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SF6 Circuit Breakers
Sulfur hexafluoride (SF6) gas is an alternative to air as an interrupting medium. SF6 is a colorless nontoxic gas, with good thermal conductivity and density approximately five times that of air. SF6 is chemically inert up to temperature of 150 C and will not react with metals, plastics, and other materials commonly used in the construction of high voltage circuit breakers.
The principle of operation is similar to the air blast breakers, except that the SF6 gas is not discharged into the atmosphere. A closed circuit completely sealed and self-contained construction is used. The equipment consists of a compressor, a storage container, a blast valve that admits gas to the interrupting chamber, and a filter through which the exhaust gas is returned to the compressor. This is called the double pressure breaker design.
Figure 15 230 kV, 15 GVA, SF6 Double Pressure Breaker (Westinghouse)
Improvement on the double pressure design is the puffer design, also sometimes called the single pressure design. SF6 gas is normally under constant pressure of about 5 ATM. During the opening operation the gas contained inside a part of the breaker is compressed by moving cylinder that supports the contacts or by a piston. This forces the SF6 through the interrupting nozzle.
An example of a SF6 puffer breaker is in Figure 16. When the contacts separate, an arc is established. If the current is not very high, it is extinguished at its first zero crossing by deionizing effects of SF6, and by the pushing the SF6 through the arc by the piston. The contact distance at this point is small and the pressure of the gas that goes through the arc is low. This feature is important because it prevents current chopping when interrupting small inductive currents. In the case of small capacitive currents, the maximum recovery voltage appears ½ cycle after the arc extinction. This give the contacts sufficient time to reach a separation that will be able to withstand the voltage. If the short circuit current is high, the arc extinction may not occur at the first zero crossing, but the gas pressure will increase sufficiently to blow the arc out.
By connecting several interrupting heads in series, SF6 breakers can be constructed for voltages of up to 765 kV.
Figure 16 107 kV, 31 kA SF6 Puffer Breaker (Magrini)
Figure 17 Sequence of Opening Operation in SF6 Puffer Breaker
(a) contacts closed
(b) contacts begin to separate - arc is established
(c) contacts continue separation - gas is blown through arc
(d) contacts fully opened - arc extinguished
+ نوشته شده توسط کمال کمالی در 87/01/22 و ساعت
13:13 |