Dead Tank — Ready for Cooldown: 3AP1 DT up to 170 kV with Mixed Gases for Lowest Temperature Applications

Secured power supply in rough environmental conditions

Reliable equipment underpins a secure global power supply, and that requirement extends to installations in harsh environments where low ambient temperatures degrade the performance of high-voltage circuit breakers.

Sulphur hexafluoride (SF6) is the most common medium for arc extinguishing and high-voltage insulation, but its key technical limitation is its condensation temperature, especially at significant overpressure. At rated lockout pressure, liquefaction begins at temperatures below approximately -30 °C, and the density of the SF6 gas remaining would no longer be sufficient for full switching performance.

The solution is a mixture of SF6 and tetrafluoromethane (CF4). This combination covers low-temperature applications without compromising switching performance: it provides a liquefaction-free insulating and arc-extinguishing medium because the SF6 partial pressure is sufficiently reduced.

Mixed-gas filling on the existing breaker design

Based on the existing circuit breaker design, the pure SF6 filling is replaced by a composition of 43 percent SF6 and 57 percent CF4 for applications down to -50 °C, or 25 percent SF6 and 75 percent CF4 for conditions down to -60 °C. In addition, the rated filling pressure is increased by 1.5 bar, to 7.5 bar.

The resulting low-temperature circuit breaker with mixed gases retains essentially the same technical key ratings as the standard circuit breaker used for low-temperature conditions down to -30 °C. The normal rated current in particular was certified at the same values as the standard breaker with pure SF6 filling.

For 60 Hz applications the short-circuit current requires an insignificant line-to-ground capacitance of 3 nF (not required for 50 Hz). This capacitance is usually already available in power grid configurations with capacitive voltage dividers, so an installation providing that value within the grid does not need external capacitors — the customer can use standard equipment without major modifications.

The new mixed-gas solution is an alternative to the proven devices with tank heaters for operation in harsh weather conditions down to -60 °C.

Global Warming Potential

The flyer compares life-cycle GWP of the 3AP1 DT 145 kV (without current transformers) in two designs. The 1995 design with tank heaters: manufacturing 26,500; operation over 40 years 480,000; end of life -2,100. The enhanced 2015 design with the mixed-gas solution: manufacturing 19,400; operation over 40 years 208,000; end of life -5,800.

CO2 footprint

Over a 100-year time horizon the mixed-gas solution also improves overall global warming potential, because the GWP of CF4 (7,400) is three times lower than that of SF6 (22,800). The new dead tank additionally offers a leakage rate of less than 0.1 percent per annum of gas losses for the complete circuit breaker, giving a markedly reduced CO2 footprint compared with other low-temperature applications using tank heaters.

Within the same voltage range, a 56 percent GWP reduction is achieved by the enhanced 2015 design combined with mixed gases. Optimized material utilization during manufacturing (for example a smaller base frame) and the use of mixed gases instead of permanent heating during low-temperature operation all contribute to the improved GWP.

Technical key ratings

Ratings for the mixed-gas 3AP1 DT at rated voltages 126 kV and 145/170 kV. All values in accordance with IEC (printed as "ICE" in the source); other values on request.

Figures & drawings

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