Theoretical basis of environmentally friendly gas insulated switchgear

Inflatable cabinet, as the name suggests, is a switchgear filled with gas. It mainly refers to a high-voltage live primary circuit sealed inside a box filled with insulating gas, with an operating mechanism on the outside. The primary circuit is connected to an external solid insulated busbar through an insulating sleeve.

Why is there a medium voltage charging cabinet? The medium voltage charging cabinet comes from high-voltage switchgear, GIE，Gas Insulated Equipment， Including circuit breakers, isolating switches, busbars, and so on.

If high-voltage equipment of 110kV and above is still like a medium voltage air insulated switchgear, with all charged bodies directly exposed to the air, the required distance between the break, phase to phase, and relative to ground will be very large.

For example, 300kV corresponds to a lightning impulse withstand voltage of 1050kV. According to the IEC60071 standard, the air clearance distance needs to be greater than 2 meters, which requires a very large switchgear. If the distance of the fracture is greater than 2 meters, the switch equipment cannot be realized.

By using high-pressure SF6 gas, the net distance can be greatly reduced.

1 pound force/square inch=0.00689 megapascals, 100 psi=0.689 MPa. It can be seen that the insulation breakdown voltage of the 0.1 inch (2.54mm) electrode of 100 psi SF6 gas reaches 48kV, and the inflation pressure of high-voltage switches is generally greater than 0.5MPa. Therefore, the insulation breakdown strength of atmospheric air is 3kV/mm, that is, the insulation strength of 2.54mm is 3 × 2.52=7.56kV. 7.56/48=0.1575=16, and the volume can be reduced to 1/6.

Gas insulated switchgear, with the main circuit completely sealed inside the high-pressure gas, is not affected by the environment and can maintain stable operation for a long time. However, air insulated switchgear is greatly affected by humidity, pollution, and other factors, resulting in a significant decrease in insulation strength. It is necessary to adopt a larger insulation distance to ensure use in complex environments.

The medium voltage switchgear adopts gas insulation, greatly reducing the size of the switchgear. From an insulation perspective, the switchgear can be air insulated. The inflation pressure of general medium voltage switchgear is 0.13MPa, which is about 20psi. The insulation breakdown voltage of 0.1-inch (2.54mm) electrode with 20psi SF6 gas reaches 17kV, 7.56/17=0.44=44%. Less than half, the compact ring main unit can be seen to have a very small size.

The insulation strength of the insulating medium, in a slightly uneven electric field, is 2.5 to 3 times that of dry air and nitrogen. In simple calculation, for air insulation 12kV, a 75kV impulse withstand voltage net distance of 125mm is required, while for SF6, the net distance is only 40 to 50mm. For air insulation 40.5kV, a 185kV impulse withstand voltage net distance of 300mm is required, while for SF6, the net distance is only 100 to 120mm.

For high current switchgear, the limiting factor for size is temperature rise, as SF6 gas is similar to air. Although SF6 is beneficial for reducing temperature rise, enclosed compartments without convection make it impossible to reduce the size of switchgear.

The environmentally friendly gas uses air insulation, and it can be seen that the air at 40psi is similar to SF6 at 20psi, which means 0.00689 × 40=0.2756MPa. This means that Nuvotula Company uses high-pressure dry air insulation for its switchgear, based on the theory of similar insulation strength under increased pressure. Dry air at a relative pressure of 0.175MPa is similar to 0.03MPa SF6.

The inflation pressure of NU1 is 0.24 MPa, which is an absolute pressure of 0.34 MPa. The pressure of a conventional SF6 inflation cabinet is 0.13 MPa, with 0.34/0.13=2.6 times.

If the pressure is not increased, it is necessary to increase the size, or use a uniform electric field, or adopt a solid+air composite insulation method. However, increasing the size and reducing the competitive advantage of air insulated switchgear are generally not accepted by customers.

Uniform electric field requires the removal of all sharp corners of the conductor, the use of circular busbars, and the addition of voltage equalization shielding rings, which increases the cost of the switchgear. Under a uniform electric field, the distance between conductors can be significantly reduced. Under the same phase spacing, an 80mm diameter bare conductor can achieve optimal electric field and meet the requirements of air insulation electric field.

And more importantly, adding solid insulation and casting epoxy resin poles. The thickness of the solid insulation layer should not be too large. As the thickness increases, the electric field does not decrease but instead increases, and the cost and temperature rise also increase

The medium voltage gas insulated switchgear follows the cabinet form, with a C-GIS CUBIC square box body, low inflation pressure, and easy on-site installation. The new environmentally friendly gas insulated switchgear requires high pressure and adopts a high-pressure GIS equipment casting cylindrical form with a three-phase cross shaped layout, especially for 72.5kV. The cylindrical structure can reduce size and increase inflation pressure.