State Space Modeling of an Offshore Wind Power Plant With an MMC-HVDC Connection for an Eigenvalue-Based Stability Analysis

Abstract

Large offshore wind power plants (OWPPs) installed far from the coastline are emerging to benefit from the strong and steady wind resources available at these locations. The high-voltage direct-current (HVDC) transmission system based on the modular multilevel converter (MMC) is the most appropriate solution to transmit the produced energy to the onshore grid, in a way that a complex power-electronic-based electrical system is formed at the OWPP. Undesired interactions can occur between the MMC-based HVDC station, the several wind-turbine (WT) converters, and the passive elements of the offshore grid. To guarantee a safe and reliable operation of the OWPP, a small-signal analysis must be performed in advance to predict possible unstable situations and their root causes, in a way as to take corrective measures to avoid them. In this paper, a state space model of an OWPP is developed in which a recently proposed model in multiple dq frames is adopted for the MMC. With the developed model, an eigenvalue-based stability assessment is carried out, where participation factors are calculated to identify the main contributors to the unstable modes that appear as a consequence of undesired low-frequency interactions at the offshore grid. As far as the authors know, an eigenvalue-based stability assessment of an OWPP with an MMC-HVDC connection is a topic rarely or never explored in the literature before and, thus, this is the main contribution of this paper. An adaptation to the MMC state space model is proposed so that the converter's dynamics in grid-forming mode can be represented, and this is another contribution of this paper. Finally, various studies are presented to prove that the adopted approximated MMC model is accurate enough for small-signal stability analyses.INDEX TERMS Power-electronic-based power system, small-signal stability analysis, HVDC transmission system, modular multilevel converter, offshore wind power plant.

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