Nowadays, SF6 gas-filled current transformers become popular modern type of CT used in high-voltage substations. It uses SF6 gas as its internal insulating medium instead of conventional insulating oil.
We normally install SF6 CTs in voltage levels above 132 kV. A typical SF6 CT is a single-unit, live-tank type instrument transformer. Because it contains pressurized SF6 gas, the CT must be hermetically sealed to prevent moisture or air from entering.
Insulation System
SF6 CTs usually have a silicon composite insulator instead of a porcelain one. manufacturers design these CTs mainly for outdoor installation in the substation yard. SF6 gas provides the internal insulation, and composite insulator housing serves the purpose of outer insulation support. The insulation system of an SF6 CT corresponds to Class E insulation.
Manufacturers generally make the live tank of the CT from aluminium alloy steel. The CT contains a rupture disk with the correct bursting pressure at the top. This disk releases excessive internal pressure during a fault, preventing the CT from exploding. Every SF6 CT includes a valve for topping up or evacuating the SF6 gas when required. An SF6 current transformer is always equipped with a gas density monitor to observe the gas pressure inside the CT.
Magnetic Cores
Manufacturers normally use non-ageing CRGO silicon steel for the CT core. Some manufacturers also use nanocrystalline material to construct the core. A nanocrystalline core can achieve the same accuracy with a smaller size, mainly because it requires very low magnetizing current.
Advantages of SF6 CT
- The most important advantage of an SF6 CT is its lightweight structure. Obviously, it is much lighter compared to an oil-filled CT of the same rating.
- Because of its high dielectric strength, the pressurized SF6 gas allows the current transformer to be smaller than an oil-immersed CT. In other words, pressurized SF6 gas optimizes the size of high-voltage current transformers.
- The silicon composite housing makes the CT hydrophobic.
- The CT becomes highly shock-resistant because, unlike porcelain insulators, silicon composite insulators do not break easily.
- The combination of composite silicon insulation and pressurized SF6 gas makes the CT highly reliable under pollution and moisture.
- The rupture-disk, pressure-release system, and the sealed construction provide greater safety under fault conditions.
- Since SF6 gas does not age easily, it preserves the insulation for 30 to 40 years.
- SF6 gas also offers excellent thermal properties, and it is non-flammable.
Lighter weight, compact and fire hazard free characters make it popular in extra-high-voltage and ultra-high-voltage applications.
Disadvantages of SF6 CT
- The major disadvantage of an SF6 CT is the gas itself. SF6 is a greenhouse gas, so any leakage during normal operation or maintenance contributes to environmental harm.
- Topping up SF6 gas is not as simple as adding oil to an oil-filled CT; it requires specialized gas-handling equipment.
- Oil leakage is visible, but SF6 leakage is invisible. Therefore, regular SF6 leakage checks and regular monitoring of the gas density meter are essential.
- The initial cost of an SF6 CT is higher than that of an oil-filled CT of the same rating.