As a new type of high-voltage electrical switchgear developed in modern times, sf6 circuit breaker has been widely used in power system because of its superior performance, safety and reliability, convenient maintenance and long service life.
However, many colleagues still lack the knowledge of sf6 gas density relay, the key component of sf6 circuit breaker. This brings some difficulties to installation, maintenance and other work. It may even affect the quality of work, and misunderstand some of the normal phenomena of the equipment in operation.
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.
Under the rated pressure at 20°C, it has a certain density value. Within the range of various allowable conditions for the operation of the circuit breaker, although the pressure of sf6 gas changes with temperature changes, the density value of sf6 gas remains unchanged.
Because the insulation and arc extinguishing performance of sf6 circuit breakers depend to a large extent on the purity and density of sf6 gas, the detection of sf6 gas purity and density monitoring are 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.
National standards stipulate that sf6 circuit breakers should be equipped with pressure gauges or sf6 gas density meters and density relays. The pressure gauge or sf6 gas density meter is used for monitoring, and the density relay is used for control and protection.
The one with pointer and scale installed on sf6 circuit breaker is called sf6 gas density meter. The one without pointer and scale is called density switch. With pointer, scale, and electrical contact, it is called density relay. They are all dedicated meters used to measure the density of sf6 gas.
The sf6 gas density meter is mainly composed of elastic metal curved tube, gear mechanism and pointer, double-layer metal belt and other parts. In fact, it is composed of double-layer metal belt added to the spring tube pressure gauge mechanism.
The internal space of the sf6 gas density meter is communicated with the sf6 gas in the circuit breaker. The end of the elastic metal curved pipe is hinged with a bimetallic belt that plays a role of temperature compensation. The double-layer metal belt is hinged with the gear mechanism and the pointer mechanism.
When the sf6 gas density meter is not installed and used, if the ambient temperature is 20°C, the pointer points to 0MPa.
However, if the double-layer metal belt stretches when the ambient temperature is higher than 20°C, the lower end of the sf6 gas density meter will be displaced, driving the gear mechanism and pointer to move in the direction of decreasing density or pressure indicating value, and the pointer reading is less than 0MPa.
Otherwise, when the ambient temperature is lower than 20°C, 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 0MPa.
After the circuit breaker is filled with sf6 gas, wait for a period of time to make the internal temperature of the sf6 circuit breaker rise to balance with the external ambient temperature, and then the sf6 gas density meter displays the rated density value.
sf6 gas density meter can only accurately measure the density of sf6 gas only after the sf6 circuit breaker is out of operation and the internal and external temperature of the circuit breaker reaches equilibrium.
When the sf6 circuit breaker is in operation, the reading error of the sf6 gas density meter depends on the size of the temperature rise caused by the load current of the circuit breaker and the loop resistance.