Guide material for gas management and detection in Sulfur hexafluoride electrical equipment

Source: Date:2023.05.30 

1、 Scope

This guideline is mainly used to provide gas management and inspection methods in equipment for departments using Sulfur hexafluoride electrical equipment.

This guideline applies to the gas detection of electrical equipment in operation by the user department.

This guideline is a general guideline. Some specific provisions on the inspection and management of Sulfur hexafluoride gas in electrical equipment shall be subject to other relevant standards

2、 Referenced standards

The provisions contained in the following standards constitute the provisions of this standard by reference in this standard. At the time of publication of this standard, all versions shown are valid. All standards will be revised, and parties using this standard should explore the possibility of using the latest versions of the following standards.

GB 5832.1-86 Determination of trace moisture in gases - Electrolytic method

GB 5832. 2-86 Determination of trace moisture in gases - Dew point method

GB 116 05-89 Humidity Measurement Methods

GB 110 23-89 Test Method for Sulfur hexafluoride Gas Sealing of High Voltage Switchgear

GB 120 22-89 Industrial Sulfur hexafluoride

DL 506-92 Field Measurement Method of Moisture Content in Sulfur hexafluoride Gas Insulated Equipment

3、 General properties of six sulfur compounds

3.1 Physical properties D

The Molecular formula of Sulfur hexafluoride gas is SF, the molecular weight is 146.07, and the molecular diameter is 4.56X1 0- ` 0m,

Sulfur hexafluoride gas is gaseous at normal temperature and pressure, and its density is 6.16 g/L (about 5 times of air) at 20 ℃ and 101325P a

The critical temperature of Sulfur hexafluoride gas is 45.6 ℃. It is liquefied after compression. It is usually transported in steel cylinders as liquid. Its saturated vapor pressure curveThe pure Sulfur hexafluoride gas is colorless, tasteless, odorless, non-toxic and non woodcutting.

3.2 Electrical properties

3.2.1 Sulfur hexafluoride is a negative gas (capable of absorbing free electrons), with good arc extinguishing and insulation properties. In the uniform electric field under the pressure of 101.3 kPa, the withstand voltage strength of Sulfur hexafluoride gas is about 2.5 times that of nitrogen.

3.2.2 Pure Sulfur hexafluoride is an inert gas that will decompose under the action of electric arc. When the temperature is above 4000 K, most of the decomposition products are single atoms of sulfur and fluorine. After the arc is extinguished, most of the decomposition products recombine into stable Sulfur hexafluoride molecules. A very small amount of the decomposition product undergoes chemical reactions with free metal atoms, water, and oxygen during the recombination process, producing metal fluorides and fluorides of oxygen and sulfur.

4、 Types and their impacts

4.1 Types of impurities

4.1.1 The new gas impurities and their allowable content should comply with the quality standards specified in Table 1.

4.1.2 The Sulfur hexafluoride gas in the operating electrical equipment contains various impurities. Due to the diffusion of residual moisture inside electrical equipment, the content of air and water will exceed the initial values corresponding to the new gas. The discharge decomposes part of Sulfur hexafluoride, which can produce fluoride and other gases containing oxygen and sulfur, as well as solid decomposition products.

4.2 Impact of impurities

Some impurities (such as nitrogen) have no significant impact on insulation and arc extinguishing performance when their content is low. It is necessary to limit the content of other types of impurities, so that whether they exist alone or in a mixture, they will not cause operational accidents to the equipment using these gases. For example, water, acidic impurities, and oxygen (especially when mixed together) can accelerate corrosion, leading to mechanical failure. When water is mixed with any acidic impurities, it will endanger the electrical safety of equipment under low temperature and High-pressure area. Therefore, the content of these impurities must be limited to prevent corrosion and condensation.

4.3 Allowable content of impurities

The allowable (impurity) limits for the use of impure gases are not clearly specified, because they largely depend on the design and installation position of electrical equipment, such as the following situations: small flashover distance, leakage path length, the existence of arcs in Closed system, the nature of various elements in contact with gases, absorption devices, etc. The humidity requirements in electrical products should be implemented in accordance with the provisions of 8.1.2. The content of other impurities shall be determined by the design department and the user department based on specific circumstances.

4.4 Toxic impurities

When Sulfur hexafluoride is used in electrical equipment, it may decompose to produce oxygen, sulfur fluoride and metal fluoride powder under fault conditions or under the action of normal breaking arc. Experience has shown that even in environments containing very small amounts of decomposition products, workers can experience pungent or uncomfortable odors, which can cause significant irritation to the nose, mouth, and eyes. This reaction occurs within a few seconds before there is a significant toxic reaction.

5、 Sampling method

5.1 Overview

The purpose of sampling is to obtain representative gas samples. In general, SF. It exists in a gaseous state, and the sample should be directly extracted from inside the equipment and should not be extracted through filters inside the equipment.

5.2 Sampling container zI

The ideal is to directly pass the gas sample into the analytical device from the inspected equipment. Items that cannot be directly detected at the operation site shall be sampled using containers made of inert materials. When the sample taken is liquid, the sampling container must undergo a pressure test of 7M Pa and must not be filled. The filling conditions should comply with the provisions of 6.1 and 6.2 in GB 12022-89. The dirt in the sampling container increases the impurities in the tested sample. The sampling bottle shall not be used to contain substances other than Sulfur hexafluoride. After the container is used, it is heated to 100 ℃ and vacuumized, filled with new Sulfur hexafluoride to normal pressure, and washed twice at least.