Analysis of behaviour of an emission-free 420 kV live-tank vacuum circuit-breaker

T&D equipment, SF6 still contributes to global warming if released into atmosphere. Both, industry and system operators have committed to phasing out SF6 in distribution and transmission systems to support societies’ net-zero targets.

One technological path for reducing the CO2 emission potential is the use of vacuum interrupter units in combination with Clean Air insulation for circuit-breaker applications, a technology that completely avoids the use of greenhouse gases.

This paper evaluates the transient responses of circuit-breakers with vacuum interrupter technology operated in 420-kV-systems and their impact on adjacent HV equipment. Substantial and critical switching requirements according to standards and specifications derived from operational experience in transmission systems serve as a basis for discussion. By means of tests and simulation studies for 420 kV vacuum circuit-breakers in typical transmission grid configurations, voltage grading with non-linear resistors (NLR), an established scheme used for SF6 circuit-breakers since decades, is discussed. It is concluded, that NLR can be used to effectively protect the internal configuration of circuit-breakers. As a side effect, transient stresses of HV components in the substation are reduced without impacting the insulation coordination of the substation.

As an outlook, a planned pilot installation in the transmission grid in Germany is discussed as next step to establishing this sustainable technology for future 420-kV-applications. The circuitbreaker will be installed in a coupler bay with bypass rail connection to investigate typical standard use cases such as switching transformers, switching of unloaded and loaded overhead lines, and coupling and commutation circuits under real grid conditions. Two accompanying measurement concepts with broadband voltage dividers allow the evaluation of over-voltages and transient recovery voltages (TRV) as well as the characterization of switch-internal processes (e.g. post-arc currents and arcing times). The pilot is deliberately distinguished from the previously analyzed special transient switching cases (e.g. partially compensated hybrid cables/overhead lines) and aims to investigate standard operating cases.