Ways to improve the electrical strength of gaseous media

    In order to reduce the size of power facilities, it is always desirable to make the air gap length or insulation distance as small as possible. For this reason, measures must be taken to improve the electrical strength of the gas medium. From a practical point of view, to increase the breakdown voltage of the air gap is nothing more than two ways; 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 inhibit the ionization process in the gas medium. The specific methods are:

    1. Improve electrode shape to improve electric field distribution

    The more uniform the electric field distribution, the greater the average breakdown field strength of the air gap. Therefore, it is possible to reduce the maximum electric field intensity in the air gap, improve the electric field distribution, and increase the breakdown voltage of the air gap by improving the shape of the electrode (increasing the radius of curvature of the electrode, eliminating the surplus and sharp corners on the electrode surface, etc.) .

    The use of shielding to increase the radius of curvature of the electrode is a common method. Taking the "stick-board" air gap with the worst electrical strength as an example, if a metal ball of appropriate diameter is added to the end of the stick, the breakdown voltage of the air gap can be effectively increased.

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

    Examples of applying the shielding principle to improve the electric field distribution on the UHV transmission line to increase the corona initiation voltage include: protection fittings (pressure equalizing ring) installed on the insulator string of UHV lines, and expanded diameter wires used in UHV lines.

    2. Use space charge to improve electric field distribution

    Because of the extremely uneven electric field, the corona discharge must occur before the air gap is broken down, so under certain conditions, the space charge generated by the discharge itself can also be used to adjust and improve the electric field distribution in the space to improve the breakdown of the air gap. Voltage. Take "wire-plate" and "wire-wire" air gaps as examples. When the diameter of the wire is reduced to a certain extent, the power frequency breakdown voltage of the air gap will increase as the diameter of the wire decreases. Thin line 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, the surface of the wire cannot be absolutely smooth, so Before a uniform overall corona occurs on the entire surface, corona and brush discharges will appear in individual parts first, so the breakdown voltage is close to the "rod-plate" or "rod-rod" air gap.

    3. adopt 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, the proper shape and location of the air gap can hinder the movement of charged particles and adjust the space charge. The distributed barrier is also an effective method to improve the electrical strength of the gas medium.

    The barrier is made of insulating material, but its insulation performance is not important, what is important is its tightness (the ability to block charged particles). It is generally installed in the corona gap, and its surface is perpendicular to the power line.

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

    The breakdown voltage of a barrier air gap has a great relationship with the installation position of the barrier. If it is a "sticky" air gap, both electrodes will have corona discharge, so barriers should be installed near both electrodes to be effective.

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

    Obviously, the barrier is difficult to play a role in a uniform or slightly uneven electric field.

    4. use high pressure

    The electric strength of air under normal pressure is relatively low, about 30kV/cm. Even if the above-mentioned 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 under normal pressure is much lower than that of ordinary solid and liquid media. However, if the air is compressed to greatly exceed 0.1MPa (1atm), its electrical strength can also be significantly improved. This is mainly because increasing the air pressure can greatly reduce the free travel length of electrons, thereby weakening and inhibiting the ionization process. If you can replace air with some high-electric-strength gas (such as sulfur hexafluoride) while using high pressure, better results can be obtained.

    5. Use high electric strength gas

    Among the many gases, there are some strongly electronegative gases containing halogen elements [for example, sulfur hexafluoride and freon have extremely high electrical strength (much higher than air), so they can be called high electrical strength gases. Using these gases to replace air, of course, can greatly increase the breakdown voltage of the air gap, and even mixing a part of this gas in the air can significantly increase its electrical strength.

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

    There are few gases that can meet the above requirements at the same time. At present, the only gas with high electrical strength that has been widely used in engineering is SF6 and its mixed gas. In addition to high electrical strength, SF6 gas also has excellent arc extinguishing. Ability, other related technical performance is also quite good. Various power equipment and enclosed combined electrical appliances made with 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, and simplifying installation and maintenance. Therefore, development The prospects are very broad.

    6. use high vacuum

    The use of high vacuum can also weaken the impact ionization process in the air gap and significantly increase the breakdown voltage of the air gap. If the breakdown process in high vacuum is completely explained by the 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 passing through the distance between electrodes, and it is difficult to cause enough collision ionization) , But the actual situation is not the case. When the distance between the electrodes is small, the electric strength of high vacuum is indeed very high, and can even exceed the compressed SF6 gas, but when the distance between the electrodes increases, the voltage increases slowly, and its electric strength is significantly lower Due to the breakdown field strength of the compressed gas, this indicates that the breakdown mechanism of the high vacuum has changed at this time, and a new physical process has appeared. Therefore, the previous gas discharge theory can no longer be simply explained.

    Vacuum breakdown studies show that: when the distance between the electrodes is small, the breakdown of high vacuum is related to the strong field emission on the cathode surface. The current it causes will cause the electrode to heat up locally and release metal gas, which will reduce the vacuum and cause the breakdown. When the distance between the electrodes is large, the breakdown will be caused by the so-called "full voltage effect". At this time, as the distance between the electrodes and the breakdown voltage increase, electrons can accumulate to a large extent when they fly over the vacuum from the cathode to the anode. The kinetic energy of these high-energy electrons will release positive ions and photons when they bombard the surface of the anode, which in turn will strengthen the surface ionization on the cathode. This repeated action will generate more and more electron currents, which will cause the electrode to vaporize locally and cause the breakdown of the gap. This is the so-called "full voltage effect".

    Because of this, as the distance between the electrodes 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 quantity and type of adsorbed gas), so the dispersion is very large.

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

(Huiputech)