Structure and Working Principle of SF6 Density Meter and Density Relay
The insulation and arc extinguishing performance of SF6 circuit breaker depends to a large extent on the purity and density of SF6 gas, so the detection of SF6 gas purity and the monitoring of density are particularly important. To monitor SF6 gas, the density value under the rated pressure at 20°C is usually used as a reference. Although the pressure of SF6 gas in operation changes with the change of external temperature, for a well-sealed SF6 circuit breaker, the density of SF6 gas should remain unchanged.
If an ordinary pressure gauge is used to monitor SF6 circuit breaker gas, it is impossible to distinguish whether the pressure change of SF6 gas is caused by gas leakage or ambient temperature change. National standards stipulate that SF6 circuit breakers should be equipped with SF6 gas density meters and density relays. The SF6 gas density meter is for monitoring, and the density relay is for control and protection. The SF6 gas density meter installed on the SF6 circuit breaker is called a density meter with a pointer and a dial, and the SF6 gas density meter also has electrical contacts, which can also be used as a density relay; the one without a pointer is called a density relay.
1 Structure and working principle of SF6 gas density meter
1.1 Structure of SF6 gas density gauge
The SF6 gas density meter shown in Figure 1 is mainly composed of elastic metal curved tube, gear mechanism and pointer, double-layer metal belt and other components. The inner space of the hollow elastic metal curved pipe communicates with the SF6 gas in the circuit breaker, and the end is connected with the double-layer metal belt, and the double-layer metal belt is connected with the gear mechanism and the pointer mechanism. Among them, the main function of the double-metal strip is temperature compensation, that is, when the ambient temperature is not 20°C, the double-layer metal strip will compensate according to the difference between the ambient temperature and 20°C.
Figure1 Schematic diagram of the structure of the SF6 gas density meter
1.2 Working principle of SF6 gas density meter
(1) When the density meter is not installed and used, if the ambient temperature is 20°C, the pointer points to 0; when the ambient temperature is higher than 20°C, the double-layer metal strip will elongate, and its lower end will move in the direction when the pressure decreases, Drive the gear mechanism and pointer to move in the direction of decreasing density or pressure indication value, and the indication reading of the pointer is less than 0; 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, The reading of the pointer is greater than 0.
(2) During the process of filling SF6 gas into the circuit breaker, as the gas pressure gradually increases, the end of the elastic metal curved pipe moves to the direction when the pressure increases, and the double-layer metal belt always compensates at 20°C , also moves in the direction of increasing pressure, driving the gear mechanism and pointer to move in the direction of increasing density or pressure indication value, and the indication value becomes larger.
(3) The indicated value of density gauge or pressure gauge is not only related to pressure, but also related to temperature. During the process of filling the circuit breaker with SF6 gas, due to the sudden expansion and depressurization of SF6 gas, the temperature generally drops from the ambient temperature to below 0°C, and the double-layer metal belt will always compensate at 20°C, and the actual temperature of SF6 gas and The temperature difference between the ambient temperature is compensated. In this case, the indicated value of the density meter can neither represent the density or pressure value at the actual temperature of SF6 gas, nor can it represent the density or pressure value at ambient temperature, nor can it represent the density or pressure value at 20 °C.
(4) After the circuit breaker is filled with SF6 gas for a period of time, the internal temperature of the SF6 circuit breaker rises to the same temperature as the external environment. After the commissioning personnel adjust the SF6 gas to the rated density or pressure value, no matter what the ambient temperature of the circuit breaker is Due to the temperature compensation effect of the double-layer metal strip, the pointer of the density gauge points to the pressure or density value at 20°C.
(5) When the circuit breaker is out of operation, if the temperature of the SF6 gas inside the circuit breaker reaches equilibrium with the external ambient temperature, the indicated density or pressure value will not change with the external ambient temperature. When the ambient temperature rises, the temperature of SF6 gas inside the circuit breaker also rises, and the end of the elastic metal tube moves in the direction of increasing pressure, which tends to drive the pointer to move in the direction of increasing density or pressure value . At this time, the double-layer metal strip elongates with the increase of the ambient temperature, and its lower end moves to the direction when the pressure decreases, and the changes of the two completely cancel each other out. The density or pressure value remains unchanged. Conversely, when the ambient temperature decreases, the indicated value of the pointer also maintains the original density or pressure value.
(6) When the quality of SF6 gas decreases and the pressure decreases due to air leakage from the circuit breaker or test gas intake, the end of the elastic metal tube moves in the direction of pressure decrease, and the pressure change caused by the ambient temperature is controlled by the double-layer metal belt. Compensation is carried out to drive the indicated value of the pointer to move in the direction of decreasing, and the result is that the density or pressure value indicated by the pointer becomes smaller. Since the density meter has two pairs of electrical contacts, it is used for sending an alarm signal and blocking the circuit breaker control circuit when the density of SF6 gas decreases.
1.3 Precautions for using SF6 gas density meter
(1) The SF6 gas density meter can only measure the density or pressure of SF6 gas accurately when the circuit breaker is out of operation and the temperature inside and outside the circuit breaker reaches equilibrium.
(2) When the SF6 circuit breaker is running, the reading error of the density meter depends on the load current of the circuit breaker and the temperature rise caused by the loop resistance.
(3) The calibration of the SF6 gas density meter is to use the instrument to automatically convert the SF6 gas pressure value at 20°C, display the density value at various temperatures and pressures, and compare it with the various pressures simulated by the instrument to judge the quality of SF6 gas density meter.
2 Structure and working principle of SF6 gas density relay
2.1 Structure of SF6 gas density relay
The SF6 gas density relay is mainly composed of two bellows, a standard SF6 gas bag, a micro switch electrical contact, a lever, etc., as shown in Figure 2. C1-L1 is the electrical contact that alarms when the SF6 gas pressure drops, and C2-L2 is the electrical contact that locks the circuit breaker when the SF6 gas pressure drops.
Figure 2 Structure diagram of SF6 gas density relay
1- Bellows 1; 2- Bellows 2; 3- Standard SF6 gas bag; 4- Micro switch electrical contacts; 5- Shaft; 6- Lever
2.2 Working principle of SF6 gas density relay
(1) The SF6 gas bag sealed outside the bellows 1 communicates with the SF6 gas of the circuit breaker, and is balanced with the standard gas bag sealed outside the bellows 2 through a lever, which drives the electrical contact of the micro switch to act, and realizes the circuit breaker to send a gas replenishment alarm signal and circuit breaker lockout function.
(2) When the circuit breaker is out of operation and the temperature at the rated density of SF6 gas is equal to the temperature of the external environment, the state of the SF6 gas outside the bellows 1 is the same as that in the standard SF6 gas bag outside the bellows 2, and the lever remains in balance state, so that the micro switch electrical contacts are in the open position. When the temperature of the external environment changes, the pressure of the SF6 gas on both sides changes at the same time. At this time, the lever remains in a balanced state, and the electrical contact of the micro switch remains in the open position.
(3) When the circuit breaker is out of operation and the temperature of the SF6 gas in the circuit breaker is equal to the temperature of the external environment, if the SF6 gas leaks, the pressure of the SF6 gas outside the bellows 1 will decrease, and the standard SF6 gas bag outside the bellows 2 will The pressure remains constant and the lever is out of balance. As a result, the two ends will rotate counterclockwise to reach a new equilibrium position. When the air leakage reaches a certain level, the electrical contacts of different functions of the micro switch will be closed respectively, and different instructions or signals will be issued.
(4) When the circuit breaker is put into operation, the standard SF6 gas package is still at the ambient temperature. Since the electric power consumed when the load current passes through the loop resistance is converted into heat energy, the SF6 gas in the circuit breaker is heated up. At this time, the SF6 gas outside the bellows 1 The pressure will increase, pushing the lever to rotate clockwise around the shaft, so that the electrical contact of the micro switch cannot be closed. In this case, if the circuit breaker leaks SF6 gas, the pressure of the SF6 gas outside the bellows 1 will decrease. However, due to the effect of temperature rise, more SF6 gas must be leaked than when the circuit breaker is out of operation to close the electrical contacts of the micro switch.
2.3 Precautions for using SF6 gas density relay
(1) The SF6 gas density relay can accurately measure the density value of SF6 gas only when the circuit breaker is out of operation and the temperature inside and outside the circuit breaker reaches equilibrium.
(2) When the circuit breaker is running, if the circuit breaker SF6 gas leaks, due to the effect of temperature rise, it is necessary to leak more SF6 gas than when the circuit breaker is out of operation, so that the electrical contacts of the density relay can be closed.
(3) For the verification of the density relay of SF6 circuit breaker, use the instrument to automatically convert the pressure value of SF6 gas at 20°C, display the density value at various temperatures and pressures, and compare it with the various pressures simulated by the instrument. Observe the contact condition of the electrical contact to see if it can send a signal or lock the circuit breaker within the specified value of low voltage to judge whether the SF6 circuit breaker density relay is good or bad.
(4) When replenishing SF6 gas to the circuit breaker, some people think that filling more SF6 gas can reduce the probability of the circuit breaker sending gas replenishment and blocking signals, which is wrong. Too much SF6 gas added to the circuit breaker will increase the burden on the sealing parts of the circuit breaker, increase the probability of damage to the seal of the circuit breaker, and increase the possibility of gas leakage. Therefore, when the SF6 circuit breaker replenishes gas, it must be strictly controlled within the standard range specified in the manufacturer's instruction manual.