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Author: Sandra SparenbergMar 12, 2024 6 min read

Transformer: Representative insulating oil sampling in 10 steps

When assessing the condition of transformers, the sampling of insulating oil is of vital importance. It provides decisive information about the oil quality and therefore about the general operating condition of a transformer. This article deals with possible errors in sampling and analysing insulating oil. In the following interview with Dr Paulina Nieves, an experienced chemist and laboratory services manager at Electrical Oil Services, you will learn more about the steps involved in sampling insulating oil and what can go wrong.

"I would say that the insulating oil sampling, in other words, the sample taking, is the most critical. This is where 90 per cent of the breakdown voltage errors occur, for example, leading to incorrect results and giving companies a false sense of security. It's good that our EOS team is well aware of this and undergoes regular training.”

Dr Paulina Nievesresponsible for EOS lab in Germany



Dr Nieves, what considerations are important when sampling insulating oil and analysing transformer oil?

Insulating oil analyses enable a good assessment of the insulating fluid in transformers with regard to its electrical, physical and chemical properties. Of course, it is not only the analyses that need to be carried out carefully; even the sampling of insulating oil from the transformer requires a careful approach.

I would say that the insulating oil sampling, in other words, the sample taking, is the most critical. This is where 90 per cent of the breakdown voltage errors occur, for example, leading to incorrect results and giving companies a false sense of security. It's good that our EOS team is well aware of this and undergoes regular training.

Four points are particularly important when sampling insulating oil:

  1. A clean environment for oil sampling
  2. Suitable equipment for oil sampling
  3. Correct identification of the oil sample
  4. Accurate handling of the oil sample


How are insulating oil samples optimally prepared?

All sampling equipment must be clean. It must not have any impurities that could distort the result. Careful cleaning and drying of the sampling equipment in advance is therefore essential.

It must also be ensured that the insulating oil sampling is carried out accurately and well documented. A report form that clearly assigns the sample to the correct transformer and provides important information is part of the process.

The person taking the insulating oil sample needs a suitable collecting tray, a special device for draining the transformer oil and an oil sample bottle with a label. The oil sample bottle is usually an aluminium bottle with a plastic screw cap and a sealing ring. The cap must be very tight so that no air is drawn in when the sample cools down.



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How is an insulating oil sample taken?

The processes must be carried out quickly and without interruption. The aim is to prevent the oil sample from coming into contact with the atmosphere as much as possible and to protect the sample from external influences. There are specific devices that we use for this purpose. Unfortunately, due to the design of transformers, this is not always so easy.

We provide the following 10 steps as a guideline. However, the insulating oil sampling may vary depending on the specific circumstances:

  1. Before starting the oil sampling, all necessary tools should be gathered and checked to ensure they are clean and in good condition. Any contamination on the equipment can lead to errors in the sample.
  2. The sampling point should be cleaned with a lint-free cloth. We strongly recommend that the routine sample be taken from the same location each time to ensure consistent insulating oil sampling.
  3. The sampling toolkit uses an oil-resistant rubber stopper with an approx. 5 mm thick stainless steel tube for insertion.  
  4. A thermocouple is attached to the end of the tube and the sampling valve must be opened carefully. The temperature of the transformer oil is recorded as approximately 1 litre of oil flows into the waste. The insulating oil sampled first should be analysed for the absence of water.
  5. The end of the hose is to be placed on the bottom of the bottle. Slowly fill the bottle to gently displace the air and minimise distortion of the transformer oil.
  6. Fill the oil sample bottle so that some insulating oil overflows. While the oil is still flowing, slowly pull out the hose.
  7. Close the sampling valve and remove the hose. 
  8. Tilt the sampling bottle so that the oil level drops to an acceptable level. Then screw the bottle cap on tightly.
  9. Immediately label the sample bottle and ensure that the documentation is clearly recorded and attached to the sample bottle.
  10. Finally, return everything well packaged to the Electrical Oil Services lab or any other authorized testing laboratory.


In your opinion, how is a representative insulating oil sampling guaranteed?

If you follow the above steps, you are already on the right track to obtain a representative sample. Also, as already mentioned, it is important to label each insulating oil sample correctly - nothing is more annoying than a sample without a label or missing documentation. Finally, safe storage is important. To this end, the insulating oil sample should be stored in a cool and dark place to minimise changes until the analysis can be carried out.


How is a correct analysis of the insulating oil sample carried out?

Once the transformer oil sample has arrived at our EOS® laboratory, we always make sure that it has acclimatised and reached room temperature. All analyses should be carried out within a short time frame. The Gas-in-Oil (DGA) analysis must be carried out as soon as the oil sample bottle is opened. In contrast to other parameters, this test cannot be repeated. In other cases such as the breakdown voltage, the measurement can be repeated the next day if necessary. The water content value increases the longer the bottle is open.


What quantity of insulating oil is recommended for an analysis?

600 ml of insulating oil is usually sufficient for routine analyses. However, it also depends on whether the breakdown voltage is to be analysed - 400 ml is required for this alone. In addition, we always take 50 ml of the insulating oil sample as a control sample and store it for two years. So you see, here's the answer: It depends!

One thing is certain, our EOS® laboratory services include more than just analysis work. We provide advice right from the start and recommend appropriate measures - including the quantity of samples to be taken and the scope of the analyses.


Dr Nieves, one last question: How long do you need to analyse the insulating oil of a transformer?

That depends, but usually one to two days. However, as we always write reports, enter them into the EOS® database and also provide recommendations for action on request, the entire insulating oil analysis and documentation often takes up to a week. However, this includes the detailed measures that guarantee preventive maintenance.

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In summary, correct insulating oil sampling is a crucial aspect of transformer condition assessment. It helps to identify problems at an early stage and maintain the longevity and performance of the transformer. It is therefore important that insulating oil sampling is carried out carefully and to the highest standards. There are specialised companies on the market that offer this service. 


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Author: Sandra Sparenberg

Sandra Sparenberg is Head of Corporate & Marketing Communications at HCS Group and is responsible for the company's strategic communications, including press relations and sales communications for the Haltermann Carless, ETS Racing Fuels and Electrical Oil Services (EOS) brands. One focus of her work is the development of online communications and Inbound Marketing. T +49 69 695 386 117 E-mail