The SF6 gas density relay, also known as the SF6 gas density controller or monitor, is a crucial monitoring device for high-voltage switchgear in the energy industry. It is mainly applied in SF6 insulated equipment such as GIS combined electrical apparatus, circuit breakers, and transformers. By integrating pressure measurement and limit switch functions, this device continuously monitors the SF6 gas density within sealed containers. When the density falls below or exceeds the set threshold, it triggers alarm, lockout, or overpressure signals to ensure the safe operation of electrical equipment.

1. Density Monitoring and Leak Detection

   The density of SF6 gas directly affects the insulation and arc-extinguishing capabilities of equipment. The density relay monitors the gas density in real time (actually the pressure compensated for temperature) to determine if there is a leak or abnormality. When the density drops below the alarm value, it sends a signal; when it drops below the lockout value, it locks out the equipment operation to prevent accidents.

2. Temperature Compensation Mechanism

   The pressure of SF6 gas changes with temperature, but the density relay compensates for the temperature effect through a double-layer metal strip or a standard gas chamber, ensuring that the reading always corresponds to the standard value at 20℃ and avoiding misjudgment.

3. Protection Triggering

   When the gas density drops to a dangerous level, the relay triggers an alarm or lockout signal, which links to the control system to cut off the power or prohibit opening and closing operations, ensuring the safety of the equipment.

According to structure and function, SF6 density relays can be classified into the following types:

• Mechanical density relay - Spring-tube or bellows structure, with temperature compensation, no power supply required
  • Suitable for harsh outdoor environments (IP65 protection), such as circuit breakers in substations
• Intelligent density relay - Integrated pressure/temperature sensor, supports 4-20mA analog output or RS485 communication
  • Can upload density values to monitoring systems in real time for remote diagnosis
• Integrated valve and gauge density relay - Integrated calibration valve, supports online calibration, reduces leakage points
  • Compact size, suitable for compact switchgear (such as gas-insulated ring main units)
• Wireless remote transmission density relay - Utilizes LoRa or NB-IoT technology, with a transmission distance of up to 1-3 kilometers
  • Suitable for distributed equipment monitoring, such as remote substations or mountainous power grids

1. Necessity of Regular Verification
   • The relay contacts may suffer from poor contact due to oxidation, oil film contamination or mechanical aging. Therefore, it is necessary to regularly verify the alarm/trip values and contact resistance (which should be ≤ 1Ω).
2. Verification Method
   • Use a dedicated verification instrument (such as an automatic SF6 density relay verification instrument), simulate pressure changes by charging and discharging gas, record the action values and automatically convert them to standard values at 20℃.
   • During verification, the secondary circuit should be disconnected to avoid interference; the equipment should be placed upright and the ambient temperature should be stable.

1. False Alarm/Lockout: This may be caused by direct sunlight leading to overcompensation of the relay. A sunshade should be installed or the installation position adjusted.
2. Contact Failure: Oxidation of the contacts or a decline in the insulation of the silicone oil may cause this issue. The relay needs to be cleaned or replaced.
3. Reading Fluctuation: Fluctuation in load current causing internal temperature rise in the equipment is a normal phenomenon. If the reading continuously drops, leakage should be checked.

• Power equipment: Used for online monitoring of SF6 circuit breakers, transformers, and GIS.
• Special environments: Oil-filled or nitrogen-filled types are suitable for harsh outdoor conditions, with a protection level up to IP65.

• Parameter Selection: Choose the appropriate model based on the equipment's rated pressure (e.g., 0.4 to 0.6 MPa) and temperature range (-20°C to +60°C).
• Installation Notes: Avoid direct sunlight and vibration, and ensure the airtightness of the airway connections.