38 Pieces of Electricity Knowledge

01Why is the switch cabinet called a "complete set"? What kind of cabinets does it consist of?

The switch cabinet is composed of incoming lines, metering, PT, outgoing lines, etc. The cabinet is equipped with primary and secondary components, which must be matched when purchased, so it is called a "complete set".

02How many levels are there in my country’s power voltage, and explain the role of classification.

Divided into 500KV, 220KV, 110KV, 35KV, 10KV, 380V, 220V and other levels.

03Why should we remove the voltage (10KV level)? What method can be used to safely remove voltage?

During a power outage, in order to prevent the capacitor from having residual power and injuring people, the voltage must be discharged.

Methods for safely removing voltage: When opening the cabinet door or panel, do not touch the busbar directly with your hands. First use a 10KV test pen to check whether there is electricity, then find a wire, connect one end to ground and the other to the front end of the test pen. Use the front end of the test pen to contact the busbar to discharge. Repair can only be done after completion to ensure personal safety. Keep this in mind.

04 What should be the voltage of 10KV test electricity? What is the difference between domestic and international standards?

Domestic: cabinet 42,000 volts, break 48,000 volts;

International: 36,000 volts

05How to understand the meaning of the letters KYN in the switch cabinet model?

K—Metal enclosed armored type

Y—removable

N—Indoor

06What is the function of the pressure relief channel?

If the cabinet explodes, a strong air wave will be generated. At this time, if there is a pressure relief channel on the top of the cabinet, the air wave can be discharged from the top without affecting the normal work of other parts; if there is no pressure relief channel, the air wave will rush away from the weakest place.

07What main primary components should the switch cabinet be equipped with?

Circuit breakers, contacts, lightning arresters, zero sequence transformers, current transformers (CT), voltage transformers (PT), high voltage fuses, etc.

08What is the function of the ring network switch cabinet? How to protect the transformer?

Its function is to open and close with load, and it relies on high-voltage fuses to protect the transformer.

09What is the national standard for air insulation distance?

The air distance between phases and ground requires 125mm for 10KV.

10What is the creepage distance of insulation parts?

The conductive distance from the high-voltage live side to the ground and crawling along the surface of the insulation is called the insulation creepage. 10KV requires 235mm.

11How to check whether the vacuum switch is vacuum?

Check with pressure test. The fracture is detected using 48,000 volts.

12. What are the functions of isolating plugs and isolating switches, and what type of cabinets are they used for?

Isolating plug is used for handcart cabinets; isolating switch is used for fixed cabinets. Its function is to open and close without load, which is convenient for maintenance.

13How many levels are the currents of high-voltage cabinets divided into? What level is the load switch?

It is divided into three levels: large, medium and small, namely: 3150A, 1250A and 630A. The load switch is of 630A level.

14What is the function of high voltage fuse?

Used as a protection transformer, PT belongs to the small transformer category.

15What is the function of a charged display?

It serves as an indicator to show whether there is high-voltage electricity in the cabinet.

16What is the function of program lock? Where is it installed?

The operation sequence of the cabinets is interlocked according to the prescribed procedures, which plays a role in preventing misoperation.

17What is the function of the grounding switch?

The grounding switch is installed at the outlet end to prevent reverse supply of high-voltage power during maintenance and ensure personal safety.

18. What is the function of the valve?

When the handcart is pulled away from the cabinet, prevent people or metal from contacting the static contacts with high voltage.

19. Why does the busbar need to be glued?

It plays an insulating role, reducing the air insulation distance and preventing small animals from entering the cabinet and causing short circuit accidents.

20What is the function of lightning arrester? Why does every group of cabinets have at least one lightning arrester?

Lightning arresters are designed according to voltage levels and must not be installed incorrectly. Some are also designed differently according to different uses. For example: power plants output electricity, have circuit operations, and have capacitors.

The lightning arrester is mainly used to absorb excessive voltage when it is generated in a certain section of the line to achieve the purpose of safe operation of the line.

For example, the incoming line cabinet has a set of lightning arresters to protect the safety of the circuits in the cabinet; and the external power supply is already equipped with a lightning arrester when it enters the cabinet. Therefore, if the external power supply generates high voltage, it will be protected by the lightning arrester when entering the cabinet. Safety; the lower end of the outlet cabinet is equipped with a lightning arrester, mainly because overvoltage will occur when the vacuum circuit breaker is opened and closed, so a lightning arrester must be installed to replace the overvoltage absorber.

21How to distinguish phases, colors and arrangement of phases.

Color: yellow, green, red and blue

Phase sequence: ABCNPE

Left and right arrangement: left, middle, right, most left, bottom

Front and rear arrangement: back, center, front, front, bottom

Arrange up and down: top, middle, bottom, bottom

22 Discuss the functions of current transformers and zero sequence transformers.

The current transformer plays the role of metering and protection;

The zero sequence transformer plays a protective role.

23Can current transformers be assembled reversely? Why?

The normal flow direction is L1→L2. If it flows from L2→L1, the secondary line design needs to be modified.

24What is the difference between VE cabinet and ring main unit?

The VE cabinet (mobile type) is disconnected by a circuit breaker to protect the transformer for 0.2 seconds.

The ring main unit (fixed type) uses a high-voltage fuse to protect the transformer for 0.02 seconds.

25What is the function of the thermostat?

The humidity is detected through the induction probe. If it exceeds the normal humidity, it is adjusted by heating the heating plate. There are two modes of operation: automatic and manual.

26What is the role of the primary plan diagram?

Represents the original components and directions assembled on the main circuit line

27What do the names PT and CT stand for?

PT—voltage transformer, CT—current transformer

28What do the names AC and DC stand for?

AC - alternating current, DC - direct current

29How many operating mechanisms do circuit breakers have?

Manual, automatic distribution, magnetic, spring, magnetic suction for contactors.

For example: low-pressure air switch is manually operated

High voltage vacuum switch uses CD10 or CD17 (electromagnetic mechanism)

Use CT8 or CT19 (spring mechanism)

30What is the function of the empty switchgear? How does it protect the transformer?

Its function is to extinguish vacuum arcs, and can open and close with load for power transmission. It relies on current transformers and instruments to cut off the current to protect the transformer. Other components can also be installed for other purposes.

31What is the function of vacuum circuit breaker?

It can open and close with load and use vacuum to extinguish the arc.

32What should be the stroke of the vacuum switch?

Stroke: 11±1 Spring compression: 4±1 VE: 8±0.5

33 Classification of circuit breakers:

(1) Vacuum circuit breaker: uses vacuum (vacuum degree is less than 100,000 mercury) as arc extinguishing medium and main insulation medium. The dynamic and static contacts for arc extinguishing are sealed in the vacuum arc extinguishing chamber. There are mainly 10VK. 35KV voltage level.

(2) Sulfur hexafluoride circuit breaker: came out late. It was only widely used in the early 1970s and is now an advanced equipment in the power system, using SF6 gas as the arc extinguishing medium. The cutting capacity is about 10 times higher than that of ordinary circuit breakers. Arc voltage is low. The arcing time is short, the contacts are less burned, and it can be operated frequently. Disadvantages SF6 is more expensive. A recovery device is required and the structure is complex. Our country has manufactured a fully enclosed combined electrical appliance with a power of 220KV and using SF6 as the main body. Put the circuit breaker, isolating switch. Transformer, lightning arrester. The main equipment of the substation such as busbars are all installed in sealed containers containing SF6 gas, which greatly reduces the space occupied by the substation. It is the main direction for the development of ultra-high voltage electrical appliances. It has been widely used abroad.

(3) Circuit breaker: uses compressed air as the arc extinguishing medium. In terms of ultra-high voltage, it is currently the main producer. China produces 380KV. 765KV has been produced abroad.

(4) Self-produced circuit breaker: The fixed medium is decomposed into gas by the action of the arc, and the arc is blown to extinguish the arc. Low voltage and small current interruption. Suitable for agricultural electronic systems.

(5) Magnetic blown circuit breaker: uses a magnetic field to blow the arc into the arc extinguishing grid.

34 power transformer

A transformer is an electrical device that transforms voltage (current). It cannot convert power.

Basic structure: It is mainly composed 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 coated with insulating paint. The thickness It is made of 0.35-0.5mm silicon steel sheets with good magnetic permeability. It forms the closed magnetic circuit in the transformer. The primary coil is called thick coil type primary side, and the secondary coil is called secondary coil type secondary side. They are connected by a three-point single-phase transformer. The three-phase transformer group is a three-phase transformer.

35 current transformer

Basic structure: The current transformer is a current transformer used to expand the measuring range.

The basic structure is the same as that of an ordinary voltage regulator. It is also composed of an iron core and a primary and auxiliary coil wound around the iron core. When used, the main primary side is connected in series with the circuit load under test, and the auxiliary side is connected in series with a 5A ammeter and a protective relay current coil. Because the primary side uses a thick wire and the number of turns is only one or a few turns, its resistance is extremely small and the voltage is very low. After being connected in series to the circuit under test, it does not change the supplementary measurement current. The secondary wire is very thin. There are many turns, but the induced potential is not high (only a few volts). The resistance of the secondary ammeter and relay coil is very small. When working, the secondary side can be regarded as a short-circuit state. According to the operating principle of the transformer: the primary current of the electrically charged transformer The ratio should be inversely proportional to the number of turns:

L1/L2=W2/W1 or L1=W2/W1L2. W2/W1=K2 transformation ratio.

36 voltage transformer

Basic structure: It is a special transformer that expands the voltage range. The basic structure is the same as an ordinary transformer. Therefore, the voltage relationship between the primary side of the voltage transformer and the secondary side is U1/U2=W1/W2.

37 lightning arrester

Basic principle: when overvoltage is transmitted. The internal and external gaps S1 and S2 break down simultaneously. The strong lightning current flows into the earth through the grounding device, which limits the amplitude of the overvoltage.

One of the largest lightning accidents in China: the huge 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 burned by the fire. Direct losses of hundreds of millions of yuan.

38 high voltage insulator

The function of high-voltage insulation: to support and fix the busbar and live conductors, and to ensure sufficient distance and insulation between live conductors or between conductors and the earth; insulator insulation is obtained by surface shape.

Insulators of different voltage levels. Except with different effective heights. The surface conditions are also different. Its surface is made into a high and low uneven skirt. to increase its surface length.