Solution of SF6 densimeter without disassembly and calibration (SF6 three-way valve)

Requirements for SF6 densitometer in the operation regulations of the State Grid, DL/T596-1996 Preventive Test Procedures for Electric Power Equipment implemented by the Ministry of Electric Power on January 1, 1997: "SF6 gas densitometer (including setting value) inspection" item is included in the No. 13 of the "switchgear" section, and the test time required in the regulations is 1-3 years, after major repair or when necessary, the specific calibration time is formulated by the pilot plants in each province, Generally, it is appropriate to check at least once every three years, which is applicable to high-voltage circuit breakers and combined circuit breaker GIS of ABB, Siemens, Schneider, Areva and other brands.

1. Steps of current conventional calibration methods:
Arrange equipment power failure, cut off the power supply of the densitometer control circuit, disassemble the densitometer and let it stand for 2 hours (to make the Tangxing transformer in the pressure gauge return to normal), replace the seal ring, resume operation of the equipment, reinstall the qualified densitometer, arrange equipment power failure, reinstall the calibrated densitometer, check the action reliability of the locking contact, test it, and resume operation.
 
2. Problems in conventional methods:
The inspection (power failure) takes a long time, and many equipment cannot be calibrated on time: the whole process of this method takes several days. At present, the power supply pressure in various provinces is increasing, and the cost of power failure is increasing, so it is quite difficult to achieve it. Therefore, the calibration cycle of a large number of SF6 densimeters is prolonged and delayed, bringing great potential safety hazards.
Risk during disassembly and assembly: SF6 gas under pressure has strict requirements for sealing process, frequent disassembly, and negligence of maintenance operation, which is very easy to damage the sealing surface of SF6 flange and cause SF6 gas leakage, leading to equipment failure (accident), and there is a large potential safety hazard in the electric network.

3. Eighteen requirements for major anti accident measures of power grid:
Considering the importance of SF6 density relay to the safe operation of high-voltage switchgear, it is stipulated in 12.1.1.6 of the Eighteen Major Anti accident Measures of State Grid Corporation of China that the connection mode between SF6 density relay and switchgear body shall meet the requirement of checking density relay without disassembly.

4. The scheme is as follows:
Technical indicators:
Pressure rating: PN64
Sealing performance: leakage rate<10-15MPa M3/s Ambient temperature: - 40~100 ℃
Impact resistance: 600m/s2 11ms
Service life: 105 times
Material: the valve body is made of stainless steel

5. Conditions and time of transformation:
Because there are valves on the original joint seat to isolate the air chamber of the body, it is theoretically not necessary to cut off the power during construction, but the on-site valves may not be able to completely close the air chamber. At the same time, considering the impact on the background alarm and locking models, it is recommended to cut off the power for construction. Each interval is reconstructed according to three meters, and it is estimated that each interval will need to stop for about 1.5-2 hours.