574 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 49, NO. 3, JUNE 2002 Compensated Synchronous PI Current Controller in Overmodulation Range and Six-Step Operation of Space-Vector-Modulation-Based Vector-Controlled Drives Ashwin M. Khambadkone, Member, IEEE, and Joachim Holtz, Fellow, IEEE Abstract—Overmodulation enhances the power utilization of the installed capacity of a voltage-source inverter. A space-vector strategy is used for constant-switching-frequency inverters. In order to achieve the overmodulation, a modified reference signal with nonuniform angular velocity is generated using a preprocessor. Such a reference wave produces low-frequency har- monics in currents. The presence of current harmonics restricts the bandwidth of the synchronous proportional plus integral current controller in the overmodulation range. A compensating current control is presented to allow for high-bandwidth current control in synchronous coordinates during overmodulation and six-step. The proposed scheme allows for an easy upgrade of a conventional vector control scheme to include overmodulation and, thus, reduce the design-to-market time. Index Terms—Current control, feedback pulsewidth modula- tion, overmodulation, space-vector modulation. I. INTRODUCTION V ECTOR CONTROL of an induction motor allows for a fast torque control over a wide operating range. It is being applied for a number of applications both in the low and high power ranges. Fast current control is essential for the vector con- trol in order to achieve decoupling and torque control. Usually, a linear current controller of the proportional plus integral (PI) type and a feedforward pulsewidth modulator are used for cur- rent vector control. A large number of pulsewidth modulation (PWM) schemes can be used to go along with a linear current control. In so doing, the pulsewidth modulator can be designed to address various performance criteria [1]. Of the various per- formance criteria, overmodulation has received considerable at- tention [2]–[5]. Its significance is that it allows the full utiliza- tion of the installed voltage capacity of the inverter. In order to achieve overmodulation, lower order harmonics other than the third harmonic have to be added to the modulated output voltage. The interaction of these harmonics with the PI-type linear current controller is the focus of our paper. Overmodulation refers to the operation of the pulsewidth modulator beyond the linear range. In space-vector modulation, Manuscript received December 14, 2000; revised November 16, 2001. Ab- stract published on the Internet March 7, 2002. A. M. Khambadkone is with the National University of Singapore, Singapore 119260 (e-mail: eleamk@nus.edu.sg). J. Holtz is with Wuppertal University, 42097 Wuppertal, Germany (e-mail: j.holtz@ieee.org). Publisher Item Identifier S 0278-0046(02)04921-3. the maximum modulation index is achieved. However, the maximum possible voltage occurs during the six-step operation and is equal to . We define this as modulation index . The region between the modulation indexes and is called overmodulation. A process of obtaining a voltage in this region was described in [6]. This technique uses a preprocessor that generates a suitable reference voltage vector. The prepocessor alters the desired reference voltage vector by adding lower order harmonics. It causes change in magnitude and angle of the resultant reference vector, producing either a uniform or nonuniform angular velocity, depending on the region of operation. Adding lower order harmonics to the reference voltage causes distorted motor current, but it also achieves higher modulated output voltage. In the normal range of operation, triplen frequency harmonics do not affect the motor phase current, hence, the space-vector modulation produces only the fundamental and the switching frequency harmonics in the motor currents. However, in the overmodulation range as proposed in [6] and [2], the motor currents will have lower order harmonics other than the triplen harmonics. A high-gain current controller tries to cancel out the harmonics that have been introduced due to overmodulation. This is because the current controller is primarily designed to control the fundamental frequency current. In so doing, it works against the overmodulation. Filters can be used to make the current controller immune to the low-frequency harmonics, but this reduces the bandwidth of the current control loop. This results in poor dynamic performance of the current controllers. In fact, during overmodulation, the best settings of the PI-type current controllers in the linear range are no longer suitable. They cause unsteady behavior which results in current and torque transients (see Fig. 1). Here, a stable operation was possible only after the bandwidth of the current controller was reduced to around 100 Hz. As fast current control is mandatory for a vector control drive, the problem of current control in the overmodulation range comes forth. How then do we deal with harmonic currents that are present in the overmodulation range? This problem is being addressed in our paper. The problem is different from the problem in [3], which deals with the compensation of the dynamic component of voltage during overmodulation. On the other hand, we deal with the compensation of the effects of harmonic voltage in the current control loop. This problem 0278-0046/02$17.00 © 2002 IEEE