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Conference programme 

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Poster session

Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Stephan Vogel DTU - Technical University of Denmark, Denmark
Co-authors:
Stephan Vogel (1) F P Tonny Rasmussen (1) Joachim Holboell (1) Walid El-Khatib (1)
(1) DTU - Technical University of Denmark, København, Denmark

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Poster
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Presenter's biography

Biographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited

Mr. Vogel has received a Bachelor degree in Renewable Energy Systems from the HTW-Berlin, Germany in 2012 and a Master degree in Wind Energy from the Technical University of Denmark in 2014. He is currently employed as a PhD student in the electrical department of the Technical University of Denmark where he focuses on lightning exposure to wind turbines and the design of a novel impulse generator for large scale wind power components.

Abstract

Dc collection network simulation for offshore wind farms

Introduction

The possibility to connect offshore wind turbines with a collection network based on Direct Current (DC), instead of Alternating Current (AC), gained attention in the scientific and industrial environment [1]. There are many promising properties of DC components that could be beneficial such as: smaller dimensions, less weight, fewer conductors, no reactive power considerations, and less overall losses due to the absence of proximity and skin effects. On the other hand, challenges arise due to low grid impedance, high input and output current ripple of DC/DC converters, and high fault currents in case of an incident.

Approach

To provide a solid foundation for a DC collection network simulation, intensive research of electrical representations of components was performed, and suitable models were identified and implemented in the transient simulation program PSCAD.

Main body of abstract

By utilizing this tool, transient effects and fault scenarios in wind turbine converters, cables, and in the High Voltage Direct Current (HVDC) link, were studied. In total, the grid is comprised of four wind turbines that are connected with a DAB (Dual active bridge) converter. The steady-state simulations of the system showed high ripple current due to the low grid impedance and the high input and output ripple of the converter. Three different solutions have been proposed to reduce the ripple. In general, the proposed DC grid shows a good transient response to disturbances, and steady-state conditions are regained after the faults. Three different fault scenarios have been studied: terminal short-circuits in the turbine, cable faults in the medium voltage grid and lightning incidents from the grid side. Peak current and voltages during fault conditions reached high magnitudes up to several times the nominal value. Every turbine and cable segment was protected with a set of DC breakers that were able to detect and isolate faulty turbines or entire cable segments with realistic trigger times. Additionally, each model of the DC grid has been evaluated regarding switching and conduction loss, and an overall efficiency of the DC collection network was found to be 94.4%. This value might be increased with a careful converter design.

Conclusion

This work provides a study about the simulation of a medium voltage DC grid in an offshore wind farm. The behavior in steady-state and during fault conditions is investigated . The efficiency of the network is determined in full-load conditions. Furthermore, key design aspects of such a grid are illustrated and issues regarding ripple current and converter design are shown.


Learning objectives
This work provides an overview of the challenges that arise in MVDC grids.



[1] C. Meyer, M. Hoeing, A. Peterson, and R. W. De Doncker, Control and design of dc grids for oshore wind farms, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, vol. 43, no. 6, pp. 14751482, 2007.