Steady-State Analysis of the DR-MMC Based Hybrid Topology for Offshore Wind Power Transmission

The hybrid topology with a modular multilevel converter and diode rectifiers connected in series (DR-MMC) has emerged as a promising solution for offshore wind power transmission. Accurate steady-state analysis is critical for the parameter design, device selection and performance evaluation of the DR-MMC. However, existing steady-state analysis methods for this topology demonstrate considerable errors due to insufficient consideration of the DR commutation process and the neglect of the intercoupling between the DR and MMC. This article presents a more elaborate description for the DR commutation process within the DR-MMC by taking into account the dc current ripple, ac-side equivalent series resistance, and the coupling from the MMC. Building upon this, a novel steady-state model constituted by double loops is developed for the DR-MMC. Specifically, the commutation process of the DR and the power conservation of this hybrid topology together form the outer loop, while the steady-state model of the individual MMC serves as the inner loop. Experimental results from a scaled-down prototype validate that the proposed steady-state model can determine the electrical quantities of the DR-MMC accurately across its entire operating range.