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Transcript

Low Frequency Current Oscillation Reduction for Six Step Operation of Threephase Inverters Abstract: This paper investigates a control method to significantly reduce the low frequency current oscillation for six step operation of a three-phase inverter. The six step operation of an AC electric machine drive system under high speeds has a few advantages compared with pulse width modulation (PWM) operation including reduced switching losses, a better utilization of the DC bus voltage, and an enhanced speed capability. In digital implementation of the six step control, power electronics may not be able to switch on and off exactly at the zero crossing points of reference voltages, which will result in a DC offset in phase voltages and cause a low frequency oscillation in phase currents. The low frequency oscillation will create issues such as higher power losses, lower efficiency, and excessive heat that may demagnetize magnets on the rotor for a permanent magnet synchronous machine. This research presents a control algorithm using asymmetric PWM to ensure that power electronics switch on and off exactly at the reference voltage zero crossing points. Both simulation and experimental results demonstrate the algorithm can reduce the low frequency oscillation in phase currents by more than 90%. Existing system: A modern AC electric machine is usually driven by an inverter using PWM control to ensure sinusoidal phase currents with a low total harmonic distortion (THD). For an increasing fundamental frequency, fewer pulses are created for each fundamental cycle and six step operation, another power electronics switching technique, can often be applied to reduce the power electronics switching losses and extend the machine speed capability as well. For the six step operation, it is desirable to have the power electronics switch on and off exactly at the reference phase voltage zero crossing points. However, the power electronics may not be able to achieve this using digital implementation. This can create a DC offset in phase voltages over one fundamental cycle, hence, a low frequency oscillation in phase currents. Low frequency is used here as the oscillation frequency is lower than the fundamental frequency of the AC voltage or current. This work prefers to use low frequency instead of subharmonic as the subharmonic, in its pure sense, is defined as f1/n, where f1 is the fundamental frequency and n is an integer. Proposed system: This paper discusses how the low frequency current oscillation is produced for the six step operation of a three-phase inverter. The contribution of this work is to develop and implement a new control algorithm to eliminate the low frequency current oscillation for the six step operation. The algorithm calculates the accurate duty ratio of the power switches if the reference phase voltage changes polarity for the next sampling point to ensure the switches can turn on and off exactly at the voltage zero crossing points. Simulation and experimental results are presented to verify the control algorithm using both a three-phase RL load and a three-phase interior permanent magnet synchronous machine load. Reference: [1] P. Dallos, The Auditory Periphery Biophysics and Physiology, Chapter 6. Nonlinear Distortion, pp. 448, Academic Press, 1973. [2] J. Holtz, W. Lotzkat, and A. M. Khambadkone, “On continuous control of PWM inverters in the overmodulation range including the six-step mode,” IEEE Transactions on Power Electronics, vol. 8, no. 4, pp. 546- 553, Oct. 1993. [3] R. J. Kerkman, D. Leggate, B. J. Seibel, and T. M. Rowan, “Operation of PWM voltage source-inverters in the overmodulation region,” IEEE Transactions on Industrial Electronics, vol. 43, no. 1, pp. 132-141, Feb. 1996. [4] Z. Peroutka and T. Glasberger, “Comparison of methods for contrinuous transition of space vector PWM into six-step mode,” EPE International Power Electronics and Motion Control Conference, pp. 925-930, Portoroz, Slovenia, Aug. 2006.