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Superconducting generator field windings are suggested for the next generation of wind turbines. The foreseen wind turbines with power ratings greater than 10 MW give raise to technical challenges. Superconducting field windings can produce strong magnetic fields with a compact design. Generators with field windings made of superconducting MgB2 wire could potentially reduce weight, material requirements, maintenance requirements and power losses, compared to conventional wind turbine generators. Losses in the superconductors increase the cooling costs of the system, hence these losses affect the economic feasibility of applying superconductor technology for wind turbine applications. Superconductors in a rotor field winding will be exposed to a strong DC magnetic field with an AC magnetic field ripple. A theoretical model of the AC losses in the superconducting material MgB$_2$ predicts increasing losses when the strength of the DC and AC magnetic field increase. However, experimental testing is required to determine the magnitude of the losses in a MgB2 superconductor. A superconductive MgB2 generator field winding demonstrator has been developed by SINTEF Energy Research in collaboration with NTNU, as part of the research project INNWIND.EU. The field winding is used to produce a strong background DC magnetic field during testing of AC losses. Custom hardware have been designed, produced and installed to produce a controlled AC field ripple and to measure the losses in a 10 cm sample of a MgB2 superconducting wire. The losses are measured using a calorimetric measurement method. The test equipment is installed in the air gap of the field winding, in the existing cryogenic cooling environment of the field winding. During testing, the field winding and the wire sample is cooled to temperatures of about 20 K. Losses are measured for different AC magnetic fields up to 21 mT peak, with different background DC fields up to 0.35 T. The losses measured increase with the 2nd power of the AC magnetic field, regardless of the strength of the background field applied. For a fixed AC field with different background fields, the AC losses peak at about 0.1 T DC field and are lowest at the highest field value of 0.35 T. After the first strong magnetization of the wire sample, it was found a significant difference in the AC losses as a function of the DC background field. Considering the magnitude and the loss patterns, it is assumed that the losses at the tested field strengths are dominated by eddy current losses and hysteresis losses in the copper and the nickel in the superconducting wire, not superconductor losses in the MgB2.

Superconducting rotor windings are considered for large offshore wind turbines to reduce weight, size and cost. The European project INNWIND.EU utilize various national projects in different European countries to accelerate the development of innovations that can help realize larger wind turbines. SINTEF Energy Research is one of the organizations that are involved in this project by designing, building and testing a superconducting rotor field winding. This could potentially reduce weight, volume and cost of future wind turbines. This master thesis describes the method and steps that were done to complete a superconducting field winding and how it was tested. Ten MgB2 double pancake coils were glued together with a thermally conductive epoxy on a thermal interface. The coils were joined electrically by a validated procedure that makes low resistance joints by using a custom made soldering tool. BSCCO high temperature superconductors were soldered at each end of the field winding to lead the current from the high temperature stage of the cryostat to the field winding. A mechanical support placed around the field winding was designed to withstand the magnetic forces acting on the field winding during testing. The field winding was placed in a cryostat and cooled down to 17.7K for testing. Seven out of ten coils were proved to be superconductive. Coil 1, 6, 10 and the BSCCO HTS were not superconductive as a voltage was measured when a current was flowing through the field winding. The quality of the joints was validated by measuring the voltage over each joint and calculating the resistance. The average resistance of the joints was 40nOhms where the highest resistance was 110nOhms. 15mW of heat would be dissipated in the joints when testing with a current of 200A. The inductance of the field winding was found to be 4.6H.

Superconducting rotor windings are considered for future large offshore wind turbines for their low weight, volume and their possibility to reduce costs. In a joint European project, INNWIND.EU, a superconducting small scale generator pole will be designed, build and tested. In this master thesis, the coil will be constructed out of ten sections, each consisting of a double pancake coil with 104 turns. These coils will be wound with a wet-winding technique, without a specific turn-to-turn insulation where the applied epoxy constitutes the insulating layer between turns. The insulating ability of the epoxy is validated by measuring the turn-to-turn resistance. Several test coils has been built, tested and investigated for possible internal contacts. The results indicate that the coil will not be damaged during normal operation, DC ramping or AC magnetic fields in the rotor windings. However, the winding method relies on the fabrication of the MgB2 wire, where irregularities has been discovered on the wire which has caused an internal contact. A splicing procedure has been developed. A custom designed high power soldering tool has been constructed and joints are tested. The joints will produce approximately 1 - 5mW during operation.

Superconductive materials can conduct DC currents without energy losses when cooled down below their critical temperature. However, under AC conditions, energy losses appear in the superconductor. In 2001 the new and inexpensive superconductor MgB2 entered the market. Currently this conductor is used in DC applications such as MRI, but before implementing the use of MgB2 in AC applications more research has to be carried out. The basic questions are how large are the AC losses, in what way do they depend on magnetic field, current and temperature, and can they be reduced to a level acceptable for commercial use?This Master?s thesis is a part of a larger work at NTNU/SINTEF. In earlier projects calorimetric AC loss measurements have been performed on an MgB2 round wire and tape, and there have been performed critical currents on the MgB2 tape.The aim for the Master?s thesis was to find the relationship between the twist pitch and the losses of a multifilamentary wire. Calorimetric AC loss measurements have been performed on seven different lengths, from 3 mm to 50 mm, of the round multifilamentary MgB2 wire. These measurements have been performed by applying AC magnetic fields from 40 mT to 210 mT with a frequency between 15 Hz to 260 Hz at three different temperatures.At 50 Hz, 100 mT and 35 K the AC losses reduces by a factor of 6 by reducing the sample length of 12 mm, 29 mm or 50 mm to 3 mm. From theoretical calculations, it is believed that by reducing the twist pitch of a long sample from 24 mm to 6 mm the benefit from twisting will be at a maximum.

Tradisjonelle induksjonsovner bruker vekselstrøm for å indusere strømmer og varme opp større aluminiumsdeler. Effektiviteten til ovnene ligger normalt mellom 55-60%, noe som gir et stort forbedringspotensial. Ved å erstatte kobberspolene med spoler av superledere og vekselstrømmen med likestrøm vil tapene i spolene bli neglisjerbare. Aluminiumsdelen som skal varmes opp må da roteres for å se et vekslende felt, noe som medfører en samlet virkingsgrad for systemet på ca 90 %. SINTEF har sammen med flere samarbeidspartnere konstruert en 200kW induksjonsovn basert på MgB2 superleder. Denne rapporten tar for seg sammensammensetning og testing av en superledende spole for bruk i den planlagte induksjonsovnen. I rapporten er alle nødvendige deler og metoder for å sette sammen spolen beskrevet og forklart. Spolen består av totalt 16 disker som ble skjøtet sammen i serie. Hver disk er 550m og er viklet i en dobbel pannekake konstruksjon med 2 ganger 75 vindinger i hver disk. Under testingen brukte spolen 120 timer på å nå en stabil temperatur som lå under den kritisk temperatur for MgB2 superlederen (37K), og det ble gjort tester ved å føre en strøm på maks 10A gjennom spolen. Under strømtestene ble spenningsoppbygningen over alle skjøtene samt over alle spolediskene målt. Strømtestene avslørte at alle skjøtene i spolen hadde en motstand på under 60nΩ. En av spolediskene skilte seg ut og hadde en motstand på 66,6uΩ ved 10A, noe som tyder på et spoledisken har en defekt. Motstanden var for høy til å kunne gjøre tester med den planlagte strømstyrken på 200A. Induktansen til spolen ble målt til 4,9H.

This Master thesis studies losses in superconductors. Losses arise when the superconductor carries alternating current or is placed in an alternating magnetic field. As most power applications involve at least one of the two, loss mechanisms and magnitudes are important when examining the possibility of making superconductor systems competitive to conventional power systems. There are two parts to the task at hand. The first part is a literature study on superconductivity and AC losses in superconductors. A division between two general types of superconductors is presented; type I and II. The Bean model for AC losses in type II superconductors is described, together with equations giving the power law between generated losses and applied magnetic field. In the Bean model, the losses are proportional to the applied field cubed below a limit called the penetration field. Above it, the exponent changes to one. Losses due to coupling of filaments are also treated. The measuring setup used in the AC loss experiments is calorimetric, and the principle behind the method is presented. The superconductor used, magnesium diboride (MgB2), is introduced. The literature study is concluded with short résumés on other AC loss studies done on MgB2, and studies done on one type of high temperature superconductor. The second part is measuring AC losses due to an applied alternating magnetic field in two superconductor samples from different manufacturers. Specific information on the two samples, details on the measuring system, preparations and the measuring procedure is described. The logged data and equations used when processing the results are also listed. Measurements have been performed at six different temperatures; 25, 28.5, 30, 31.5, 35 and 45 K. The magnitude of the applied magnetic field was varied between 3 and 150 mT. Both parallel and perpendicular field directions were applied. Generated losses lead to a temperature increase in the superconductor. The rise in temperature was detected as increased resistivity of a thin copper wire glued onto the sample, as the copper resistivity is temperature dependent. The obtained results are examined in double logarithmic (loglog) and normal axis diagrams, where the main aim is to find loss slopes and penetration fields at the different temperature levels, and to compare these to the Bean model loss equations. In addition, the results are compared to theoretical loss equations for cylindrical conductor geometry. This is done in order to look at the accuracy of the fittings and to compare the penetration fields obtained here to the ones found in loglog diagrams. The results have also been compared to various studies on MgB2 and other superconductor types. The measured loss slopes at fields below the penetration field, found from loglog diagrams, do not fit the Bean model. The slopes are here lower than the applied field cubed. At fields greater than the penetration field, losses are proportional to the applied field, as in the Bean model. Two reasons for the deviations have been discussed; measuring errors and losses being coupling losses. Even if the measuring errors may be considerable due to human reading errors, they would have to be systematic for the losses to fit the Bean model. This is the reason why measuring errors are seen as unlikely to be the grounds for the non-fitting results. The results do also not fit the coupling loss slope and as only one field frequency has been used, the obtained results are not enough to support or reject this theory. Due to the deviation from the Bean model loss slopes the curve fittings to the cylindrical conductor loss equations were mostly poor, as they have the same loss slopes as the Bean model. The penetration fields found from loglog and normal axis diagrams and the curve fitting are not equal. It is establish that the ones found from loglog diagrams should be used. Two of the other studies done on MgB2, which have been summarised in the thesis, fit the Bean model, and the last case did not. The authors found no explanation to the non-fitting results, and have ruled out coupling losses as a viable reason. Studies on the other type of superconductor also represented both cases. Here, some non-fitting results were explained by coupling losses. When comparing loss magnitudes, only one of the samples used in these experiments had as low results as found in two other studies.