How does the insulation property of SF6 gas affect its insulation strength?- SF6在线监测系统_SF6检漏仪_SF6传感器_SF6密度继电器

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How does the insulation property of SF6 gas affect its insulation strength?

发布时间：2016-01-22 来源：ydlcb 浏览次数：48次

SF6 gas has excellent insulation properties, in a relatively uniform electric field, its insulation strength is about 2.5 to 3 times that of air, and its insulation strength can reach the level of insulating oil in 3 gauges. The factors that affect its insulation strength are as follows:

(1) The influence of electric field uniformity. The insulation strength is particularly sensitive to the uniformity of the electric field. Under a uniform electric field, the insulation strength increases linearly with the increase of the distance between contacts. When the distance increases, the insulation strength increases and saturation occurs due to the non-uniformity of the electric field. Under an uneven electric field, its insulation strength is even close to the level of air.

(2) The relationship with stress. Under a more uniform electric field, the insulation strength increases with the increase of SF6 gas pressure, but it is not proportional.

(3) The influence of electrode surface positive. Generally, the rougher the surface of the electrode, the lower the breakdown voltage. The larger the electrode area, the greater the probability of accidental factors, thus reducing the breakdown voltage.

(4) The influence of voltage polarity. The effect of voltage polarity on the breakdown voltage of SF6 gas is related to the uniformity of the power plant. In a uniform electric field, there is no polarity effect because the electric field strength is equal everywhere. In a slightly uneven electric field, when the electric field with a larger curvature is negative, the field strength near it is larger, and it is easy to produce negative electron emission, so that the breakdown voltage of the air gap is reduced. Because the insulation structure of the SF6 circuit breaker is a slightly uneven electric field form, its insulation level is often determined by negative polarity voltage.

In addition, under the influence of space charge generated by corona discharge of rod electrode, the breakdown voltage of positive polarity is lower than that of negative polarity.

The factors that affect SF6 gas breakdown voltage are as follows:

(1) Uniformity of electric field. The breakdown voltage of SF6 varies greatly in different uniform electric fields. In a uniform electric field, the breakdown voltage of SF6 is larger than that in a highly non-uniform electric field, and this effect is much greater than that of air.

(2) SF6 working pressure. Increasing the working pressure of SF6 is one of the ways to increase the breakdown voltage of SF6, but the working pressure should not be too high, otherwise SF6 will liquefy. At the same time, it should be noted that the breakdown voltage of SF6 tends to be saturated with the increase of air pressure, and the more uneven the electric field, the easier it is to saturate. Therefore, it is only effective to increase SF6 working pressure under the condition that SF6 is liquefied and the electric field is fairly uniform.

(3) SF6 gas is an impurity. When the SF6 is mixed with air, nitrogen and other impure gases or conductive particles, its insulation capacity can be significantly reduced. The conductive particles of SF6 gas move under the action of electric field, and gradually form a "conductive bridge", resulting in a decrease in breakdown voltage.

(4) Electrode surface. The breakdown voltage of SF6 decreases with the increase of electrode surface area. The rougher the electrode surface, the lower the breakdown voltage. After multiple discharge treatment, the electrode surface is smooth, and the breakdown voltage has a tendency to increase.

The influence of the non-uniformity of electric field on the electrical strength of SF6 is much greater than that of air.

Sulfur hexafluoride has a high electrical strength, mainly because it has a strong electronegativity, easy to capture free electrons and form negative ions (electron attachment process), after the electron becomes a negative ion, its ability to cause collision ionization becomes very weak, thus weakening the discharge development process.

Compared with the breakdown voltage in the uniform electric field, the breakdown voltage of SF6 in the non-uniform electric field drops much more than that of air. The excellent insulation properties of SF6 can only be fully utilized in the case of relatively uniform electric field.

When designing various electrical equipment with sulfur hexafluoride gas as insulation, the electric field in the air gap should be homogenized as far as possible, and measures such as shielding should be adopted to eliminate the extremely uneven electric field at all sharp corners, so that the excellent insulation performance of SF6 can be fully utilized.

Sulfur hexafluoride (SF6 gas)

(1) Breakdown of SF6 in uniform and slightly uneven electric fields Breakdown of SF6 gas in uniform and slightly uneven electric fields

The impact ionization and discharge processes in SF6 electronegative gases, in addition to considering the α process, should also take into account the electron attachment process, which can be expressed by an electron attachment system η similar to the definition of the electron impact ionization coefficient α, η is the average number of electron attachment times occurring in the direction of an electron moving 1cm in the direction of the electric field. It can be seen that the effective impact ionization coefficient in a electronegative gas is α = αη

Electron avalanche growth in uniform electric field: na = n0e (αη) d

Where: n0- the initial number of electrons at the cathode surface; na- The number of electrons at the anode

At this time, it should be noted that in the general gas, the number of positive ions is equal to the newly added electricity. In a electronegative gas, the number of positive ions is equal to the sum of the number of new electrons and the number of negative ions.

Because the conditions of strong electronegative gas in practice belong to the category of stream discharge, the conditions of self-sustaining stream discharge are not discussed here, but directly discussed. Therefore, the flow self-sustaining discharge condition of electronegative gas in a uniform electric field can be written as: (αη) = K by reference to equation (120)

Experimental studies show that for SF6 gas, the constant K=10.5, the corresponding breakdown voltage is Ub = 88.5pd + 0.38

(kV) type: p- pressure, Mpa, d- distance between electrodes, mm

In engineering applications, pd is usually <1MPa mm, so the above formula can be approximately written as: Ub ≈ 88.5pd

Equations (216) and (217) show that the breakdown of SF6 gas in a uniform electric field also follows Baschen's law. Its breakdown field strength at 0.1MPa(1atm) is Eb = Ub ≈ 88.5kV/cm, almost 3 times that of air.

Common in gas-insulated electrical equipment is a slightly non-uniform electric field air gap, such as the air gap between cylinders of the same axis.

In a slightly non-uniform electric field, the effect of polarity on the breakdown voltage of the air gap is opposite to that in a non-uniform electric field, and the breakdown voltage under negative polarity is about 10% lower than that under positive polarity. The impact coefficient is very small, about 1.25 during lightning impact, and even smaller during operation impact, only 1.05 ~ 1.1.

(2) Breakdown of SF6 in a very uneven electric field

The abnormal phenomenon of SF6 gas breakdown in a very heterogeneous electric field is related to the motion of space charge.

The shielding effect of space charge on the rod will increase the breakdown voltage, but under the action of lightning impulse voltage, the space charge can not move to the favorable position, so the breakdown voltage is lower than the static breakdown voltage. When the air pressure is increased, the space charge diffuses slowly, so when the gas pressure exceeds 0.1-0.2mpa, the shielding effect is weakened, and the power frequency breakdown voltage will decrease.

(3) Other factors affecting the breakdown field strength

The design field strength of sulfur hexafluoride gas insulated electrical equipment is much lower than the theoretical breakdown field strength, because there are many influencing factors that will reduce its breakdown field strength. Only two of the main influencing factors are described here, namely, electrode surface defects and conductive particles.

1. The electrode surface defect diagram shows the influence of electrode surface roughness Ra on SF6 and gas electrical strength Eb.

It can be seen that the higher the working pressure of GIS, the greater the influence of Ra on Eb, and therefore the higher the technical requirements for electrode surface processing.

When the electrode surface roughness is large, the local electric field strength at the surface protrusion is much larger than the average electric field strength of the air gap, so the breakdown can be induced when the average field strength has not reached the critical value at the macro level.

In addition to surface roughness, there will be other sporadic random defects on the electrode surface, and the larger the electrode surface area, the greater the probability of such defects. Therefore, the larger the electrode surface area, the lower the breakdown field strength of SF6 gas, which is called the "area effect".

(2) Conductive particles in the equipment there are two types of conductive particles, namely fixed particles and free particles, the former has a similar effect to the electrode surface defects, and the latter will bounce between the electrodes and have a greater adverse effect on the insulation performance of SF6 gas.