Low Frequency Oscillation Reduction Algorithm for Six-step Operation of Three-Phase Inverters Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ns064962m

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  • A power inverter, or inverter, is an electronic device or circuitry that converts direct currents (DC) to alternating currents (AC) in the power electronics family. A three-phase inverter converts DC current to three-phase AC current. Inverters, especially three-phase inverters, have been widely used in many fields including renewable energy applications such as interface circuits between wind turbines and the electric grid, transportation electrification such as machine drives in electric and hybrid electric vehicles, ships and aircrafts, general purpose drives for industrial applications such as pumps, fans and compressors, and the power converter circuits in medical equipment such as Magnetic Resonance Imaging (MRI) and medical power supplies. In AC machine drive systems, the three-phase inverters can be operated by the Pulse Width Modulation (PWM) control or six-step control. With the PWM control, the input reference voltages are modulated by a high frequency carrier wave. The top switch of the phase leg will be turned on when the reference voltage is greater than the carrier. In six-step operation control, the top switch of the phase leg will be turned on when the reference voltage is greater than zero. The advantages of using PWM control include low Total Harmonic Distortion (THD) in phase currents and controlled current. Compared to the PWM control, the six-step operation has advantages containing reduced switching losses, better utilization of DC bus voltage, and extended speed capability. In analog implementation of the six-step control, the control signals of the switches are generated by a comparator in the electronic circuit. Hence, the switches will be turned on and off at the phase voltage zero crossing points. In digital implementation, however, power electronics may not be able to switch on and off exactly at the zero crossing points of the phase voltage due to the limited resolution of the sampled phase voltage command, which will result in a DC offset in the phase voltage and cause a low frequency oscillation in the phase current. The low frequency oscillation will create issues such as higher power loss, lower efficiency, and excessive heat that may permanently demagnetize permanent magnets on the rotor for a permanent magnet (PM) synchronous machine. This research investigates a control method to significantly reduce the low frequency oscillation phase current for the six step operation of a three-phase inverter in digital implementation. The proposed control method using asymmetric PWM ensures that power electronics switch on and off exactly at the voltage zero crossing points. The algorithm calculates the duty ratio when the phase voltage command crosses zero from positive to negative or from negative to positive. In addition, the control algorithm was simulated and tested on a three-phase RL load and a PM synchronous machine. Both simulation and experimental results demonstrate that the proposed control algorithm can reduce the low frequency oscillation components in the phase current by more than 90%.
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