Air around us is a good insulator. It is abundance and self restored after electrical breakdown process. Air is commonly used as insulator in switchgears and overhead transmission line. However as demand for electricity supply increases especially in dense urban area, the need for a smaller and compact high voltage apparatus arise. Researchers realize that air is not the viable to fulfill this needs. Thus a new gas insulation technology is born - The Sulphur Hexaflouride (SF6). Air has Critical Breakdown Voltage of 24.5kV/cm which means that for one centimeter gap between a different phase, or between phases and ground, air can withstand 24.5kV. If the voltage exceeds this value, breakdown will occur.
 |
| Breakdown Voltage of Air |
On the other hand, SF6 gas has Critical Breakdown Voltage of 88.4kV/cm which means that for one centimeter gap between a different phase, or between phases and ground, air can withstand 88.4kV. Breakdown will occur if voltage exceeds this value.
In the previous section, I have stated that the breakdown voltage of SF6 is higher than air. In this world everything happens for a reason and same goes to this. SF6 has added characteristics which makes it more superior than air in terms of breakdown voltage. The reason is SF6 has higher dielectric strength and has arc quenching capability.
The Townsend Breakdown criterion for air is given by:
where gamma is the Townsend Second Ionization Coefficient and alpha is Townsend Second Ionization. However for SF6 gas, the Townsend Breakdown Criterion is given by:
which we can see there is an additional coefficient added and it is called the attachment coefficient. The attachment coefficient is introduced to Townsend Breakdown Criteria in order to describe the behaviour of SF6 gas molecules which will attach itself to excess electrons during the first ionization and the second ionization. This behavior is the one which contribute to more superior dielectric strength and higher arc quenching capability compare to air.
In 1889, Friedrich Paschen made a discovery when he was experimenting the breakdown voltage of a parallel plate in a gas with respect to gap distance and pressure. He realize that as pressure of gas decrease, the density of gas will also decrease thus providing more space for electron to move. At some point, increment in gap distance will also increase breakdown voltage. Below is the Paschen's Law
where p is the gas pressure and d is the gap distance. The graph below shows different Pachen's Curve for CO2, Air and H2.
 |
| Breakdown Voltage of SF6 gas |
Explanation on why SF6 has higher breakdown voltage than air
In the previous section, I have stated that the breakdown voltage of SF6 is higher than air. In this world everything happens for a reason and same goes to this. SF6 has added characteristics which makes it more superior than air in terms of breakdown voltage. The reason is SF6 has higher dielectric strength and has arc quenching capability.
Molecular Structure of SF6 Gas
The Townsend Breakdown criterion for air is given by:
Effect of Pressure Towards Gas Breakdown Capability (Paschen's Law)
In 1889, Friedrich Paschen made a discovery when he was experimenting the breakdown voltage of a parallel plate in a gas with respect to gap distance and pressure. He realize that as pressure of gas decrease, the density of gas will also decrease thus providing more space for electron to move. At some point, increment in gap distance will also increase breakdown voltage. Below is the Paschen's Law
Noted that each gasses shows increment in breakdown voltage one the (pd) value increase. In my experience, 33kV GIS switchgear has a pressure of 1.2 bar compared to 132kV GIS switchgear which could reach to 5 bar.
No comments:
Post a Comment