Online Monitoring Technology of SF6 Gas in Power Equipment

SF6 gas, as an excellent insulating medium and arc extinguishing medium, is widely used in power transmission and substation equipment. For SF6 gas in high-voltage circuit breakers, transformers and other major transmission and substation equipment, two requirements should be met: firstly, the pressure of SF6 gas should reach the rated level; secondly, the water content in SF6 gas should meet the specified requirements. Therefore, the annual leakage rate and water content rate of SF6 gas should be tested regularly in operation.

1. Gas monitoring technology of SF6 monitor at early stage

When poor sealing of electrical equipment causes SF6 gas leakage, the pressure gauge pointer will be deflected as the pressure drops. When the pressure drops to the yellow area, the electrical contact connected to the signal circuit of the SF6 monitor circuit breaker is turned on and an alarm signal is issued. If SF6 gas is not added in time and the pressure continues to drop to the red area, the electrical contacts connected to the circuit breaker control circuit are connected to start the intermediate relay and block the opening and closing operation of the circuit breaker. For non-integrated automation substations, there is no data collection of SF6 gas pressure signals to enable the remote transmission of data. At the same time, the pressure gauge cannot reflect the water content of SF6 gas. Currently, the widely used method is to use a dew point meter for testing. Since the test is executed on an annual cycle, it is monitored off-line. Also, since SF6 gas emission is generated during the testing process, it inevitably causes environmental pollution.

2. The current application of gas online monitoring technology of SF6 monitor

(1) SF6 gas density of online monitoring technology

Recent research shows that the insulation arcing performance of SF6 gas is not only related to air pressure, but also closely related to density. In a series of tests on the SF6 monitor, the minimum arc time increased by only 0.35m on average when the SF6 gas density was constant and the pressure was reduced. In the case of constant gas pressure, reduced SF6 gas density, the minimum arc time is increased by an average of 1.2 m. Therefore, the density of SF6 gas is more reflective of the absolute green interrupting and arcing performance of the circuit breaker than air pressure. The principle of SF6 gas online monitoring is to measure the gas pressure and temperature with certain accuracy by pressure and temperature sensors, and to calculate the gas density by SF6 gas state parameter equation.

(2) SF6 gas moisture content of online monitoring technology

When the moisture content of SF6 gas is at 20℃, the new equipment and the air chamber communicated with the arc extinguishing chamber is not more than 150uL/L according to the switching test standard of electrical equipment; In accordance with the preventive test procedure of power equipment stipulated in this Law, the air chamber communicated between SF6 operating equipment and arc extinguishing chamber shall not be greater than 300ul/L. Recent studies have found that the SF6 gas rising moisture content and the increase in ambient temperature has a relationship, but there is no specific allowable standard, so the test results are often not comparable and far apart.

Some manufacturers of SF6 high-voltage electrical equipment have given curves of SF6 gas water content with empirical temperature, which makes the detection results at different temperatures comparable and has some industrial application value.

The humidity sensor is the core of the whole data acquisition system. Compared to air, the moisture content in SF6 gas is very low, so a hybrid sensor should be used for the measurement, which can be either an alumina polymer type sensor or a capacitive humidity sensor. The wet-sensing material in these sensors absorbs and releases water molecules in proportion to the relative humidity of the surrounding environment, changing the dielectric constant and reflecting the different moisture contents through the circuit.

Online monitoring of SF6 gas density can monitor the insulation performance of SF6 gas in a short period of time and analyze the leakage rate of SF6 gas. Online monitoring of SF6 gas humidity can detect the problem of excessive moisture content in SF6 gas in time. SF6 monitor has a positive effect on improving the reliability of power equipment operation and power supply reliability of power system.

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