Arne Hejde Nielsen

Advanced simulation of windmills in the electric power supply

An advanced model of a grid-connected windmill is set up where the windmill is a complex electro-mechanical system. The windmill model is implemented as a standardised component in the dynamic simulation tool, PSS/E, which makes it possible to investigate dynamic behaviour of grid-connected windmills as a part of realistic electrical grid models. This means an arbitrary number of wind farms or single windmills within an arbitrary network configuration. The windmill model may be applied to the study of electric power system stability and of power quality as well. It is found that a grid-connected windmill operates as a low-pass filter, whereby the two following observations are made: (1) interaction between the electrical grid and the mechanical systems of grid-connected windmills is given by a low frequency oscillation as the result of disturbances in the electric grid; (2) flicker, which is commonly explained by the dynamic wind variation, may also be caused by mechanical eigenswings in the windmill mechanical construction.

Modelling and Transient Stability of Large Wind Farms

The paper is dealing with modelling and short-term voltage stability considerations of large wind farms. A physical model of a large offshore wind farm consisting of a large number of windmills is implemented in the dynamic simulation tool PSS/E. Each windmill in the wind farm is represented by a physical model of grid-connected windmills. The windmill generators are conventional induction generators and the wind farm is ac-connected to the power system. Improvements of short-term voltage stability in case of failure events in the external power system are treated with use of conventional generator technology. This subject is treated as a parameter study with respect to the windmill electrical and mechanical parameters and with use of control strategies within the conventional generator technology. Stability improvements on the wind farm side of the connection point lead to significant reduction of dynamic reactive compensation demands. In case of blade angle control applied at failure events, dynamic reactive compensation is not necessary for maintaining the voltage stability.

Turbulence and intermittent transport at the boundary of magnetized plasmas

Numerical fluid simulations of interchange turbulence for geometry and parameters relevant to the boundary region of magnetically confined plasmas are shown to result in intermittent transport qualitatively similar to recent experimental measurements. The two-dimensional simulation domain features a forcing region with spatially localized sources of particles and heat outside which losses due to the motion along open magnetic-field lines dominate, corresponding to the edge region and the scrape-off layer, respectively. Turbulent states reveal intermittent eruptions of hot plasma from the edge region, propagating radially far into the scrape-off layer in the form of field-aligned filaments, or blobs. This results in positively skewed and flattened single-point probability distribution functions of particle density and temperature, reflecting the frequent appearance of large fluctuations. The conditional fluctuation wave forms and transport statistics are also in a good agreement with those derived from the experiments. Associated with the turbulence bursts are relaxation oscillations in the particle and heat confinements as well as in the kinetic energy of the sheared poloidal flows. The formation of blob structures is thus related to profile variations, which are here triggered in a quasiperiodic manner by a global dynamical regulation due to the self-sustained sheared flows.

Mechanism and scaling for convection of isolated structures in nonuniformly magnetized plasmas

Large-scale radial advection of isolated structures in nonuniformly magnetized plasmas is investigated. The underlying mechanism considered is due to the nonlinear evolution of interchange motions, without any presumption of plasma sheaths. Theoretical arguments supported by numerical simulations reveal an inertial scaling for the radial velocity of isolated structures in the ideal limit. This velocity increases as the square root of the structure size relative to the length scale of the magnetic field. The magnitude of the radial advection velocity, as well as the dynamical evolution of the structures, compares favorably with recent experimental measurements of radially propagating blob structures in the scrape-off layer of magnetically confined plasmas.

Fluctuations and transport in the TCV scrape-off layer

Fluctuations and particle transport in the scrape-off layer of TCV plasmas have been investigated by probe measurements and direct comparison with two-dimensional interchange turbulence simulations at the outer midplane. The experiments demonstrate that with increasing line-averaged core plasma density, the radial particle density profile scale length becomes broader. The particle and radial flux density statistics in the far scrape-off layer exhibit a high degree of statistical similarity with respect to changes in the line-averaged density. The plasma flux onto the main chamber wall at the outer midplane scales linearly with the local particle density, suggesting that the particle flux here can be parameterized in terms of an effective convection velocity. Experimental probe measurements also provide evidence for significant parallel flows in the scrape-off layer caused by ballooning in the transport of particles and heat into the scrape-off layer. The magnitude of this flow estimated from pressure fluctuation statistics is found to compare favourably with the measured flow offset derived by averaging data obtained from flow profiles observed in matched forward and reversed field discharges. An interchange turbulence simulation has been performed for a single, relatively high density case, where comparison between code and experiment has been possible. Good agreement is found for almost all aspects of the experimental measurements, indicating that plasma fluctuations and transport in TCV scrape-off layer plasmas are dominated by radial motion of filamentary structures.

Monitoring of power system events at transmission and distribution level

Monitoring of events in the power system provides a great deal of insight into the behavior of the system. Events with impact on the entire power system typically occur in or near the transmission network and are therefore best monitored at transmission system substations. This puts great demands on the equipment and in general restricts the use of the data since it becomes property of the transmission system operator. An alternative is to measure at lower voltage levels. This paper documents the close agreement between voltage and frequency at low and high voltage levels during a remote and a nearby fault in the transmission network. The very high quality of the data suggests that valuable information about the transmission system can be obtained at lower voltages.

Variable-speed wind turbines with multi-pole synchronous permanent magnet generators. Part I: Modelling in dynamic simulation tools.

Variable-speed wind turbines are a promising concept for large offshore wind farms. The variable-speed concept can be realised with multi-pole synchronous generators (MPSG) excited by permanent magnets. The main advantages of this concept are that (i) the wind turbines are gearless and (ii) the electrical excitation system is replaced by permanent magnets. The electromagnetic construction of the permanent magnet generators (PMG) is, however, more complex than in conventional concept generators. The PMG are grid-connected through and controlled by their frequency converters. The converter control can be used to stabilise the large wind farm at transient events in the grid to complete the grid specifications of the power system controllers. This article gives an overview of the structural concepts of PMG and explains modelling details of the PMG and their frequency converters.

Coherent structures in two-dimensional plasma turbulence

Low‐frequency, flute‐type electrostatic fluctuations propagating across a strong, homogeneous magnetic field are studied experimentally. The fluctuations are generated by the Kelvin–Helmholtz instability. The presence of relatively long‐lived vortexlike structures in a background of wide‐band turbulent fluctuations is demonstrated by a conditional sampling technique. Depending on plasma parameters, the dominant structures can appear as monopole or multipole vortices, dipole vortices in particular. The importance of large structures for the turbulent plasma diffusion is discussed. A statistical analysis of the randomly varying plasma flux is presented.

Shear flow generation and energetics in electromagnetic turbulence

Zonal flows are recognized to play a crucial role for magnetized plasma confinement. The genesis of these flows out of turbulent fluctuations is therefore of significant interest. Here the relative importance of zonal flow generation mechanisms via the Reynolds stress, Maxwell stress, and geodesic acoustic mode (GAM) transfer in drift-Alfven turbulence is investigated. By means of numerical computations the energy transfer into zonal flows owing to each of these effects is quantified. The importance of the three driving ingredients in electrostatic and electromagnetic turbulence for conditions relevant to the edge of fusion devices is revealed for a broad range of parameters. The Reynolds stress is found to provide a flow drive, while the electromagnetic Maxwell stress is in the cases considered a sink for the flow energy. In the limit of high plasma β, where electromagnetic effects and Alfven dynamics are important, the Maxwell stress is found to cancel the Reynolds stress to a high degree. The geodesic ...

Formation and temporal evolution of the Lamb-dipole

The formation and dynamics of dipolar vortex structures in two-dimensional flows are studied. Localized initial structures possessing a finite linear momentum are found to develop into dipoles by direct numerical solutions of the two-dimensional Navier-Stokes equations. The detailed structure of the evolving dipoles depend on the initial condition. However, the gross properties of their evolution are only weakly dependent on the detailed structure and can be well-described by the so-called Lamb-dipole solution. The viscous decay of the Lamb-dipole, leading to an expansion and a decreasing velocity, is well described by an adiabatic theory. During the expansion the dipole is found to trap fluid as it evolves.