Blockchain Papers

Christophe Lebrun, Oetske Leroux-Fankhauser, Natkamon Tovanich, Thibault Vatter · 5 authors

No abstract is available for this record.

Fahim Abid

An increasing number of wind farms and mining operations located far off the coast will lead to the development of offshore substations. To avoid the large costs associated with platforms and floaters, such a substation can be placed on the seabed and controlled remotely. The conventional solution is to place the power components, e.g. switchgear placed inside thick-walled pressure-proof vessels to protect them from water and high pressure on the seabed. For current switching in medium voltage applications, there are mainly two options: vacuum circuit breakers or gas circuit breakers (filled at atmospheric or slightly elevated pressure). Whichever option is chosen, power cable feed-throughs or penetrators from the high-pressure water environment into the low pressure inside the vessel are required. These features add substantial technical complexity and costs, in particular at large sea depths. A novel concept is used in this thesis, where the interruption chamber of the circuit breaker can be gradually filled as the switchgear is lowered until finally reaching the same pressure as on the seabed. Reducing the differential pressure on the encapsulation will reduce the overall cost and complexity of such subsea substations. The gas pressures in this case may be in the range of up to tens of bars.\nIf the temperature and pressure of a gas exceed its critical point, it enters a supercritical state. In this state, the physical properties are between that of a gas and a liquid. The properties include high diffusivity, high heat conductivity, high heat capacity, high dielectric strength and an absence of vapour bubbles. These properties of the supercritical fluid are believed to be in favour of a successful current interruption medium. However, there is a distinct lack of knowledge on arc properties and the current interruption capability of extremely high-pressure gasses as well as on the supercritical region. In this thesis, nitrogen (N2) is chosen for its low critical point (33.5 bar, 126 K), good insulation strength and environment-friendly nature. As the critical temperature of N2 is lower than room temperature, the transition to supercritical state can be achieved by pressurizing N2 above 33.5 bar.\nThis thesis reports on the experimental investigation of the characteristics of N2 arc as a function of filling pressure as well as in the supercritical state. For the bulk of the study, filling pressures of 1, 20, 40 and 80 bar are investigated, the latter two being in the supercritical state. A fixed electrode arrangement is used where the arc is initiated by the melting of a copper ignition wire. The investigated arc current amplitude is in the range of 85 A to 450 A at a frequency of 190 Hz to 950 Hz. Based on the focus area of different phases of the arc, this thesis can primarily be divided into three major parts. First, the arc properties during the high-current phase, i.e. during current peak time, are investigated for free-burning and tube-constricted arcs. In the second phase, the investigation is focused near the current zero (CZ) where the thermal phase of the arc is studied. In the final phase, the post-arc dielectric recovery characteristics are studied. The effect of the forced gas flow is investigated in both the thermal and dielectric phase of the arc.\nBased on the experimental results, the arc voltage is found to increase with the filling pressure without any abrupt change during the transition from gas to the supercritical state. Increased current density due to the constriction of the arc at high filling pressure turned out to be the dominant factor for the high arc voltage. When the free-burning arc is physically constricted by means of burning inside a tube, an inverse relation between the arc voltage and the inner tube diameter is observed at 1 bar, as expected. At higher filling pressures, however, such a simple relationship does not exist. The reduced arc radius and the increased absorption of radiation at high filling pressures may limit the interaction between the arc and the tube.\nThe energy deposition in the arc increases while the arc radius decreases with increasing filling pressure. The arc gets constricted and as a result the temperature of the arc core increases. In the free-burning arc, in the absence of forced cooling, the arc core fails to dissipate the stored thermal energy quickly. As a result, without efficient cooling a high post-arc current is often observed at a high filling pressure compared to at 1 bar. The high energy deposition in the post-arc channel due to increased post-arc current causes an early re-ignition at high N2 pressure compared to at 1 bar. A forced gas flow, however, significantly enhances cooling at high filling pressures and improves the interruption performance.\nIn the free-burning arc arrangement, the post-arc dielectric strength of the gap increases rapidly with increasing filling pressure, only after a critical time delay following CZ. This critical time delay is probably linked to the temperature decay of the gap. Below the critical time delay, however, the dielectric strength of the gap is lower at a higher filling pressure in contrast to at 1 bar, similar to what is observed in the thermal re-ignitions of the freeburning arc. Forced gas flow significantly enhances the dielectric recovery of the arc channel at a high filling pressure, also in the thermal phase. The experiments indicate that although the thermal phase is the critical phase of the ultrahigh-pressure N2 arc interruption, the dielectric phase is inherently superior at a high filling pressure compared to atmospheric pressure. With the help of efficient cooling, the thermal phase can be improved, and hence the ultrahigh-pressure N2 reveals its potential to be used as a current interruption medium.