In many projects, the filling of new transformers is often regarded as a “formal final step”. In reality, it is a technically sensitive process that has a decisive impact on the quality of the insulation system – and therefore on operational reliability and service life. Transformer size, oil type and temperature conditions directly influence the outcome. Anyone who oversimplifies the process or works under time pressure risks problems at a later stage.
In this interview, Peter Klausen, Field Service Manager at Electrical Oil Services, explains what project managers should pay particular attention to when filling new transformers.
“Filling is not just filling. Depending on transformer size, oil type and temperature conditions, the technical requirements change significantly. Precisely controlled vacuum management, an adapted filling speed and controlled oil quality are crucial for long-term operational reliability.”
— Peter Klausen, Field Service Manager, Electrical Oil Services GmbH
Because from the outside, filling a transformer appears relatively simple: insulating oil is filled and the transformer is ready for operation. In practice, however, three key factors interact – the size of the transformer, the type of oil used and the ambient conditions, particularly temperature.
These parameters determine how long the vacuum process takes, what filling speed is permissible and which limit values must be met. A one-size-fits-all approach does not ensure process reliability.
The risk of excessive moisture in the paper insulation, insufficient dielectric strength or trapped air is always present.
With smaller distribution transformers, oil volumes are manageable, geometries are simpler and filling times are relatively short.
With medium-sized power transformers, complexity increases significantly:
At this stage, it is no longer just about introducing insulating oil, but about fully impregnating the solid insulation. Insufficient degassing or an excessive filling speed can result in residual gas or moisture – with direct consequences for dielectric strength.
When working with large power transformers, the process is clearly defined by process engineering. The interaction between vacuum, filling speed and degassing must be precisely coordinated. Time pressure is a frequent source of error.
A very significant one. Mineral oils and esterbased or biooils differ in several physical properties, including viscosity and moisture behaviour.
Ester fluids are more hygroscopic, meaning they absorb moisture more easily. As a result, storage, handling and the filling process must be controlled even more carefully. The temperature window is also narrower, as flow properties can change more significantly.
In general, the filling process must be adapted to the specific characteristics of the insulating oil used. In the context of sustainability, the use of highquality regenerated transformer oils is also becoming increasingly important. Modern processing methods now enable recycled oils that meet the requirements for new insulating oil and offer technically equivalent performance.
Temperature directly affects the viscosity of transformer oil. If the oil is too cold, it flows poorly, penetrates the insulation more slowly and can trap moisture or air. If it is too warm, material properties and process parameters also change.
This aspect is often underestimated, particularly during outdoor operations or in colder climates. Yet correct temperature control has a direct impact on impregnation quality and therefore on future operational reliability.
Practical Tip: Decisionmakers should consider 3 topics when filling a new transformer for the first time:1. Plan the process early Filling should not be treated as the final project step under time pressure. 2. Assess oil type and transformer size individually Standard solutions do not work in every scenario. 3. Consistently demand quality assurance Measurement data and process documentation are not a formality – they are a safety factor. |
This always comes down to optimal quality assurance, compliance with standards and proper documentation.
Immediately after filling new transformers, several quality parameters are key, including:
Breakdown voltage
Water content
Gas content
Particle cleanliness
For mineral insulating oils, the key international standard is IEC 60296, which defines requirements for new and regenerated oils, including electrical and chemical properties.
Additional standards used for testing and evaluation include IEC 60422 (assessment of oil quality in service), DIN EN 60156 (breakdown voltage), DIN EN 60814 (water content) and IEC 60247 (dissipation factor). These standards define test methods and quality criteria and provide a reliable basis for technical assessment. In addition, projectspecific manufacturer specifications are binding, particularly for alternative insulating fluids such as ester oils.
Crucially, all relevant requirements must be verifiably met and documented.
Comprehensive documentation ensures that the filling process was carried out in compliance with standards and with full process reliability. This includes, in particular, records of vacuum management, temperature profiles during filling, and analysis results of the transformer oil used, such as water content, breakdown voltage and gas content.
Information on the equipment used and the measurement methods applied is equally important. A central database solution allows all process and analysis data to be stored in a structured way, tracked over the long term and used for audits, warranty issues or conditionbased decisions.
I see it as a major advantage when operators can rely on such a structured data basis. The transparency we provide to our customers via our central EOS® database is highly valued in practice – especially when it comes to verification, benchmarking or longterm condition assessments.
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FAQs on the topic Filling new transformersWhy is filling a transformer such a critical process? Does the filling process differ depending on transformer size? Are there differences between mineral oil and ester oils during filling? Which measurements are particularly important after filling? What role does temperature play during filling? |