The Function and Principle of SF6 Gas Density Meter

1. The role of SF6 gas density meter and density relay

The so-called density refers to the mass per unit volume of a specific substance under specific conditions. The SF6 gas in the SF6 circuit breaker is sealed in a fixed container. It has a certain density value under the rated pressure at 20℃. Within the range of various allowable conditions for the operation of the circuit breaker, although the pressure of SF6 gas changes with temperature, the density value of SF6 gas is always the same. Because the insulation and arc extinguishing performance of SF6 circuit breakers depends to a large extent on the purity and density of SF6 gas, the detection and density monitoring of SF6 gas purity is particularly important. If an ordinary pressure gauge is used to monitor the leakage of SF6 gas, it will be difficult to distinguish whether the pressure change of SF6 gas is caused by the actual leakage or the change of the ambient temperature. In order to achieve the purpose of frequent monitoring of density, SF6 circuit breakers should be equipped with SF6 gas density meters or density relays. The SF6 gas density meter is for monitoring, and the density relay is for control and protection.

SF6 gas density meter installed on SF6 circuit breaker, with pointer and scale is called density meter; without pointer and scale but with electrical contact is called density switch; with pointer, scale and electrical contact is called a SF6 relay. They are all dedicated meters used to measure SF6 gas.

2. The working principle of SF6 gas density meter

1. When the density meter is not installed and used, if the ambient temperature is 20℃, the pointer points to 0MP, but if the ambient temperature is not 20℃, because the double-layer metal belt is compensated according to the difference between the ambient temperature and 20℃, so, when the ambient temperature is higher than 20°C, the double-layered metal belt stretches, and its lower end will be displaced in the direction of movement when the pressure is reduced, driving the gear mechanism and pointer to move in the direction where the density or pressure indication value decreases, and the reading of the pointer is less 0MP; otherwise, when the ambient temperature is lower than 20℃, the gear mechanism and pointer will move in the direction of increasing density or pressure indication value, and the reading of the pointer is greater than 0MP.

2. During the process of charging the circuit breaker with SF6 gas, as the gas pressure gradually increases, the end of the elastic metal curved pipe is displaced in the direction of the pressure increase, and the double-layer metal belt is always compensated at 20°C. With the displacement in the direction of movement when the pressure increases, the gear mechanism and the pointer are driven to move in the direction where the density or pressure indication value increases, and the indication value becomes larger. The indicated value of the density gauge or pressure gauge is not only related to pressure, but also related to temperature. In the process of charging the circuit breaker with SF6 gas, due to the sudden expansion and pressure drop of SF6 gas, the temperature generally drops from the ambient temperature to below 0℃. The double-layer metal belt is always compensated at 20℃, and the actual temperature and environment of SF6 gas cannot be adjusted. The temperature difference between the temperatures is compensated. Therefore, in this case, the indicated value of the density meter cannot represent the density or pressure value of the SF6 gas at the actual temperature, nor can it represent the density or pressure value at the ambient temperature, let alone represent the density or pressure value at 20°C.

3. 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 rise to equilibrium with the external environment temperature, then adjust the SF6 gas to the rated density or pressure value. At this time, regardless of whether the pressure of SF6 gas is increased or decreased due to the influence of the ambient temperature, the pointer of the density meter always points to the rated pressure or density value at 20°C due to the temperature compensation effect of the double-layer metal belt.

4. After the circuit breaker is out of operation, if the temperature of the SF6 gas inside the circuit breaker is in equilibrium with the external environment temperature, the indicated density or pressure value will not change with the change of the external environment temperature. When the ambient temperature rises, the temperature of the SF6 gas inside the circuit breaker also rises, and the pressure also increases. The end of the elastic metal tube moves in the direction of movement when the pressure increases, which drives the pointer to the density or pressure. However, since the double-layer metal belt stretches as the ambient temperature increases, and its lower end moves in the direction of movement when the pressure decreases, the changes in the two are completely offset, and the result is the indicator value of the pointer remains unchanged, that is, the density or pressure value when automatically converted to 20°C remains unchanged. On the contrary, when the ambient temperature decreases, the indicator value of the pointer also remains unchanged at the original density or pressure value.

5. When the circuit breaker due to some reasons, such as gas leakage or taking gas during the test, reduces the quality of SF6 gas and the pressure decreases, and the pressure change caused by the ambient temperature is compensated by the double metal belt and drives the pointer to move in the direction where the indicated value decreases. As a result, the density or pressure value indicated by the pointer becomes smaller. Since the density meter has two pairs of electrical contacts, it is used to signal and lock the circuit breaker when the SF6 gas density decreases. When the pointer drops to a certain position, it sends a gas supplement signal or locks the circuit breaker.

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