PhD Oral Exam - Seydeh Nazanin Afrasiabi, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Power-hardware-in-the-loop (PHIL) based machine emulation is increasingly being used as an effective approach for simplifying the testing of electric drive systems. The PHIL-based machine emulator systems control the power converters or power amplifiers in order to mimic machine behavior. Control of the machine emulator is achieved with the help of a machine model running on a real-time controller. Utilizing a motor emulator allows performance testing of the physical machine and its controller before the manufacturing process, reducing risks and costs. This methodology enables testing of different drive inverter faults, including diode rectifier faults, transistor switch faults, and line-to-line faults, without risking damage to the machine. Moreover, it can be applied to test various faulty machines and grid faults, such as open circuits, short circuits, and unbalanced faults. Therefore, this approach finds extensive application in diverse industrial sectors, including military, aerospace, and electric vehicles.

Various open research challenges exist in this domain, all of which aim to improve the accuracy and utility of this test methodology. Emulation accuracy depends on various factors, namely the detailed mathematical model of the electrical machine, the design and selection of the control for the emulation system, and the selection of emulator hardware technologies. Since PHIL-based machine emulation is a relatively novel testing methodology, there is a need to develop a high-performance, highly accurate test bench. Therefore, this PhD work aims to develop an accurate motor emulation test bench to truly validate the performance of the motor, drive inverter, and drive inverter control prior to manufacturing the prototype motor.

A reliable and flexible grid is crucial for PHIL-based machine emulation system test benches. To achieve this, grid emulators are introduced to enhance the reliability and flexibility of motor emulation systems. Currently, power amplifiers are popular choices for emulators due to their high bandwidth, compact size, and ease of control. However, amplifiers are designed for specific voltage ratings, and when higher voltage is required, a step-up transformer is added to the amplifier output. The connection of the transformer at the output of the amplifier can impact the performance and characteristics of the grid emulator system. Therefore, this research work aims to characterize the grid emulator system with an amplifier and transformer under various operating conditions.

Machine emulation accuracy depends on various factors, with one of the key factors being the selection of emulator hardware technology. Thus, this research compares three types of machine emulation systems: one utilizing a conventional IGBT-based hard switching power converter, another employing a high-bandwidth soft switching power converter, and the third using a linear amplifier. Detailed configurations and control descriptions for these systems are provided in this work. Comparative experimental studies are conducted, and the obtained results are then compared with those of the physical induction motor (IM) to verify the emulation performance.

The typical voltage source inverter is employed as an emulator converter in motor emulation systems. However, this converter introduces various harmonics into the motor emulation system, primarily attributed to dead time, switching components, and control signals. These harmonics can deteriorate motor emulation accuracy. Therefore, it is important to investigate and compensate for emulator converter harmonics in the motor emulation system. Hence, this PhD work will also explore this topic.

The induction motor drive with an LC filter configuration enables smooth pulse width modulation (PWM) output voltages of the motor drives. Moreover, employing high-performance, high-bandwidth class D power amplifiers as emulator converters enhances the system's bandwidth, thereby ensuring accurate machine emulation. Another open research topic that could improve motor emulation systems is testing electric drives with filters. Testing such a machine drive using conventional emulator structures would be challenging. Therefore, this PhD work will also investigate this topic.