Characterization of controlled Switching of SF6 free 420 kV Live Tank Circuit-Breaker

A critical technological advancement in this context is the development of the world’s first SF₆free 420 kV Live Tank circuit Breaker (LTB), which is capable of handling complex switching conditions and has successfully undergone type testing [1]. This circuit breaker uses an environmentally friendly gas mixture composed of 90 % CO₂ and 10 % O₂. Its design incorporates two series-connected breaking chambers and standard grading capacitors, enabling it to meet the stringent dielectric and thermal requirements of high-voltage applications.

Controlled switching is a key enabler for reducing transient phenomena associated with switching operations under the above-mentioned operating conditions. By switching at a precise point on the voltage waveform across the circuit breaker (CB), controlled switching minimizes inrush current and transient overvoltage. This eliminates the need for traditional 1 mitigation techniques such as pre-insertion resistors, thereby reducing mechanical and electrical stress on equipment and enhancing the longevity of network assets. Furthermore, suitability for controlled switching requires CB to maintain stable performance across a range of operating conditions. This includes minimizing statistical variation in operating times and accounting for the influence of external parameters such as coil operating voltage, temperature, idle time etc.

Considering these factors enables accurate optimization of the switching targets and effective compensation for external influences. These aspects are addressed under the parameter definition tests outlined in the recently published technical specification IEC TS 62271-319 [2].

Controlled switching performance depends on key dielectric and mechanical parameters [3].

The Rate of Decay of Dielectric Strength (RDDS) during closing predicts pre-strike behavior, while the Rate of Rise of Dielectric Strength (RRDS) during opening indicates the circuit breaker's ability to avoid restrikes and reignitions during low-current interruptions. Together, these parameters define dielectric recovery dynamics which are crucial for reliable switching operations.

In this study, a comparative analysis is presented between the RDDS and RRDS characteristics of the newly developed SF₆-free 420 kV live tank circuit breaker and those of a conventional

SF₆-based breaker. The evaluation is based on a series of type and development tests designed to simulate real-world switching scenarios – such as symmetric and asymmetric short circuit duties, capacitive switching, and shunt reactor switching – in accordance with IEC TS 62271319.

The results confirm that the SF₆-free breaker exhibits comparable performance in terms of dielectric behavior and timing precision. The CB also complies to key requirements according to applicable product standards, thereby validating its suitability for controlled switching applications in modern high-voltage networks. This advancement not only supports the transition to greener technologies but also addresses the operational demands imposed by the evolving energy landscape, characterized by increased switching frequency and distributed generation.