The insulation structure of high-voltage switchgear can be divided into insulation within the cabinet space and insulation of internal components, depending on their location.
The most common insulation within the cabinet space is air, meaning that air is used as the insulating medium for both live parts and ground, and between phases. Because the insulation distance between phases in a gas-filled switchgear is significantly reduced, the switchgear cabinet size is significantly reduced. Therefore, compared to air-insulated switchgear, insulation within the cabinet space offers advantages such as environmental immunity, compact size, high reliability, improved operator safety, and maintenance-free primary circuits, thus improving safety and reliability. However, its disadvantage is that the use of SF6 gas can cause environmental problems.
Insulation of internal components primarily involves solid insulating materials made of ceramic, plastic, rubber, glass, and other materials. For example, vacuum circuit breakers, key components in switchgear, utilize glass or ceramic arc extinguishing chambers. The insulating components of vacuum circuit breakers are commonly made of 4330 glass fiber reinforced phenolic compression molding for electrical insulation. However, this material has poor aging resistance and moisture resistance, and its insulation strength decreases during use. Therefore, glass fiber reinforced unsaturated polyester compression molding (SMC and DMC) is now being used to increase creepage distance and improve insulation performance. With the development of science and technology, specialty plastics are increasingly becoming the primary solid insulation material in medium-voltage switchgear.
In order to give full play to the advantages of various insulation materials, a technology that combines several insulation methods in a single insulation type has emerged, which is called composite insulation technology. There are mainly three types:
(1) Composite insulation technology with air insulation as the main body. This technology uses air insulation as the main body and sets solid insulation partitions such as epoxy resin on the discharge path to shorten the minimum discharge gap. Typical applications include: putting heat shrinkable insulation sleeves on busbars, applying epoxy resin on contacts, etc. The principle of this technology is that the solid insulation partition has the function of preventing discharge. Applying a charge with the same polarity as the voltage on the partition surface can improve the electric field and obtain a withstand voltage 1.5 times that of air insulation.
(2) Composite insulation technology with gas insulation as the main body. The composite insulation in low-pressure SF6 gas close to atmospheric pressure is the same as that in gas insulation. Inserting solid insulation partitions in the discharge path can improve the electric field and increase the withstand voltage. The only difference is that the partition effect in SE6 gas is related to the shape of the partition. The medium-voltage switchgear can reduce the equipment volume to 1/3 of the air switchgear by combining SF6 gas and solid insulation.
(3) Solid-gas-vacuum composite insulation technology. The characteristic of this technology is that a cast solid insulation member with a surface grounding layer is used as an external closed container, a vacuum interrupter is placed inside the container, and the remaining space of the container is filled with SF6 gas. Through this method, the volume of the switch cabinet can be reduced to 1/3 of that of pure air insulation.
So, how to improve the insulation level of high-voltage switchgear? There are mainly the following measures:
(1) Ensure that the pure air gap meets the requirements. For 10kV switchgear with air insulation, the minimum air gap between phases and to the ground should be no less than 125mm; the air gap between the conductor and the insulating partition should be no less than 30mm.
(2) Reasonably configure the external insulation creepage distance. The necessary porcelain creepage distance is the basis for the porcelain to prevent pollution flashover. The external insulation creepage distance must be reasonably configured according to the pollution conditions in the area. For indoor switchgear operating under condensation and severe pollution (indoor equipment level II pollution) environment conditions, the minimum nominal external insulation creepage distance is 18mm/kV for porcelain materials and 20mm/kV for organic materials.
(3) Select components with good insulation properties. Components with good insulation properties should be selected, especially voltage transformers. Their volt-ampere characteristics must meet the requirement of no significant saturation under the line voltage. In terms of connection method, it is best not to connect one or two phase voltage transformers between the phase line and the ground to ensure the symmetry of the three-phase impedance to the ground and avoid neutral point displacement or resonance.
(4) Use interphase insulating partitions For trolley-type switchgear, if the interphase distance cannot meet the insulation requirements, the solution can be to install interphase insulating partitions. The insulating partitions used should be integral adhesive insulating partitions, and their materials are: ① Epoxy resin laminated glass cloth board made by electric heating and curing; ② DMC and SMC unsaturated polyester glass fiber reinforced plastic board.
(5) Perform insulation sealing Perform insulation sealing between the busbar compartments of each switchgear. Once an accident occurs in a switchgear, the accident can be confined to the accident cabinet, thereby minimizing the loss. Secondly, sealing and isolating the various unit rooms of the switchgear is also an effective measure to limit the scope of the accident. For the busbar, a flame-retardant heat-shrinkable insulating tube set can be used. Arc-resistant and flame-retardant insulating boards or insulating sleeves should also be used to seal the busbar compartments of adjacent cabinets to prevent the occurrence of "fire burning the camp" accidents. The bottom of the cabinet must be tightly sealed to prevent small animals from entering and to prevent moisture from the cable trench from invading and causing ground or interphase short circuit accidents. The primary isolating static contact of the trolley cabinet should be covered with an insulating cover. When the trolley exits the switch cabinet, an insulating baffle should be used to separate the live part of the static contact from the trolley compartment to prevent maintenance personnel from accidentally getting an electric shock. An insulating curtain can also be used, but after the trolley switch is pushed into the operating position, the air gap between the live part and the edge of the socket must be greater than 30 mm. However, this reduces the degree of sealing between the compartments.
(6) Make full use of insulating materials. For insulating partitions, insulating materials with good insulation performance, non-flammable or flame-retardant, and low hygroscopicity should be selected. Adding flame-retardant heat shrink tubing to the busbars and other exposed conductors in the switch cabinet can improve the insulation level of their air gaps. In addition, the use of single-component RTV silicone, the installation of climbing skirts, and the painting of busbars with silicone flame-retardant and thermally conductive high-voltage insulating paint are also effective measures to improve the insulation level of the switch cabinet.
(7) Manage and improve the operating environment. The management and improvement of the operating environment are of great significance to the safe operation of the switchgear. Substation equipment should adhere to the principle of "cleaning every time it is stopped". Switch rooms with high humidity can adopt measures such as heating and installing dehumidifiers to improve the operating environment of the switch equipment.
