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In this paper, a dc–ac buck converter is proposed as a grid-side converter in high voltage dc (HVdc) transmission systems for high-power renewable energy source grid integration. The proposed architecture consists of multimodules of half-bridge voltage source converters (HB-VSCs). The dc terminals of the HB-VSCs are connected in series across the entire dc link (i.e., capacitor tapped), whereas their ac outputs are connected to multiwinding transformers to provide the three-phase terminals for ac grid integration. The proposed grid-connected capacitor-tapped multimodule converter (CT-MC) architecture, inspired from the HVdc shunt tap proposed by ABB, provides a dc–ac conversion with a relatively moderate voltage rating of semiconductor devices. It also provides operation with a lower number of semiconductor devices, gate driver circuits, voltage sensors, and lower total MVA rating of semiconductor devices (67%) compared with the conventional three-phase HB modular multilevel converter, which positively affects the system cost and reduces the computational burden of the employed controller. In this paper, the operational concept and control of the CT-MC are presented along with a capacitor voltage balancing approach. Simulation results are presented during normal and abnormal conditions to show the viability of the proposed architecture. Finally, a scaled down prototype for the CT-MC is employed to validate the converter operation. [ABSTRACT FROM AUTHOR]

Offshore wind energy is now seen as a key contributor for the future renewable energy mix. HVDC technology is among the chief technologies enabling widespread use of offshore wind. Thanks to their numerous advantages, voltage-source converter-based-HVDC (VSC-HVDC) systems are expected to be the technology of choice. Unfortunately, most of VSCs are defenseless against dc-side faults, such as two-level VSCs, and half-bridge modular multilevel converters. This paper proposes the buck-boost inverter-based-HVDC system (BBI-HVDC) as a means to overcome the limitations of the classical VSC-HVDC systems. The proposed configuration does not only provide sinusoidal three-phase voltage, but also provides complete blocking capability of ac-side contributions during a dc-side fault. The latter is achieved by simply disabling the gating signals to the switches upon fault detection. The performance of the proposed system is illustrated during normal conditions, ac-side faults, and dc-side faults. A simulation case study comparing the performance of the conventional HVDC converters with the proposed system during dc-side faults is conducted. The simulation results reveal the promising performance under normal operation as well as a significant decrease in the dc fault current due to the ac-side contribution blocking capability of the BBI-HVDC system during dc-side faults. [ABSTRACT FROM AUTHOR]