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Density refers to the mass per unit volume of a specific substance under specific conditions.SF6 gas in circuit breakers is sealed in a fixed container. It has a certain density value at the rated pressure of 20 degrees Celsius. Within the range of various permissible conditions for circuit breaker operation, although the pressure of SF6 gas changes with temperature, the density value of SF6 gas remains the same. Because the insulation and arc extinguishing performance of SF6 circuit breakers largely depend on the purity and density of SF6 gas, the detection of SF6 gas purity and density monitoring are particularly important. If a common pressure gauge is used to monitor leaks in SF6 gas, it is difficult to differentiate whether the pressure changes in the SF6 gas are caused by actual leaks or by changes in environmental temperature. In order to achieve the purpose of frequent monitoring of density, SF6 circuit breakers should be equipped with SF6 gas density meter or density relays. SF6 gas density meter is used for monitoring, while density relays are used for control and protection.
SF6 gas density meter is installed on SF6 circuit breakers, with pointers and scales called density meters; those without pointers and scales but with electrical contacts are called density switches; those with pointers, scales, and electrical contacts are called SF6 relays. They are all special instruments for measuring SF6 gas.
When installing and using SF6 gas density meter, if the ambient temperature is 20 degrees Celsius, the pointer points to 0MPa. However, if the ambient temperature is not 20 degrees Celsius, the double-layer metal strip inside the density meter will compensate based on the difference between them, so that the pressure inside the mechanism is compensated to 20 degrees Celsius conditions. Therefore, when the density meter is not in use, if the ambient temperature is higher than 20 degrees Celsius, the double-layer metal strip will stretch, and its lower end will move in the direction of movement, driving the gear mechanism pointer to move in the direction of decreasing the density or pressure indication value, and the reading of the pointer is less than 0MP. Otherwise, when the ambient temperature is lower than 20 degrees Celsius, the gear mechanism and pointer will move in the direction of increasing the density or pressure indication value, and the reading of the pointer is greater than 0MP.
During the filling process of SF6 gas into the circuit breaker, as the gas pressure gradually increases, the end of the elastic metal bending tube moves in the direction of increasing pressure, and the double-layer metal strip always compensates for the pressure when deviating from 20 degrees Celsius. With the displacement in the moving direction as the pressure increases, the gear mechanism and pointer move in the direction of increasing the density or pressure indication value, and the indication value becomes larger. SF6 gas density meter's indication value is not only related to pressure but also to temperature. During the filling of SF6 gas into the circuit breaker, due to the sudden expansion and pressure drop of SF6 gas, the temperature usually drops from ambient temperature to below 0 degrees Celsius. The double-layer metal strip always compensates at 20 degrees Celsius and cannot adjust the temperature difference between the actual temperature of SF6 gas and ambient temperature. Therefore, in this case, the indication value of the density meter does not represent the density or pressure value of SF6 gas at actual temperature, nor does it represent the density or pressure value at ambient temperature, let alone 20 degrees Celsius.
After the circuit breaker is filled with SF6 gas, wait for a period of time to allow the internal temperature of the SF6 circuit breaker to reach equilibrium with the external ambient temperature, and then adjust the SF6 gas to the rated density or pressure value. At this time, whether the pressure of SF6 gas increases or decreases due to the influence of ambient temperature, the pointer of SF6 gas density meter always points to the rated pressure or density value at 20 degrees Celsius due to the temperature compensation.
After the circuit breaker stops running, if the temperature of SF6 gas inside the circuit breaker is in equilibrium with the external ambient temperature, the indicated density or pressure value will not change with the variation of the external ambient temperature. When the ambient temperature rises, the temperature of the SF6 gas inside the circuit breaker will also rise, and the pressure will increase. When the pressure increases, the end of the elastic metal tube moves in the direction of movement, driving the pointer to the density or pressure. However, since the double-layer metal strip stretches with the increase in ambient temperature, and its lower end moves in the direction of movement when the pressure decreases, the two changes are completely offset, resulting in the pointer indicating value remaining unchanged, i.e., automatically converted to the density or pressure value at 20 degrees Celsius remaining constant. Conversely, when the ambient temperature decreases, the indicator value also remains unchanged from the original density or pressure value.
When the circuit breaker reduces the mass of SF6 gas due to some reason (such as leaking or sucking in during testing) and the pressure decreases, the pressure change caused by the ambient temperature can cause the double metal strip and driving pointer to move in the direction of decreasing the indicated value. As a result, the indicated density or pressure value decreases. Since the density meter has two pairs of electrical contacts, it is used to send a signal to the circuit breaker and lock it when the density of SF6 gas decreases. When the pointer drops to a certain position, it sends a signal to fill the gas or lock the circuit breaker.
