SF6 gas has a history of one hundred years, it was synthesized by two French chemists Moissan and Lebeau in 1900 as an artificial inert gas, around 1940, the U.S. military used it in the Manhattan Project (nuclear military). 1947, the commercial use. Currently, SF6 gas is mainly used in the electric power industry. SF6 gas is used in four types of electrical equipment as insulation or arc extinguishing; SF6 circuit breakers and GIS, in this case sulfur hexafluoride enclosed combination appliances, internationally known as "gas insulated switchgear, SF6 load switchgear, SF6 insulated transmission lines, SF6 transformers and SF6 insulated substations. The gas-isolated switchgear is used in 80% of the total gas consumption. In terms of gas consumption, 80% of them are used for high and medium voltage power equipment.
The SF6 gas in the SF6 relay is sealed in a fixed container, and it has a certain density value at the rated pressure at 20℃. Within the various permissible conditions of circuit breaker operation, the density value of SF6 gas always remains the same, although the pressure of SF6 gas changes with the change of temperature. Because the insulation and interrupting performance of SF6 circuit breaker depends largely on the purity and density of SF6 gas, it is especially important to detect the purity and monitor the density of SF6 gas. If an ordinary pressure gauge is used to monitor the leakage of SF6 gas, it will not be possible to distinguish whether the pressure of SF6 gas changes due to the real existence of leakage or due to the change of ambient temperature. In order to achieve the purpose of monitoring density frequently, the national standard stipulates that SF6 circuit breaker should be equipped with pressure gauge or SF6 gas density gauge and density relay. The pressure gauge or SF6 gas density gauge is to play a monitoring role, and the density relay is to play a control and protection role.
The SF6 gas density meter installed on the SF6 circuit breaker with pointer and scale is called density meter; without pointer and scale is called density switch; some SF6 gas density meters also have electrical contacts, that is, also used as SF6 relay. They are all special meters used to measure SF6 gas.
SF6 relay is the only means installed on the SF6 switch to monitor the change of SF6 gas density, and it realizes automatic alarm and blocking action according to the change of SF6 gas, so the SF6 relay is directly related to the normal operation of the switch. Calibration is mainly to detect the accuracy of the density relay, whether the current display value is correct, and whether its action signal for alarm and protection is normal. If the display of the meter is incorrect, it may lead to incorrect SF6 pressure in the switch and cause insulation problems. If the action signal is not output correctly for alarm or protection, it will lead to a serious safety accident of the switch.