1. How many voltage levels are there in my country, and what are their functions?
They are categorized as 500kV, 220kV, 110kV, 35kV, 10kV, 380V, and 220V. Since voltage is related to line loss, the higher the voltage, the lower the line loss. Therefore, the longer the distance the power needs to be transmitted, the higher the voltage level required.
2. How can I safely remove the voltage?
During a power outage, it is necessary to remove the voltage to prevent residual charge in capacitors and possible injury. Dian Laohu.com teaches you how to safely remove the voltage: When opening a cabinet door or panel, do not touch the busbar directly with your hands. First, use a 10kV tester to check for voltage. Then, connect a wire with one end grounded and the other to the tip of the tester. Touch the tester tip to the busbar to discharge the voltage. Only after this is complete can repairs be performed to ensure personal safety. Remember this.
3. What should the voltage of the 10kV test line be?
Domestic: 42,000V cabinet, 48,000V breakout; International: 36,000V
4. How do I understand the meaning of the KYN letters in switchgear model numbers?
K—Metal-enclosed armored type, Y—Removable type, N—Indoor type
5. What is the function of a pressure relief duct?
If a cabinet explodes, a strong air wave will be generated. If there is a pressure relief duct on the cabinet roof, the air wave can be discharged through the top without affecting the normal operation of other components. If there is no pressure relief duct, the air wave will rush to the weakest point.
6. What are the main primary components that should be installed in a switchgear?
Circuit breakers, contact blocks, lightning arresters, zero-sequence transformers, current transformers (CTs), voltage transformers (PTs), high-voltage fuses, etc.
7. What is the function of a ring main unit? How does it protect the transformer?
It is designed to open and close the switch under load, and it uses high-voltage fuses to protect the transformer.
8. What is the national standard for air insulation distance?
The air distance between phases relative to ground is 125mm for 10kV.
9. What is creepage distance?
The creepage distance is the conductive distance from the high-voltage live side to ground, creeping along the surface of the insulation. The creepage distance for 10kV is 235mm.
10. How is a vacuum switch checked for vacuum?
Tested by withstand voltage test. A 48,000-volt test is used for the break.
11. What is the function of an isolating switch, and in which type of cabinet is it used?
Isolating plugs are used in trolley cabinets; isolating switches are used in fixed cabinets. Both function to open and close the circuit breaker without load, facilitating maintenance.
12. What are the current ratings of high-voltage cabinets? To which rating do load switches belong?
They are classified into three levels: large, medium, and small, namely: 3150A, 1250A, and 630A. Load switches belong to the 630A rating.
13. What is the function of a high-voltage fuse?
Used to protect transformers, PTs are small transformers.
14. What is the function of a live indicator?
It indicates whether high voltage is present in the cabinet.
15. What is the function of a program lock? Where is it installed?
It interlocks the operating sequence of cabinets according to a prescribed procedure to prevent incorrect operation.
16. What is the function of a grounding switch?
A grounding switch is installed at the outgoing line end to prevent reverse flow of high voltage during maintenance, ensuring personal safety.
17. What is the function of a valve?
When a trolley is pulled away from the cabinet, it prevents people or metal from coming into contact with the high-voltage static contacts.
18. Why are busbars coated with rubber?
It provides insulation, reducing the air insulation distance and preventing small animals from entering the cabinet and causing short-circuit accidents.
19. What is the function of a lightning arrester?
Lightning arresters are designed according to voltage levels and must not be installed incorrectly. Some are designed differently depending on the intended use. For example, power plants output electricity, operating lines, and capacitors.
Lightning arresters primarily absorb excessive voltages generated at certain points in a line, ensuring safe operation.
For example, incoming line cabinets have a lightning arrester installed to protect the lines within the cabinet. External power sources are already equipped with a lightning arrester when they enter the cabinet, so if they generate high voltages, they are protected by the lightning arrester, ensuring safe entry.
Outgoing line cabinets have a lightning arrester at the bottom to prevent overvoltages generated when vacuum circuit breakers are opened and closed. Therefore, lightning arresters must be installed instead of surge absorbers.
20. How to distinguish phase colors and phase arrangement?
Yellow, green, red, blue, alternating yellow and green
A, B, C, neutral
Left, center, right, far left
Back, center, front, far front
Top, center, bottom, bottom
21. Explain the functions of current transformers and zero-sequence transformers.
Current transformers provide metering and protection; zero-sequence transformers provide protection.
22. Can current transformers be installed in reverse?
The normal flow direction is L1 → L2. If the flow goes from L2 → L1, the secondary line design needs to be modified.
23. What is the difference between a VE cabinet and a ring main unit?
A VE cabinet (hand-operated type) uses a circuit breaker to disconnect and protect the transformer for 0.2 seconds. A ring main unit (fixed type) uses a high-voltage fuse to protect the transformer for 0.02 seconds.
24. What is the function of a hygrostat?
It detects humidity using a sensor probe. If the humidity exceeds the normal value, the heating plate generates heat to adjust the humidity. There are two operating modes: automatic and manual.
25. What is the function of the primary circuit diagram?
It represents the components and their routing on the main circuit.
26. What do the terms PT and CT stand for?
PT—voltage transformer, CT—current transformer.
27. What do the terms AC and DC stand for?
AC—alternating current, DC—direct current.
28. How many operating mechanisms are there for circuit breakers?
Manual, automatic, magnetic, spring, and contactor-operated. For example, low-voltage air switches use manual operation, while high-voltage vacuum switches use CD10 or CD17 (electromagnetic mechanism) and CT8 or CT19 (spring mechanism).
29. How does an empty switchgear protect the transformer?
Its function is to extinguish arcs with a vacuum. It can open and close the switch under load for power transmission. It uses current transformers and instruments to cut off the current to protect the transformer. It can also be equipped with other components for other purposes.
30. What is the function of a vacuum circuit breaker?
It can open and close the switch with load and use vacuum arc extinguishing.
31. What should be the stroke of the vacuum switch?
Stroke: 11±1 Spring compression: 4±1 VE: 8±0.5
32. Classification of circuit breakers:
(1) Vacuum circuit breaker: It uses vacuum (vacuum degree is less than 100000 Hg) as the arc extinguishing medium and the main insulating medium. The moving and static contacts of the arc extinguishing are sealed in the vacuum arc extinguishing chamber. The main voltage levels are 10VK and 35KV.
(2) Sulfur hexafluoride circuit breaker: It came out relatively late. It was not widely used until the early 1970s. It is now an advanced equipment in the power system and uses SF6 gas as the arc extinguishing medium.
The breaking capacity is about 10 times higher than that of ordinary circuit breakers. The arc voltage is low. The arcing time is short and the contacts are less burned. It can be operated frequently. Disadvantages: SF6 is expensive. A recovery device is required and the structure is more complicated.
my country has manufactured 220KV fully enclosed combined electrical appliances with SF6 as the main component. The main equipment of the substation, such as circuit breakers, disconnectors, transformers, lightning arresters, busbars, etc., are all installed in sealed containers filled with SF6 gas, which greatly reduces the space occupied by the substation. It is the main development direction of ultra-high voltage electrical appliances. It has been widely used abroad.
(3) Circuit breaker: Compressed air is used 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: The fixed medium is decomposed into gas by the arc to blow the arc to extinguish the arc. The voltage is low and the interruption current is small. It is suitable for agricultural electronic systems.
(5) Magnetic blow circuit breaker: The magnetic field is used to blow the arc into the arc extinguishing grid.
33. Power transformer
A transformer is an electrical device that converts voltage (current). It cannot convert power.
Basic Structure: It primarily consists of a closed iron core and two insulated coils (i.e., windings) wound around it with different numbers of turns. To reduce eddy current and hysteresis losses, the core is constructed from laminated silicon steel sheets coated with insulating varnish and 0.35-0.5mm thick for excellent magnetic conductivity.
This forms the closed magnetic circuit in the transformer. The primary coil is called the thick-coil primary, and the secondary coil is called the secondary. A three-phase transformer consisting of three separate single-phase transformers is called a three-phase transformer.
34. Current Transformer
Basic Structure: A current transformer is a current transformer used to extend the measuring range. Similar to a conventional transformer, its basic structure consists of an iron core and two primary and secondary coils wound around it. In operation, the primary is connected in series with the load of the circuit being measured, while the secondary is connected in series with a 5A ammeter and the current coil of a protective relay.
It has only one or a few turns, so its cathode is small and the voltage is very low. Therefore, when connected in series to the measured circuit, it does not change the measured current. The secondary conductor is very thin and has many turns, but the induced potential is not high (only a few volts). The resistance of the secondary ammeter and relay coil is also very small.
During operation, the secondary can be considered short-circuited. According to the operating principle of transformers, the ratio of the primary current of the voltage transformer should be inversely proportional to the number of turns: L1/L2 = W2/W1 or L1 = W2/W1L2. W2/W1 = K2 transformation ratio.
35. Voltage Transformer
Basic Structure: This is a special transformer that increases the voltage range. The basic structure is the same as that of a conventional 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 internal and external gaps S1 and S2 break down simultaneously. The powerful lightning current flows into the earth through the grounding device, limiting the magnitude of the overvoltage.
One of the largest lightning accidents in China: the massive fire at the Huangdao oil depot in Shandong on August 12, 1989. 19 people died and 78 were injured. 36,000 tons of crude oil were lost in the fire, resulting in direct losses of hundreds of millions of yuan.
37. High-voltage Insulators
High-voltage insulators support and secure busbars and live conductors, ensuring adequate distance and insulation between live conductors or between conductors and the ground. Insulator insulation is achieved through surface shape.
Insulators of different voltage levels not only have different effective heights but also differ in surface conditions. Their surfaces are designed with raised and lowered skirts to increase their surface length.
Post time: Aug-28-2025
