Introduction of Indoor Switchgear – Advantages and Limitations of Indoor Switchgear

Introduction of Indoor Switchgear

Switchgear is a generalized term given to a unit which is a combination of protective control and metering types of equipment as a whole. Switchgear can be of different forms depending on the kind of applications such as protection of feeders, protection of motors, rail traction purposes, measurements etc. For example, the switchgear installed in an electrical power substation perform various functions such as connections of incoming feeders and transformer units with the busbar, interruption of short circuit current using combinations of circuit breakers and relaying devices, switching of capacitor banks, measurement of current and voltage, monitoring and control, battery storage, etc. Depending on the voltage levels as well as economic viability, switchgear can be of indoor or outdoor types. The indoor switchgear sits inside a building or metal housing and is commonly used for medium voltages, also called MV switchgear, with the live parts enclosed in a grounded metal case. Voltage levels ranging from 3-36 kV are often termed medium voltages in IEC practice. Indoor GIS and hybrid gear are also used above 36 kV where space is tight. This page stays on indoor switchgear.

Design of Indoor Switchgear

Indoor switchgear may be air-insulated metal-clad gear in a building, or gas-insulated (GIS). GIS is used where land costs are high and air is corrosive or dusty. The switchgear is enclosed in a grounded metal case with ventilation and inspection openings. SF6 gas has been widely used for GIS insulation because its dielectric strength is about three times that of air at the same pressure. F-gas rules now push many new designs toward SF6-free mixtures. This metal enclosure keeps live parts off the weather and off casual contact, which outdoor open terminals do not. Indoor switchgear can be divided into two types based on metal enclosure and switching methods.

1. Metal-enclosed indoor switchgear.
2. Metal-clad indoor switchgear

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Metal Enclosed Switchgear

The switchgear arrangements are enclosed on all sides with metal sheets containing primary interrupting devices and fuses, and all the pieces of equipment are encased in a central assembly. Doors and removable coverings provide the access to the interior of the enclosure. Fig.1 shows an image of a metalenclosed type indoor switchgear system.

Metal Clad Switchgear

Metal-clad switchgear typically uses vacuum-type circuit breakers that can be withdrawn or drawn out. Relaying and metering instruments are separately isolated by grounded metal barriers. Metal-clad switchgear is commonly rated at voltage levels from 4.76 kV to 38 kV, with main bus continuous ratings of 1.2 kA, 2 kA, 3 kA and 4 kA as typical IEEE published bands, not every panel.

In general, all metal-clad switchgears are metal enclosed but all metal-enclosed are not metal clad switchgears. The metal-enclosed switchgear is of same voltage rating as that of metal-clad but is of simpler construction. The metal-clad switchgear has more internal barriers and a withdrawable breaker than a simple metal-enclosed assembly.

Advantages of Indoor Switchgear

Compared to outdoor switchgear, the indoor switchgear system has the following advantages :
The list below is typical for indoor metal-clad and GIS, not a guarantee for every site.

• Weather protection and grounded metal covers reduce casual contact with live parts.
• Takes lesser floor area than an equivalent outdoor air-insulated yard.
• Easier indoor access for maintenance when the building is dry and clean.
• Operating costs can be lower when weather damage and outdoor inspections drop.
• Grounded metal enclosures reduce the chance of accidental contact.
• Locked indoor rooms limit public access to the gear.
• Less exposed to rain, dust and pollution than open outdoor gear.

Limitations of Indoor Switchgear

The main disadvantage of indoor switchgear is often a higher installed cost than outdoor air-insulated gear, because of the building or the GIS enclosure. For many high-voltages substations, outdoor or air-insulated systems remain the cheaper layout.

Conclusion

We discussed and compared indoor and outdoor switchgear. Indoor switchgear keeps the apparatus in a building or metal housing, so weather and public access are easier to control, and the footprint can be smaller. Metal-clad indoor gear also allows a withdrawable breaker. Cost and use at the highest voltages still often favour outdoor air-insulated yards.