Vacuum switch cabinet type and technical requirements

Overview
In actual use, the vacuum circuit breaker is often installed in a metal cabinet to form a high-voltage switch cabinet. In addition to the circuit breaker, the switchgear must be equipped with an isolating switch for isolating the circuit, a grounding switch for safety protection, a current transformer and a voltage transformer for measurement or protection, and a lightning arrester or lightning arrester for overvoltage protection. RC absorber, but also install the secondary circuit components and lines for relay protection, and lead cables or overhead lines can enter the cabinet, making the switch cabinet a power distribution device with relatively independent combined functions. A large number of switchgears are used in switch stations of power plants, substations of transmission lines, and user terminal substations that receive electrical energy.

About 10 years ago, switchgear equipped with low-oil circuit breakers almost occupied a monopoly in the country. However, with the rise of vacuum circuit breakers, low-oil switches refused to gradually take 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 even banned the continuous use of fuel-less switches in the construction of urban power grids and important users’ stations and stations. Gradually use vacuum switchgear to replace oil-less switchgear.

At present, the technical standards for vacuum switchgear in my country are:

GB3906-91 3～35kV AC metal enclosed switchgear
DL404-91 Indoor AC High Voltage Switchgear Ordering Technical Conditions

The technical parameters of the switch rejection are similar to those of the circuit breaker. It depends on the parameters of the installed circuit breaker. The only difference is that the switch rejection rated current is based on the minimum of the electrical components (such as disconnectors or current transformers) in the main circuit. Rated current value.

2. The type of high voltage vacuum switch

There are three types of high-voltage vacuum switchgear, each of which has several basic types, each with its own characteristics. See Table 1 for details.

Classification Method Basic Type Structural Features Advantages and Disadvantages
According to the circuit breaker installation method: fixed type ①The circuit breaker is installed
②Isolating switch is installed in the cabinet ①The space in the cabinet is relatively spacious, and the maintenance is easy
②Easy to manufacture and low cost
③Poor safety
Removable type The circuit breaker can be moved out of the cabinet with the removed parts (handcart). ①The circuit breaker can be moved out of the cabinet for easy replacement and maintenance.
②The isolating switch is omitted
③Compact structure
④The processing precision is higher and the price is more expensive
According to the composition of the compartment in the cabinet, semi-enclosed type. The front and sides of the cabinet are closed, and the back of the cabinet and the bus bar are not closed. ①Simple structure and low cost
②Poor safety
Box type The number of compartments is small, or the protection level of the partition is lower than IP1X ①The bus bar is also closed, and the safety is better
②The structure is more complicated and the price is slightly higher
Interval type ①The circuit breaker and the components connected at both ends have compartments
②The partition is made of non-gold back plate ①It is safer
②The structure is complicated and the price is more expensive
The armored type has the same structure as the spacer type, but the partition is made of grounded metal plates. ①The safety is the best
②The structure is more complicated and the price is higher
According to the insulating medium in the cabinet, air insulation, the insulation between the poles and the pole to the ground is guaranteed by the air gap ① stable insulation performance ② low cost
③The cabinet is larger
Composite insulation The insulation between poles and pole to ground is ensured by small air gaps and solid insulation materials. ①The cabinet is small in size, but the anti-condensation performance is not reliable enough.
②Higher cost
SF6 gas insulation All circuit components are placed in a closed container and filled with SF6 gas. ①Complex technology ②High processing precision requirements
①High price
Semi-enclosed switch cabinets (such as GG-1A(F)Z) (see Figure 1) and box-type switch cabinets (such as XGN and GGXZ) (see Figure 2) are both fixed types, while compartmentalized types (such as JYN type) and Armored switches (KYN type) are all removable (for the appearance drawings of JYN type, KYN type and other related switch cabinets, please refer to our company's product catalog and specific instructions on the website). Among the removable switch cabinets, a kind of middle cabinet has been introduced from abroad in recent years. Its removable parts (commonly known as handcarts) are smaller and lower in height. It is almost the base of the vacuum circuit breaker that adds some parts to it. Handcart.






At present, most domestic switches use air insulation. Although they are larger, they are still the mainstream due to the advantages of reliable insulation, easy maintenance, and low cost. As for composite insulation, although some models are used in the vacuum circuit breaker body, it is not widely used in switch cabinets. At most, solid insulation materials are added to the main busbar of the cabinet to increase its insulation reliability. In addition, for individual parts of the cabinet where the insulation level is slightly insufficient, glass fiber insulation boards or sleeves of insulation heat shrink tubing are often added as auxiliary measures.

In addition to air insulation and composite insulation, there are switches using SF gas insulation abroad. This switchgear contains the entire conductive circuit, including busbars, disconnectors, vacuum circuit breakers, current, voltage transformers, arresters, and even wire heads, etc., in a closed cabinet, filled with SF slightly higher than atmospheric pressure The gas, with SF as the insulating medium, is called an inflatable cabinet. The advantage is that the volume is reduced, and it has nothing to do with external environmental conditions (such as humidity, altitude, dust, fog, pollution, etc.). The disadvantage is that the technology is complex, the precision is high, the reliability of the electrical components itself is extremely demanding, and the price is abnormally high. my country has not yet launched such products on the market

3. Technical requirements for high-voltage switch rejection

In my country's GB3906 and DL404 standards for high-voltage switchgear, the technical requirements for switchgear have been specified in detail. Please refer to relevant professional books. The more important and practical items are:

1) Requirements for insulation distance between phases and relative to ground
2) Requirements for creepage distance to prevent condensation
3) Requirements for protection level:

In order to prevent the human body from approaching the high-voltage live parts of the high-voltage switchgear or touching the moving parts, as well as to prevent solid foreign objects from entering the shell, the IEC standard unifies the IP protection code. The fourth letter of the protection level indicates the protection ability of electrical equipment against rain; There is no rainproof requirement for the inner cabinet, and no grading is required, so it is indicated by "X". The outer surface can usually be IP3X, and if necessary, it can also be IP4X. The protection requirements of the inner partition can be slightly lower, ranging from IP0X to IP3X.

4) Requirements for the five-prevention interlocking function

The five-prevention interlock plays a great role in preventing misoperation, reducing man-made accidents, and improving operation reliability. The five prevention functions refer to five types of electrical misoperations that can be prevented. The five misoperation prevention functions are:

a) Prevent the wrong opening and closing of the circuit breaker.

b) Prevent closing or disconnecting the isolating switch (or isolating plug) with load.

c) Prevent live operation of the grounding switch or hanging the grounding wire-this function involves the system's artificial short-circuit grounding or personal safety, so it is set as a mandatory interlock. It requires: ①When the circuit breaker or the upper and lower isolating switches are closed, the grounding switch must not be closed; ②Only after the grounding switch is closed, the rear door and lower front door of the cabinet can be opened, and it may be temporarily clamped. Ground wire. This usually adopts a mechanical interlocking method to make a. The grounding switch can be operated only when the isolating switch is in the open position; b. Only the front and back doors can be opened and connected to the temporary grounding wire. As an auxiliary measure, a high-voltage live display device can be installed on the front and rear doors as a warning.

d) Prevent the power supply with temporary grounding wire or when the grounding switch is closed-this item is also mandatory, the purpose is to ensure the safety of the system and the person.

e) Prevent entering the electrified interval by mistake-it is also mandatory to ensure personal safety.
In summary, the implementation of the five defense functions includes non-mandatory (item a) and mandatory (item b, c, d, e); there are active defense and passive defense. Passive defense refers to the use of high-voltage live display devices, which are only indicative and not reliable enough. Active defense can be roughly divided into three categories: ① Using mechanical interlocking device, using mechanical parts to drive and generate constraints, the highest reliability (unless the parts are damaged or broken), it should be recommended first; ② Flip (plug) and mechanical The reliability of the program pin is slightly lower, because the lock and the key are not absolutely one-to-one correspondence; ③The electrical interlock is used, and the reliability is worse, because the electromagnetic lock and the wire may be damaged, and the power supply is also required (required Separate from the relay 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 switch cabinets, high-voltage live display devices are often used. When the main circuit has high voltage, it outputs a low voltage signal through the principle of capacitor voltage divider, ignites a neon lamp, and sends out a reminder with a light signal (the low voltage signal can also be transformed to control the electromagnetic lock to form a mandatory lock). Most of the sensing elements for charged display are hidden in insulators or porcelain bottles, the latter can also play a supporting role. The operator can understand which section of the main circuit is running with power by observing the indicator light. This reminder signal is even more important when maintaining or testing the components in the cabinet. The price of the high-voltage live display device itself is not high, about 600 to 700 yuan per set (three-phase), which accounts for a very small proportion of the whole cabinet price, and its effect on personal safety is not small. Although neon bulbs are sometimes damaged, the possibility of damage at the same time is extremely high. Therefore, it is recommended that the design institute design the main circuit connection, draw more high-voltage live display devices, if the main connection diagram is drawn at the end, the manufacturer Generally, it is not proactively added, because it is not included in the project pricing, which will inevitably reduce production profits.

There should generally be a live display device on the back of the switch cabinet. In addition, for the bus from the upper end of the lower isolating switch to the lower terminal of the circuit breaker in the main circuit of the incoming cabinet, it is best to install a live display if possible. Because this section of bus ① is where the current transformer is installed. The current transformer may be damaged, causing grounding. ③ Directly reflect the isolation state of the lower isolating switch. If the transmission part of the isolating switch fails, it may happen that the operating handle is hit to the opening position, but the switch of the isolating switch is not disconnected. ③When repairing or testing circuit breakers or current transformers and exhausting temporary grounding, grounding is often implemented on this section of the bus. One more set of live display means one more monitoring method.

6) Circumstances where power failure is not allowed on the main incoming line

I have encountered a situation where the main incoming line of the power supply in the distribution room is not allowed to cut out. The reason is that the transformer of the substation is three-winding, one high-voltage power supply incoming line (with circuit breaker), one for the distribution room, and the other for the substation. Even if the power distribution room fails, the station power cannot be stopped, that is, the high-voltage side circuit breaker cannot be tripped. For this special situation, the switch cabinet of the power distribution room must be declared to the manufacturer in advance when ordering, otherwise the manufacturer will make it according to conventional equipment, and it will be brought during the maintenance and testing of the main input cash cabinet breaker or current transformer. Considerable danger.

The solution is to isolate the entire lower isolating switch. If it is the lower cable entry method, another cabinet should be added. This cabinet is only equipped with the lower isolating switch (ie the incoming line isolating switch), and the outgoing line can be led to the main incoming circuit breaker cabinet by busbars or high-voltage cables. Of course, the interlocking of the two cabinets needs to be solved to meet the function of preventing errors (program lock can be considered). If the overhead line enters from the rear cabinet, the incoming line (lower) isolating switch can be moved into the rear cabinet, and the iron plate at the rear of the main cabinet will seal and isolate, but the depth of the rear rejection must be deepened at this time.

7) Long-term heating current carrying capacity

Since the electrical components of the primary circuit are in a sealed cabinet (except for semi-closed cabinets) running at the same time, and each emits heat, the temperature of the air in the cabinet must be higher than the ambient temperature outside the cabinet, which makes it difficult for electrical components to dissipate heat and increase the temperature rise. Moreover, according to the requirements of the DL404-91 standard, the switchgear should pass a temperature rise test of 110% of the rated current. Therefore, the electrical components in the cabinet, in addition to the current transformer for measurement and protection needs, other components such as circuit breakers, disconnectors (or disconnectors), bushings, etc., should be selected products that are one grade larger than the long-term rated current specifications. For example, when the cabinet is 630A, choose 800A, 800A choose 1000A, and 1000A choose 1250A electrical components.

8) Busbar contact surface treatment

There are many conductive connection surfaces in the switch cabinet, and there are the connection of the outlet end of the electrical component to the bus bar, and the connection of the bus bar to the bus bar. When there is current, the conductive connection surface will heat up. The amount of heat generated is related to the current density and is also closely related to the physical state of the contact surface.

From a microscopic point of view, even the finely processed contact surface is uneven. The conductive effect is achieved by the touch of several tips, while the non-tip parts are left with air gaps. Moisture in the humid air and dust in the atmosphere will seep into the air gap. Under the action of heat and moisture, the contact surface will be oxidized and contaminated, the contact state will become worse, the contact resistance will rise, and the heat will also increase. Therefore, the electrical connection surface must be protected. The easiest way is to apply neutral petroleum jelly or conductive paste, which occupies the gap between the connectors and prevents air. Moisture and dust enter, reducing the degree of oxidation. The better way is to tin the contact surface. The tin layer is soft and stretches under bolt pressure, increasing the actual electrical contact area, and tin is not easily oxidized. In addition, under the protection of the tin layer, the allowable heating temperature of the joint is increased from 90°C to 105°C, which is quite beneficial.

9) Dynamic stability

The main bus and down conductors in the switchgear will generate larger electromotive force when the fault current flows, and the magnitude of the electromotive force is proportional to the square of the short-circuit current. There are two types of electric power: one is generated between the three-phase bus, the force is trying to expand and reduce the distance between the three-phase bus, the direction is alternating, and the frequency is twice the work rate; the other is generated on the same-phase bus, As long as the bus bar is not in a straight line, each segment has a force on each other, and the direction of the force is to try to make the bus bar "straight."

In order to resist electric power, the bus bar must have sufficient dynamic stability, which must be supported by supporting insulators. Generally speaking, the terminal board of the isolating switch should not be used as the main support point, but only as an auxiliary, especially when its parameters are low. The distance between two insulators should normally not exceed 800mm. Supporting insulators can sometimes be used as high-voltage live display insulators.

10) There should be isolation between the switch cabinets

In substations or switch stations, many switchgear cabinets are often arranged side by side into a row, and the main bus bars are used to connect the cabinets to form a power distribution network. In the past, the busbar compartment of the old switchgear was directly connected without any partition or obstruction. In this way, when one of the switchgears has a flashover failure, the arc burns along the bus channel of the Unicom while burning the refusal, burning down a series of switchgears one after another, causing the whole station to be paralyzed and blackout, causing heavy losses. The phenomenon is commonly known as "burning company".

In order to prevent "fire burning", in addition to semi-enclosed (GG-1A(F)) fixed cabinets after the 1990s, all other types of box, partition and armored switchgear are adopted between the cabinet and the cabinet. Separate measures, each cabinet is sealed with grounded metal plates, and the main busbars are connected through wall bushings. When a cabinet has a fault in the cabinet, the accidental arc cannot be arbitrarily spread longitudinally after moving to the busbar room. It is basically limited to the cabinet, and the impact on adjacent cabinets is greatly reduced, and it will not burn down in a series, which reduces the loss. When connecting two cabinets with a bus duct, remember to install partitions and bushings.

11) Installation height of cable head

Most of the cables that enter the cabinet from the bottom cross the bottom plate and then branch. In addition, after the phase separation, the outer edge also has insulation requirements. Therefore, the installation point of the cable head must have a certain height, usually greater than 500mm. When it is too low, it will bring difficulties to the production, construction and installation of the cable head. If it is necessary to install a zero-sequence current transformer, the height will be insufficient.

When two or more cables are connected in parallel, the split-phase cable cores cannot be interleaved with each other, and the insulation distance between the different-phase cable cores must be paid attention to.

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12) Switchgear should consider or have measures to reduce internal failures

The common internal faults, causes and preventive measures of the switchgear are shown in the following table:

Locations prone to internal failures Possible causes of internal failures Examples of measures
The circuit breaker is poorly maintained, the mechanism screws are loose, and the insulation margin is insufficient. Regularly maintain according to the regulations, formulate regulations, and add insulation partitions
Isolation switch, load switch, grounding switch Misoperation, poor contact, or serious heating. Add interlock and formulate regulations; carefully grind contacts, adjust carefully, and apply conductive paste
Transformer ferromagnetic resonance Adopt suitable circuit design to avoid this kind of electric effect
The cable compartment is not properly designed, choose the right size
Improper layout to avoid cable cross connection, installation quality check
Damage of solid or liquid insulation (defect or loss) on-site quality inspection, insulation withstand voltage test
Screw connection surface and contact surface electrochemical corrosion, improper assembly Use anti-corrosion ballast layer or conductive paste to check the assembly quality
Five-prevention interlock failure, loose position, damaged parts Trial operation during maintenance and routine inspection, analyze the cause, and replace parts
All parts of the staff’s mistakes Use barriers to restrict the access of personnel, live parts are wrapped with insulation, regulations are drawn up, and warning signs are put up
Aging under the action of electric field
Pollution, moisture, dust, and entry of small animals, etc. Take measures to improve operating environmental conditions
Overvoltage lightning protection, on-site insulation withstand voltage test, plus lightning arrester or RC absorption device
Flashover discharge, the cabinet has a pressure release window.