SF6 gas is increasingly used in various high voltage electrical equipment due to its excellent insulation and arc extinguishing performance. It has become the insulation and arc extinguishing medium of high voltage equipment.
In order to ensure the safe operation of high voltage equipment, strict monitoring of parameters such as density and moisture content of SF6 gas is necessary. SF6 relay is a device used to monitor the density of SF6 gas in high voltage electrical equipment. It timely issues alarm signals and blocking signals in response to gas leakage in the electrical equipment. It can prevent accidents and ensure the safe operation of high voltage electrical equipment. However, the on-site installed SF6 relay often remains inactive over time, causing phenomena such as poor flexibility of action and poor contact of contacts. Some may even experience decreased temperature compensation performance due to environmental conditions, resulting in unintentional operation of the density relay. Therefore, it is necessary to regularly (usually every 1 to 3 years) conduct on-site inspections of SF6 relay and take preventive measures.
In a general sealed container, the pressure of SF6 gas at a certain temperature can represent the density of SF6 gas. In order to standardize, the pressure of SF6 gas at 20°C is commonly used as the representative value of its corresponding density. Therefore, the SF6 relay uses the pressure of SF6 gas at 20°C as the reference value.
SF6 relay inspection is actually to test the action pressure at 20°C. SF6 relay inspection can be done by the following methods.
This method is mainly used on-site. After the SF6 equipment installation is completed, during the inflation process of the main body, the density relay is calibrated using the inflation process (the action of the contacts/the indication value of the pressure gauge). This method is simple and practical, and canpreliminary judgment of the quality of density relay can be made, but there are several issues:
Great influence of temperature. The SF6 gas in the gas cylinder is stored in liquid form. When the device is filled with SF6 gas, it undergoes a phase change from liquid to gas, absorbing energy. The SF6 gas filled into the device within a short period of time will have a significant temperature difference compared to the ambient temperature. However, the SF6 relay is often exposed to outdoor conditions for a long time, which is the same as testing the relay with SF6 gas at different temperatures, making it difficult to ensure the accuracy of the test.
Using the reading of SF6 pressure gauge as a reference for comparison is a method with large errors.
SF6 relays are mainly used in the process of voltage reduction, and testing them during the filling process does not match the actual conditions and affects accuracy.
Testing the SF6 relay by releasing gas using SF6 equipment.
This method has the same issues as the first two methods during the filling process, and the released gas needs to be recovered using specialized gas recovery equipment, which is a complex procedure and is generally not used.
This testing method is conducted in a constant temperature (20℃) environment, eliminating the error caused by temperature and providing high accuracy. However, the test duration is long and requires a dedicated constant temperature test chamber, which is not generally available on-site and is mainly used by manufacturers. Currently, imported or domestically produced density relays undergo this type of testing.