Sulfur hexafluoride gas test

        (1) After the installation of SF6 electrical equipment is completed, the humidity and air content in the SF6 gas chamber and the leak detection of the equipment should be re-examined before being put into operation (after 24 hours of inflation).

        (2) After the equipment is energized, the SF6 humidity test and SF6 leak detection in the SF6 gas are repeated once every 3 months, and the humidity and leak detection are detected every 1~3 years after stabilization. When the gas quality index is found to change significantly, it should be reported to the “SF6 Testing Center” for processing.

        (3) For SF6 electrical equipment with a gas pressure lower than 0.35 MPa and less gas consumption (such as a circuit breaker below 35kV), as long as the leak detection proves that there is no air leakage, the gas humidity is acceptable during the handover, and the gas humidity is not detected during the operation, but Check the gas humidity when it is abnormal.

        In accordance with the above requirements, this section mainly describes the test method for gas humidity and equipment leakage.

        Gas humidity refers to the amount of water vapor in the gas. When SF6 gas contains moisture, it will corrode the insulation and metal parts of the equipment due to the chemical corrosion of moisture. At the same time, condensed water is generated on the surface of the insulation, causing the surface of the insulation to flash on the surface. In order to reduce and eliminate the harmful effects of SF6 under arcing, it is necessary to limit the moisture content of the SF6 gas to a minimum. The Pre-Regulation specifies the humidity of the SF6 gas in operation, see Table 2-10.

        The method used in the Pre-Regulation is electrolysis. The principle is that the SF6 gas to be measured is guided into the electrolytic cell, and the moisture in the gas is absorbed and electrolyzed. The amount of water in the SF6 gas is determined based on the relationship between the amount of electricity required to electrolyze the water and the amount of water. Calculate the amount of electrolysis current in the SF6 gas and the water content in the gas according to the following formula, ie

I = CPT ₀ F _q /3P _o TV _o *10 ^-4

        Where I - electrolysis current, μA;

               C——gas moisture content, μL/L;

               F——Faraday constant (96485C);

               P ₀ - standard atmospheric pressure (101.325kPa);

               T ₀ - critical absolute temperature (273K);

               v ₀ - molar volume (22.4 L / mol);

               P——run gas pressure, Pa;

               T——operating gas temperature, K;

               q——Gas flow rate during test, mL/min.

        (5) The sealing performance of the measuring system should be good to prevent moisture infiltration.

Second, the gas leakage rate test (ie leak detection)

        Leak detection can be divided into qualitative and quantitative. Qualitative is to directly detect the joints of the equipment and find the leak point. The quantification is to determine the leakage rate of the leak point by the dressing method or the pressure folding algorithm, thereby obtaining the annual leak rate of the gas chamber.

        Bandaging

        The dressing method uses a plastic film to seal the flange joints and pipe joints of the equipment, and measure or estimate the volume of the dressing space. After a period of time, the SF6 concentration leak detector is used to measure the concentration of SF6 in the dressing body. 2-10) and formula (2-11) calculate annual air leakage G and annual air leakage rate M

        Formula κ - concentration of SF6 gas in the dressing;

                V——the volume of the dressing space (excluding the volume of the dressing equipment), L;

                p——SF6 gas density, 6.16g/L (20°C, 0.1MPa)

                T——year hours, take 8760h;

                △t——binding time, h;

                Q——The amount of inflation of the equipment air chamber, g.

        The pressure folding algorithm records the pressure and temperature of each chamber of the equipment during operation. According to the pressure and temperature, the corresponding density is found on the gas state diagram (see Figure 2-26). The time is the abscissa and the density is the ordinate. , mark the density change at different times, and get the leakage situation and trend from the curve change.

        Annual leakage rate M=(p ₀ -p _t ) /p ₀ * T _r /T ₀ * 100%

        Where P _{0 is the} initial gas density, g/L;

               p _t ——the gas density measured after _t time, g/L;

               T ₀ - the time interval between two densities (the same units as Tr);

               T _r - the time of year (12 months or 365 days).

        Figure 2-28 shows the test device for checking the operating pressure value of the density relay on site. The operating procedure is as follows:

        (1) Open the valve on the exhaust valve 7 and the SF6 cylinder to slowly fill the SF6 gas. Because of the high density of the SF6 gas, the air is discharged to the outside through the exhaust valve, passed through the gas drying device for 5 minutes, and then the exhaust valve and the SF6 cylinder are closed.

        (2) Gently open the valve on the SF6 cylinder and set the pressure to the rated value. Then open the exhaust valve to slowly lower the pressure, the bell is connected to the df or bc terminal, and the pressure value when the bell rings is p. Value or p, value.

        (3) Close the exhaust valve, and then slowly increase the pressure. The electric bell connects to the ae contact. When the bell rings, the pressure value is Ps.

(4) Compare the detected pressure value with the standard value (see Table 2-11). If it is suspected that there is a problem with the density relay, it should be sent to the manufacturer for inspection and adjustment.