Estimates are that circa 40GW of offshore wind power capacity is going to be installed throughout Europe by the end of this decade. In this scenario, a pan-European offshore grid network is needed in order to efficiently integrate large amounts of offshore wind into the different European countries' transmission networks. In this paper, the dynamic model of multi-terminal HVDC (MTDC) transmission system composed of voltage-source converters is presented. Afterwards, the dynamic models are used to compare four different methods for controlling the DC voltage inside MTDC networks, viz.: droop control, ratio control, priority control and voltage margin control. Lastly, a case study is performed in a four-node MTDC network and the different control strategies are compared.@en

EeFarm-II is used to evaluate 13 different electrical systems for a 200 MW wind farm with a 100 km connection to shore. The evaluation is based on component manufacturer data of 2009. AC systems are compared to systems with DC connections inside the wind farm and DC connection to shore. Two options have the best performance for this wind farm size and distance: the AC system and the system with a DC connection to shore. EeFarm-II is a user friendly computer program for wind farm electrical and economic evaluation. It has been built as a Simulink Library in the graphical interface of Matlab-Simulink. EeFarm-II contains models of wind turbines, generators, transformers, AC cables, inductors, nodes, splitters, PWM converters, thyristor converters, DC cables, choppers and statcoms.@en

Doubly fed induction generator (DFIG) still shares a large part in today's wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively.@en

Today, doubly fed induction generator (DFIG) gains the dominant market power in the wind energy industry. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor side voltage source converter (VSC) from transient over current during grid voltage dip. But under unsymmetrical voltage dip, the severe problems are not the transient over current, but the electric torque pulsation and DC voltage ripple in the back to back VSCs. This paper develops dynamic models in Matlab/Simulink, investigates the behavior of DFIG during an unsymmetrical voltage dip, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controller limit the torque pulsation and DC voltage ripple effectively.@en

The increasing penetration of variable speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will lead to a reduction of inertia in the grid, as the rotational speed of a variable speed turbine and the grid are decoupled by a power electronic converter. A lower system inertia will result in larger and faster frequency deviations after occurrence of abrupt variations in generation and load. It is possible to implement an additional control loop in the power electronic converter of the turbine, which connects the turbine inertia directly to the grid. It is even possible, by control, to make a `virtual inertia¿ that is larger than the real inertia. During a frequency drop additional power can be released to the grid with this control. This behaviour will be called inertial response. The additional power is obtained from the kinetic energy that is stored in the rotating mass of the turbine. In this contribution two different control strategies are investigated and compared with each other. A type of control that is equal to the primary frequency control of large power plants shows the best results, both with respect to the power that is needed to limit the frequency drop and with respect to electrical and mechanical stress of the turbine. Keywords: Frequency control; Inertia; Stability; Wind power generation@en