Medium-Voltage Switchgear
Medium-voltage (MV) metal-enclosed power switchgear is defined as a switch- gear assembly completely enclosed on all sides and top with sheet metal (except for ventilating and inspection openings) containing primary power circuit switching and/or interrupting devices with buses and connections, and may include control and auxiliary devices. The rated voltage and insulation levels for MV-class metal-enclosed switchgear assemblies are shown in Table 7.2.
7.2.1 Construction Features
Switchgear is a general term used to define switching (and/or interrupting), protective, regulating, and metering devices, including all associated con- trols and interconnections, and accessories used for generation, transmis- sion, and distribution of electrical power. As shown in Table 7.1, switchgear equipment comes in various forms and ratings that can be used to perform particular functions. There are some fundamental differences among the various types of equipment available in the MV class. These differences are important from a maintenance and operation point of view and are dis- cussed in the following sections.
• The interrupting and switching device (i.e., breaker) is removable and can be physically moved into disconnect, test, or operating position.
• The primary bus conductors and connections are insulated.
• All live parts are enclosed within grounded metal compartments.
• Automatic shutters close off and prevent exposure of the primary circuit elements when the circuit breaker is removed from operating position.
• The primary and secondary contacts are self-aligning and self- coupling.
• Mechanical interlocks are provided to ensure a proper and safe operation.
• The circuit breaker housing cell door may be used to mount relays, instruments, and wiring. The relays, control devices, and associated wiring are isolated by grounded metal barriers from primary conductors.
• The elements of primary circuits such as circuit breakers, buses, and potential transformers are completely enclosed by grounded metal barriers.
Metal-Enclosed Interrupter Switchgear
Metal-enclosed interrupter switchgear consists of indoor and outdoor types with or without power fuses, voltage rating from 4.16 to 34.5 kV, and current rating of upto 1200 A. This switchgear is characterized by the following features:
• The primary buses and connections are uninsulated.
• The disconnecting device is an interrupter switch that may be remov- able or stationary.
• This switchgear may have instrument transformers, control wiring, and accessory devices.
Station-Type Cubicle
The station-type cubicle switchgear consists of indoor and outdoor types with power circuit breakers rated from 14.4 to 34.5 kV, 1200 to 5000 A, 1500 to 2500 MVA interrupting capacity. This switchgear has the following features:
• Power-operated stationary circuit breaker
• Bare buses having continuous carrying capacity equal to the service required
• Bare connections having current-carrying capacity equal to that of the power circuit breaker
• Each phase of primary is segregated and enclosed by metal
• Three-pole, single-throw, group-operated disconnect switches that are interlocked between power circuit breaker and front door giving access to primary compartment
• A set of instrument transformers
• Control wiring, terminal blocks, ground bus, and accessory devices