In 35kV and higher voltage substations, SF6 gas density relays are installed on various substation equipment such as circuit breakers, combination devices, high-voltage switchgear, current transformers, voltage transformers, and bushings, where SF6 is used as the insulating medium. SF6 density relays consist of two main functions: pressure indication display and contact signal operation. They are vital components for monitoring the insulation strength of SF6 electrical equipment at all times. According to the "Preventive Test Regulations for Electrical Equipment" (DL/T 596-1996), SF6 gas density monitors (including their settings) should be inspected when necessary, during major maintenance, or every 1-3 years.
It is important to note that the verification of SF6 gas density relays imposes specific requirements on the treatment of metering equipment. Firstly, the equipment with the SF6 gas density relay to be verified must be in a de-energized state (except for non-removable ones). Secondly, the equipment should have the necessary conditions for verifying SF6 gas density relays, such as having shut-off valves and check valves in the piping, and the meter being verified should be isolated from the main gas circuit. Regarding the verification instruments and equipment selected for verification, it is required that the SF6 gas density relay verification instrument, once inspected and approved, should have a measurement accuracy not lower than 0.25 grade. Performing this work on in-service equipment carries risks such as the possibility of gas leakage in the SF6 equipment (which may damage the original sealing during on-site verification and meter removal/installation), relay secondary wiring restoration errors (recovery errors may occur when disconnecting/reconnecting signal lines), and omission risks during site restoration (forgetting to restore shut-off valves to normal working status). Based on this analysis, it is of great significance to reasonably arrange the verification cycle of density relays for electrical equipment that meets the verification conditions, and prompt corrective measures should be taken for electrical equipment that does not meet the verification conditions.
The State Grid Corporation of China's "Five Communications and One Measure" management regulations include various requirements for SF6 gas density relay verification at different stages and for different categories of equipment objects. Whether it meets the requirements for density relay verification in terms of design and manufacturing, whether it strictly checks and discovers and rectifies problems during handover acceptance, whether it pays attention to chamber pressure changes during substation operation and maintenance inspections, and whether corrective measures are well-implemented during maintenance and defect clearance—all these factors determine whether the density relay verification work needs to be conducted, whether the necessary conditions are met, and whether the work proceeds smoothly.
The "State Grid Corporation of China Substation Testing Management Regulations" provide detailed specifications for testing items, cycles, and standards for different types of equipment. The verification standards for gas density relays in SF6 circuit breakers and combination devices are separately specified. As shown in Table 1, it outlines the verification standards for density relays in combination devices and SF6 circuit breakers.
Table 1 Calibration regulations for SF6 density relays
SF6 Circuit Breaker
Gas Density Relay Verification
When abnormal data is displayed
When the recommended verification cycle by the manufacturer is reached
Gas density meter calibration
power outage experiment
When data display is abnormal
When the calibration cycle recommended by the manufacturer is met and the requirements of the equipment technical documentation are met