Effect of Moisture inside SF6 Gas Insulated Switchgear

Water is important to us and other living things in this world. Without water, life could not be sustained. Water can be found anywhere even in the air around us in the form of moisture. That is why we can see condensation on a cold glass of water. The water is not from the inside of the glass but it is from the moisture in the air. We use the term Humidity in order to measure the content of moisture in the air.

A glass of cold coke with a slice of lemon on top and condensation at the outer glass. You want it, don't you? Too bad it's already inside my stomach...except for the glass.

I live in Malaysia and my country is one of the country which has a very high humidity. In other words, Malaysia has a high moisture content in its air. This humid condition affects not only people and other living things but also non living things such as electrical equipment, your boss's car, your friend's stinky stocking, and other examples which I don't want to list here in order to prevent taking up more space on your computer RAM.

In my experience working in the electricity industry as an electrical engineer, various ways are being applied in order to reduce the effect of moisture to High Voltage (HV) Equipment. Moisture is conductive in nature so the existence of moisture on HV equipment will not only lower the effectiveness of the insulation coordination but it will also cause deterioration on the HV equipment insulation. The deterioration will lead to a catastrophic damage to the HV equipment. In this blog entry I will explain on the Effect of Moisture inside SF6 Gas Insulated Switchgear.

Moisture maybe bad for HV equipment but women use moisturizer to increase  moisture content on their skin so they always look radiance, subtle and fresh. Please don't ask for the phone number of the woman in the picture, she's not my office colleague.

All SF6 Gas Insulated Switchgear (GIS) in Tenaga Nasional Berhad and most parts of the world are fully encapsulated in order to hold the SF6 gas. To connect one encapsulation to the other components, heavy duty gaskets (or O-Rings) are used. However because of the aging factor, thermal or mechanical stress, these gaskets lost its flexibility and deform. The deformation of these gaskets not only leads to SF6 leakage, but also the entrance of air to the encapsulated SF6 gas chamber. These air will bring moisture along with them.

One of my apprentice engineer asked me about the effect of moisture entering the SF6 GIS. I answer to him:

"Do you think i'm a walking google website?" 

Just joking. Of course I explained to him all the effects of moisture to the SF6 GIS. It is good to share knowledge with other fellow engineers. The next paragraph is what I have explained to him.

Men also use moisturizer. Please don't be shy to admit. The guy above is not a gay.

When switching is done in a SF6 Gas insulated switchgear, arching will occur. This is because there is a high impedance on the breaker switch when the switch is opened and high voltage current tries to flow through the impedance. The arching release immense heat and SF6 gasses will quench the arching in order to prevent it from spreading.

It is generally known that the rule of chemistry said any substance (either solid, liquid or gas) will experience chemical reaction in the presence of heat. Same as SF6, the immense heat will break the chemical bonding of SF6 and create Sulfur atom and Florin molecule. Because this chemical reaction takes place in an encapsulated area filled with SF6, the Sulfur atom and Fluorine molecule will combine back to become SF6 as usual.

However when there is a leakage and air enters the SF6 gas compartment, it will bring along the moisture. Air and moisture will prevent the Sulfur atom and Fluorine molecule to combine back. In immense heat, highly mobile hydrogen molecules will combine with Fluorine molecule and form Hydrogen Fluoride which is corrosive in nature. Hydrogen Fluoride can be seen as a thin layer of white precipitation on busbars. Because of its corrosive nature, the busbar will be corroded and metal ions will be released into SF6 compartment. This metal ions will combine with Fluorine molecule and becomes Metal Fluoride. Metal Fluoride will also precipitate on the busbar.

Both precipitation will create uncertainties on the electric field of the busbar which result in Ultrasound generation. At the same time busbar will slowly becomes corroded. In times the busbar will fail and cuase catastrophic damage to the whole switchgears.

Moisture - good for women and some men but bad for HV equipment.