Transformers are the lifeline of every single electrical grid and network system. They work with voltage levels to amp them up or bring them down according to the requirements of the system, at a certain point of the network.
With the need for varying voltage levels in an electrical system, it is necessary for a transformer to perform switching action efficiently without any errors.
A transformer has two windings that are called the primary and the secondary winding. Current passes through the primary winding and gets converted into electric flux which is an invisible component invisible to the human eye. This flux is absorbed by the secondary winding that produces a current value in return.
The primary and the secondary winding are wrapped with insulation that is made up of cellulose. For further protection, the windings are submerged in transformer oil with high dielectric constant, that acts as a coolant while also suppressing corona discharges.
The presence of moisture in a transformer can reduce the dielectric strength of oil as well as increase the number of surges in the transformer. Excess moisture is a surefire way to deteriorate the cellulose in the winding, ultimately decreasing the life of the transformer.
Methods of Measuring moisture in a transformer:
Residual moisture inside a transformer can be measured in a number of ways. Below is a list of the 3 most commonly used methods of measurement.
Dielectric test strength of oil:
Oil is immersed in a setup that has two electrodes separated at high voltages.
The level of voltage at which the sparks occur is called the BDV ( Breakdown Voltage).
The lower the BDV, the greater is the moisture content present in the oil.
Insulation resistance test of winding:
This test is performed by connecting one end of the megger to the line terminal of winding under test.
The other terminal of the megger is connected to the ground.
The handle is Rotated, or a button is pressed down to get the reading of the insulation resistance of the conductors.
Water in oil analysis:
The ratio of water in the oil mixture also affects the insulation properties of the power transformer.
When more water is present in the void, the transformer is easily affected by the voltage impulses.
99% of water is found in the paper cellulose in the winding.
But here’s an intriguing question:
How does water get into the transformer?
It is fascinating to see how moisture enters the tightly sealed environment of a power transformer. Especially when transformers are specifically designed to keep out moisture for the safe functioning of the transformer.
Water generally comes from the paper that is scarred from high voltage and current levels. After scarring, it separates from cellulose to become free water. Ageing of cellulose and oil also decompose them to result in the formation of free water which results in the accumulation of excessive moisture content inside the transformer.
But the steps to get oil out of your transformer is pretty basic. The most commonly used methods are:
Hot oil flush:
Fresh oil is passed on through paper filters and then sprayed on the insides of the transformer.
As hot oil has a greater affinity for moisture, it attracts all the free water molecules from the core and the winding.
Spray hot oil:
Hot oil is sprayed in the transformer with the surroundings maintained at a vacuum-based atmosphere.
With the effect of hot oil, the free water is immediately converted to water vapour and simultaneously sucked out by the vacuum pump.
Excessive moisture content is always dangerous and detrimental to the functioning of the transformer. We these simple steps you can easily tackle the moisture of your transformer.
