Common and differential mode of dc-bias in three-phase power transformers

Dc magnetization due to the geomagnetically induced currents (GICs) and HVDC (High Voltage Direct Current) systems may cause core saturation and result in serious destruction in the transformer performance as well as the power system stability. Based on susceptibility, transformers are classified into different groups. For instance, a three-phase, three-limb transformer is considered less vulnerable to effects of GIC compared to a single-phase or a three-phase, five-limb transformer. However, our study shows that such classifications do not apply to the dc magnetization caused by converter modulation. In this article, we introduce the concept of common mode and differential mode to distinguish dc-bias caused by different mechanisms. Main focus is given on differential mode dc current since it has rarely been reported in any literature. The differential mode dc current was demonstrated by system simulations of classic three-level voltage source converters as well as modular multilevel converter. Detailed experimental investigations were made on a three-phase, three-limb transformer, where the loss impact as well as reactive power consumption were studied. The test shows a significant difference in stray loss between the two modes in three-phase power transformers. Finally, we discuss the effect of delta winding on dc-bias of different modes.Common and differential mode of dc-bias in three-phase power transformerspublishedVersio

Modelling of Internal Pressure Dynamics in Mass-Impregnated Non-Draining HVDC Cables

Changing the current loading of mass-impregnated non-draining power cables causes the pressure in the insulation to vary over a wide range. It is generally recognized that under certain conditions—typically after a large load reduction—the pressure becomes so low that shrinkage cavities (voids) form in the insulation, and that partial discharges that may damage the insulation ignite. A finite element model for temperature, electric field and pressure dynamics throughout the insulation layer has been developed. Comparisons with previously obtained measurements on full-scale HVDC subsea cable samples subjected to a variety of loading patterns, external pressures (sea depths) and ambient temperatures, show that the model replicates the pressure behavior of the insulation reasonably well. Most importantly, the complicated viscoelastic properties of the lead sheath, which have a profound effect on the internal pressure dynamics and thereby on the prospects of forming potentially harmful cavities, appear to be sufficiently accurately modelled.acceptedVersio

Potential of MgB₂ superconductors on direct drive generators for wind turbines

Topologies of superconducting direct drive wind turbine generators are based on a combination of superconducting wires wound into field coils, copper armature windings, steel laminates to shape the magnetic flux density and finally structural materials as support. But what is the most optimal topology for superconducting wind turbine generators? This question is investigated by assuming some unit cost of the different materials and then minimizing the cost of the active materials of a 10 MW and 9.65 rpm direct drive wind turbine generator intended to be mounted in front of the INNWIND.EU King-Pin concept nacelle. A series of topologies are investigate by adding more iron components to the generator, such as rotor back iron, field winding pole, magnetic teeth and armature back iron. This method is used to investigate 6 topologies and to determine the optimal cost of the different topologies by using the current cost of 4 €/m for the MgB2 wire from Columbus Superconductors and also a possible future cost of 1 €/m if a superconducting offshore wind power capacity of 10 GW has been introduced by 2030 as suggested in a roadmap. The obtained topologies are compared to what is expected from a permanent magnet direct drive generators and the further development directions are discussed. Finally an experimental INNWIND.EU demonstration showing that the current commercial MgB2 wires can be wound into functional field coils for wind turbine generators is discussed

Design of an MgB₂race track coil for a wind generator pole demonstration

An MgB2 race track coil intended for demonstrating a down scaled pole of a 10 MW direct drive wind turbine generator has been designed. The coil consists of 10 double pancake coils stacked into a race track coil with a cross section of 84 mm x 80 mm. The length of the straight section is 0.5 m and the diameter of the end sections is 0.3 m. Expanded to a straight section of 3.1 m it will produce about 1.5 T magnetic flux density in the air gap of the 10 MW 32 pole generator and about 3.0 T at the edge of the superconducting coil with an operation current density of the coil of 70 A/mm2.Electrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc

Play-Responsive Teaching in Early Childhood Education

This open access book develops a theoretical concept of teaching that is relevant to early childhood education, and based on children’s learning and development through play. It discusses theoretical premises and research on playing and learning, and proposes the development of play-responsive didaktik. It examines the processes and products of learning and development, teaching and its phylogenetic and ontogenetic development, as well as the ‘what’ of learning and didaktik. Next, it explores the actions, objects and meaning of play and provides insight into the diversity of beliefs about the practices of play. The book presents ideas on how combined research and development projects can be carried out, providing incentive and a model for practice development and research. The second part of the book consists of empirical studies on teacher’s playing skills and examples of play with very young as well as older children

Fabrication of a scaled MgB2 racetrack demonstrator pole for a 10 MW direct drive wind turbine generator

Field windings made of MgB2 wires or tapes are considered for their potential to reduce volume, weight and cost of large offshore wind turbine generators. To gain experience of how to use this relatively new material in full-scale generators, tests of different winding methodologies and techniques are needed. In this paper, we describe in detail the steps used to wind a racetrack coil with a length of 1 m and a width of 0.5 m out of 4.5 km of MgB2 superconducting tape. The width corresponds to a full-scale pole of a 10 MW generator, whereas the length of the straight section is shorter than the corresponding full-scale pole. The coil was built up of 10 double pancake coils. Each double pancake coil was wet-wound using a semi-automatic winding process, where Stycast 2850 was applied directly to the MgB2 tape without any other dedicated electrical insulation. The strengths and weaknesses of the winding process are discussed and compared to the dry-winding method.acceptedVersio