Current interruption in air-filled medium voltage load break switches

Switchgears are used for connecting and disconnecting parts of the electrical power grid during e.g. maintenance and repair. Switchgears are critical to the reliability of the electrical supply and constitute a key element in the infrastructure of modern society A switchgear unit contains several types of electrical switches and disconnectors, one of which is the load break switch (LBS), used for interrupting currents up to 650 A at voltages of up 36 kV (medium voltage). Today, most load break switches are based on the excellent properties of sulphur hexafluoride (SF6) as a current interruption medium. This gas has the ability to "quench" the current arc that is created when the two poles of the LBS are pulled apart. In addition, most compact switchgear units are also filled with SF6 to provide electrical insulation between the components. SF6 is one of the strongest greenhouse gases we know of, and even though switchgears are hermetically sealed, reduction of SF6 usage is desired. In this project, we aim to develop a compact and inexpensive load break switch based on air rather than SF6. The project is a collaboration between NTNU, SINTEF, USN and ABB. The proposed innovation may lead to a wide-ranging and sweeping technology shift for ABB's plants, placing them at the forefront of medium voltage switchgear technology and at the same time contributing significantly to the international efforts of reducing usage and emission of SF6 to the atmosphere. Since the start in 2014, SINTEF and NTNU have upgraded their laboratories to include new measurement methods for interruption of load currents. A completely new test-setup allows more design parameters to be investigated. New, promising concepts for current interruption have been identified and explored, and a new model has been established to explore design parameters for a full scale LBS. PhD student Henning Taxt has constructed his own experimental set-up and completed several experiments. The purpose is the explore fundamental principles and correlations between ablation and current interruption. Nina Aanensen has completed her post.doc project on numerical analysis of experimental data in cooperation with ABB Switzerland. Three papers were presented at scientific conferences and one paper is submitted to IEEE T-Power Delivery The University of South-East Norway (form: TUC) have assembled laboratory equipment for thermal measurements and conducted several test campaigns. Several MSc projects and one MSc thesis have been conducted in collaboration with Post.Doc Elin Fjeld and the academic staff. The focus is on critical design parameters for unwanted temperature increase in air insulated load break switches. An empirical, parametric model for temperature rise is completed. This provides both insight into the fundamental physics, as well as is becoming an important design tool for future switchgear. The model has been used to predict the change in temperature when different test parameters are changed, with promising results. Post.Doc. Elin Fjeld has been an active member in the Cigré international working group A3.36 ?Application and benchmark of Multiphysics simulations and engineering tools for temperature rise calculations?, and has provided valuable input to the group based on the result from this project. ABB Switzerland has completed several test campaigns and finalized a numerical model that provides insight into the behavior of current arcs in air and other SF6-alternatives. ABB Norway is working with design- and development work for a complete bay with a new LBS. All the critical development test were performed successfully. The results of this project presented in several publication on full scale interruption in SF6-alternatives and thermal management during CIRED 12th-15th June 2017 in Glasgow and also in XXII Symposium on Physics of Switching Arc.

Load currents (typically up to 630 A) in metal enclosed switchgears for 12, 24 and 36 kV are today most economically interrupted using sulphur hexafluoride (SF6) breakers. SF6 is also used for electrical insulation inside such switchgear cabinets. There a re major environmental concerns associated with this, as SF6 is the strongest "greenhouse gas" we know of. A new compact and inexpensive load break switch based on air instead of SF6 is the enabling technology that would pave the way for medium voltage metal enclosed switchgears completely without SF6. This is an environmentally benign solution and would be a highly competitive product for the world market. In this project, we propose to perform fundamental scientific studies of current interruption in air and develop the technology needed to successfully utilize air as the insulation medium in load break switches and metal enclosed switchgears. The project is a collaboration between ABB Power Products in Norway (Skien), SINTEF, NTNU, TUC and ABB Switz erland Corporate Research. The proposed innovation may lead to a wide-ranging and sweeping technology shift for ABB PP's plants in both Skien, China and India, placing them at the forefront of medium voltage switchgear technology and at the same time cont ributing significantly to the international efforts of reducing usage and emission of SF6 to the atmosphere.