**Distribution Knowledge**
**Switchgear** refers to a complete set of power distribution equipment assembled by primary and secondary devices according to a specific line scheme. It is used for controlling and protecting lines and equipment, as well as for inserting and removing. Voltage levels can be classified into high-voltage switchgear (handcart and fixed types) and low-voltage switchgear (fixed and drawer types). The structure of switchgear is generally similar, typically divided into a busbar room, circuit breaker room, secondary control room (instrument room), and feeder room. Steel plates are often used to isolate these compartments.
Internal components include busbars, circuit breakers, measuring instruments, isolating switches, indicator lights, conventional relays, integrated Relay protection devices, and grounding switches.
From an application perspective, switchgear can be categorized into several types:
**Line Counter Cabinet**
Also known as the incoming cabinet, it is used to receive electrical energy from the grid (from the incoming line to the busbar). It usually includes circuit breakers, isolating switches, PT (voltage transformer), and CT (current transformer).
**Outgoing Cabinet**
Also called the feeder or distribution cabinet, it is used to distribute electrical energy from the busbar to each outlet. It typically has circuit breakers, CT, PT, and isolating switches.
**Busbar Connection Cabinet**
Also referred to as the busbar sectioning cabinet, it is used to connect two busbars. In single busbar and double busbar systems, it helps meet user requirements for different operating modes and enables selective load removal during faults.
**PT Cabinet**
The voltage transformer cabinet is installed directly on the busbar to detect bus voltage and provide protection. It mainly includes PT, isolating switches, fuses, and lightning arresters.
**Isolation Cabinet**
Used to isolate busbars at both ends or to separate power receiving equipment from the power supply. It allows operators to have a visible disconnection point for maintenance. Since it cannot handle load current, the handcart cannot be moved when the circuit breaker is closed. An interlock mechanism is usually required between the circuit breaker’s auxiliary contact and the isolation cart.
**Capacitor Cabinet**
Also known as the compensation cabinet, it is used to improve the power factor of the grid or for reactive power compensation. It includes capacitor banks, switching control circuits, and fuses. After being disconnected, capacitors need time to discharge, so direct contact should be avoided. Re-closing after a short period is not allowed to prevent overvoltage damage.
**Metering Cabinet**
Mainly used for electric energy metering (kWh), it includes isolating switches, fuses, CT, PT, active and reactive power meters, relays, and other secondary equipment like load monitors.
**GIS Cabinet**
Also known as a gas-insulated switchgear, it combines a circuit breaker, isolating switch, grounding switch, CT, PT, and busbar in a metal enclosure filled with SF6 gas for insulation. It is suitable for high-voltage and high-capacity power grids.
**Circuit Breaker**
Under normal conditions, the circuit breaker is in the closed position. It can open and close the circuit under control of a protection device. It can handle normal load currents and short-circuit currents for a certain time, making it essential for fault isolation.
Due to arcing during operations, arc extinguishing media such as oil, SF6, vacuum, or air are used. The state of the circuit breaker is usually determined through indicators or auxiliary devices.
**Isolating Switch**
An isolating switch provides a visible break to ensure safe maintenance. It cannot interrupt load current or fault current and must be operated without load.
**Fuse**
A simple protective device that melts when current exceeds a set value, cutting off the circuit. It is not reusable and requires replacement after operation. Fuses come in various types, including fast-acting and delayed-action.
**Load Switch**
It has a basic arc-quenching system and can handle certain loads but cannot interrupt short-circuit currents. It is often used with fuses for cost-effective protection.
**PT/CT**
Voltage and current transformers are used to isolate the primary circuit from the secondary control equipment. They expand the use range of secondary devices and prevent high voltages/currents from damaging them.
**Handcart/Drawer**
Used in high and low voltage switchgear, they allow easy maintenance. They have three positions: working, testing, and withdrawal.
**Grounding Switch**
Used for grounding during maintenance and to create a ground fault for protection. It ensures safety and enables quick tripping of upstream circuit breakers.
**Contactor**
An electrical device for frequent switching of main circuits and control circuits. It is commonly used for motors, lighting, and capacitor banks. It has a high operational frequency but lower voltage capability than circuit breakers.
**Relay**
Used to control other devices in the control loop or for protection and signal conversion. It is suitable for small-capacity control circuits.
**Testing**
Common tests include type tests, routine tests, sampling tests, life tests, tolerance tests, and operation tests. These ensure product quality, reliability, and performance under various conditions.
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