Electrical insulation with low-GWP gases

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. Compact switchgears contain live, high voltage conductors and switches. Electrical insulation between these parts is typically provided by sulphur hexafluoride (SF6) gas. SF6 is an excellent electrical insulator, but unfortunately a strong greenhouse gas. Even though switchgears are hermetically sealed and SF6 treated in a closed cycle, reduction of SF6 usage is desired. Currently, no alternative gas has been found that can match the insulation level of SF6 at a reduced global warming potential (GWP). In this project, we aim to develop the knowledge and technology required to replace SF6 as electrical insulator in compact gas insulated switchgear with low-GWP alternatives. For this, we need to understand the underlying mechanisms of the electrical breakdown in gases. This will be addressed through experimental and theoretical studies in a collaboration between ABB AS, ABB Switzerland, NTNU and SINTEF Since the start in 2015, SINTEF has developed an experimental set-up to study the withstand levels of triple points and completed a numerical model of electrostatic fields and electron avalanches in complex geometries. SINTEF together with ABB Norway finalized the set-up for experiments in controlled atmospheres and the first series of lightening impulse tests in different gases have been finished. The experimental work have been conducted in collaboration with MSc students at NTNU and ETH and the modelling work together with scientists from ABB Switzerland. Under the scope of this project, SINTEF has developed a scalable computer code for simulations of discharge processes in complex geometries. This cutting-edge work has been scaled up to thousands of computer cores on national and international supercomputers and enable, for the first time, large scale transient 3D simulations in engineering-relevant geometries. This code has been extensively verified and has already been used to validate engineering tools. At NTNU, PhD candidate Hans Kristian Hygen Meyer has performed several experiments and simulations in scope of this research project, and has now delivered his thesis for evaluation by the phd committee. The goal of his project was defined as an experimental study and modelling of effect of surface charges on dielectric media after lightning impulse. Several papers were presented at scientific conferences and one paper is accepted in IEEE Transactions on Dielectrics and Electrical Insulation Journal (additionally one is submitted and under review). His PhD thesis results were also presented in Energiforskningskonferansen in Oslo, 30th May 2018. Meyer had spent five months in ABB Switzerland and he published the outcomes of his sabbatical in collaboration with ABB scientists. ABB Switzerland have launched- and expanded upon the platform for simulation and modelling of discharges in gases. The platform opens for testing different physical models in a virtual test lab, and is a critical building block for the collaboration project. Several conference contributions describing the concept was well received by the scientific community. In order to transfer the generated scientific knowledge and evaluate the outcomes of simulation code, VHV Lab, one engineer from Scandinavian Technology Center had been in ABB Switzerland for one month. In this period, details of calculation models, accuracy of results and understandability of interface of the model were discussed. ABB Norway are developing complete solutions with clean air and other SF6-free insulation media. One paper was submitted to CIRED 2015 on this work. The first MV switchgear for secondary distribution insulated with the SF6 alternative labeled "AirPlus" was unveiled at the 2016 Hannover Fair in April and launched for selected markets in July 2016. One new module for SafeRing AirPlus was completed during Q1 2017 and additional products and portfolio expansions are under development. In November 2015, Liander and ABB started a long term field experience program and four SafeRing AirPlus ring main units (RMUs) were installed in Lianders network in Flevoland, Netherlands. The outcome of the measurements and observations of insulating gas quality during the first year (2015-2016) of service was presented in CIRED 2017. The status of the unit after three years operation will be presented in CIRED 2019. Several workshops and meetings between partners have been carried out in the reporting period. In In February 2018, all the partners presented the results and progress in the project during a two days workshop in Skien.

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The ABB medium voltage (MV) plant in Skien (ABB Power Products division, ABB PP) manufactures compact, state-of-the art 12 kV to 40.5 kV gas insulated switchgear (GIS), where more than 90% of their final products are exported abroad. The insulation gas used in these switchgears is sulphur hexafluoride (SF6), a remarkably good insulator but also a potent greenhouse gas. The global warming potential (GWP) of SF6 is roughly 23,900 and a replacement is therefore being sought by the industry. Air represents one alternative to SF6. ABB PP has successfully introduced an air-insulated 12 kV switchgear unit by careful optimisation of an SF6-based design. However, this development process has also shown that replacing SF6 with air as insulation gas in switchgear compartments for higher voltages, e.g. 24 and 40.5 kV, is not feasible with current technology. In parallel, a rapid progress in the synthesisation of low-GWP, electrically insulating gases has been made. Promising candidates are types of fluoronitriles and perfluorinated ketones. The use of these gases in MV equipment has only very recently been disclosed to the public. In order to pave the way for a new generation of environmentally friendly MV GIS, we propose a combined theoretical and experimental study of the insulation properties and breakdown dynamics of air and other low-GWP gases in switchgear. This project is a collaboration between ABB PP, SINTEF, NTNU and ABB Corporate Research in Switzerland. The anticipated conclusion of this project is that ABB PP will possess the technology and knowledge required for the design, prototype production, testing, and eventually mass production of market competitive SF6-free metal enclosed MV switchgears for the voltage range 12 to 40.5 kV.