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With lower construction costs and power losses, the DC wind farm (DCWF) based on series-connected DC wind turbines (DCWT) is a potential collection scheme for large-scale offshore wind power. However, energy curtailment caused by unequal wind speed exists in DCWTs and results in energy waste, weakening the advantages of DCWTs. The energy curtailment mechanism is analyzed for DCWF, where single active bridge (SAB) is used as DC/DC converter in DCWTS and a full-bridge modular multilevel converter (FB-MMC) as the receiving end converter. The linear relationship between input and output of single active bridge (SAB) is interpreted, and a novel coordinate control strategy for DCWF is proposed to solve the wind energy curtailment caused by unequal wind speed. In addition, an auxiliary control strategy for receiving end converter is proposed to reduce the loss on HVDC link. A complete DCWF is built in PSCAD/EMTDC software package, and the results show that the proposed control strategy makes DCWF can operate well without energy curtailment under unequal wind speeds.

The series–parallel DC wind farm composed of a large number of DC wind turbine (DCWT) is a potential scheme for offshore wind power collection with lower construction costs and power losses. However, enhanced coupling issues, such as energy curtailment and fault isolation, exist in the DC wind farm. To overcome the difficulties, a series–parallel DC collection system is proposed, where DCWTs based on the single active bridge (SAB) are connected in series and then are connected in parallel via DC/DC converters. The coupling mechanism among series–parallel strings is analyzed first, and a coordinate control strategy for the whole system is proposed based on the characteristics of SAB. DC fault located at different strings is analyzed and strategies are proposed to protect the whole system. Simulation results in PSCAD/EMTDC verifies the proposed control strategy with the less coupling among series–parallel DC wind farms under unequal wind speeds and fault.

This paper proposes an analytical model for the modular multilevel converter (MMC) based on the theory of harmonic balance, which removes the iteration process for traditional models. The proposed model considers dynamic harmonic interactions between the modulation of converters and passive elements in the MMC. By transferring the equations into frequency-domain as complex Fourier series, a steady-state harmonic model is created with singular coefficients matrix. Then, through the elimination of all DC components in dynamic model, the singular coefficient matrix can be derived to nonsingular one, and the order of it can be reduced. Finally, the typical open-loop analytical model with DC neutral grounded MMC is derived aim, and the time-domain simulations is used to validate the effectiveness of proposed model. The proposed model can be used to accelerate the simulation and analyze the influence of parameters to guide the design of MMC.