How SF6 works as electrical insulation

People knows what is H2O. Lots of people knows what is OMG. And some even know WTF is..haha. But what is SF6? Why is it so good that some electrical engineers swear that SF6 is their best friend?

Before I explain further, let me go to the toilet first and release my own "SF6" gasses first.

Just joking. In my previous posts, i have explained thoroughly what is SF6 and its capability of quenching any arching activity. In this post, i will give a real life situation which explains how SF6 works as electrical insulation.

Tamco VY30M25D 33kV Switchgear

The picture on the left shows a single switchgear feeder with 3 phase 33kV supply in PPU National Semiconductor substation in Pulau Pinang. The brand of the switchgear is Tamco VY30M25D and it uses air insulated busbar. The width of the switchgear around 1.2m and the length is 1.75m. It is considered very big as it will take 2 person to rack out the breaker from switchgear. Imagine it to be as big as a 2 door wardrobe. You can easily fit 3 person inside the breaker compartment.

Tamco GIS Switchgear

The picture on the right is also a single switchgear feeder with 3 phase 33kV supply fitted in PPU Hospital Substation in Pulau Pinang. It is also made by Tamco and uses SF6 gas as insulation for busbars. Usually it is called Gas Insulated Switchgear (GIS). The width of the switchgear around 0.5m and the length is 1m. The built is very compact and has less floorprint compared to the switchgear in Pic 1 although both of them operate at the same voltage level. 

The usage of SF6 as insulation gives advantage of equipment spacing inside a switchgear. This will lead to a lesser floorprint which means that the whole land area of a substation can be greatly reduced. In a country where land is considered a scarcity such as Japan or in a metropolitan area where land is considered a premium like Kuala Lumpur, GIS technology is preferred. Breakdown voltage of SF6 is higher than breakdown voltage of air so the busbar of each phase can be placed as close as the breakdown voltage permits. You can refer to my post on Gas Insulation (Saturday, 9th April 2011) to know more on this. In the future with the current R&D done throughout the world, new insulation gas will come out and eventually will further reduce the size of switchgears with its superior insulation capabilities.

SF6 Gas Insulated Switchgear also has a longer lifetime compared to their air insulated switchgear counterpart. According to electrical gas breakdown phenomena, we have 2 types of discharge: i) Non-sustaining and ii) Self-Sustaining. The discharge will start as non-sustaining at first and will soon develop to self sustaining discharge. That is why we will see an increase of partial discharge activity if we do a trending on any switchgear with partial discharge irregularities. This non-sustaining discharge if not properly controlled or rectify will lead to self sustaining discharge and further leads to breakdown avalanche. By then it is already to late. 

In Air Insulated Switchgear, air cannot absorb the electrons created by the non-sustaining discharge. The abundance of electrons available from the non sustaining discharge will further propel the process of creating a self sustaining discharge thus less time is needed for a self sustaining discharge to form. This will greatly reduce the equipment lifetime.

In SF6 Gas insulated Switchgear, SF6 gas has a tendency to attach to electrons created by the non-sustaining discharge so longer time is needed for a self sustaining discharge to form thus the equipment lifetime will not be reduced. SF6 gas will 'arrest' any free electrons created by the non-sustaining discharge on the spot. The SF6 and electrons attachment will prevent these free electrons from creating any self sustaining discharge.

3 electrons being captured by SF6 Special Squad (I bet the red one is the leader)

The picture above is only a gimmick to entertain your mind after reading my long explanation. You will not find these guys inside your switchgears...

Except if they are crazy enough to steal the copper busbars.