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 is mostly used for medium voltages also called MV switchgear and is enclosed in a metal enclosure that is grounded. Voltage levels ranging from 3-36 kV can be termed as medium voltages. However, in recent times indoor switchgear has also been successfully employed for high voltages higher than 36 kV. In the coming sections, we will try to restrict our discussion mainly to the indoor switchgear.
Design of Indoor Switchgear
Compared to outdoor switchgear, the indoor switchgear is mostly gas insulted system or GIS, as compared to an air-insulated system. GIS is employed where the cost of land is more, and air is corrosive and dust-laden. The switchgear arrangement is enclosed in a grounded metal enclosure on all the sides provided with openings for ventilation and inspection. The switchgear units are insulated in a gas environment. For the purpose of insulation, SF6 gas is preferred due to its superior dielectric properties. The dielectric strength of SF6 is about three times that of air. The metal enclosure provides extra safety compared to the outdoor system. There are two types of indoor switchgear in terms of metal enclosure partitioning and switching methods:
- Metal-enclosed indoor switchgear.
- Metal-clad indoor switchgear
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. The fig.1 shows an image of a metalenclosed type indoor switchgear system.
Metal Clad Switchgear
In metal-clad switchgear, the circuit breakers are normally vacuum type and are withrawable or drawnout type. Relaying and metering instruments are isolated by grounded metal barriers separately. Voltage levels for metal-clad switchgear ranges from 4.76kV to 38kV with main bus continuous ratings of 1.2kA 2kA 3kA and 4kA.
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 is advantageous over metal-enclosed switchgear because the former is highly customizable.
Advantages of Indoor Switchgear
Compared to outdoor switchgear, the indoor switchgear system has the following advantages :
More reliable and safer.
- More reliable and safer.
- Takes lesser space than the outdoor system.
- Easier maintenance and durability.
- Lower operating costs.
- Reduced risk of electrocuting due to grounded metal enclosures.
- More security.
- Lesser prone to environment conditions.
Limitations of Indoor Switchgear
The main disadvantage of indoor switchgear compared to the outdoor one is the higher installation costs. Also, due to economic reasons and at high voltages, the outdoor system or the air insulated system becomes more viable economically.
In the preceding sections, indoor switchgear system was discussed and compared with the outdoor switchgear. The indoor switchgear arrangements are highly reliable, secure and cost effective. They are highly customizable and durable as compared to the outdoor counterparts. These are able to withstand harsh atmospheric conditions. However economic viability and high voltage levels limit the use of indoor switchgear systems.