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This paper introduces the control of the parallel operation of two voltage source converter (VSC)-HVdc links interconnecting an offshore wind farm. The aim of the study is to propose and validate a control system that allows the parallel operation of two VSC-HVdc links by controlling the currents injected by the VSC converters. The currents set points are established by a voltage controller in order to maintain constant voltage and frequency in the capacitor of the output filter and therefore within the offshore wind farm (OWF). It is demonstrated that the decoupled control of the d-q component of the voltage at the capacitor allows achieving the direct control of voltage and frequency, respectively. The voltage and frequency control is implemented by orienting the capacitor voltage toward a synchronous axis that is generated within the controller and therefore is not subjected to any grid disturbance. Both converters collaborate therefore in maintaining constant voltage and frequency, achieving in this way the parallel operation of the converters. The validation of this approach is demonstrated by simulation where the OWF and the VSC-HVdc rectifier have been modeled. Simulation results demonstrate that the proposed control system allows the parallelization of the converters while maintaining constant voltage and frequency within the OWF, even during transient faults. [ABSTRACT FROM AUTHOR]

A modular multilevel converter control system, based on converter energy storage, is proposed in this paper for two different control modes: active power and dc voltage. The proposed control system decouples the submodule (SM) capacitor voltages from the dc bus voltage. One of the practical applications is the management of active redundant SMs. A practical HVDC system with 401-level MMCs, including 10% redundancy in MMC SMs, is used for validating and demonstrating the advantages of the proposed control system. This paper also presents a novel capacitor voltage balancing control based on \max–\min functions. It is used to drastically reduce the number of switchings for each SM and enhances computational efficiency. [ABSTRACT FROM AUTHOR]

This paper presents a transformerless high-voltage dc–dc converter based on cascaded sub-modules. It is intended for interconnecting high-voltage or medium-voltage dc grids. The proposed dc–dc converter consists of two phase legs, each of which consists of an upper arm and a lower arm with their middle nodes crossly connected through a branch of active sub-modules. By properly controlling the output voltage of the cross-connected branch, a trapezoidal ac current is induced to interact with the ac voltage components in the upper and lower arm for rebalancing the power amongst converter arms. The features of modular design, single-stage power conversion, and transformerless structure make the proposed dc–dc converter gain the outstanding merits of wide voltage ratio range, high system efficiency, and light converter weight. A control scheme is also elaborated for guaranteeing the normal operation of the proposed dc–dc converter. A 100-kV 100-MW simulation model performed in MATLAB/Simulink verifies the feasibility of the proposed dc–dc converter. Experimental results obtained from a 100-V 1-kW laboratory setup also confirm the validation of the proposal. [ABSTRACT FROM PUBLISHER]