Several hurdles had to be overcome to get there. Compared with existing vacuum interrupters in the medium voltage range, the interruption at high voltage required an increased contact gap and a contact configuration providing an axial magnetic field. Furthermore, intensive investigations into contact materials were necessary to pass low current switching tests (e.g. for capacitive currents), and dielectric optimization was necessary to be able to use dry air for the insulation between the terminals and to achieve similar dimensions to the GL309-type circuit breaker. To respect the modified mechanical energy balance of the circuit breaker system itself and the lower required stroke of the vacuum interrupter, an optimisation of the mechanical chain was carried out, making it possible to keep Alstom’s established FK-type spring-operated mechanism.
It uses vacuum as the interrupting technology and dry air for insulation.
At the same time, different configurations were also investigated: 2 vacuum interrupters in series, or 1 vacuum interrupter.
“We decided to choose the 1 vacuum chamber solution, because it enables us to stick with the current ‘candlestick’ design so that it can be fully interchangeable with existing 72 kV SF6 circuit breakers; additionally, this solution does not require grading capacitors and it allows for higher mechanical reliability.”
(1) Circuit breakers are “live tank” when the enclosure of the interrupting unit is at line potential, or “dead tank” when the enclosure is at earth potential.