Switchgear is a complete set of power distribution equipment that combines primary and secondary devices according to a specific line scheme. It is used for controlling and protecting electrical lines and equipment. Switchgear can be classified into fixed and handcart types, with the ability to connect and disconnect from the power supply. Voltage levels divide switchgear into high-voltage (fixed and handcart) and low-voltage (fixed and drawer) switchgear. The structure of switchgear is generally similar, consisting of a busbar room, circuit breaker room, secondary control room (instrument room), and feeder room, with steel plates isolating the different sections.
Internal components include busbars, circuit breakers, conventional relays, integrated protection devices, metering instruments, isolation switches, indicator lights, and grounding switches.
From an application perspective, switchgear is divided into several categories:
(1) Incomer Cabinet: Also known as the incoming cabinet, it receives electrical energy from the grid, typically equipped with circuit breakers, CTs, PTs, and isolation switches.
(2) Feeder Cabinet: Also called a distribution cabinet, it distributes electric energy from the busbar to various outlets, usually including circuit breakers, CTs, PTs, and isolation switches.
(3) Busbar Connection Cabinet: Used to connect two busbars, often found in single or double busbar systems, allowing flexible operation modes and fault handling.
(4) PT Cabinet: Contains voltage transformers for measuring busbar voltage and providing protection functions, along with isolation switches, fuses, and surge arresters.
(5) Isolation Cabinet: Designed to isolate busbars or power equipment, ensuring safe maintenance by providing visible disconnection points. It cannot handle load currents and requires interlocking mechanisms with circuit breakers to prevent errors.
(6) Capacitor Cabinet: Used for power factor correction, improving system efficiency. Components include capacitor banks, switching circuits, and protective devices. After disconnection, capacitors require time to discharge, so direct contact should be avoided. Re-closing after a short period is not allowed to prevent overvoltage damage.
(7) Metering Cabinet: Designed for measuring electric energy (kWh), featuring isolation switches, fuses, CTs, PTs, active and reactive energy meters, and other secondary equipment like load monitors.
(8) GIS Cabinet: A closed-type combination of circuit breakers, disconnectors, grounding switches, CTs, PTs, and busbars, enclosed in a metal housing. It uses gas (like SF6) for insulation and arc quenching, suitable for high-voltage applications.
(9) Circuit Breaker:
Under normal conditions, the circuit breaker is closed. It can open and close the circuit under control from protection devices. It can cut off both normal loads and short-circuit currents. Different types include oil, SF6, vacuum, and air circuit breakers, which are commonly used in engineering.
To extinguish arcs during operation, circuit breakers use various media such as oil, gas, or vacuum. The state of the circuit breaker is usually determined through auxiliary devices like position indicators.
(10) Isolation Switch:
Isolation switches have a visible break point, used for safely isolating high-voltage supplies. They cannot handle fault currents or load currents and must be operated when no current is present.
(11) Fuse:
Fuses are simple protection devices that melt when current exceeds a certain threshold, cutting off the circuit. They are not reusable and must be replaced after activation. Fuses come in various types, including delayed, fast, and super-fast action.
(12) Load Switch:
Load switches have basic arc suppression features, capable of handling small currents but not short-circuit currents. They are often used with fuses for overload protection in cost-effective solutions.
(13) Transformer:
Transformers convert voltages using electromagnetic induction without changing frequency. Common types include power transformers, rectifier transformers, and CT/PT units. Key parameters include rated capacity, voltage, losses, and tap settings.
(14) PT/CT:
Voltage and current transformers isolate the primary circuit from the control circuit, enabling safe measurement and protection. CTs measure current, while PTs measure voltage, both crucial for monitoring and control.
(15) Handcart/Drawer:
Handcarts and drawers are parts of high- and low-voltage switchgear, respectively. They allow easy access for maintenance and inspection, with three positions: work, test, and out.
(16) Grounding Knife:
Grounding knives ensure safety during maintenance by grounding lines and equipment. They can also be used to create controlled ground faults for protection purposes.
(17) Main Electrical Appliances:
Main electrical appliances include transfer switches, buttons, rotary switches, and signal lights, used to control and monitor electrical systems.
(18) Contactor:
Contactors are used to frequently switch on and off AC/DC main circuits and large-capacity control circuits. They are often used for motors, lighting, and capacitor banks, with high operating frequency and breaking capacity at lower voltages.
(19) Relay:
Relays are used in control loops to manage other electrical devices or provide protection and signal conversion in the main circuit. They are suitable for long-distance control of small-capacity circuits.
(20) Testing:
Common tests include type testing, routine testing, sampling testing, life testing, tolerance testing, and commissioning testing. These ensure product quality, reliability, and performance under various conditions.
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