Abstract
Offshore Direct Current (DC) collector grids are a promising technology for decreasing the installation and operation costs of offshore wind parks. Nevertheless, the stability properties and hence the design of such DC collector grids is not common or standardised. Hence, this paper describes an attempt to fill these gaps by analysing the stability of two different types of DC collector grids—star and string—by considering identical operating conditions. The approach follows a non-parametric formulation of the impedance based Nyquist Stability Criterion. The hyperbolic Π equivalent formulation of the telegraph equation is adopted for modelling the submarine cable due to high capacitance that is distributed and thus the conventional 50 Hz Π-model is not sufficient anymore. Furthermore, the paper shows how to integrate the complex dynamics of wind turbines into the overall stability assessment through an impedance building algorithm. Finally, it is shown how to stabilise the collector grids by means of active control parameter changes and it has been observed that the star configuration of wind turbines is more favourable on account of stability and controllability.
Highlights
This paper investigates the collector grid stability of direct current (DC) offshore wind farms and performs a comparative study of star and string topologies on the aspect of stability
The stability of systems has been studied against the variation of wind turbine (WT), DC/DC converter control parameters, capacitance of WT and DC/DC
It can be concluded that the star arrangement is favourable compared to the string configuration w.r.t. stability, as the star point voltage is extremely stiff due to its proximity to the DC/DC converter, and the impedance of the DC/DC converter can be actively reduced to create a barrier for disturbances to circulate through the system
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. It is possible that the internal parameters of certain systems are not known or, due to the complex non-linearity involved, it may not be possible to derive linearised analytical transfer function For such systems, the frequency response of individual subsystems can be extracted using wideband impedance measurement devices [16,17]. To formulate the impedance frequency response of the source and load, a non-parametric impedance building algorithm based on two-port network theory using the transmission matrix is developed Through this algorithm, the complex dynamics of WTs, submarine cables and DC/DC converter can be integrated to form Thevenin equivalent non-parametric impedance models at the desired node where stability needs to be evaluated. A non-parametric network impedance building algorithm based on two-port network theory via transmission matrices; Application of non-parametric impedance-based stability analysis for offshore DC collector grids; An effective comparison between star and string offshore WT typologies on the aspect of system stability
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