H8 Converter: A Three-Phase Transformerless Inverter with Reduced Common-Mode Voltage

This thesis presents the development of a novel architecture of DC/AC three-phase converter, addressed to as “H8”. Key feature of the project is the reduction of the common mode voltage (CMV) in a transformerless converter. This follows the trend of recent years of introducing novelties that deal with the CMV-related problems in transformerless inverters. Despite the fact that good ideas were already proposed, this work was motivated by the necessity of exploring the simple idea of decoupling DC and AC sides of the inverter with switching devices, which is a strategy that has already been demonstrated to be effective in single-phase converters. Alongside the novel architecture, also the modulation was developed with the CMV reduction in mind, exploiting the degrees of freedom given by space vector (SV) modulation. Deriving from the H6 traditional structure and having a total of eight active devices, it was named “H8”. It differs from a traditional H6 for the presence, at the DC side, of a network composed by a three-levels voltage divider, two active devices and two clamping diodes. A model of the inverter has been designed with the toolbox PLECS, allowing to develop and test the H8 features in a MATLAB/Simulink control environment. Simulations were carried out to develop and verify the proposed architecture and modulation; electro-thermal simulations were also carried out for evaluating the impact of the DC-decoupling hardware on the efficiency and the harmonic content at the inverter output. A prototype of the converter was also designed and built from the ground up. In order to prevent EMI problems control and power circuitry were separated on two different boards, interconnected by an interface card. Experimental results reflect simulations, proving the feasibility of the proposed inverter.