30 Essential Tips of High and Low Voltage Switchgear

High and low voltage switchgear, simply put, is an integrated system where various electrical components are pre-designed, assembled, and thoroughly tested.

It's a staple in power systems, primarily used to control and protect electrical energy while distributing it to different locations. This equipment is crucial for power systems, ensuring stable operation and more reliable, safer power supply. Based on function and application, high and low voltage switchgear comes in various types. Switching equipment includes circuit breakers and load switches; control equipment features contactors and relays; protection equipment encompasses fuses and surge arresters; and measurement equipment includes voltmeters and ammeters. These devices function as a cohesive team, working together to ensure the seamless operation of the power system. So, what are the essential fundamentals of high and low voltage switchgear that must be thoroughly understood? Don't worry—the following article has compiled all the key points for you. We recommend bookmarking it for future reference.

1. Why is switchgear called “complete sets”?

Switchgear cabinets are assembled from components like incoming lines, metering devices, PTs, and outgoing lines. They house primary and secondary components that must be used together, hence the term “complete set.”

2. How many voltage levels exist in China's power grid, and what is the purpose of these classifications? Answer: Voltage levels include 500kV, 220kV, 110kV, 35kV, 10kV, 380V, and 220V. Voltage levels correlate with line losses—higher voltages result in lower losses. Consequently, power grids commonly employ high-voltage transmission, with voltage levels increasing for longer transmission distances.

3. Why is discharge required (for 10kV systems)? What methods ensure safe discharge? Answer: During power outages, residual capacitive charge must be safely discharged to prevent personnel injury. Safe discharge procedures: When opening cabinet doors or panels, never touch busbars directly with bare hands. First, use a 10kV voltage tester to check for electricity. Then, take a conductor wire, ground one end, and attach the other end to the front of the voltage tester. Use the tester's tip to discharge the busbar. Only proceed with maintenance after discharge is complete to ensure personal safety. Remember this point.

4. What should the voltage be for a 10kV test? What are the differences between domestic and international standards? Answer: Domestic: 42kV for cabinet body, 48kV for busbar joints; International: 36kV.

5. What is the function of a pressure relief channel? Answer: In the event of a cabinet explosion, a powerful blast wave would be generated. If the cabinet top has a pressure relief channel, the blast wave can be vented through the top without affecting other components' normal operation. Without a relief channel, the blast wave would rupture at the weakest point. 6. What primary components should be installed in switchgear? Answer: Circuit breakers, surge arresters, zero-sequence transformers, current transformers (CTs), voltage transformers (PTs), high-voltage fuses, etc.

7. What is the function of ring main unit switchgear? How does it protect transformers? Answer: Its function is to enable load-breaking and closing operations. It relies on high-voltage fuses to protect transformers.

8. What is the national standard for air insulation distance? Answer: The air distance between phases and between phase and ground is 125mm for 10kV systems.

9. What is the creepage distance of an insulator? Answer: The creepage distance is the conductive path along the outer surface of an insulator from the high-voltage live side to ground. The requirement for 10kV is 235mm.

10. How is the vacuum integrity of a vacuum switch verified? Answer: Through withstand voltage testing. The break points are tested at 48kV. 11. What is the function of a disconnect switch, and in which cabinet types is it used? Answer: Disconnect switches are used in fixed cabinets. They operate without load, provide a visible break point, and facilitate maintenance.

12. What is the function of a high-voltage fuse? Answer: To protect transformers. Note: PTs belong to the category of small transformers.

13. What is the function of a live display indicator? Answer: It indicates whether high voltage is present inside the switchgear.

14. What is the purpose of a sequence lock? Answer: It interlocks the operating sequence between switchgear units according to specified procedures, preventing misoperation.

15. What is the function of an earthing switch? Answer: Installed at the outgoing end, it prevents backflow of high voltage during maintenance, ensuring personnel safety. 16. What is the function of the live door? Answer: Prevents personnel or metal objects from contacting live static contacts when the drawout unit is withdrawn from the cabinet.

17. Why are busbars insulated with rubber? Answer: Provides insulation, reduces air clearance requirements, and prevents small animals from entering the cabinet and causing short circuits.

18. What is the function of a surge arrester? Why does each cabinet group have at least one? Answer: Surge arresters are designed for specific voltage levels and must never be installed incorrectly. Some are also designed for different applications, such as power plant output, line operation, capacitor use, or power station use. Their primary purpose is to absorb excessive voltage generated at any point in the line, ensuring safe operation. https://www.dgdqw.com如: The incoming switchgear cabinet contains one set of surge arresters to protect internal circuitry. Since external power sources already incorporate surge arresters at their entry point, any high voltage generated externally is safely mitigated by these arresters before entering the cabinet. The lower end of the outgoing switchgear cabinet is equipped with surge arresters, primarily to address overvoltage phenomena that occur during the opening and closing of vacuum circuit breakers. Therefore, surge arresters must be installed to replace overvoltage absorbers. 19. Describe the functions of current transformers and zero-sequence transformers. Answer: Current transformers serve measurement, metering, and protection functions; zero-sequence transformers serve protection functions.

20. Can current transformers be reversed in installation? Why? Answer: The normal current flow is L1→L2. If reversed to L2→L1, the secondary wiring design requires modification. 21. What is the function of a humidity controller? Answer: It detects humidity via sensing probes. If humidity exceeds normal levels, it regulates by heating through heating plates. It operates in two modes: automatic and manual.

22. What is the purpose of the primary schematic diagram? Answer: It represents the components installed along the main circuit route and their routing.

23. What do PT and CT denote? Answer: PT—Voltage Transformer, CT—Current Transformer

24. What do AC and DC denote? Answer: AC—Alternating Current, DC—Direct Current

25. What is the function of a vacuum circuit breaker? Answer: Capable of load-breaking and closing operations, utilizing a vacuum arc extinguishing chamber (vacuum bulb) for arc extinction. 26. Classification of circuit breakers: Answer: (1) Vacuum circuit breakers: Employ vacuum (vacuum level below 100,000 mm Hg) as both the arc-extinguishing medium and primary insulation medium. Both moving and stationary contacts are sealed within the vacuum arc extinguishing chamber. Primarily available in 10kV and 35kV voltage ratings. (2) SF6 Circuit Breakers: Developed later. Widely adopted since the early 1970s, they are now advanced equipment in power systems, using SF6 gas as the arc-quenching medium. Their breaking capacity is approximately 10 times higher than conventional circuit breakers. They feature low arc voltage, short arc duration, minimal contact wear, and support frequent operation. Disadvantages include the higher cost of SF6, the need for recovery equipment, and a more complex structure. China has manufactured fully enclosed switchgear assemblies primarily using SF6 for voltages up to 220kV. These enclose key substation equipment—circuit breakers, disconnectors, instrument transformers, surge arresters, and busbars—within a single SF6-filled sealed container, significantly reducing substation footprint. This represents the primary development direction for ultra-high voltage equipment and is already widely adopted internationally. (3) Circuit Breakers: Utilizing compressed air as the arc-quenching medium, these are currently the primary products for ultra-high voltage applications. China manufactures 380kV units, while internationally, 765kV units are already in production. (4) Gas-blown Circuit Breakers: These utilize a fixed medium that decomposes into gas under arc exposure to extinguish the arc. Suitable for low-voltage, low-current applications, they are ideal for agricultural electrical systems. (5) Magnetic-blown Circuit Breakers: These employ a magnetic field to blow the arc into an arc-extinguishing grid. 27. Power Transformers Answer: A transformer is an electrical device that converts voltage (and current). It does not convert power. Basic Structure: Primarily consists of a closed iron core and two mutually insulated coils (windings) with different turns wound around the core. To minimize eddy current and hysteresis losses, the core is laminated from silicon steel sheets coated with insulating varnish, each 0.35-0.5mm thick and possessing excellent magnetic permeability. This forms the closed magnetic circuit within the transformer. The primary coil is called the thick-wound primary, The secondary coil is called the secondary winding. A three-phase transformer is formed by connecting three single-phase transformers in a specific configuration. 28. Current Transformer Answer: Its basic structure is similar to that of a standard voltage transformer, consisting of a primary winding, core, and secondary winding. During operation, the primary side is connected in series with the load of the circuit being measured, while the secondary side connects in series with a 5A ammeter and protective circuit. The primary side uses thick wire with only one or a few turns, whereas the secondary side uses very thin wire with many turns. According to transformer operating principles: The ratio of primary to secondary current in a current transformer should be inversely proportional to the ratio of primary to secondary turns: IN1/IN2 = N2/N1, (where IIN1 and IN2 represent primary and secondary currents, respectively; N1 and N2 represent primary and secondary coil turns, respectively).

29. Voltage Transformer Answer: Basic Structure: A specialized transformer designed to extend the voltage measurement range, sharing the fundamental structure with standard transformers. Thus, the voltage ratio between the primary and secondary sides of a voltage transformer is U1/U2 = N1/N2 (where N1 and N2 represent the number of turns in the primary and secondary coils, respectively).

30. High-voltage insulator Answer: Function: Supports and secures busbars and live conductors while maintaining sufficient clearance and insulation between live conductors or between conductors and ground. Insulation is achieved through surface configuration. Insulators for different voltage levels vary not only in effective height but also in surface design. Their surfaces feature raised, undulating flanges to increase effective surface length.