Development and Testing of HV SF6 Circuit Breaker for Minimum Ambient Temperature of −40 °C

In order to prevent the liquefaction of SF6 gas in circuit breakers intended to operate at very low temperatures, the filling pressure must be low enough. At certain ambient temperature, depending on filling pressure, the liquefaction of SF6 gas starts and the density and pressure drop sharply. This process will jeopardize insulation characteristics and breaking capability of the circuit breaker. In order to prevent the lique-faction of SF6 gas down to -40 °C of ambient temperature, filling pressure at 20 °C must not be higher than 0.45 MPa. Lower filling pressure will result in lower pressure rise in heating chamber during arc quenching. This is the reason why breaking capability of the circuit breaker (live tank, 123 kV 40 kA, manufactured by Končar) with lower filling pressure (0.45 MPa) must be proven even though the same interrupter unit has proven breaking capability (in GIS) with filling pressure of 0.65 MPa. The focus of the research was test duty L90 according to IEC 62271-100, which is considered as highly challenging for modern SF6 circuit breakers.

The simulations (performed in HV CB Simulation) of interruption process with starting variant of interrupter unit (filling pressure 0.45 MPa) revealed that certain changes in the design and setting of circuit breaker are needed in order to satisfy existing empirical criteria in pressure of SF6 gas in heating chamber and in the distance between arcing contacts at instant of current zero. Proposed design and setting changes were evaluated through the estimation of breaking capability by using calculated parameters of black box arc model in Matlab model of test circuit. The circuit breaker with improved interrupter unit for minimum ambient temperature of -40 °C has successfully passed the test duty L90 in high power laboratory KEMA. During the testing, special current zero measurements have been carried out. The results of current zero measurements were used for additional verification of the method for the estimation of breaking capability based on calculated parameters of black box arc model.