High Voltage DC-biased Oil Type Medium Frequency Transformer; A Green Solution for Series DC Wind Park Concept

The electric energy generated by remote offshore wind parks is transported to the consumers using high voltage submarine cables. On the generation site, such transmissions are realized today by collecting the energy produced by several wind turbines in a bulky and expensive transformer placed on a dedicated platform. An alternative solution has been proposed recently, which allows to reduce the installation and maintenance costs by eliminating such a platform. It is suggested to equip each wind turbine in the wind park by an individual DC/DC converter and connect them in series to reach the DC voltage level required for an efficient HVDC energy transportation to the shore. The DC/DC converter is supposed to be a Dual Active Bridge (DAB) converter, which can be made reasonably small to be placed on the wind turbine tower or even in its nacelle. The key element of the converter defining its size and mass is a special transformer, which operates at voltages comprising a high (switching) frequency component superimposed on a high DC offset voltage. DC insulation design of such a transformer and investigation of the effects of a high DC insulation level on the other electromagnetic properties of the transformer is the subject of the present research.In order to verify the concept a prototype of the transformer was built, and its evaluation presented. The unit has been manufactured for the rated power of 50 kW and rated voltages 0.4/5 kV including DC offset of 125 kV and square-shaped oscillations with the frequency of 5 kHz. The magnetic system was made of ferrite material and consisted of 10 shell-type core segments. The magnetic properties have been verified by measuring magnetization and losses at various frequencies in the range 1-10 kHz to cover the operational range of the DAB. The types and dimensions of the windings and their conductors were chosen to minimize the proximity and eddy current effects at higher frequencies. To reduce the size of the transformer and to allow for its efficient cooling, the active part was immersed in oil and cellulose-based materials (paper and pressboard) were used to build the high voltage insulation system. The principles for dimensioning the insulation of the transformer are discussed. The criteria used for selecting insulating distances were based on the consideration of the electric field strength obtained from FEM simulations and using the non-linear Maxwell-Wagner model accounting for local variations of the electric field caused by accumulation of interfacial charges induced by DC stresses. The properties of the materials needed for the calculations were obtained by measuring their dielectric constants and electric conductivities. The methodology used for the measurements conducted for conventional mineral oil and eco-friendly biodegradable transformer oils and, respectively, for oil-impregnated paper/pressboard, is presented. The methodologies used for obtaining parameters of the built transformer prototype needed for its integration in the power electric circuit of the DAB are introduced. A method developed for accurate calculations of the leakage inductance for the shell-type multi core transformers with circular windings is described. Two innovative methods for evaluations of parasitic capacitances based on high frequency equivalent circuits of the transformer are presented. The results of their verifications against performed Frequency Response Analysis measurements and FEM calculations as well as their accuracy are discussed.Thermal performance of the developed transformer prototype is analysed based on the results of computer simulations of heat transfer in its active part under rated load. Identified hot spots and solutions for their elimination are presented.Finally, the expected dimensions, weight, and efficiency of an actual DC/DC converter with the rated parameters corresponding to a 6 MW, 1.8 kV real wind turbine having a 250 kV offset DC voltage are estimated assuming that the developed transformer prototype is scalable. It is shown that the proposed solution allows for installing the full-scale converter having 2.2 Tons in weight and 1.8 m3 in volume on the bottom of the wind turbine’s tower

Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

Flux switching machines using segmental rotors

Flux switching machines (FSM) employing a segmental rotor have field and armature systems on the stator and a presentation of an unexcited rotor with isolated segments. The single-tooth winding arrangement on the stator provides a potential for material and energy savings. The principle for producing bipolar flux in the armature stator teeth relies on the natural switching of the armature tooth flux, accomplished by the moving segments of the rotor. Three phase configurations have been studied, from conception and design to construction and testing, with field excitation provided by either a field winding or permanent magnets (PM). Flux switching machines have shown characteristics that are peculiar when employing a segmental rotor, significantly affecting the symmetry of the induced armature EMF waveform and parity of magnitudes of the positive and negative torques. For three phase operation, six topologies are feasible when employing a 12-tooth stator and two other topologies may be produced on a 24-tooth stator. An optimum topology on the 12/8-configuration and another proof-of-principle topology on the 12/5-configuration, using field-windings and permanent-magnets, have been designed and constructed, while applying modern practices and considerations for manufacture. The characteristics of FSMs employing a segmental rotor, initially predicted by finiteelement (FE) modelling, have been verified by measurements. The FSM employing a field-winding is found to have a specific torque output which is similar to the conventional switched reluctance motor and still substantially higher than that of the synchronous reluctance motor. Although the PM adaptation of the FSM produces specific torque output which is nearly twice that of the wound-field FSM and about 64% that of an equivalent permanent-magnet synchronous motor (PMSM) with surface or insert magnets, accounting for the usage of the magnets reflects its specific torque output to be about 1.48 times higher than the PMSM. Although the FSM is operated as an AC machine with sinusoidal three-phase currents, its dq-equivalent representation shows significant differences from that of the conventional AC machine. In the prediction of the performance, it is found, in both the wound-field and PM configurations, that the dq model is more dependable if the coupling dq inductance is taken into account.EThOS - Electronic Theses Online ServiceCommonwealth Scholarship Commission of UKGBUnited Kingdo

Magnetostriction and magnetic anisotropy in non-oriented electrical steels and stator core laminations

Magnetostriction is a source of vibration and acoustic noise of electrical machines and it can be highly anisotropic even in non-oriented electrical steel. Understanding of magnetostriction under magnetisation and stress conditions present in stator core laminations can help predict the core vibration and radiated noise. Anisotropy of magnetostriction of a 0.50 mm thick non-oriented steel investigated in Epstein strips cut at angles to the rolling direction was much higher than the anisotropy of its magnetic and elastic properties because magnetostriction arises directly from magnetic domain processes. Magnetostriction of a disc sample of the 0.50 mm thick steel was measured under ID and 2D magnetisation and compared with that of a 0.35 mm thick steel with different anisotropy level. A 2D magnetostriction model and an analytical simple domain model were used to explain the experimental results. 2D magnetostriction is dependent on the magnetostrictive anisotropy and the ratios of the transverse to longitudinal magnetostriction. AC magnetostriction measured in the disc samples was larger than in the Epstein strips due to the form effect. An induction motor model core was constructed from the 0.50 mm thick steel for measurements of localised flux density and deformation. Core deformation due to Maxwell forces was calculated. Magnetostriction and specific power loss of the core material under magnetisation conditions present in the core was measured. The localised loss in the stator teeth, tooth roots and back iron differed from their average value by 52%, 19% and 36% due to the magnetic anisotropy. Magnetostriction was estimated to be about 55% and 80% of the radial deformation at the tooth root and back iron regions respectively. Stator teeth deformed asymmetrically and the magnitude of the space harmonics increased due to the magnetostrictive anisotropy. The measurement results inferred that 2D magnetostriction can be predicted from the magnetostrictive anisotropy and vice versa. Also, core deformation and vibration of large machines, where segmented stator core laminations are used, can be estimated analytically with the knowledge of 2D magnetostriction of the core material

Optical packaging of microlens over UV-LED array

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HASTECS: Hybrid Aircraft: reSearch on Thermal and Electric Components and Systems

In 2019, transportation was the fastest growing sector, contributing to environmental degradation. Finding sustainable solutions that pollute less is a key element in solving this problem, particularly for the aviation sector, which accounts for around 2-3% of global CO2 emissions. With the advent of Covid-19, air traffic seems to have come to a fairly permanent halt, but this pandemic reinforces the need to move towards a "cleaner sky" and respect for the environment, which is the objective of the Clean Sky2 program (H2020 EU), the context in which the HASTECS project has been launched in September 2016