What is transformer oil filtration?
The function of the transformer oil is to cool and insulate the transformer. It must be chemically stable with high thermal conductivity and dielectric strength. It is also important that the oil retains these properties at high and low temperatures. Large transformer which are in danger of overheating may have special mechanism for oil flow to ensure proper cooling.
The use of oil in a transformer also requires certain precautions, such as detection of gas buildup and oil purification. This is because the oil undergoes both mechanical and electrical stress during the working of a transformer. The oil also gets contaminated as it flows in the transformer. The oil is in regular contact with other insulating components and the windings. With time it gets contaminated and its properties change.
To avoid this, we need to carry out regular testing of the oil. We also have to ensure regular oil purification. Also known as oil filtration, oil purification is the process of removing impurities, dissolved moisture and gases from the transformer oil. Both testing and transformer oil purification must be carried out as per a regular schedule.
There are many methods of oil testing, which are broadly divided into:
- Furan analysis
- Dissolved gas analysis
- PCB analysis
- General electrical & physical tests
Of these, general and physical tests are carried out on a bi-yearly basis, dissolved gas analysis once an year and furan testing is done once in two years. The last two tests are more comprehensive and give us an indication of the impurities present in the oil.
Transformer oil is bound to get dirty with time. There are many reasons why we need to ensure the regular cleaning of this oil. Some of these are:
- To maintain the insulation property of the oil, which will, in turn, ensure the insulation of other media.
- To maintain the insulation within the transformer. Better insulation means that there are fewer chances of transformer breakdown. This will eventually increase the lifespan of the transformer.
- A smooth running transformer means no cuts in the output. Ultimately, regular filtration is imperative to ensure an uninterrupted power supply – a critical requirement of any modern city.
When do we carry out the filtration process? Ideally, it must be followed as per the testing. Regular testing will expose the buildup of moisture and gases. This is an inevitable development with regular use. Maintenance and cleaning must immediately follow to ensure no hiccups. There are standards that must be maintained at every step of the process, from selecting the machine to following the procedure. Some of the most common methods here are dehydration by vacuum and solids and water separation by centrifugation.
The different steps in the process are:
The three main steps in the transformer oil purification are heating of the oil, removal of sludge and dehydration/degasification.
Heating the oil
First, we bring the oil temperature to a specified level, typically upto 65º C. The heater design must also prevent pressure build-up and temperature rise beyond a specific level. There are two reasons for heating the oil:
- Building up the latent heat, which will later help in removing the moisture and gases from the oil.
- Reducing the viscosity of the oil drops. This aids in better filtration.
Removal of sludge
Next comes the removal of sludge from oil. This is the dirt that accumulates in the oil. There are two main methods deployed for this:
- Filter candles
- Centrifugal force
Dehydration & Degasification
This is the final step in the Transformers repairing and the objective is to separate the oil from moisture or gas. These processes are carried out in the degassing chamber. However, we have to ensure that the hydrocarbons are retained so as to maintain the properties of the oil. Once all these steps are carried out, the oil is ready to be used again in the transformer.