C-GIS is the abbreviation of gas insulated switchgear, which is defined according to the insulation form of the main switch, and corresponds to air insulated switchgear and solid insulated switchgear. That is, the main circuits of the switchgear, such as switches, circuit breakers and connections, are sealed inside the gas box filled with insulating gas, and the external main busbars and cable connections are insulated in a solid manner. The current transformer is directly installed on the outlet bushing or cable in a through-type manner. There are also epoxy pillar-type current transformers installed inside the gas box, and the wiring terminals are reserved outside the gas box. The voltage transformer is connected to the cone bushing plug plate in the gas box through an American cable head. There is no part of the high-voltage main circuit and auxiliary circuit exposed to the natural air.
Ring main unit RMU is defined according to the application. Ring main unit power supply is hand-in-hand power supply mode, also known as secondary power distribution equipment. Typical ring main unit uses two load switches as ring in and ring out as connection equipment for hand-in-hand power supply. Load switch cabinet is also equipped with load switch fuse combination or circuit breaker as transformer outlet switch. For transformers of 1250kVA and below, load switch fuse combination is the most economical and effective protection solution. Transformers greater than 1250 kVA are separated and combined by circuit breakers.
The ring main unit is also equipped with a voltage transformer cabinet to provide operating power, a metering cabinet for high-voltage meters to measure electricity charges, and a direct line cabinet as a lead-in unit.
The current of the ring main unit is generally 630A, and the short-circuit breaking current is 20kA. It is a small current switchgear. The air-insulated mid-mounted switchgear can also be used for ring network applications, but the air-insulated switchgear is larger in size, while the ring main unit is a compact switchgear specially used for ring network applications. The early ring main units were common box type oil-insulated or gas-insulated switchgears, which were integrated with the transformer, small in size and light in weight.
Currently, ring main units generally adopt the form of compact gas-filled cabinets, that is, this compact ring main unit is a type of gas-filled cabinet. In fact, ring main units also have air-insulated forms and semi-insulated forms. There are air-insulated ring main units with vacuum load switches and semi-insulated ring main units with SF6 gas-bag insulation. Therefore, it cannot be said that ring main units are gas-filled cabinets.