Transformers: Methods of insulating oil testing

Ensuring the quality of insulating oil in transformers is an essential aspect of a preventive maintenance programme. To extend the service life of transformers, it is therefore advisable to regularly take and test a sample of insulating oil. Such insulating oil tests serve to detect impurities and degradation products at an early stage. The most important aim is to avoid transformer performance losses or even failures. Read here which methods and key figures are important for these insulating oil tests. 

Transformers are important components of a power grid. They are manufactured to the highest standards and are designed for a long service life. However, many transformers are already older and are increasingly being operated at their performance limits. This increases their vulnerability to faults.

Ensure a long transformer service life and counteract failure susceptibility with insulating oil tests

No more unplanned shutdowns: Checking the insulating oil or carrying out insulating oil tests - even during operation - helps to recognise and counteract faults in good time.

With suitable insulating oil tests and targeted diagnostics, you can

• identify faults and malfunctions
  
• recognise impending failures at an early stage and thus prevent them 
  
• enable strategic maintenance planning and condition assessment of entire transformer fleets
  
• targeted insulating oil management can be set up to ensure the performance of transformers and increase their service life 

Insulating oil testing methods: these tests are important

There are a total of 17 common transformer oil tests. Some of the tests are part of the required routine tests, while others such as furans, PCBs and corrosive sulphur help to delve deeper into a diagnosis or are necessary to obtain certain certificates.  

Insulating oil tests always analyse the electrical, chemical and physical properties of the transformer oil. Below are some examples as well as an overview of routine tests:

1. Electrical properties of the insulating oil:

• Electrical resistivity
  
• Dielectric strength 
  
• Dielectric dissipation factor
  
• Viscosity

2. Chemical properties of the insulating oil:

• Water content
  
• Acid content (acidity)
  
• Sludge content
  
• Furan analysis (ageing of the cellulose)
  
• Gas-in-oil analysis (DGA)

3. Physical properties of the insulating oil:

• Interfacial tension
• Viscosity
• Flash point
• Pour point
• Colour

Overview of the routine tests of an insulating oil test:

• Colour - used to observe the darkening of the insulating oil.
  
• Dielectric breakdown voltage - is a measure of the insulating oil's ability to withstand electrical stress without failure.
  
• Interfacial tension - measures the presence of soluble impurities and oxidation products in the insulating oil.
  
• Moisture content - indicates the water content in the insulating oil.
  
• Acidity - measures the content of acids formed by oxidation and impurities.
  
• Dielectric dissipation factor - measures the electrical insulation properties.

Which insulating oil tests are carried out depends on the particular customer and the transformer fleet to be analysed. What exactly does the company want to have analysed? How much should an insulating oil analysis cost? How detailed should the analysis be? How large is the transformer fleet? How old are the individual transformers? For old transformers in particular, it is necessary to determine the PCB content in addition to the parameters mentioned above.

Use certified companies and laboratories for every insulating oil test

It is important for all insulating oil tests and their analyses to ensure operational safety at all times and to avoid malfunctions. The quality of the analysis results must also be guaranteed. Certified expert companies and laboratories ensure high-quality analyses and make a decisive contribution to minimising potential risks. On top, they have highly skilled staff, state-of-the-art technology and strict quality control mechanisms.

How to choose the right laboratory service provider? Check out our blog:

• Laboratory services for insulating oils: what to look out for

Or read this blog article to find out exactly how sampling works during operation:

• Transformer: Representative insulating oil sampling in 10 steps

Conclusion

Regular insulating oil tests based on certain parameters enable an accurate assessment of the condition and performance of transformers. The associated inspection of the transformers ensures a high level of operational safety and reliability. Authorised companies and certified laboratories carry out high-quality insulating oil tests and provide reliable results that help to identify and prevent potential transformer issues at an early stage.

You may also be interested in this blog article:

• Oil change cost check: When should the insulating oil in transformers be replaced?