High-voltage switch and high-voltage switch cabinet

1.1 Working principle and common types of high-voltage switch

1.1.1 Working principle of different types of high-voltage switch

Oil circuit breaker is a kind of high-voltage switch widely used in the early days. Its working principle is based on the gas produced by the decomposition of insulating oil under the high temperature of the arc to extinguish the arc. During the opening process, the moving and static contacts are separated to generate an arc. The arc causes the surrounding insulating oil to decompose rapidly, forming a large amount of hydrogen, acetylene and other gases. These gases form high pressure in the arc extinguishing chamber, pushing the oil flow to blow towards the arc, causing the arc to cool rapidly, stretch and finally extinguish.

The insulating oil of the oil-rich circuit breaker is not only used for arc extinguishing, but also as an insulating medium between the live part and the grounded shell. Its structure is relatively simple, but it is bulky and uses a lot of oil, which poses a risk of fire and explosion. The oil-less circuit breaker reduces the amount of insulating oil used, and only uses insulating oil as the arc extinguishing medium. The insulation between the live part and the grounded shell uses other materials such as insulators. Compared with the oil-rich circuit breaker, its volume is smaller and the safety is improved, but the insulating oil still needs to be regularly inspected and maintained.

The vacuum circuit breaker uses a high vacuum environment to extinguish the arc. The arc extinguishing chamber is made of insulating materials such as ceramic or glass, and the interior is evacuated to a high vacuum state, and the moving and static contacts are sealed in the arc extinguishing chamber. When the circuit breaker is opened, the arc generated by the separation of the contacts spreads rapidly in the vacuum. Since there are almost no gas molecules in the vacuum, the arc cannot be maintained and is quickly extinguished. The vacuum circuit breaker has the advantages of small size, light weight, fast action, short arcing time, long life, and no explosion hazard. It is suitable for occasions with frequent operations, such as the distribution room of industrial enterprises and the switchgear of urban distribution networks.

SF6 circuit breakers use SF6 gas as an insulating medium and arc extinguishing medium. SF6 gas has excellent insulation properties and arc extinguishing ability, and its insulation strength is about 2.5-3 times that of air. When the circuit breaker is opened, the high pressure of SF6 gas is used to blow out the arc. SF6 circuit breakers usually use compressed air, self-energy or mixed arc extinguishing methods.

The compressed gas SF6 circuit breaker compresses SF6 gas through a piston to form a high-speed airflow blowing toward the arc; the self-energy SF6 circuit breaker uses the energy of the arc itself to heat and compress SF6 gas to generate an arc-blowing airflow; the hybrid SF6 circuit breaker combines the advantages of compressed gas and self-energy. SF6 circuit breakers have the characteristics of strong breaking capacity, high break voltage, convenient operation and maintenance, and long maintenance cycle. They are widely used in high-voltage and ultra-high-voltage power systems.

1.1.2 Applicable scenarios and performance differences

Different types of high-voltage switches have significant differences in applicable scenarios and performance. Due to the risk of fire and explosion and the large maintenance workload, oil circuit breakers are rarely used in newly built power systems. Only some of them are still in operation in some old substations. However, in some special occasions, such as small industrial enterprises or rural power grids that do not have high requirements for fire protection and are more sensitive to equipment costs, a small number of oil circuit breakers may still be seen.

Vacuum circuit breakers are widely used in medium voltage power systems (3 - 35kV) due to their small size, fast action and long life. Especially in places where frequent operation is required, such as the power distribution room of the factory workshop, the ring network cabinet and switchgear of the urban distribution network, vacuum circuit breakers are the preferred high-voltage switchgear. In some places with high environmental requirements, such as hospitals, shopping malls, office buildings, etc., the explosion-free characteristics of vacuum circuit breakers give them obvious advantages.

SF6 circuit breakers are mainly used in high-voltage and ultra-high-voltage power systems (110kV and above). At these voltage levels, the insulation performance and breaking capacity of the switchgear are extremely high. SF6 circuit breakers can meet the requirements of the system with their excellent electrical performance and arc extinguishing ability.

In key power facilities such as booster stations in large power plants and substations of high-voltage transmission lines, SF6 circuit breakers are the core high-voltage switchgear. Compared with vacuum circuit breakers, SF6 circuit breakers have higher breaking voltage and can withstand larger short-circuit currents, but their equipment cost is higher and the management and maintenance requirements for SF6 gas are also stricter. If SF6 gas leaks, it may have a certain impact on the environment, so it is necessary to equip professional gas detection and recovery equipment.

1.2 Structural composition and functional division of high-voltage switchgear

1.2.1 Cabinet structure and material properties

The cabinet structure design of high-voltage switchgear must take into account the requirements of electrical performance, mechanical performance, and protection performance. Common cabinet structure forms include metal armored, interval and box types. The metal armored switchgear uses a metal shell to completely enclose each functional unit. It has good mechanical strength and protection performance, can effectively prevent the spread of internal fault arcs, and ensure the safety of personnel and equipment. The interval switchgear separates different functional areas through partitions. Although the protection performance is slightly inferior to the metal armored type, it is more flexible in structural design and convenient for equipment installation and maintenance. The box-type switchgear has a relatively simple structure and is usually used in places with high space requirements and relatively low protection level requirements.

The selection of cabinet materials has a crucial impact on the performance of high-voltage switchgear. At present, the cabinet of high-voltage switchgear is mainly made of high-quality cold-rolled steel plates or stainless steel plates. Cold-rolled steel plates have good mechanical processing properties and conductivity, and are relatively low in cost. After surface treatment, they can effectively improve their corrosion resistance.

During the cabinet manufacturing process, cold-rolled steel plates are precisely processed by CNC equipment to ensure the dimensional accuracy and structural strength of the cabinet. Stainless steel plates have stronger corrosion resistance and are especially suitable for humid and corrosive environments, such as seaside substations, chemical companies and other places. Its surface does not require additional anti-corrosion treatment and can maintain good appearance and performance for a long time. However, the cost of stainless steel plates is high and the processing difficulty is relatively large, which to a certain extent limits its wide application.

In order to improve the insulation performance of the cabinet, some high-voltage switchgears will also use insulating materials for isolation and protection inside the cabinet. Common insulating materials include epoxy resin, polyvinyl chloride, etc. Epoxy resin has excellent electrical insulation properties, mechanical strength and chemical corrosion resistance, and is often used to make insulating partitions, insulators and other components. Polyvinyl chloride has good insulation properties and flexibility, and can be used to make cable sheaths, insulating tapes, etc. The rational application of these insulating materials can effectively improve the insulation level of the cabinet and prevent electrical accidents.

1.2.2 The role and relationship of each functional partition

The high-voltage switch cabinet is usually composed of multiple functional partitions such as the circuit breaker room, busbar room, cable room, relay instrument room, etc. Each functional partition has its own unique role and is closely related to each other to jointly ensure the normal operation of the high-voltage switch cabinet.

The circuit breaker room is the core part of the high-voltage switch cabinet, mainly used to install circuit breakers and their operating mechanisms. As a key device for controlling and protecting circuits, the circuit breaker can connect and disconnect the load current under normal circumstances, and quickly cut off the short-circuit current in the event of a fault to protect the safety of the power system.

The design of the circuit breaker room must meet the installation, operation and maintenance requirements of the circuit breaker, and is usually equipped with a special guide rail to facilitate the entry and exit and operation of the circuit breaker trolley. At the same time, the circuit breaker room should also have good ventilation and heat dissipation conditions to ensure that the heat generated by the circuit breaker during operation can be dissipated in time to prevent overheating and damage to the equipment. In addition, in order to ensure the safety of operators, the circuit breaker room is also equipped with a complete interlocking device to prevent misoperation.

The busbar room is used to install and arrange the busbar. The busbar is an important carrier of power transmission and is responsible for transmitting electrical energy from the power supply side to each load side. The busbar room usually adopts a closed structure to improve the insulation performance and safety of the busbar. The material of the busbar is generally copper or aluminum. The copper busbar has good conductivity and mechanical strength, but the cost is high; the aluminum busbar has a lower cost and a lighter weight, and is widely used in some cost-sensitive occasions. The design of the busbar room needs to consider factors such as the current carrying capacity and dynamic thermal stability of the busbar to ensure that the busbar can work reliably under normal operation and short-circuit fault conditions. At the same time, the busbar room is isolated from other functional partitions by insulating partitions to prevent the spread of internal fault arcs.

The cable room is mainly used to connect cables and install cable terminals, as well as to arrange current transformers, grounding switches and other equipment. The space in the cable room should be spacious enough to facilitate the laying and maintenance of cables. Current transformers are used to measure the current in the circuit and provide current signals for relay protection devices and measuring instruments; grounding switches are used to reliably ground the circuit when repairing equipment to ensure the safety of maintenance personnel. The bottom of the cable room is usually equipped with ventilation holes and drainage facilities to prevent the cable from getting damp and accumulating water, which affects its insulation performance. In addition, the cable room is also isolated from the circuit breaker room and the busbar room by insulating partitions to ensure the electrical independence between the functional partitions.

The relay instrument room is used to install various relays, instruments, control switches and secondary circuit equipment. Relays are important components for realizing power system protection, control and signal transmission. They can issue trip signals or control instructions in time according to changes in parameters such as current and voltage in the circuit. Instruments are used to monitor various electrical parameters in the circuit, such as voltage, current, power, etc., to provide real-time equipment operation information for operators. Control switches are used to realize manual operation of equipment such as circuit breakers and disconnectors. The design of the relay instrument room needs to consider factors such as the rationality of equipment layout, wiring convenience and anti-interference ability to ensure the reliable operation of the secondary circuit. The relay instrument room is connected to other functional areas through cables to achieve the transmission and control of electrical signals.

The functional areas of the high-voltage switchgear are interrelated and mutually influential. The circuit breaker in the circuit breaker room is connected to the power supply side and the load side through the busbar to achieve the distribution and control of electric energy; the busbar in the busbar room transmits electric energy to each functional area to provide power for the equipment; the cable in the cable room connects the high-voltage switchgear with the external power system to achieve the transmission of electric energy; the relay instrument room monitors and controls the equipment in other functional areas to ensure the safe and stable operation of the high-voltage switchgear. In the design and operation of the high-voltage switchgear, it is necessary to fully consider the relationship between the functional areas to ensure the overall performance and reliability of the equipment.