Graphical Analysis and Control of Cascaded H-Bridge Multilevel Inverter Using Clarke Transformation With Neutral-Shift Strategy Under Nonideal Conditions

This article proposes a graphical approach using a 3-D space vector representation to diagnose the unbalanced operation of a three-phase cascaded H-bridge (CHB) inverter operating under nonideal conditions. Under ideal conditions, the application of the Clarke transformation to the inverter voltage phasor results in a circular trajectory within the stationary and orthogonal (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula>)-frame, enabling the inverter to supply balanced three-phase phase/line voltages and currents to the load. In contrast, when nonideal conditions arise, such as dc voltage imbalances among the bridges or failed cells, the Clarke transformation of the inverter voltage phasor forms an elliptical path, leading to unbalanced operation. This article discusses and highlights the characteristic differences between inverter-balanced and unbalanced operation modes. Additionally, a generalized neutral-shift method has been suggested to address unequal dc voltage sources simultaneously and failed cells without differentiation. The proposed control strategy ensures the inverter maintains balanced line-to-line voltages and currents, even when its phase voltages remain unbalanced. Simulation and experimental results from a three-cell (per phase) CHB inverter with a symmetrical RL load are provided to validate the effectiveness of the proposed approach. Finally, the study explores the impact of nonideal conditions on modifying inverter voltage and current spectra.