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ENERGIX-Stort program energi

Current interruption in supercritical fluids

Alternative title: Strømbrytning i superkritiske fluider

Awarded: NOK 7.7 mill.

An increasing number of windfarms located far off the coasts and also a growing demand for electric power supply to oil and gas installations on the seabed will lead to a development of an off-shore electric power transmission infrastructure. To avoid the large costs of platforms or floaters, power switching equipment and other components will be placed on the seabed and be remotely controlled. The conventional solution is to use thick-walled pressure proof steel vessels to provide an environment for the power components similar to that of on-shore installations (air at atmospheric pressure). Power cable feed-throughs or penetrators from the high-pressure water environment and into the more "normal" ambient inside are also required. These features add substantial technical complexity and cost, in particular at large sea depths. The current project aimed at developing a novel subsea circuit breaker concept where the interrupting chamber contains nitrogen in its supercritical state at the same (high) pressure as the surrounding water. This has the potential of huge cost savings as no pressure-safe enclosure is required and power cable feed-throughs become simpler. Supercritical fluids have certain physical properties similar to those of liquids and others similar to what is seen in gases. For arc extinction purposes - which is what current interruption in switchgear is about - the properties may be advantageous. In this project, the concept of using supercritical fluids for current interruption purposes is explored. The explosion safe high voltage lab at Tiller, Trondheim, has been substantially upgraded. Now, current interruption experiments with a current of approximately 2000 A (50 Hz) and a system voltage of 24 kV can be carried out in nitrogen pressures up to 300 bar. The conducted experiments have contributed to determine the characteristics of electric arcs burning in supercritical nitrogen, both free-burning, wall-constricted, and under force cooling from ablation, self-blast or puffer concepts. The higher the nitrogen pressure, the better the dielectric strength becomes. Still, the energy dissipated in the arc during the interruption process also increases, which makes cooling more challenging. Thus, the switchgear must be designed specifically for supercritical nitrogen to make effective high pressure interrupters. In addition to the experimental work, the postdoc candidate at NTNU has worked two years to develop a numerical model of the high-pressure arcs to better understand the physical phenomena related to electric arcs and current interruption. The model includes the ignition of the electric arc by explosion of ignition copper wires, and the interaction between the arc and solid materials in the test switch. For more information, contact NTNU professor Kaveh Niayesh: Kaveh.niayesh@ntnu.no. Overall, the project has resulted in 10 journal or conference papers, and efforts are being made to apply for further funding through e.g. EU. More information on the experimental results from the project can be found in the publication list, or by e-mail: nina.stoa-aanensen@sintef.no. Both the lab infrastructure and the knowledge obtained through the project will act as an important foundation for further research. In addition to subsea applications, the project has received interested within the medium voltage DC switchgear research community.

Prosjektet har bidratt til å skape en plattform for forskning på subseabrytere med høyt fyllingstrykk, som vil brukes til videre forskning og utvikling av mer kostnadseffektive brytere for havbunnen. Plattformen består av utdannet personale (PhD og postdoktorkandidat ved NTNU og forskere i SINTEF Energi), fysisk laboratorieinfrastruktur og en simuleringsmodell for høytrykkslysbuer. Laboratoriet er fleksibelt, og legger til rette for ikke bare superkritisk nitrogen, men også andre fluider som karbondioksid, trykksatt luft eller til og med olje. Prosjektet har gjennom publikasjoner og konferansedeltagelse nådd flere tusen lesere. Jobben videre blir å utvikle kompetansen i det eksisterende prosjektteamet, samt å involvere industri i videre arbeid mot produktutvikling.

A novel subsea circuit breaker concept where the interrupting chamber contains a supercritical fluid at the same (high) pressure as the surrounding water is proposed. This has the potential of huge cost savings as no pressure-safe enclosure is required and power cable feed-throughs become simpler. Supercritical fluids have certain physical properties similar to those of liquids and others similar to what is seen in gases. For arc extinction purposes - which is what current interruption in switchgear is about - the properties may be advantageous. Their high density yields a high dielectric strength, whereas high heat conductivity and low viscosity yield a large heat transfer capability, leading to a fast dielectric recovery after a current interruption. The concept of using supercritical fluids for current interruption purposes will be explored by experimental work in SINTEF's Safety Cell laboratory. Arcing tests with gradually increasing pressure of nitrogen and possibly also hydrogen, and with increasing currents and arcing times will be carried out. Arc voltage, thermal time constants and other characteristic parameters will be recorded, and used to establish a physical model for such arcs.

Publications from Cristin

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ENERGIX-Stort program energi