Mitsubishi Electric Corporation has developed two new technologies which may help reduce the use of the planet’s most potent greenhouse gas, sulphur hexafluoride (SF6), commonly used in high-voltage power systems and switchgear.
Mitusbishi’s arc-cooling technology achieves a 25 percent improvement in the interruption of electrical current in sulphur-fluoride (SF6) gas-insulated switchgears used in high-voltage power systems, and a high-density dielectric coating technology that improves insulation performance by 30 percent in high-voltage conductors. The two technologies will contribute to the further miniaturisation of switchgears and help to reduce the use of SF6 gas, which has global-warming potential that is 22,800 times greater than that of CO2. China currently produces the largest global proportion of SF6 emissions, and researchers have warned China’s SF6 emissions by 2050 could increase to 249 kt, or 6 billion tonnes CO2e, if mitigation measures are not adopted.
The key features of the new switchgear technologies are as follows.
1) Arc-cooling technology for interrupter improves current-interruption performance by 25 percent
- High-pressure gas generated with a unique coolant effectively cools and then extinguishes the conductive arc (conductive plasma) during current interruption.
- Current flow through the arc is reduced by the arc’s change into insulated gas, thereby improving current interruption by 25 percent compared to conventional non-coolant method.
The interrupter has two pairs of electrodes that remain closed when electric power is supplied. When the electrodes are opened the current cannot be interrupted immediately due to the conductive arc. In conventional method, the arc is extinguished by flowing the arc with gas to lower its temperature. Mitsubishi Electric’s new arc-cooling technology uses a unique coolant to generate a high-pressure gas jet to effectively cool and then extinguish the arc.
2) High-density dielectric coating technology improves dielectric performance by 30 percent
- The high-density dielectric coating technology densifies the dielectric coating layer on high-voltage conductors and thereby improves dielectric performance by 30 percent compared to operation without this dielectric coating layer.
In the switchgear, compressed SF6 gas is injected between a metal high-voltage conductor and a grounded tank. If the conductor’s metal surfaces are not coated, surface roughness of even just several μm can lead to electric discharges and thereby diminish the dielectric performance of the SF6 gas. Mitsubishi Electric’s new dielectric coating technology inhibits such discharges for improved dielectric performance. In addition, densifying the dielectric coating layer suppresses discharges due to air in the dielectric coating layer.
3) Downsizing enables reduced use of SF6 gas
- Improved current interruption achieved with the new arc-cooling technology enables the number of interrupters to be reduced from two to one.
- The new high-density dielectric coating technology suppresses discharges from the conductor surfaces, making it possible to reduce the size of the SF6 gas tank.
A gas-insulated switchgear incorporates an SF6 gas tank, which also contains an interrupter and high-voltage conductor. Conventional designs required two interrupters to conform to Japan’s JEC-2300 standard and the IEC 62271-100 international standard. However, Mitsubishi Electric’s new gas-insulated switchgear requires only one interrupter thanks to the new arc-cooling technology for improved current-interruption performance. In addition, incorporation of a dense coating layer in the high-voltage conductor improves dielectric performance and thereby allows the distance between the conductor and the grounded enclosure to be shortened, which enables a smaller enclosure to be used. As a result, reducing the number of interrupters and tank size enables less SF6 gas to be used.
Without such technologies and reductions in SF6, researchers suggest China’s SF6 emissions from the electrical sector – created by leakage, disposal, could reach