PhD Defence Regis Nibaruta | Common Mode Elimination strategies in Voltage Source Converters for Compact Module Designs

Common Mode Elimination strategies in Voltage Source Converters for Compact Module Designs

Regis Nibaruta is a PhD student in the Department of Power Electronics. (Co)Promotors are prof.dr. T. Batista Soeiro and dr.ir. P. Venugopal from the Faculty of Electrical Engineering, Mathematics and Computer Science (UT) and prof.dr. V. Havryliuk from the Ukrainian State University of Science and Technology.

The accelerating adoption of electric vehicles (EVs) has intensified the demand for efficient, compact, and cost-effective charging solutions. While galvanic isolation through line-frequency or high-frequency transformers remains the industry standard for safety compliance, these components significantly increase the size, cost, and complexity of EV chargers. Transformerless charging architectures offer a promising alternative by eliminating isolation transformers, thereby enhancing power density and reducing system volume. However, such configurations introduce new challenges, most critically, the generation of common-mode voltage (CMV), which leads to high-frequency leakage currents through parasitic paths between the EV chassis and ground. These leakage currents can violate international safety standards such as IEC 61851 and UL 2202, trigger residual current devices (RCDs), and compromise electromagnetic compatibility (EMC). This dissertation investigates advanced modulation strategies for three-phase three-level converters, particularly T-type topologies, to enable transformerless EV chargers with effective CMV suppression. The central objective is to eliminate CMV at its source through a novel space vector modulation (SVM) scheme that restricts switching to medium vectors and the zero vector only. The implemented modulations ensure that the CMV remains ideally zero throughout each switching cycle, lowering the need for bulky EMI filters or galvanic isolation components. To further improve practicality and implementation efficiency, a switching sequence optimization method is developed to ensure that no phase switches more than once per PWM cycle. This approach significantly reduces switching losses and thermal stress while maintaining output voltage quality. Additionally, a dead-time compensation technique is integrated into the modulation scheme to prevent the reintroduction of CMV spikes caused by non-ideal switching intervals. To demonstrate a practical application of the proposed CMV suppression methods, a two-stage transformerless EV charger prototype is implemented. The front-end stage employs the adopted zero-CMV SVM technique, while the back-end stage consists of a bidirectional buck-boost DC–DC converter operating with a synchronous switching scheme. This coordinated control ensures that the high-frequency switching events in the back-end do not reintroduce CMV across the EV battery terminals, further extending CMV suppression throughout the full power path. The adopted strategies are analytically modeled, simulated under various operating conditions, and validated experimentally using a 5kW three-phase three-level T-type converter prototype built with Silicon Carbide (SiC) devices. The hardware results demonstrate a significant reduction in CMV fluctuation and leakage current levels compared to conventional SVM techniques. It is important to note, however, that the experimental validations were conducted under laboratory conditions using high-resolution but non-certified instrumentation. While the findings are promising and suggest the proposed methods can support safety and EMC compliance, formal validation using certified test procedures and equipment is required for regulatory certification. Overall, this research advances the design of transformerless EV charging systems by providing a modulation-based approach to CMV elimination that is both experimentally validated and compatible with emerging wide bandgap device technology. The outcomes contribute to the broader goal of scalable, efficient, and compact EV infrastructure, and lay the foundation for future commercial adoption of transformerless charger designs.