SF6 Circuit Breakers

Sulfur hexafluoride circuit breaker

High-voltage circuit breakers have greatly changed since they were first introduced in the mid-1950s, and several interrupting principles have been developed that have contributed successively to a large reduction of the operating energy. These breakers are available for indoor or outdoor applications, the latter being in the form of breaker poles housed in ceramic insulators mounted on a structure.

Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as sulfur hexafluoride (SF6), having excellent dielectric and arc-quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.

A gas blast applied to the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. Sulfur hexafluoride is generally used in present high-voltage circuit-breakers at rated voltage higher than 52 kV.

Into the 1980s, the pressure necessary to blast the arc was generated mostly by gas heating using arc energy. It is now possible to use low energy spring-loaded mechanisms to drive high-voltage circuit-breakers up to 800 kV.

Brief history

The first patents on the use of SF6 as an interrupting medium were filed in Germany in 1938 by Vitaly Grosse (AEG) and independently later in the United States in July 1951 by H. J. Lingal, T. E. Browne and A. P. Storm (Westinghouse).

The first industrial application of SF6 for current interruption dates to 1953. High-voltage 15 kV to 161 kV load switches were developed with a breaking capacity of 600 A. The first high-voltage SF6 circuit-breaker built in 1956 by Westinghouse, could interrupt 5 kA under 115 kV, but it had six interrupting chambers in series per pole.

In 1957, the puffer-type technique was introduced for SF6 circuit breakers, wherein the relative movement of a piston and a cylinder linked to the moving part is used to generate the pressure rise necessary to blast the arc via a nozzle made of insulating material (Figure 1). In this technique, the pressure rise is obtained mainly by gas compression.

The first high-voltage SF6 circuit-breaker with a high short-circuit current capability was produced by Westinghouse in 1959. This dead tank circuit-breaker could interrupt 41.8 kA under 138 kV (10,000 MV·A) and 37.6 kA under 230 kV (15,000 MV·A). This performance was already significant, but the three chambers per pole and the high pressure source needed for the blast (1.35 MPa) was a constraint that had to be avoided in subsequent developments.

The excellent properties of SF6 lead to the fast extension of this technique in the 1970s and to its use for the development of circuit breakers with high interrupting capability, up to 800 kV.

Several characteristics of SF6 circuit breakers can explain their success:

• Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber;
• Autonomy provided by the puffer technique
• The possibility to obtain the highest performance, up to 63 kA, with a reduced number of interrupting chambers
• Short break time of 2 to 2.5 cycles
• High electrical endurance, allowing at least 25 years of operation without reconditioning;
• Possible compact solutions when used for gas insulated switchgear or hybrid switchgear
• Integrated closing resistors or synchronized operations to reduce switching over-voltages
• Reliability and availability
• Low noise levels

The reduction in the number of interrupting chambers per pole has led to a considerable simplification of circuit breakers as well as the number of parts and seals required.