Medium voltage Secondary Switchgear is electrical equipment used to control, protect, and isolate power circuits in distribution networks.
Unlike primary switchgear, which handles bulk power, secondary switchgear is located closer to the end-user and is responsible for delivering power to residential areas, commercial buildings, and industrial facilities.
Key functions
- Power distribution: Secondary switchgear manages the flow of electricity from primary distribution networks to downstream transformers. It takes a feeder line from a utility provider and branches it out to supply various consumers.
- Protection: It detects and clears faults, such as short circuits and overloads, to prevent damage to electrical equipment like transformers. This protection is achieved using circuit breakers or fuses.
- Isolation: The switchgear provides safe isolation of circuits for maintenance and repair work. Disconnectors and earthing switches allow for the de-energizing and grounding of a circuit, protecting personnel.
- Measurement: Instrument transformers for current and voltage are often included to monitor electrical parameters for energy management and billing.
- Transformation: In many secondary substations, the switchgear operates in conjunction with a transformer to step down the medium voltage to a low voltage for the final consumer.
Common components
- Circuit breakers and switches: Devices that interrupt the flow of current. Circuit breakers automatically trip during faults, while switches (like load break switches) are used to manually open and close circuits.
- Fuses: Provide passive protection by melting and breaking the circuit when current exceeds a specific level.
- Protection relays: Intelligent devices that sense abnormal electrical conditions and send a signal to a circuit breaker to open and clear the fault.
- Busbars: Conductive bars that carry electricity and distribute it to different circuits within the switchgear.
- Instrument transformers: Include current transformers (CTs) and voltage transformers (VTs) for accurate measurement of electrical parameters.
- Enclosure: A metal-clad enclosure houses all the components, protecting them from environmental factors and ensuring operator safety.
Main types
Secondary MV switchgear is classified primarily by its insulation and construction.
By insulation
- Air-Insulated Switchgear (AIS): Uses atmospheric air as the insulating medium between live components. AIS is a well-established and cost-effective technology that is often modular and easy to maintain.
- Gas-Insulated Switchgear (GIS): Utilizes an insulating gas, traditionally sulfur hexafluoride (𝑆𝐹6) or pure air in modern versions, to insulate live parts. GIS units are more compact than AIS, making them ideal for urban areas where space is limited.
- Solid-Insulated Switchgear (SIS): Uses solid materials to insulate the live parts, providing a compact, maintenance-free, and environmentally friendly alternative to gas-insulated systems.
By construction
- Modular: Made of separate, functional bays that are joined together to form the complete switchgear. This design is highly flexible for customization and expansion.
- Compact (Ring Main Unit – RMU): Integrates all functional bays and equipment into a single, compact, sealed enclosure. RMUs are commonly used in secondary distribution for supplying urban and residential networks.
| Item | Unit | Value | ||||
|---|---|---|---|---|---|---|
| Rated voltage | kV | 12 | 17.5 | 24 | 40.5 | |
| Rated frequency | Hz | 50 or 60 | ||||
| Rated power frequency withstand voltage (1 min) | Phase to phase, phase to earth | kV | 28 | 38 | 50 | 95 |
| Across isolating distance | 32 | 45 | 60 | 110 | ||
| Rated lightning impulse withstand voltage (peak) | Phase to phase, phase to earth | kV | 75 | 95 | 125 | 185 |
| Across isolating distance | 85 | 110 | 145 | 215 | ||
| Rated current | A | 630 or 1250 | ||||
| Rated short time withstand current | kA | 21 or 25 | ||||
| Rated duration of short circuit | s | 3 or 1 | ||||
| Rated peak current | kA | 54.6 or 65 | ||||
| IP degree | Tank with HV parts | IP67 | ||||
| Indoor | IP41 | |||||
| Outdoor | IP54 | |||||
| Annual leakage rate | % / year | ≤ 0.01 | ||||