So why is the switch cabinet called a "complete set", and what cabinets does it consist of?
A switch cabinet consists of incoming lines, metering, PTs, and outgoing lines. The cabinet contains primary and secondary components, which must be purchased as a set; hence the term "complete set."
There are many more electrical tips to learn:
1. How many voltage levels are there in my country's power grid? Explain the function of each level.
It is divided into levels such as 500KV, 220KV, 110KV, 35KV, 10KV, 380V, and 220V.
Since voltage levels are related to line losses, higher voltage results in lower line losses. Therefore, the farther the power transmission distance, the higher the voltage level required.
2. Why is it necessary to discharge the voltage (10KV level)? What methods can be used to safely discharge the voltage?
When a power outage occurs, the voltage must be discharged to prevent residual charge in capacitors from injuring people.
Electric Tiger Network teaches you the safe method of voltage discharge: When opening cabinet doors or panels, never touch the busbar directly with your hands. First, use a 10KV test pen to check if there is electricity. Then, find a wire, ground one end and connect the other end to the front end of the test pen. Use the front end of the test pen to touch the busbar to discharge the voltage. Only after this is completed can you perform maintenance to ensure personal safety. Remember this.
3. What should the voltage of the 10KV test circuit be? What are the differences between domestic and international standards?
Domestic: Cabinet body 42,000 volts, break point 48,000 volts;
International: 36,000 volts
4. How to understand the meaning of the letters KYN in the switch cabinet model?
K—Metal-enclosed armored type
Y—Removable type
N—Indoor
5. What is the function of the pressure relief channel?
In the unlikely event of an explosion, a powerful blast wave will be generated. If there is a pressure relief channel on the top of the cabinet, the blast wave can escape from the top without affecting the normal operation of other parts; if there is no pressure relief channel, the blast wave will rush towards the weakest point.
6. What are the main primary components that should be installed in the switch cabinet?
Circuit breakers, contact blocks, surge arresters, zero-sequence transformers, current transformers (CT), voltage transformers (PT), high-voltage fuses, etc.
7. What is the function of a ring main unit? How does it protect the transformer?
Its function is to allow for opening and closing under load, and it protects the transformer by using a high-voltage fuse.
8. What is the national standard for air insulation distance?
The air distance between phases relative to the ground is required to be 125mm for 10KV.
9. What is the creepage distance of insulation components?
The conductive distance from the live side of a high-voltage circuit to ground, creeping along the outer surface of an insulating component, is called the creepage distance. For 10kV, this distance is required to be 235mm.
10. What method can be used to check if a vacuum switch is in a vacuum?
Tested using a withstand voltage test. The fracture surface was tested using 48,000 volts.
11. What are the functions of isolating plugs and isolating switches, and what types of cabinets are they used in?
Isolating plugs are used in handcart cabinets; isolating switches are used in stationary cabinets. Their function is to open and close the circuit without load, facilitating maintenance.
12. How many levels are there in the current rating of a high-voltage switchgear? Which level does a load switch belong to?
The load switches are categorized into large, medium, and small sizes: 3150A, 1250A, and 630A. The load switch belongs to the 630A class.
13. What is the function of a high-voltage fuse?
Used as a protective transformer, PT belongs to the category of small transformers.
14. What is the function of a live display?
It serves as an indicator, showing whether there is high voltage inside the cabinet.
15. What is the function of a program lock? Where is it installed?
The operation sequence of the cabinets is interlocked according to the prescribed procedure to prevent misoperation.
16. What is the function of a grounding switch?
The grounding switch is installed at the outgoing end to prevent reverse high-voltage power supply during maintenance and ensure personal safety.
17. What is the function of a trapdoor?
When the handcart is pulled away from the cabinet, prevent people or metal from coming into contact with the static contacts that carry high voltage.
18. Why do busbars need to be coated with rubber?
It serves as an insulator, reducing the air insulation distance and preventing short circuits caused by small animals entering the cabinet.
19. What is the function of a surge arrester? Why does each cabinet have at least one surge arrester?
Surge arresters are designed according to voltage levels and must not be installed incorrectly. Some are even designed differently depending on their intended use, such as those for power plant output, those operating on power lines, or those with capacitors.
Surge arresters are primarily used to absorb excessively high voltages generated in a section of a power line, ensuring safe operation. For example, an incoming line cabinet has a surge arrester to protect the wiring inside; external power supplies already have surge arresters installed when they enter the cabinet, so if high voltage is generated from the external power supply, it is relatively safe to enter the cabinet due to the protection provided by the surge arrester; outgoing line cabinets have surge arresters at the bottom, mainly to address the overvoltage phenomenon that may occur when the vacuum circuit breaker opens and closes, thus requiring the installation of surge arresters to replace overvoltage absorbers.
20. How to distinguish hues and their arrangement.
21. Discuss the functions of current transformers and zero-sequence transformers.
Current transformers serve a metering and protection function;
The zero-sequence current transformer serves a protective function.
22. Can a current transformer be assembled in reverse? Why?
The normal flow direction is L1→L2. If the flow direction is L2→L1, the secondary line design needs to be modified.
23. What is the difference between a VE switchgear and a ring main unit?
The VE switchgear (handcart type) is tripped by a circuit breaker, protecting the transformer for 0.2 seconds.
A fixed-type ring main unit uses a high-voltage fuse to protect the transformer for 0.02 seconds.
24. What is the function of a humidity controller?
The humidity is detected by a sensor, and if it exceeds the normal level, it is regulated by heating the heating plate. There are two operating modes: automatic and manual.
25. What is the function of a single-scheme diagram?
This represents the components assembled on the main circuit and their routing.
26. What do PT and CT stand for?
PT—Voltage Transformer, CT—Current Transformer
27. What do AC and DC stand for?
AC—Alternating current, DC—Direct current
28. What are the different types of operating mechanisms in a circuit breaker?
Manual and automatic circuit breakers use magnetic, magnetic, spring, and contactor-type switches. For example, low-voltage air switches use manual operation.
High-voltage vacuum switches use CD10 or CD17 (electromagnetic mechanism).
Use CT8 or CT19 (spring mechanism).
29. What is the function of the open circuit switchgear, and how does it protect the transformer?
Its function is vacuum arc extinguishing; it can be used for switching on and off under load, for power transmission, and for protecting transformers by cutting off current through current transformers and instruments. It can also be equipped with other components for other purposes.
30. What is the function of a vacuum circuit breaker?
It can operate under load and uses vacuum to extinguish arcs.
31. What should be the travel distance of a vacuum switch?
Stroke: 11±1 Spring Compression: 4±1 VE: 8±0.5
32. Classification of circuit breakers:
(1) Vacuum circuit breaker: Utilizes vacuum (vacuum degree below 100,000 mercury column) as the arc-extinguishing medium and main insulation medium. Both the moving and stationary contacts for arc extinguishing are sealed within the vacuum arc-extinguishing chamber. Mainly available in 10KV and 35KV voltage levels.
(2) Sulfur hexafluoride (SF6) circuit breakers: These were introduced relatively late, only becoming widely used in the early 1970s. Currently, they are advanced equipment in power systems, using SF6 gas as the arc-extinguishing medium. Their breaking capacity is about 10 times higher than ordinary circuit breakers. They have low arc voltage, short arcing time, and fewer contact burnouts, allowing for frequent operation. Disadvantages include the relatively high cost of SF6, the need for a recycling system, and a more complex structure. my country has already manufactured 220kV fully enclosed combined electrical appliances with SF6 as the main component. These appliances house the main equipment of substations, such as circuit breakers, disconnect switches, instrument transformers, surge arresters, and busbars, all within a sealed container filled with SF6 gas, significantly reducing the space required for substation operation. This is a major direction for the development of ultra-high voltage electrical appliances. They are widely used abroad.
(3) Circuit breaker: uses compressed air as the arc-extinguishing medium. It is currently the main product in the ultra-high voltage field. China produces 380KV. Foreign countries have produced 765KV.
(4) Self-produced circuit breaker: It utilizes the decomposition of a fixed medium into gas by an electric arc to extinguish the arc. It has low voltage and small breaking current. It is suitable for agricultural electronic systems.
(5) Magnetic blow circuit breaker: It uses a magnetic field to blow the electric arc into the arc-extinguishing grid.
33. Power Transformer
A transformer is an electrical device that changes voltage (current). It cannot change power.
Basic structure: It mainly consists of a closed iron core and two coils (i.e., windings) with different numbers of turns wound on the iron core and insulated from each other. In order to reduce eddy current and hysteresis losses, the iron core is made of silicon steel sheets with good magnetic permeability, coated with insulating varnish and with a thickness of 0.35-0.5mm. It forms the closed magnetic circuit in the transformer. The primary coil is called the thick coil type primary side, and the secondary coil is called the secondary coil type secondary side. A three-phase transformer group composed of three single-phase transformers is a three-phase transformer.
34. Current transformer
Basic structure: A current transformer is a converter used to expand the measurement range.
The basic structure is the same as that of a regular transformer, consisting of an iron core and primary and secondary coils wound around it. In use, the primary side is connected in series with the load of the circuit being tested, while the secondary side is connected in series with a 5A ammeter and a protective relay current coil. Because the primary side uses thick wire with only one or a few turns, its cathode is small and the voltage is very low. Therefore, it does not change the measured current after being connected in series with the circuit being tested. The secondary side has very thin wire with many turns, but its induced electromotive force is not high (only a few volts). The resistance of the secondary side ammeter and relay coil is very small. During operation, the secondary side can be considered as a short circuit. According to the transformer operating principle, the ratio of the primary current to the secondary current of the current transformer should be inversely proportional to the number of turns.
L1/L2 = W2/W1 or L1 = W2/W1L2. W2/W1 = K2 ratio.
35. Voltage transformer
Basic structure: It is a special transformer that extends the voltage range, and its basic structure is the same as that of an ordinary transformer. Therefore, the voltage relationship between the primary and secondary sides of the voltage transformer is U1/U2 = W1/W2.
36. Lightning arrester
Basic principle: When an overvoltage is transmitted, the inner and outer gaps S1 and S2 break down simultaneously. The powerful lightning current flows into the ground through the grounding device, limiting the amplitude of the overvoltage.
One of the largest lightning strikes in China: the massive fire at the Huangdao oil depot in Shandong Province on August 12, 1989. 19 people died and 78 were injured. 36,000 tons of crude oil were swept away by the fire. Direct losses amounted to hundreds of millions of yuan.
37. High-voltage insulators
The function of high-voltage insulators is to support and fix busbars and live conductors, and to ensure sufficient distance and insulation between live conductors or between conductors and the ground; the insulation of insulators is achieved by their surface shape.
Insulators of different voltage ratings not only have different effective heights, but also different surface conditions. Their surfaces are often made with raised, uneven skirts to increase their surface length.
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