Basic knowledge of the hottest vacuum switchgear

Basic knowledge of vacuum switchgear

1 Overview

in practical use, vacuum circuit breakers are often installed in metal cabinets to form high-voltage switchgear. In addition to the circuit breaker, the switchgear should also be installed with disconnectors for isolating circuits, grounding switches for safety protection, current transformers and voltage transformers for measurement or protection, lightning arresters or RC absorbers for overvoltage protection, as well as secondary circuit components and lines for relay protection. The incoming cables or overhead lines can enter the cabinet, Make the switch cabinet a power distribution device with relatively independent combined functions. All kinds of switchgear are widely used in switchyards of power plants, substations of transmission lines, and user terminal substations that receive electric energy

about 10 years ago, the switchgear equipped with low oil circuit breaker almost occupied a monopoly position in the country, but with the rise of vacuum circuit breaker, low oil switch refused to gradually retreat to the second place. Since the requirement of "oil-free transformation" was put forward in 1993, this trend has been further promoted. Some provinces and cities have even banned the continued use of oil-free switches in the construction of urban electricity and stations of important users, while the old stations have gradually replaced the oil-free switch cabinets with vacuum switch cabinets

at present, the technical standards for vacuum switchgear in China include:

GB 3 ~ 35kV AC metal enclosed switchgear

DL indoor AC high voltage switchgear ordering technical conditions

the technical parameters of the switch are similar to those of the circuit breaker, which depends on the parameters of the installed circuit breaker, the only difference is, The rated current of the switch is taken according to the minimum rated current of each electrical component (such as disconnector, or current transformer) in the main circuit

２. Types of high voltage vacuum switchgear

high voltage vacuum switchgear can be classified in three ways, each of which has several basic types, each of which has its own characteristics. See Table 1 for details

classification method basic type structural characteristics advantages and disadvantages according to the installation method of circuit breaker fixed ① circuit breaker fixed installation

② disconnector is installed in the cabinet ① the space in the cabinet is spacious, easy to repair

② easy to manufacture, low cost

③ poor safety removable circuit breaker can be moved out of the cabinet with removable parts (handcart) ① the circuit breaker is moved out of the cabinet, replaced Convenient maintenance

② omit the disconnector

③ compact structure

④ higher processing accuracy and price. The front and side of the semi enclosed cabinet are closed according to the composition of the compartments in the cabinet, and the back of the cabinet and the bus are not closed ① simple structure, low cost

② poor safety. The number of box compartments is small, or the protection grade of the partition is lower than ip1x ① the bus is also closed, with better safety

② complex structure, The price is slightly higher. ① the circuit breaker and the components connected at both ends have compartments

② the clapboard is made of non gold back plate ① the safety is better

② the structure is complex and the price is more expensive. The armored structure is the same as the spacer type, but the clapboard is made of grounding metal plate ① the safety is the best

② the structure is more complex, The price is higher. According to the insulating medium in the cabinet, the air insulation between poles and pole to ground insulation is guaranteed by the air gap ① the insulation performance is stable ② the cost is low

③ the cabinet body is larger, and the composite insulation between poles and pole to ground insulation is guaranteed by the smaller air gap and solid insulation materials ① the cabinet body is small, but the anti condensation performance is not reliable

② the cost is higher, and all circuit components of SF6 gas insulation are placed in closed containers, Filling SF6 gas ① complex technology ② high processing accuracy requirements

① high price

semi enclosed switchgear (such as GG-1A (f) z) (see Figure 1) and box type switchgear (such as XGN and ggxz) (see Figure 2) belong to fixed type, while interval type (such as jyn type) and armored type switchgear (KYN type) belong to movable type (for the appearance drawings of jyn type, KYN type and other relevant switchgear, please refer to our product samples and the specific instructions on the station). In the removable switchgear, a kind of so-called mid mounted cabinet has been introduced from abroad in recent years. Its removable parts (commonly known as handcart) are smaller and lower in height, almost adding some parts to the base of the vacuum circuit breaker as a handcart

at present, most domestic switches are in the form of air insulation. Although the body of the switch is larger, it is still the mainstream because of its reliable insulation performance, convenient maintenance and low cost. As for composite insulation, although some models are used in the vacuum circuit breaker body, they are not widely used in the switchgear. At most, solid insulation materials are added to the main bus of the cabinet to increase its insulation reliability. In addition, if the insulation level of some parts in the cabinet is slightly insufficient, glass fiber insulation board or insulating heat shrinkable sleeve are often added as auxiliary measures

in addition to air insulation and composite insulation, there are switches with sf gas insulation abroad. This kind of switch cabinet encloses the whole conductive circuit, including busbar, disconnector, vacuum circuit breaker, current, voltage transformer, lightning arrester, even wire head and other components in a closed cabinet, which is filled with sf gas slightly higher than the atmospheric pressure, and SF is used as the insulating medium, which is called an inflatable cabinet. Its advantage is that the volume is reduced, which has nothing to do with the external environmental conditions (such as humidity, altitude, dust, fog, pollution, etc.). Its disadvantage is that the technology is complex, the accuracy is high, the reliability of electrical components is very high, and the price is extremely high. China has not yet launched such products on the market

3 Technical requirements for high-voltage switchgear

the technical requirements for switchgear have been specified in GB3906 and dl404 standards of high-voltage switchgear in China. Please refer to relevant professional books, The more important and practical items are:

1) requirements for the insulation distance between phases and phase to ground

2) requirements for the creepage distance against condensation

3) requirements for the protection grade:

in order to prevent human bodies from approaching the high-voltage live parts or touching the moving parts of the high-voltage switchgear, and to prevent solid foreign matters from entering the shell, IEC standards unify the IP protection code, The fourth letter of the protection grade indicates the protection ability of electrical equipment against rainwater; Since there is no rain proof requirement for indoor cabinets and there is no need to grade them, they are all represented by "X". The outer surface can usually achieve IP3X level, and can also achieve IP4X level when necessary. The protection requirements of inner diaphragm can be slightly lower, from ip0x to IP3X

4) requirements for five prevention interlocking function

five prevention interlocking plays a great role in preventing misoperation, reducing human accidents and improving operation reliability. Five prevention functions refer to the prevention of five types of electrical misoperation, which are:

A) the prevention of false opening and closing of circuit breakers

b) prevent the disconnector (or isolation plug) from being closed or disconnected with load

c) prevent the live operation of the grounding switch or hanging the grounding wire -- this function involves the artificial short-circuit grounding of the system or personal safety, so it is defined as mandatory interlocking. It requires: ① when the circuit breaker or the upper and lower disconnectors are in the on state, the grounding switch must not be operated; ② Only when the grounding switch is closed, the rear door and lower front door of the cabinet can be opened, and it is possible to hang and clamp the temporary grounding wire. This usually adopts mechanical interlocking mode, so that A. the grounding switch can operate the console only when the disconnector is in the open position; b. Only the front and rear doors can be opened and connected with temporary grounding wires. As an auxiliary measure, high voltage live display devices can be added on the front and rear doors for warning

d) prevent power transmission when there is a temporary grounding wire or grounding switch on and off -- this is also mandatory to ensure the safety of the system and personnel

e) it is also mandatory to prevent entering the live space by mistake to ensure personal safety

to sum up, there are non mandatory (a) and mandatory (B, C, D, e) in the implementation of the five prevention functions; There are active defense and passive defense. Passive defense refers to the use of high-voltage live display devices, which are only suggestive and unreliable. Active defense can be roughly divided into three categories: ① mechanical interlocking devices are used, and mechanical parts are used to drive and generate constraints. The reliability is the highest (unless the parts are damaged or broken), which should be preferred; ② Using flip (plug) and mechanical program pin, the reliability is slightly inferior, because the lock and key are not absolutely shiny pair-to-one correspondence; ③ With electrical interlocking, the reliability is worse, because the electromagnetic lock and wire may be damaged, and the power supply is also required (it must be separated from the relay protection circuit power supply), but the advantage is that it can be transmitted over a long distance

5) high voltage live display device

in various types of switchgear, high voltage live display devices are often used. When the main circuit has high voltage, it outputs a low voltage signal through the capacitive voltage division principle, lights the neon lamp, and sends a prompt with the light signal (it can also change the low voltage signal to control the electromagnetic lock, forming a mandatory locking). The inductive elements of live display are mostly hidden in insulators or porcelain bottles, which can also play a supporting role. The operator can know which section of the main circuit is in live operation by observing the indicator light. This prompt signal is more important when maintaining or testing components in the cabinet. The price of high-voltage live display device itself is not high, about 600 or 700 yuan a set (three-phase), which accounts for a small proportion of the price of the whole cabinet, and plays a great role in ensuring personal safety. Although neon bubbles are sometimes damaged, the possibility of three-phase damage at the same time is extremely high. Therefore, it is suggested that the design institute should appropriately draw more high-voltage live display devices when designing the main circuit connection. If the main connection diagram is drawn at the end, the manufacturer will generally not take the initiative to add it, because it is not included in the project pricing, which will inevitably reduce the production profit

there is usually a live display device on the back of the switchgear. In addition, for the busbar in the main circuit of the incoming cabinet from the upper end of the lower disconnector to the lower terminal of the circuit breaker, it is best to install a live display if possible. Because this section of bus ① is the location where the current transformer is installed. The current transformer may be damaged, causing grounding. ③ Directly reflect the isolation state of the lower disconnector. If the transmission part of the disconnector fails, it may occur that the joystick is turned to the opening position and the knife switch of the disconnector is not disconnected. ③ When the circuit breaker or current transformer is overhauled or tested and the temporary ground wire is completely connected, the grounding is often implemented on this section of bus. One more set of live display means one more monitoring means

6) the main incoming line is not allowed to be powered off

there has been a case that the main incoming line of the power supply in the distribution room is not allowed to be powered off, because the transformer in the substation is three winding, one high-voltage power incoming line (with circuit breaker), one power supply and distribution room, and the other power for the substation. Even if the distribution room fails, the station power cannot be stopped, that is, the circuit breaker at the high-voltage side cannot trip. For this special case, the switch cabinet in the distribution room needs to be declared to the manufacturer in advance when ordering, otherwise the manufacturer will produce it according to conventional equipment, which will bring considerable danger during the maintenance and testing of the circuit breaker or current transformer of the main cash in cabinet

the solution is to isolate the lower disconnector. If it is the way of lower cable incoming, another cabinet should be added. This cabinet is only installed with the lower disconnector (i.e. incoming disconnector), and its outgoing line can be led to the main incoming circuit breaker cabinet with busbar or high-voltage cable. Of course, it is also necessary to solve the interlocking of the two cabinets to meet the error prevention function (program lock can be considered). If the overhead line enters from the rear cabinet, the incoming line (lower) disconnector can be completely moved in the rear cabinet, and the LED