Single-phase ac-dc-ac topology for grid overvoltage and voltage harmonic mitigation

A single-phase ac-dc-ac topology to improve power quality is proposed in this study. This configuration is obtained from the addition of a floating capacitor H-bridge converter on the grid side of the conventional ac-dc-ac three-leg topology. The role of the structure is not only to ensure regulated load voltage with fixed amplitude and frequency, grid current with low harmonic content and unitary power factor, but also to mitigate fundamental overvoltage and voltage harmonics at the grid. As a consequence, the proposed topology is suitable to operate as unified power quality compensator or uninterrupted power supply. It does not use isolation transformer and is capable of generating multilevel input voltages due to the cascaded H-bridge. The operation with asymmetrical dc-link voltages is considered to increase the number of voltage levels and reduce the harmonic content of the generated input voltages. Two pulse-width modulation techniques and a control system are developed to regulate the dc-link voltages and to decrease the harmonic distortion (HD), lowering the switching stress and also the power losses. The proposed configuration is compared with the conventional one with respect to HD and semiconductor losses. Simulated and experimental results are presented for validation purposes.