Modeling and Analysis of Common-Mode Voltages Generated in Medium Voltage PWM-CSI Drives

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 18, NO. 3, MAY 2003 873 Modeling and Analysis of Common-Mode Voltages Generated in Medium Voltage PWM-CSI Drives José Rodríguez, Senior Member, IEEE, Luis Morán, Senior Member, IEEE, Jorge Pontt, Member, IEEE, Ruben Osorio, and Samir Kouro Abstract Common-mode voltages appear in pulse-width modulated current-source inverters (PWM-CSIs) drives due to the operating principles of the input rectifier and the output inverter. This paper presents the modeling and analysis of a medium voltage current-source inverter drive, using the Matlab software. Simulated results of the model are in close agreement with experimental waveforms obtained from an industrial ac drive, which shows overvoltages of up to 100%, generating important insulation stresses at the motor stator terminals. Index Terms Common-mode voltages, medium voltage ac drives, PWM-CSI. NOMENCLATURE Three phase source terminals. Motor terminals. Ground of the three-phase source. Neutral point of the motor. Rectifier output terminals. Inverter input terminals. Voltage of phase of the source with respect to the ground. Voltage of phase of the motor with respect to neutral. I. INTRODUCTION IN THE last decade, an important development in medium voltage drives has been observed [1] [3]. A very important aspect in the use of these converters is the voltage stress presented at the motor terminals, especially in retrofit applications. Usually, the isolation of the stator windings in medium voltage motors is not so oversized as in low voltage machines. An important source of overvoltages, are the common-mode voltages generated by rectifiers and inverters when they change their topology, due to the commutation of the power semiconductors. This aspect is especially important in pulse-width modulated current-source inverter (PWM-CSI) drives and has received increasing attention lately [4] [6]. In addition, PWM converter-fed drives have been found to be a very important Manuscript received June 15, 2001; revised November 1, 2002. Recommended by Associate Editor A. M. Trzynadlowski. J. Rodríguez, J. Pontt, and S. Kouro are with the Electronics Engineering Department, Universidad Técnica Federico Santa Maria, Valparaíso, Chile (e-mail: jrp@elo.utfsm.cl; jpo@elo.utfsm.cl; skouro@terra.cl). L. Morán is with the Electrical Engineering Department, University of Concepción, Concepción, Chile (e-mail: lmoran@manet.die.udec.cl). R. Osorio is with the Metallurgical Department, Mantos de Oro, Santiago, Chile (e-mail: osorio@ctcreuna.cl). Digital Object Identifier 10.1109/TPEL.2003.810855 source of bearing failures in motors [7]. These failures are generated by bearing currents originated by high in the motor voltages [8]. This paper presents the modeling and analysis of this problem using Matlab and shows experimental current and voltage waveforms obtained in a 4.16 kv industrial drive. II. DESCRIPTION OF THE DRIVE A. Power Circuit Fig. 1 shows the power circuit of the ac medium voltage PWM-CSI drive that will be analyzed in this paper. The phasecontrolled rectifier at the input side has six thyristors and controls the magnitude of the dc-link current, delivered to the inverter. The inverter has six gate turn off thyristors and generates the PWM current. Fig. 2 illustrates the waveform of the output current of phase, generated by the inverter. The dc-link filter composed by inductances and is used to attenuate the dc-link ripple current delivered by the rectifier. The capacitor filter connected at the output of the inverter avoids the generation of overvoltages in the stator leakage inductances, due to fast commutation of the inverter current. In addition, these capacitors generate a more sinusoidal voltage at the motor terminals. B. Drive Control Scheme In this paper, the trapezoidal modulation method was used for low frequencies and the selective harmonic elimination method was applied for higher frequencies, in order to generate the PWM current shown in Fig. 2 [1]. The field-oriented method, was used to control the torque and speed of the motor. This method operates with a model of the motor expressed in a rotating frame oriented with the rotor flux, controlling separately the stator current component that is proportional to the torque and the stator current component that is proportional to the flux [9]. The dc-link current is changed using a closed loop control system that includes the input rectifier. III. VOLTAGES IN THE DRIVE SYSTEM A. Voltages Generated by the Rectifier Referring to Fig. 1, and are the voltages of points 1 and 2 with respect to the ground. Fig. 3 shows the voltage waveforms when the rectifier is connected to a three-phase source of 4160 V between lines and operates with a firing angle of. This figure also shows the output voltage of the rectifier, which is obtained from (1) 0885-8993/03$17.00 2003 IEEE

874 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 18, NO. 3, MAY 2003 Fig. 1. Power circuit of PWM-CSI drive. Fig. 2. PWM output current i (t). Fig. 4. Voltages generated at the inverter input terminals with the motor running at 1400 rpm without load: (a) Inverter dc input voltage, v ; (b) inverter dc input voltage, v ; (c) inverter input voltage v ; (d) inverter common-mode voltage v -. Fig. 3. Voltages generated by the rectifier with =90 : (a) rectifier dc output voltage v ; (b) rectifier dc output voltage, v ; (c) rectifier output voltage v ; (d) rectifier common-mode voltage v -. rent source inverter without modulation, when the motor is operating without load at 1400 rpm. In addition, Fig. 4 shows the input voltage of the inverter, and the common-mode voltage - generated by the inverter. Voltage is obtained from In addition, Fig. 3 presents the common-mode voltage of the rectifier, defined by On the other hand, voltage - is derived from (3) - (2) - (4) B. Voltages at the Input of the Inverter Fig. 1 illustrates voltages and that represent the voltages of points 3 and 4 at the input of the inverter with respec to the neutral of the motor. Fig. 4 shows these voltages in a cur- C. Neutral to Ground Voltages The analysis of the voltage of the neutral point of the load ( ) with respect to ground ( ) contributes to understanding the behavior of the drive.

RODRÍGUEZ et al.: MODELLING AND ANALYSIS OF COMMON-MODE VOLTAGES 875 Fig. 5. SIMULINK block diagram of the drive. TABLE I SIMULATION RATED VALUES Fig. 6. Steady-state waveforms of the input rectifier at rated load: Rectifier output voltage, v (t); output current, i (t); Rectifier input voltage, v (t); input current i (t). The voltages at the dc reactor terminals are given by (5) Referring to Fig. 1, the voltage of the neutral is given by to ground The addition of (7) and (8), and considering (5) and (6), gives the total ground to neutral voltage between the ac source and the motor, that is (6) (7) (8) (9) Fig. 7. Inverter steady-state waveforms at rated load: Inverter output current i (t); motor voltage v (t); motor current i (t); capacitor current i (t).

876 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 18, NO. 3, MAY 2003 Fig. 8. Transient waveforms of the drive in response to a step change in the load torque: Motor speed! (t); motor torque T (t); inverter output current i (t); motor voltage v (t), motor current i (t). Fig. 9. Voltage waveforms of the drive at rated load with L = L =57mH. Phase to neutral motor voltage v (t); drive neutral to ground voltage v (t); motor phase to ground voltage v (t). And considering the definitions of common-mode voltages given by (2) and (4), the following equation is obtained: - - (10) Effect of the DC Reactors: Equation (10) shows clearly that the use of only one reactor ( ) increases the value of. The influence of the filter reactors in the neutral-ground voltage can be eliminated by choosing. Effect of the Common-Mode Voltages: Equation (10) illustrates that the difference of the common-mode voltages affects the voltage. Apparently, this produces a reduction in the neutral to ground voltage, but this is not correct. In effect, the commutations in the common-mode voltages - and - are completely asynchronous [see Figs. 3(d) and 4(d)], and so, during some intervals they present different polarities. This produces an additive effect that increases the neutral-ground voltage, which can have the same magnitude of the motor phase voltage. D. Phase to Ground Voltages It is well known that the PWM-CSI inverter delivers almost sinusoidal phase to neutral voltages to the motor, equal to the inverter output capacitor filter voltage, which is considered to be a very important advantage of this topology, compared to the voltage-source topology. However, the phase to ground voltage is the variable that gives the information necessary to evaluate the voltage stress across the motor terminals. The motor isolation must withstand the phase to ground voltage. The motor phase to ground voltage can be expressed as (11) As shown in (11), the phase-ground voltage depends on the phase to neutral voltage and on the neutral-ground voltage. The frequency of voltage is completely independent to the frequency of voltage. Therefore, the phase to ground voltage can reach a magnitude as high as two times voltage. Fig. 10. Voltage waveforms of the drive at rated load with L =114mH and L =0mH. Phase to neutral motor voltage v (t); drive neutral to ground voltage v (t); motor phase to ground voltage v (t). IV. SIMULATION OF THE DRIVE The power and control circuits of the drive are simulated using Matlab, in order to obtain the associated current and voltage waveforms. In this analysis, the converter power semiconductors (SCRs and GTOs) are considered as ideal switches. Fig. 5 shows the SIMULINK block diagram of the drive. Block 1 includes the dc-link current controller, the speed controller and the flux controller. This block generates the mean value of the voltage that must be generated by the rectifier ( ), and the angle of the current vector generated by the inverter. Block 2 represents the rectifier and generates the voltage applied to the dc-link reactor. The inverter (Block 4) uses the dc-link current, and the angle to produce the modulated currents,, and. Block 5 corresponds to the capacitors used to filter the current received

RODRÍGUEZ et al.: MODELLING AND ANALYSIS OF COMMON-MODE VOLTAGES 877 Fig. 11. Twelve pulse CSI-Drive power circuit. TABLE II EXPERIMENTAL RATED VALUES from the inverter, delivering nearly sinusoidal voltages and currents to the motor. The three-phase induction motor represented by block 6, receives the phase voltages,,, and delivers the rotor speed, the line currents,,, and the electromagnetic torque. Block 7 calculates the input voltage of the inverter delivered to block 3. Block 3 is the dc-link and calculates the dc-link current delivered to the inverter. Block 8 is a flux estimator, necessary to control the flux. Block 9 calculates the stator currents in the rotating frame. Fig. 12. Six pulse input current i (t). V. SIMULATION RESULTS An induction motor connected to a 4.16-kV 50-Hz threephase source, fed by a PWM-CSI inverter has been studied. The data of the motor and the inverter are shown in Table I of the Appendix. Fig. 6 illustrates the behavior of the input rectifier with the drive operating at rated conditions and at 1485 rpm. Fig. 6 also shows the output voltage of the rectifier, the DC-link current, the input voltage and the input current. Under the same operating conditions, Fig. 7 presents the behavior of the load side inverter, including the inverter output current, the motor voltage, the motor current and the capacitor current. It can be observed that the motor voltage has a reduced distortion, due to the connection of the output filter capacitor. Fig. 8 presents the transient behavior of the drive in response to a step change in the load torque from 0 Nm to the rated value of 2000 Nm. This figure includes the rotor speed, the electrical torque, the inverter current, the motor voltage Fig. 13. DC-link current I (t). and the motor current. This figure shows that the drive has a good dynamic behavior. Fig. 9 shows the behavior of the different voltages with the drive operating with rated load at 1400 rpm. It can be observed that the phase-neutral voltage is quite sinusoidal, but due to the presence of the neutral-ground voltage, the motor phase to ground voltage has an important distortion. In addition voltage has a maximum value of 2990 V and voltage has a higher value of 5990 V. In this case, the dc-link has two reactors mh. This figure must be compared with Fig. 10, obtained under similar operating conditions, but

878 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 18, NO. 3, MAY 2003 Fig. 14. Motor voltage v (t). Fig. 16. Neutral-ground voltage v (t). Fig. 15. Motor Current i (t). Fig. 17. Neutral-ground voltage v (t), experimental data obtained from [6]. with only one reactor: mh and mh. The motor phase to ground voltage of Fig. 10 reaches a peak value of 6700 V. VI. EXPERIMENTAL RESULTS Experimental results were obtained from an industrial drive using a 12-pulse input rectifier as shown in Fig. 11. The inverter was used to feed an induction motor rated 2.3 kv and 373 kw. Additional information of the drive can be observed in Table II of the Appendix. The drive was used in a pump, and the measurements were taken with an output frequency of 40 Hz. The input filter has three branches, tuned to the fifth, seventh, and eleventh harmonics. The first task of the input filter is to compensate the reactive power generated by the input rectifier. Filters tuned to the fifth and seventh harmonics are included to avoid parallel resonance at the line side. The filter of eleventh order reduces the propagation of current harmonic components 11 and 13. Figs. 12 15 were obtained with voltage and current transducers. Fig. 12 shows the input current waveform ( ) at the six-pulse rectifier, and Fig. 13 presents the dc-link current ( ). Fig. 14 confirms that the line motor voltage ( ) is highly sinusoidal due to the connection of the output filter capacitor. Also motor current ( ) shown in Fig. 15 is free from high frequency ripple, as predicted by the simulation. Fig. 18. Phase-ground voltage v (t), experimental data obtained from [6]. The neutral-ground voltage ( ) measured in Fig. 16, has an important value of 4.33 kv, which significantly increases the magnitude of the phase-ground voltage ( ). In addition, experimental measurements obtained in [6] are discussed. These results have been obtained for the same topology shown in Fig. 1, in a drive of 4.16 kv operating with an output frequency of 60 Hz. Fig. 17 shows that the neutral to ground voltage ( ) of the drive has a peak value of 2.5 kv and a frequency of 180 Hz, which corresponds to three times the output frequency. This result is in close agreement with the simulated waveform shown in Fig. 9.

RODRÍGUEZ et al.: MODELLING AND ANALYSIS OF COMMON-MODE VOLTAGES 879 Finally, both, simulation and experimental measurements presented in Fig. 9 and Fig. 18, respectively, show that the motor terminals have an important overvoltage with respect to ground ( ) of approximately 5.4 kv. This value is due to the presence of the common mode effect. VII. CONCLUSION The modeling and analysis of overvoltages in PWM-CSI drives due to common mode effects has been presented. A set of equations has been found, demonstrating, in a very simple form, the relation between neutral to ground voltage and common-mode voltages in the rectifier and at the input of the inverter. The drive produces a highly sinusoidal phase to neutral voltages at the motor terminals. However, there is an important displacement between motor neutral and ground. This produces overvoltages with amplitude equal to two times the phase to neutral motor voltage, originating an important stress to the stator windings isolation. The equations and simulations results presented in this paper, show that it is more advantageous to use equal filter inductances in the positive and in the negative busbars of the dc-link, producing smaller overvoltages. Experimental results obtained in the field with an actual industrial drive, confirm the presence of overvoltages generated by the drive and the validity of the model developed in this paper. REFERENCES [1] B. Wu, G. Slemon, and S. Dewan, PWM-CSI induction motor drives with phase angle control, in Proc. IEEE Ind. Applicat. Soc. Annu. Meeting, vol. 1, 1989, pp. 674 679. [2] J. K. Steinke and M. K. Buschmann, Robust and reliable medium voltage PWM inverter with motor friendly output, in Proc. Eur. Power Electron. Conf. (EPE 97), Norway, 1997, pp. 3.502 3.507. [3] P. Hammond, A new approach to enhance power quality for medium voltage AC drives, IEEE Trans. Ind. Applicat., vol. 33, pp. 202 208, Jan./Feb. 1997. [4] R. Quirt, Voltages to ground in load-commutated inverters, IEEE Trans. Ind. Applicat., vol. 24, pp. 526 530, May/June 1988. [5] B. Wu and F. DeWinter, Voltage stress on induction motors in medium voltage PWM GTO CSI drives, IEEE Trans. Power Electron., vol. 12, pp. 213 220, Mar. 1997. [6] J. Das and R. Osman, Grounding of AC and DC low voltage and medium voltage drive systems, IEEE Trans. Ind. Applicat., vol. 34, pp. 205 216, Jan./Feb. 1998. [7] D. Busse, J. Erdman, R. Kerkman, D. Schlegel, and G. Skibinski, Bearing currents and their relationship to PWM drive, IEEE Trans. Power Electron., vol. 12, pp. 243 252, Mar. 1997. [8] K. Ratnayake and Y. Murai, A novel PWM scheme to eliminate common-mode voltage in three-level voltage source inverter, in Proc. PESC 98 Conf., Japan, May 1998, pp. 269 274. [9] W. Leonhard, Control of Electrical Drives. Berlin, Germany: Springer-Verlag, 1985. Jose Rodríguez (M 81 SM 94) received the M.S. degree from the University Federico Santa Maria, Valparaiso, Chile, in 1977 and the Dr.Ing. degree from the University of Erlangen, Erlangen, Germany, in 1985, both in electrical engineering. Since 1977, he has been with the University Federico Santa Maria where he is currently a Professor and Head of the Department of Electronic Engineering. During his sabbatical leave in 1996, he was responsible for the mining division of the Siemens Corporation, Chile. He has a large consulting experience in the mining industry, especially in the application of large drives like cycloconverter-fed synchronous motors for SAG mills, high power conveyors, controlled drives for shovels, and power quality issues. His research interests are mainly in the area of power electronics and electrical drives. In the last few years, his main research interests are in multilevel inverters and new converter topologies. He has authored and co-authored more than 100 refereed journal and conference papers and contributed to one chapter in the Power Electronics Handbook (New York: Academic, 2001). Luis Morán (S 79 M 81 SM 94) was born in Concepción, Chile. He received the M.S. degree in electrical engineering from the University of Concepción, Concepción, Chile, in 1982, and the Ph.D. degree from Concordia University, Montreal, QC, Canada, in 1990. Since 1990, he has been with the Electrical Engineering Department, University of Concepción, where he is a Professor. He has written and published more than 20 papers in Active Power Filters and Static Var Compensators in IEEE TRANSACTIONS. He has extensive consulting experience in mining industry, especially in the application of medium voltage ac drives, protection systems, and power quality issues. His main areas of interests are in power quality, active power filters, FACTS, medium ac drives and power protection systems. Dr. Moran received the IEEE Outstanding Paper Award from the Industrial Electronics Society for the best paper published in the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS during 1995 and the City of Concepción Medal of Honor for achievement in applied research, in 1998. From 1997 to 2001 he was Associate Editor of the IEEE. Jorge Pontt (M 00) received the B.S. and M.S. degrees in electrical engineering from the Universidad Técnica Federico Santa María (UTFSM), Valparaíso, Chile, in 1977. Since 1977, he has been a Professor with the Department of Electrical Engineering and the Department of Electronic Engineering, UTFSM. He has had scientific stays at the Technische Hochschule Darmstadt (1979/80), University of Wuppertal (1990), and University of Karlsruhe (2000/2001), Germany. He has authored and co-authored more than 90 refereed journals and conference papers. Since 1980, he has been engaged with applied research and extensive consulting in mining industry and industrial applications. His current research interests include harmonic analysis, power electronics, drives, and mineral processing. Ruben Osorio received the M.S. degree in electronic engineering from the Universidad Técnica Federico Santa María, Valparaíso, Chile in 1998. He is a Process Engineer with the Mantos de Oro Mining Company, Copiapó, Chile. Samir Kouro is pursuing the M.S. degree in the Department of Electronics, Universidad Técnica Federico Santa María, Valparaíso, Chile.