The critical temperature is the high temperature at which sulfur hexafluoride gas liquefaction occurs. The critical pressure indicates the gas pressure required for liquefaction to occur at this temperature. Sulfur hexafluoride can only remain gaseous when the temperature is above 45 degrees, under normal conditions of use, it has the possibility of liquefaction, so sulfur hexafluoride can not be used at low temperatures and too low pressure.

The state parameters of sulfur hexafluoride and the temperature of liquefaction or solidification can be calculated easily by using the state parameter curve.

1. Calculate the filling volume of sulfur hexafluoride gas in the circuit breaker

For example, a sulfur hexafluoride circuit breaker, the working pressure at 20 ° C is 0.45MPa, (gauge pressure), the sulfur hexafluoride gas filling amount is 31kg, and the internal inflation volume of the circuit breaker is required.

When the working pressure is 0.45MPa at 20℃, the absolute pressure is 0.55MPa. The working point S in the break linear cluster can be obtained from the pressure of 0.55MPa at 20℃. It is estimated that the density of the oblique line parallel to the oblique linear cluster passing through the point S is 35kg/ m3

The gas filling volume of the sulfur hexafluoride circuit breaker is: 31/35=0.886 m3.

2. The allowable range of the internal inflation pressure of the sulfur hexafluoride circuit breaker changing with the external temperature

For example, at 20 ° C, the above inflatable working pressure is 0.45 MPa, and the absolute pressure is 0.55 MPa. When the ambient temperature rises to 30 ° C and the density =35 kg/ m3 is kept unchanged, the absolute pressure is 0.58 MPa and the working pressure is 0.48MPa when 30 ° C is obtained along the diagonal line to the right of point S in Figure 1-4. When the temperature drops to -10℃, along the density =35 kg/ m3 oblique line can be found on the left side of point S at -10℃, the absolute pressure is 0.49 Mpa, and the working pressure is 0.39MPa. The results show that when the external temperature changes between -10℃ and 30℃, the working pressure of sulfur hexafluoride circuit breaker can change between 0.39 MPa and 0.48 MPa. (Inflation pressure 0.45MPa at 20℃).

3. Understand the temperature of sulfur hexafluoride gas liquefaction under different working pressures

In the above example, the sulfur hexafluoride circuit breaker has a working pressure of 0.45MPa at 20 ° C, a density of 35 kg/ m3, a working point S and a diagonal line crossing the point S intersect the AMB curve at point T, and the temperature at this point t=-33 ° C, and the corresponding working pressure is 0.35MPa. That is, the sulfur hexafluoride gas in this circuit breaker begins to liquefy at -33 ° C. The T point indicates the liquefaction point where condensation occurs as the temperature drops.

Once the sulfur hexafluoride gas begins to liquefy, as the temperature continues to drop, the sulfur hexafluoride gas continues to condense into a liquid, the density of the gas no longer remains constant but constantly decreases, and the pressure of the gas drops faster. When the temperature drops to the liquefaction point, it does not mean that all the gas is immediately condensed into a liquid, but only the beginning of condensation. However, when the temperature continues to decrease and the pressure and density of the gas decline faster, the insulation and arc extinguishing performance of the sulfur hexafluoride gas decline rapidly, so the sulfur hexafluoride circuit breaker does not allow the operating temperature to be lower than the liquefaction point.

It can be seen from the curve AMB that the higher the working pressure (gauge pressure) of the sulfur hexafluoride circuit breaker, the higher the liquefaction temperature. The liquefaction temperature is related to the working pressure of the circuit breaker. If the liquefaction temperature is not high and -20℃ is calculated, the corresponding absolute pressure at 20℃ should not be higher than 0.82 MPa, and the working pressure (gauge pressure) should not be higher than 0.72 MPa.

When the operating pressure of the circuit breaker is very low, the temperature may not be liquefied and directly condensed into a solid.

It can be seen that the operating environmental conditions of sulfur 6 fluoride circuit breaker are -30℃ ~ +40℃, rated pressure 0.45MPa, and locking pressure 0.4MPa. The analysis results can also be calculated and verified by Boil-Mallott gas equation of state PV/T=P1V1/T1. Where: P is pressure; V is the volume; T is the temperature (absolute temperature); P1 is the pressure after the change; V1 is the changed volume; T1 is the temperature after the change. When the volume is unchanged and the sulfur hexafluoride gas pressure changes with the change of temperature, the sulfur hexafluoride gas pressure change value of LW8-35 circuit breaker can be calculated, and the parameters are substituted into the formula to obtain: P1=P×T1/T0.45MPa×(273℃-39℃)/(273℃+20℃)=0.36MPa When the temperature of sulfur hexafluoride gas changes from 20℃ to -39℃, the pressure of sulfur hexafluoride gas changes from 0.45MPa to 0.36MPa, which is lower than the locking pressure of 0.4MPa.

Conclusion:

1, the insulation and arc extinguishing performance of sulfur hexafluoride mainly depends on its purity and density. It has nothing to do with stress.

2, most of the current sulfur hexafluoride switch gas status monitoring device for the "gas density meter", in fact, it reflects the "converted to 20℃, the pressure of the gas in the switch Mpa", rather than the real pressure in the current barrel. It is composed of a sensing device for the current pressure inside the switch and a temperature compensation device composed of a bimetal strip.

3, "switch inflation pressure", "rated pressure on the switch nameplate" and other usually called pressure refers to the "gas density indicator pressure". In fact, it is the difference between the absolute pressure of the gas inside the switch and the external atmospheric pressure. Therefore, the indicating pressure of the gas density meter (referred to as "gauge pressure" or "working pressure") plus the external atmospheric pressure is the absolute pressure in the switch.

4, the absolute pressure of the gas refers to "the orthogonal pressure of the gas on the container wall".

5, 1 standard atmospheric pressure (atm) = 101325Pa

1 Engineering Atmospheric Pressure (ata) = 98000Pa

1Bar=100000Pa

6, the switch due to liquefaction caused by the reduction of gas density and gas leakage on the insulation and arc extinguishing performance of the gas is exactly the same. Therefore, when an exact decrease in gas density due to liquefaction occurs and the switch emits a "lock signal", the lock should not be manually removed.

7, because the measuring temperature range of the gas density table does not meet the actual requirements of the site, the appropriate density table should be replaced. For example, the nominal temperature range of the density table is -20 to 40 degrees, and the actual temperature range of the switch installation site may be -35 to 45 degrees, so that in extreme cases, the density table will not truly reflect the density of the gas. In short, it is necessary to make the temperature range of the density table cover the actual possible ambient temperature.

8, In order to enable the safe operation of sulfur hexafluoride switches at extremely low temperatures, there are two practical ways:

A Heat the switch bucket at the bottom of the switch; Some areas have adopted this method, and the effect is good. However, it is necessary to install heating devices and insulation measures. It's very troublesome.

B The use of sulfur hexafluoride - nitrogen mixed gas switch can reduce the gas liquefaction temperature to about -42 degrees under working pressure. Greatly adapted to low temperature environment.