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The density relay is an SF6 gas package that is sealed on the outside of the bellows and communicates with the SF6 gas in the circuit breaker. It compares with the standard gas package sealed inside the bellows through the lever with the shaft as the support point, and drives the electric contact of the micro switch to act, so as to realize its signaling and locking functions.
When the circuit breaker is out of operation and the temperature of the SF6 gas density in the circuit breaker at the rated density or pressure is equal to the ambient temperature, the state of the SF6 gas outside the bellows is the same as that of the standard SF6 package inside the bellows, with the shaft as the support point. The lever remains in a certain equilibrium position. This puts the microswitch electrical contacts in the open position.
As the ambient temperature changes, the pressure of the SF6 gas on both sides changes simultaneously. Therefore, the lever acting on the shaft as the support point remains in a certain equilibrium position, and the electrical contact of the microswitch remains in the open position.
When the circuit breaker is out of operation and the temperature of the SF6 gas in the circuit breaker is equal to the ambient temperature, the pressure of the SF6 gas outside the bellows will decreaseif the circuit breaker leaks SF6 gas, and the pressure of the standard SF6 gas density on the outside of the bellows remains the same, the leverage is out of balance.
The result is that the ends will rotate counterclockwise, reaching a new equilibrium position. When the air leakage reaches a certain level, the electrical contacts of different functions of the micro-electric contact will be closed respectively, and different commands or signals will be issued to realize their different functions.
The SF6 relay can accurately measure the density of SF6 gas only after the circuit breaker is out of operation and the temperature inside and outside the circuit breaker reaches equilibrium.
When the circuit breaker is running, if the circuit breaker leaks SF6 gas, due to the effect of temperature rise, more SF6 gas will leak than when the circuit breaker is out of operation, so that the electrical contacts of the SF6 relay can be closed.
The calibration of the SF6 relay is to use the instrument to automatically convert the SF6 gas pressure value at 20°C, display the density value under various temperatures and pressures, and compare it with the various pressures simulated by the instrument to observe situation of the electrical contact. According to whether it can send a signal or block the circuit breaker within the specified value of low voltage, the quality of the SF6 density relay is judged.
During the verification process of SF6 relay, it was found that the density relays of several circuit breakers were unqualified. The reason for this may be that the SF6 standard gas bag carries a long thin copper tube. During the installation or maintenance process, the pressure in the standard SF6 gas bag increases due to the bending or collision of the copper tube.
It may also be due to the damage to the bellows of the SF6 relay, which causes the standard gas package of the density relay to leak. When the circuit breaker leaks SF6 gas, the C1-L1 and C2-L2 contacts cannot be connected, resulting in the failure of the density relay, which seriously threatens the safe operation of the equipment and even the safety of the system.
In the actual work on site, when filling the circuit breaker with SF6 gas, some people often think that filling more SF6 gas can prevent the gas supply and the blocking signal. Indeed, if the gas pressure is a little higher, it will reduce the chances of a fill-up and lockout signal.
However, this will increase the burden on the seals of the circuit breaker, which may damage the seals of the SF6 relay and cause air leakage. Therefore, it is not recommended to increase the pressure of SF6 gas, and it should be strictly controlled within the standard.
SF6 circuit breaker is an important protection and control element in the power system. If the circuit breaker fails, it will cause great economic losses. To ensure the reliability of the circuit breaker operation, it is necessary to constantly monitor the various indicators of the circuit breaker, especially the SF6 gas density, which must meet the relevant standards to keep the SF6 circuit breaker in good working condition for a long time.
