A hybrid HVDC converter based on M2C and diode rectifiers without DC capacitors for offshore wind farm integration

•Proposing a hybrid power converter using modular-multilevel converter and diode rectifiers for HVDC transmission systems.•Eliminating energy storage components of diode rectifiers.•Enhancing AC voltage control by feedback linearization for the voltage-controlled converter.•Offering lower cost and loss of the HVDC converter compared to the conventional ones.•Simulation and experimental verification. In this paper, a new topology of high-voltage direct current (HVDC) converter system for transmitting the power from offshore wind farms (OWF) to the mainland grid is proposed. The presented HVDC converter is composed of a hybrid series connection scheme of voltage-source converters (VSC) based on M2C (modular multilevel converter) and twelve-pulse diode rectifiers (12PD) without DC capacitors integrating with the WFs in the offshore station and a full-scaled M2C in an onshore station. In this scheme, the HVDC-link voltage is controlled by the M2C in the onshore station, while the M2C in the offshore side is operated as controlled-voltage source to maintain the offshore AC voltages. A feedback linearization technique is applied to the M2C in the offshore station for regulating the offshore AC voltages, which offers a better performance for a nonlinear model of the voltage-controlled VSC when compared to schemes based on the linear control theory. Furthermore, with the utilization of the diodes and elimination of DC capacitors at the output terminal of the 12PD, the proposed HVDC converter topology offers advantages over the full-scaled M2C HVDC systems in terms of efficiency, capital investment costs, maintenance, and footprint. The proposed topology is proved through the simulation tests for a 525-kV 600-MW HVDC system integrating with wind turbine generators. Fundamental operation of the converter is also demonstrated by the experimental results for a reduced-scale HVDC system in laboratory.