Sf6 Circuit breaker
Sulfur hexafluoride circuit breakers may be used as self-contained apparatus in outdoor air-insulated substations or may be incorporated into gas-insulated switchgear which allows compact installations at high voltages
Operating principle
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.[1]
SF6 gas is electronegative and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high-pressure flow of sulfur hexafluoride gas, and an arc is struck between them. The gas captures the conducting free electrons in the arc to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc.
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
High-voltage circuit breakers have changed since they were 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. 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. Strom (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. 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 circuit breaker in a grounded tank (called a dead tank), 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 led 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
The achievement around 1983 of the first single-break 245 kV and the corresponding 420 kV to 550 kV and 800 kV, with respectively 2, 3, and 4 chambers per pole, led to the dominance of SF6 circuit breakers in the complete range of high voltages.
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. As a direct consequence, the reliability of circuit breakers improved, as verified later on by International Council on Large Electric Systems (CIGRE) surveys
