**Preface to "Environmental Compatible Circuit Breaker Technologies"**

Changing demands on power transmission and distribution grids have initiated comprehensive research and innovations in the field of circuit breaker technology for high and medium voltages. General trends toward higher voltage levels and increased reliability are supplemented by challenges of more distributed power generation and the need for large distance transmission due to the increasing use of renewable energies. An additional demand is the environmental compatibility of the components. Circuit breakers as a key component of electric grids have to safely cut off not only the normal load current but also the high fault currents caused by short circuits or ground faults on the load side.

Recent research and development have focused on attractive topics like the replacement of environmentally harmful SF6 in gas breakers and new solutions for high-voltage direct current (DC) switching, which is required for large distance transmission and the connection of offshore wind farms for example. The expansion of the application range of vacuum switchgear to higher voltage levels offers alternatives for SF6 gas breakers. Vacuum switchgear is also an interesting option for hybrid circuit breaking concepts. Despite extensive research recently, satisfactory and feasible solutions are largely missing.

This Special Issue comprises eight peer-reviewed papers, which address recent findings in the field of environment compatible switching and circuit breaker technologies. The first three papers deal with the issue of the replacement of SF6 by CO2 in gaseous insulated switchgear. The first paper considers the gaseous insulation and the experimental observation of the electrical breakdown in contact gaps filled with CO2 in comparison with SF6 under the impact of the occurrence of protrusions at the electrodes. The second and third papers concern ablation dominated arcs in a circuit breaker operated in CO2 focussing on the experimental study of plasma properties around the current zero-crossing, as well as focussing on the spectroscopy of molecule radiation and their potential for the further study of such arcs and the ablation process. A modeling study of ultra-fast switches in air with relevance for hybrid fault current limiters and hybrid high-voltage DC interrupters is presented in the fourth paper. It focuses on the high-speed elongation of the arc to reach rapid voltage increases.

The next four papers deal with vacuum circuit breakers as an environmentally compatible technology, where the first paper in this series presents fundamental studies of the vacuum arc for radial magnetic field contacts and the dependence of the arc properties and post-arc parameters on the arc ignition point. The next three papers consider the challenge of DC current interruption based on vacuum arcs. The sixth paper presents a study of the idea to create a zero-crossing of the current by applying an external ultra-fast transverse magnetic field to the vacuum arc in parallel to a capacitor. A hybrid DC circuit breaker is studied in the seventh paper, focussing on the vacuum arc commutation and its dependence on the switching parameters. The eighth paper considers doublebreak vacuum circuit breakers as an option for handling higher DC voltages and reports on experimental studies of the prestrike probability characteristics.

> **Dirk Uhrlandt** *Editor*
