Switchgear refers to a complete set of power distribution equipment assembled from primary and secondary equipment according to a certain scheme, mainly used to control and protect lines and equipment. Switchgear can be classified by incoming and outgoing line voltage levels into high-voltage switchgear (fixed type and handcart type) and low-voltage switchgear (fixed type and drawer type).




The structure of high-voltage switchgear is generally similar, mainly divided into busbar chamber, circuit breaker chamber, secondary control chamber (instrument chamber), and feeder chamber, with steel plates generally isolating each chamber. Low-voltage switchgear is mainly classified based on the installation method of components.
Internal components include: busbars (busbar assemblies), circuit breakers, conventional relays, integrated protection devices, metering instruments, disconnectors, indicator lights, earthing switches, etc.


1. Incoming Line Cabinet


Also called the receiving cabinet, it is equipment used to receive electrical energy from the grid (from the incoming line to the busbar), generally equipped with components such as circuit breakers, CTs, PTs, and disconnectors.


2. Outgoing Line Cabinet


Also called the feeder cabinet or distribution cabinet, it is equipment used to distribute electrical energy (from the busbar to various outgoing lines), generally also equipped with components such as circuit breakers, CTs, PTs, and disconnectors.


 



3. Busbar Coupling


Also called the busbar section cabinet, it is equipment used to connect two busbar sections (from busbar to busbar). In single-busbar sectionalized and double-busbar systems, busbar coupling is often used to meet user requirements for selecting different operating modes or to ensure selective load shedding under fault conditions.


4. PT Cabinet


Voltage transformer cabinet, generally installed directly on the busbar to detect busbar voltage and provide protection functions. Internally, it mainly installs voltage transformers (PTs), disconnectors, fuses, and surge arresters.


5. Isolation Cabinet


It is used to isolate two busbar sections or to isolate receiving equipment from power supply equipment, providing operators with a visible break point for convenient maintenance and repair work. Since the isolation cabinet cannot break or make load currents, when the associated circuit breaker is closed, the handcart of the isolation cabinet cannot be pulled out or pushed in. In general applications, interlocking between the auxiliary contacts of the circuit breaker and the isolation handcart is required to prevent misoperation by operators.


6. Capacitor Cabinet


Also called the compensation cabinet, it is used to improve the power factor of the grid, or for reactive power compensation. The main components are parallel-connected capacitor banks, switching control circuits, and protective devices such as fuses. It is generally installed side by side with the incoming line cabinet, and one or more capacitor cabinets can operate in parallel. After the capacitor cabinet is disconnected from the grid, since the capacitor banks need time to discharge, it is not allowed to directly touch the components inside the cabinet, especially the capacitor banks; within a certain time after power-off (depending on the capacity of the capacitor banks, e.g., 1 minute), re-closing is not allowed to avoid overvoltage damaging the capacitors. When automatic control is used, attention should also be paid to reasonably distributing the switching times of each capacitor bank to avoid one bank being damaged while others are rarely switched.


 



7. Metering Cabinet


 


Mainly used for measuring electrical energy (kWh), it is also divided into high-voltage and low-voltage types. Generally installed with disconnectors, fuses, CTs, PTs, active energy meters (traditional or digital), reactive energy meters, relays, and other auxiliary secondary equipment (such as load monitoring devices).


8. GIS Cabinet


Also called a gas-insulated switchgear cabinet, it encloses circuit breakers, disconnectors, earthing switches, CTs, PTs, surge arresters, and busbars in a metal enclosure, using gas with excellent insulation and arc-extinguishing properties (generally sulfur hexafluoride SF6) as insulation between phases and to ground. It is suitable for high-voltage and high-capacity power grids for power receiving, distribution, and control.


9. Circuit Breaker


Under normal operating conditions, the circuit breaker is in the closed state (except for special applications), connecting the circuit. When automatic control or protection control operations are performed, the circuit breaker can be controlled by the integrated protection device to open or close the circuit. The circuit breaker can not only make and break normal load currents but also withstand short-circuit currents for a certain period (several times or even dozens of times the normal operating current), and can break short-circuit currents to clear faulty lines and equipment. Therefore, the main function of the circuit breaker is to make and break circuits (including making and breaking normal currents and breaking short-circuit currents).


During the process of making and breaking circuits, arcing inevitably occurs between the moving and fixed contacts of the circuit breaker. To protect the contacts, reduce contact material loss, and reliably break circuits, measures must be taken to extinguish the arc as quickly as possible. One method is to use different arc-extinguishing media filled between the moving and fixed contacts of the circuit breaker.


According to the arc-extinguishing medium, circuit breakers can be classified into oil circuit breakers (bulk oil and minimum oil), sulfur hexafluoride (SF6) circuit breakers, vacuum circuit breakers, air circuit breakers, etc. The main primary equipment in high and low voltage switchgear we often encounter in engineering is the circuit breaker. Since the moving and fixed contacts of the circuit breaker are generally enclosed in a container filled with arc-extinguishing medium, the open/closed state of the circuit breaker cannot be directly judged; it is usually determined through auxiliary devices of the circuit breaker (such as open/close position indicators).