Supervisor:
Stig Munk-Nielsen

Co-Supervisor:

Assessment Committee:
Amir Sajjad Bahman (Chair)
Associate Professor, Dong Dong, Virginia tech
Prof. Dr. Ing., Marc Hiller, Dean of Studies

Moderator:
Tamas Kerekes

Abstract:

Silicon-carbide (SiC) MOSFETs, a wide bandgap (WBG) technology, with breakdown voltages of 10 kV have attracted increasing research interest from both industry and academia in recent years. However, unsolved technological challenges at both device- and module-level still limit their commercialization and applicability. Even though the fabrication process is advancing to optimize their performance, currently available devices are limited to engineering samples with high costs, limited quantities, and scarce availability.

As of now, the medium voltage (MV) SiC MOSFET technology is still considered immature despite notable progress throughout its voltage range (at and above 3.3 kV in breakdown voltage). Since 2014, 10 kV SiC MOSFETs have gained momentum, particularly research on auxiliary components (such as gate drivers, magnetic components, sensing, etc.), potential converter topologies, and converter control among academic and industrial stakeholders.

This PhD thesis aims to address unsolved technological challenges related to closed-loop current control of three-phase, two-level power electronic converters (PEC) enabled by 10 kV SiC MOSFET engineering samples. The research identifies critical components within volt-second compensation and digital implementation of a closed-loop current controller:

1) Accurate compensation of the volt-second error introduced by dead time and other converter nonlinearities occurring every switching event is essential to obtain a satisfactory controller performance and current quality. To accurately model and derive the volt-second error for MV SiC MOSFET power modules, a first-order analytical model of the SiC MOSFET switching behavior was utilized. 

A simple piece-wise linear compensation strategy is proposed that balances controller performance and implementation complexity. 

2) To ensure a secure digital implementation of the current controller, a "digital twin" of the RL plant was implemented on the digital signal processor (DSP) to validate the closed-loop controller's stability and performance before applying it to the 10 kV SiC MOSFET three-phase, two-level PEC. Finally, closed-loop current control of a three-phase, two-level PEC enabled by 10 kV SiC MOSFETs was experimentally demonstrated with satisfactory controller performance during steady-state and dynamic operations. 

The work presented in this PhD thesis intends to support the maturing process of the 10 kV SiC MOSFET technology by systematically identifying, analyzing, and solving technological challenges related to the control of three-phase, two-level PECs enabled by 10 kV SiC MOSFETs.