Methods for improving the electrical strength of gaseous media

In order to reduce the size of power facilities, it is always hoped that the air gap length or insulation distance will be as small as possible. For this purpose, measures must be taken to improve the electrical strength of the gas medium. From a practical point of view, there are two ways to increase the breakdown voltage of the air gap: one is to improve the electric field distribution in the air gap to make it as uniform as possible; the other is to try to weaken or suppress the ionization process in the gas medium. The specific methods are:

1. Improve the shape of the electrode to improve the electric field distribution

The more uniform the electric field distribution, the greater the average breakdown field strength of the air gap. Therefore, the maximum electric field strength in the air gap can be reduced, the electric field distribution can be improved, and the breakdown voltage of the air gap can be increased by improving the electrode shape (increasing the curvature radius of the electrode, eliminating the hair and sharp corners on the surface of the electrode, etc.).

Using shielding to increase the curvature radius of the electrode is a common method. Taking the "rod-plate" air gap with the worst electrical strength as an example, if a metal ball with an appropriate diameter is installed at the end of the rod pole, the breakdown voltage of the air gap can be effectively increased.

The high-voltage outlets of many high-voltage electrical devices (such as the upper end of the high-voltage bushing guide rod of power equipment) have sharp shapes, and shielding covers are often required to reduce the maximum field strength in the space near the outlets and increase the corona inception voltage. The shape and size of the shielding cover should be selected so that its corona inception voltage is greater than the maximum working voltage of the device to the ground. The simplest shielding cover is of course a spherical shielding pole.

Examples of applying the shielding principle to improve the electric field distribution on ultra-high voltage transmission lines to increase the corona inception voltage include: protective fittings (equalizing rings) installed on insulator strings of ultra-high voltage lines, and expanded diameter conductors used on ultra-high voltage lines.

2. Using space charge to improve the electric field distribution

Since corona discharge must occur before the air gap of extremely uneven electric field is broken down, under certain conditions, the space charge generated by the discharge itself can also be used to adjust and improve the electric field distribution in space to increase the breakdown voltage of the air gap. Taking the "wire-plate" and "wire-wire" air gaps as examples, when the wire diameter is reduced to a certain extent, the power frequency breakdown voltage of the air gap will increase with the reduction of the wire diameter, resulting in the so-called "thin wire effect". The reason is that the uniform space charge layer formed by the corona discharge of the thin wire can improve the electric field distribution in the air gap, resulting in an increase in the breakdown voltage; when the wire diameter is large, since the wire surface cannot be absolutely smooth, corona and brush-shaped discharge will appear in some parts before the uniform overall corona occurs on the entire surface, so its breakdown voltage is similar to that of the "rod-plate" or "rod-rod" air gap.

3. Use of barriers

Since the electric field distribution in the air gap and the development process of gas discharge are closely related to the generation, movement and distribution of charged particles in the air gap space, placing a barrier with a suitable shape and position in the air gap that can hinder the movement of charged particles and adjust the spatial charge distribution is also an effective way to improve the electrical strength of the gas medium.

The barrier is made of insulating material, but its own insulation performance is not important. What is important is its sealing (the ability to block charged particles). It is generally installed in the corona gap, and its surface is perpendicular to the electric lines.

The effect of the barrier depends on the space charge of the same sign as the corona electrode that it blocks, so that the spatial electric field strength between the corona electrode and the barrier can be reduced, thereby making the electric field distribution of the entire air gap uniform. Although the spatial electric field strength between the barrier and the other electrode is increased at this time, its electric field shape becomes more like the uniform electric field between two flat electrodes, so the electrical strength of the entire air gap is improved.

The breakdown voltage of the barrier air gap has a lot to do with the installation position of the barrier. If it is a "stick-stick" air gap, both electrodes will have corona discharge, so the barrier should be installed near both electrodes to achieve results.

Under impulse voltage, the effect of the barrier is smaller because the spatial charge accumulated on the barrier is less at this time.

Obviously, the barrier is difficult to play a role in the case of uniform or slightly non-uniform electric field.

4. Use high air pressure

The electrical strength of air at normal pressure is relatively low, about 30kV/cm. Even if the above measures are taken to improve the electric field as much as possible, the average breakdown field strength cannot exceed this limit. It can be seen that the electrical strength of air at normal pressure is much lower than that of general solid and liquid media. However, if the air is compressed and the pressure is much greater than 0.1MPa (1atm), its electrical strength can also be significantly improved. This is mainly because increasing the air pressure can greatly reduce the free path length of electrons, thereby weakening and inhibiting the ionization process. If the air can be replaced by some high electrical strength gases (such as sulfur hexafluoride) while using high pressure, better results can be achieved.

5. Use high electrical strength gas

Among the many gases, some highly electronegative gases containing halogen elements [such as sulfur hexafluoride and Freon have particularly high electrical strength (much higher than air), so they can be called high electrical strength gases. Using these gases to replace air can of course greatly increase the breakdown voltage of the air gap, and even mixing some of such gases into the air can significantly increase its electrical strength.

It should be pointed out that for this type of gas to be practically applied in engineering, it is not enough to rely solely on its high electrical strength. They must also meet certain other requirements, such as: ① Low liquefaction temperature (so that high gas pressure can be used at the same time); ② Good chemical stability, not easy to decompose, burn or explode, and not produce toxic substances when discharge occurs in the gas; ③ Not too difficult to produce and not too expensive.

There are very few gases that can meet all of the above requirements at the same time. At present, the only high electrical strength gas that has been widely used in engineering is SF6 and its mixed gas. In addition to its high electrical strength, SF6 gas also has excellent arc extinguishing ability and other related technical properties are also quite good. Various power equipment and closed combination electrical appliances made of SF6 gas as an insulating medium and arc extinguishing medium have a series of outstanding advantages, such as greatly saving floor space and space volume, safe and reliable operation, simplified installation and maintenance, etc., so the development prospects are very broad.

6. Use high vacuum

The use of high vacuum can also weaken the collision ionization process in the air gap and significantly increase the breakdown voltage of the air gap. If the breakdown process in high vacuum is explained completely by gas discharge theory, the resulting breakdown voltage will be extremely high (at this time, it is difficult for electrons to collide with neutral molecules when crossing the inter-electrode distance, and it is difficult to cause enough collision ionization). However, the actual situation is not so. When the inter-electrode distance is small, the electrical intensity of high vacuum is indeed very high, and can even exceed that of compressed SF6 gas. However, when the inter-electrode distance increases, the voltage increases slowly, and its electrical intensity is significantly lower than the breakdown field strength of compressed gas. This indicates that the breakdown mechanism of high vacuum has changed at this time, and a new physical process has emerged, so it can no longer be simply explained by the previous gas discharge theory.

Vacuum breakdown research shows that when the inter-electrode distance is small, the breakdown of high vacuum is related to the strong field emission on the cathode surface. The current caused by it will cause local heating of the electrode and release metal gas, causing the vacuum degree to drop and cause breakdown; when the inter-electrode distance is large, the breakdown will be caused by the so-called "full voltage effect". At this time, as the inter-electrode distance and the breakdown voltage increase, the electrons can accumulate a lot of kinetic energy when they fly from the cathode across the vacuum to the anode. These high-energy electrons will release positive ions and photons when bombarding the anode surface, which will strengthen the surface ionization on the cathode. This repeated action will produce more and more electron flow, causing the electrode to be locally vaporized and leading to the breakdown of the gap. This is the so-called "full voltage effect".

Because of this, as the inter-electrode distance increases, the average breakdown field strength will become smaller and smaller. The breakdown voltage of the vacuum gap is related to many factors such as electrode material, surface finish and cleanliness (including the amount and type of adsorbed gas), so it is very dispersed.

There are not many cases where high vacuum is actually used as an insulating medium in power equipment, mainly because various solid or liquid media are mostly used in the insulation structure of various equipment. They will gradually release gas in the vacuum, making it difficult to maintain high vacuum for a long time. At present, high vacuum is only used in vacuum circuit breakers. Vacuum not only has good insulation performance, but also has strong arc extinguishing ability, so it is still very suitable for vacuum circuit breakers in distribution networks.