Fault current levels in land-based power systems are generally rising because of the increase in renewable generation capacity. Once the fault current level exceeds the capacity of the existing protection equipment, expensive upgrades become necessary. In order to avoid excessively expensive equipment upgrades, many fault current limitation techniques have been investigated. This thesis presents the work conducted on the design, manufacture and testing of a resistive superconducting fault current limiter (SFCL) with an integrated fast-acting vacuum interrupter. The practical application of magnesium diboride (MgB2) in round wire form was also investigated. A single-strand MgB2 SFCL coil was investigated and demonstrated repeatable and reliable current-limiting action. In practical power system applications, the development of SFCLs needs a considerable scale-up of the current-carrying capability of the MgB2 wire samples. One option is to use parallel wires in order to carry current levels in the kA range. The behaviour of a prototype three-strand MgB2 SFCL coil was assessed, which showed that each of the three wire strands shared the current approximately equally and demonstrated reliable and repeatable behaviour during testing. The MgB2 SFCL coil with multiple wire strands in parallel shows considerable potential as a practical method for scaling-up the current levels required for power system applications. One of the significant operational issues for resistive SFCLs is the temperature recovery time after a fault is cleared. A vacuum interrupter was integrated therefore into the SFCL system to quickly remove the superconducting coil from the circuit during a fault condition and allow the superconducting coil to recover whilst a bypass resistor acted as a current limiting resistor. A fast-acting actuator and its control circuit were designed and manufactured to control the operation of the vacuum interrupter. The SFCL with a prototype vacuum interrupter was successfully tested to validate the design process. The energy dissipated in the superconducting coil was significantly reduced by the fast operation of the vacuum interrupter and the recovery time significantly reduced. This research demonstrates the potential of a cost-effective and compact SFCL for the power system applications.

To ensure the safe operation of high-voltage direct current grids, circuit breakers are used to disconnect a faulty link from the rest of the grid. Incorporating vacuum interrupters as a part of these circuit breakers constitutes an outstanding technology for such DC interruptions. However, testing the interrupters take a long time and can be very expensive. Hence, to reduce the time and cost of testing the interrupters, the purpose of this project was to find the most important parameters to test in a vacuum interrupter to evaluate it for use in a DC circuit breaker. This was done by modeling the particle density, before and after current-zero, and the post-arc current using a new model along with existing ones. Review of existing research was also included to support the models in order to draw conclusions regarding reignitions and restrikes. dI/dt before current-zero, dV /dt after current-zero, and contact gap length were found to be the key contributors for reignition, while temperature, contact surface condition and contact gap length were of great importance for restrikes. These breakdowns should occur around the center of the contact surface, or at surface protrusions. The following parameters should be varied when testing vacuum interrupters: dI/dt before current-zero, ranging from 10 A µs−1 to 800 A µs−1; arcing current, from 1 kA to 20 kA; arcing time, from 1 ms to 4 ms before current-injection; dV /dt after current-zero, from 0.5 kV µs−1 to 20 kV µs−1; maximum TRV from 5 kV to 25 kV, to find the threshold voltage for failed interruption; and gap length, from 1 mm to 10 mm, to find the critical gap length. Furthermore, temperature should also be measured, though measuring postarc current seems to be of lesser importance. To minimize damage to the interrupter, it was recommended to start with higher gap lengths with low values on everything else. / För att säkerställa ett stabilt kraftflöde för högspända likströmsnät används brytare för att isolera defekta delar från resten av nätet. Vakuumbrytare kan utgöra en viktig komponent i sådana likströmsbrytare. Att testa brytarna tar dock lång tid och kan bli mycket dyrt. För att minska på tiden och kostnaderna för att testa brytarna, var syftet med detta projekt att hitta de viktigaste parametrarna att testa i en vakuumbrytare för att utvärdera den för användning i en likströmsbrytare. Detta gjordes genom att modellera partikeldensiteten före och efter nollgenomgången i strömmen, och strömmen som uppstår efter den nollgenomgången genom att använda en ny modell med hjälp av äldre modeller. Granskning av befintlig forskning inkluderades också för att stödja modellerna och dra slutsatser om nytändning och återtändning. dI/dt före strömnollgenomgången, dV /dt efter strömnollgenomgången, bågströmmen och kontaktseparationen visade sig vara nyckelparametrar för nytändning. Temperaturen, kontaktytstillståndet och kontaktseparationen var av stor betydelse för återtändning. Dessa elektriska urladdningar hade en högre sannolikhet att ske närmare mitten av kontaktytan eller vid små spetsiga ojämlikheter. Följande parametrar bör varieras vid testning av vakuumbrytare: dI/dt före strömnollgenomgången, från 10 A µs−1 till 800 A µs−1 ; strömmen före strömnollgenomgången, från 1 kA till 20 kA; ljusbågstiden, från 1 ms till 4 ms; dV /dt efter strömnollgenomgången, från 0.5 kV µs−1 till 20 kV µs−1 ; maximala TRV, från 5 kV till 25 kV, för att hitta tröskelspänningen för misslyckat brytning; och kontaktseparationen, från 1 mm till 10 mm. Att mäta strömmen efter strömnollgenomgången verkade vara av mindre betydelse. Dock föreslogs det att mäta temperaturen på vakuumbrytaren. För att minimera skador på brytaren rekommenderades det att börja testandet med högre kontaktseparationer med låga värden på allt annat.

A vacuum interrupter is a switching device used in vacuum circuit breakers, which are widely employed in medium voltage circuits for interrupting the short-circuit fault currents. The vacuum interrupter is the chamber in which the arc extinction and hence the current interruption takes place. On the occurrence of a fault, the breaker mechanism separates the contacts of the vacuum interrupter. As the contacts separate, an arc is established between the contacts. The arc evolves in the contact space and extinguishes at or near the current zero, thus interrupting the current. The processes of arc ignition, evolution and extinction are very complex. These processes are fundamental to the design and the performance of the vacuum interrupter and hence the circuit breaker. The evolution of the arc predominantly depends on the short-circuit current, the design and metallurgy of the contacts. The evolution of the vacuum arc has been the focal point of considerable research activity. Significant effort has been concentrated to understand the various modes of the arc, the transition between the modes, the arc movement and the dependency on the contact design and finally the effect of the arc evolution on the current interruption performance of the vacuum interrupter. The voltage across the contacts during the arcing, termed as the arc voltage, has been a focal point of several research projects. Research has shown that the arc voltage depends strongly on the mode and the evolution process of the arc. The dependency is observed with respect to the magnitude and the nature of the arc voltage. This dependency has been established through the comparison of the arc voltage trace and the actual arc photographs. The arc voltage is thus an important parameter in understanding the arcing process in the interrupter. Arc voltage could also be utilised to compare the arcing behaviour in vacuum interrupters with different contact geometries and metallurgies. Having understood how the arc voltage depends on the arc modes and how it can be used to analyse the arcing performance of the interrupter, this work aims to establish experimentally the dependency of the arc voltage on fundamental parameters of the short-circuit current and the contact design. The variation of the arc voltage is studied with respect to the magnitude of the short-circuit current. It is seen that the magnitude of the arc voltage is higher, for a higher short-circuit current. This dependency is also reflected in the nature of the arc voltage waveform. The effect of cumulative short- circuit operations has been understood through the study of arc voltage variation with respect to the accumulated arcing time. It has been found that the arc voltage consistently decreases as the accumulated arcing time increases. The effect of the contact diameter on the arc evolution has been studied by comparing the arc voltage variations for contacts of different diameters for the same short-circuit current. It is observed that the variation of arc voltage with respect to the contact diameter depends on the type of contact. In the case of radial magnetic field contacts, it has been observed that the arc voltage is lower for a contact with lower diameter. Whereas in the case of axial magnetic field contacts there is an inverse relation between the contact diameter and the arc voltage. Finally, the effect of the type and distribution of the magnetic field on the arc voltage variation as well as the contact erosion has been studied. In general, the observations show that the arc voltage magnitude for the radial magnetic field geometry is higher than the axial magnetic field geometry. Also, there is a significant difference in the appearance of the arc voltage waveforms for the arcs under the two types of magnetic fields. Finite element simulations and short-circuit evaluations have shown that the axial magnetic field contact system with 90 deg coil orientations yield a more uniform distribution of the flux density and hence lower erosion of the contacts. These results show a clear dependence of the arc voltage on the various above mentioned parameters. Thus the arc voltage could be utilised as a diagnostic parameter during the evaluation of the vacuum interrupter. In the present scenario, significant research is being done to increase the breaking capacity of the interrupters. This calls for optimization of design of the existing contacts and the design of novel contact geometries. The arc voltage would be used as an important diagnostic tool in this process. Also, the utilization of vacuum interrupter in high voltage and extra high voltage circuits is being explored. This application requires increase in the contact gap or series connection of gaps. The arc behaviour in longer gaps and gaps connected in series would be an important research area. Again the arc voltage could be used to study the arc evolution in these specialised conditions. The experiments in this research work have been performed on commercial vacuum interrupters. For a dedicated research on vacuum arcs and vacuum interrupter contacts, development of a vacuum arc research facility has also been attempted as a part of this research work.

The aim of the present thesis is to improve and optimize a Thomson coil based actuatorfor medium voltage vacuum interrupters. The Thomson coil based actuator’s concept isdiscussed. The thesis presents analytical as well as finite element models of the actuatoralong with a comparison of their results. Several experimental setups have been built forthis degree project and they are described in the thesis. Measurements from these setupsare also compared to simulation results. The thesis concludes by drawing conclusionsfrom the compared results and proposes possible directions for additional work in thenear future. / Målsättningen för denna uppsats är att förbättra och optimera en aktuator för mellanspänningsvakuumbrytare baserad på en Thomsonspole. Aktuatorkonceptet analyserasoch diskuteras. Uppsatsen presenterar analytiska modeller såväl som numeriska modellerför FEM av aktuatorn, samt jämförelser av resultaten från simuleringar gjorda av dessa.Flera experimentuppställningar har byggts under detta examensprojekt och beskrivs idenna uppsatsen. Mätningar från dessa uppställningar jämförs också med resultaten frånsimuleringarna. Uppsatsen drar slutligen slutsatser utifrån resultaten och föreslår möjligavägar för ytterligare arbete på området inom en snar framtid.