ISIE '99. Proceedings of the IEEE International Symposium on Industrial Electronics (Cat. No.99TH8465), Bled, Slovenia, 1999, pp. 13. [7] B. Shi, G. Venkataramanan, and N. Sharma, "Design Considerations for Reactive Elements and Control Parameters for Three Phase Boost Rectifiers," in Proceedings of International Electric Machines and Drives Conference, Antonio, Texas, 2005, pp. 1757-1764. [8] J. Zubek, A. Abbondanti, and C. J. Norby, "Pulsewidth modulated inverter motor drives with improved modulation," IEEE Transactions on Industry Applications, vol. IA11, pp. 695, 1975. [9] Y. Zhao, Y. Li, and T. A. Lipo, "Force commutated three level boost type rectifier," IEEE Transactions on Industry Applications, vol. 31, pp. 155, 1995. [10] G. Venkataramanan and B. Wang, "Dynamic modeling and control of three phase pulse width modulated power converters using phasors," in 2004 IEEE 35th Annual Power Electronics Specialists Conference (IEEE Cat. No.04CH37551), Aachen, Germany, 2004, pp. 2822. [11] M. Malinowski, M. Jasinski, and M. P. Kazmierkowski, "Simple direct power control of three-phase PWM rectifier using space-vector modulation (DPC-SVM)," IEEE Transactions on Industrial Electronics, vol. 51, pp. 447, 2004. [12] P. Barrass and M. Cade, "PWM rectifier using indirect voltage sensing," IEE Proceedings: Electric Power Applications, vol. 146, pp. 539, 1999. Figure 14. Exeprimental results of the dyanmic response of the pole-to-pole voltage vAB (upper panel), ac line current ia and dc bus voltage Vdc (lower panel) to a load step change: (a) load step-up; (b) load step-down. IAS 2005 1683 0-7803-9208-6/05/$20.00 © 2005 IEEE