Power transformers may have several types of insulation media. Some can be insulated with gases like air or SF6, others with solids like insulation paper, and some can be insulated with oil.
In this article, we will approach the theory behind the dielectric breakdown in liquids that allows us to understand the reason behind electrical discharges inside oil transformers. Let’s begin.
A little reminder of gas breakdown theory
As we could see in gas breakdown theory, the ionization by avalanche of Towsend occurs due to the crashes of electrons (with initial kinetic energy) with other electrons and atoms that ends up creating a chain reaction that creates the dielectric breakdown through the electrons reaching from one side with a specific voltage to another side with lower voltage or ground.
Due to the structure of matter in gases, electrons have a wider distance between them than what they do in liquids. If we remember from gas breakdown theory, the space between the electrons was directly related to the kinetic energy that they could absorb and transmit to other electrons that they crashed with.
However, in liquids, those distances between them reduce radically when compared to gases, and there are less free electrons than in gases. Therefore, following the theory applied in gases, liquids should have their dielectric breakdown in MV, but instead, the breakdown occurs in kV. Why does this happen?
Liquids and Contaminants
The conclusion that came out was that the extra moving electrons that made the dielectric breakdown possible in kV, had to come from another source. When doing research, it was found that contaminants (liquid contaminants, solid particles or dissolved gases) inside the liquid were responsible for those extra electrons. The reason is that those particles have a lower density, thus, they float, polarize and move, aligning themselves with the direction of the electrical field.
This creates a path of higher conductivity that increases the probability of an electrical discharge between two electrodes.
In other words, the higher the proportion of contaminants inside a liquid, the lower kV is needed to create a dielectric breakdown.
Types of Contaminants that Cause the Dielectric Breakdown
We can divide those impurities in the liquid according to their dielectric strength in the following categories:
- Impurities with lower dielectric strength than the liquid
- Impurities with unstable characteristics in the presence of an electrical field
- Impurities that increase the electrical field’s intensity in the liquid
- Thermal Breakdown
An example of these impurities could be bubbles of gases. When the bubble pops-up it can lead to an electrical breakdown.
Water bubbles are an example of this. Instability creates a little bridge of low resistance between two electrodes that can lead to a dielectric breakdown.
Conductive particles can increase the electrical field and lead to small dielectric breakdowns that can be the spark that initiates the electrical discharge between the two electrodes.
The excessive heat inside a transformer due to several sources can boil the oil. This evaporates contaminants that in a highly ionized gas state can increase their volume and lead to an electrical discharge.
We have now understood the true reason behind a dielectric breakdown inside a liquid. This is very useful to understand the purpose of quality tests for oils inside transformers which we will approach in another article.
What can be done to minimize the dielectric breakdown in oils?
Installing insulating barriers, forced and natural circulation of oil to cool it down, and installing radiators to control temperature and bubbles.