Skip to content
Kilian GerblMar 27, 2023 2:35:19 PM4 min read

Valuable tips when you have to replace the oil in the transformer

You know this situation: An insulating oil sample was taken from your transformer and tested in the laboratory. The result of this transformer oil analysis suggests - in order to keep the transformer's performance stable, among other things - that the insulating oil should be changed. You can read here what the next steps are.


The insulating oil analysis is an important criterion for determining the condition of the transformer. This is because the insulating oil quality decreases in the course of a transformer's operating time and thus influences its performance. The inhibitors present in the insulating oil are depleted over time, the acid content in the transformer oil increases and has a negative effect on the electrical and thermal properties. 


The demand plan: Profound recommendations are based on regularly documented analysis data 

Because transformer insulation and cooling are equally important, operators should have the insulating oil in their transformers checked regularly as part of their asset management programme. 

Ideally, results of previous insulating oil analyses are available and can be used to decide whether and when the transformer oil needs to be processed or changed. This means that precise documentation of the regular checks is necessary, preferably in a transformer database. 

The demand plan resulting from this data helps the experts to professionally justify necessary measures. And to recommend exactly those steps that make technical and economic sense. Depending on the condition of the insulating oil and the inspected transformer, there are different ways to improve the quality of the insulating oil.


3 options to improve the transformer oil quality: 

1. The transformer oil may be dried, filtered and degassed using an oil processing system.

2. The transfomrer oil may be replaced by new or recycled insulating oil. 

3. An insulating oil regeneration is recommended, which contributes to the deep cleaning of the insulating oil AND the entire insulating system. 


You can read more about the mobile regeneration process in this blog article:


25 Questions and more: What to consider when you have to replace the insulating oil in the transformer 

In case any of the above three steps are recommended in the asset management plan, you should consider the following points to assess the service company to be selected:


Specialist disposal company

Is the company you hire to change the insulating oil authorised to transport and dispose of the used oil? Is it a certified waste management company? If this is not the case, does it agree to a take-over certificate? Does the company have the appropriate equipment and tools to do the work? Have the hoses, for example, been tested? Is the equipment regularly serviced and flushed with fresh oil? Does the process plant give a clean and well maintained impression? Have the operators been adequately trained, demonstrate the required competencies and undergo  regular training sessions associated with this type of work?


Cost estimate / quotation

Have you received a cost estimate or quotation from the service provider and what do they contain? Is the nature and scope of the work clearly presented? Does the estimated labour time seem realistic? How high are the estimated labour costs and the costs for the required materials? Can deadlines or commitments be made? How long is the estimate / quotation valid? Is there a clear contingency should the work or site experience delays?


Emergency plan / risk assessment

What dangerous situations can occur? What mistakes can be made? To what extent is the service provider prepared for emergencies? How does the service provider deal with contamination by hazardous substances? Or what happens if, for example, the insulating oil is not heated or not heated enough and then processed? Does your selected service provider provide accurate and representative RAMS (Risk Assessment & Method Statement)?


Project planning 

Has the service provider received all the information he needs? Are further preliminary investigations or appointments necessary? Have the responsibilities on all sides been defined? Where are the priorities? What is the time sequence of the measures? Does it fit in with your own procedures? Is an emergency generator on site? How are the experiences documented so that they can be used for future measures? To what extent does the project plan allow for scheduling flexibility, e.g. for supplementary work?


Is new or recycled transformer oil recommended for replacement?

By answering above questions, you will find out whether you can trust the service provider with your transformers or your insulating oils and how reliable the company appears. You will then also know whether to follow the recommendation in the demand plan.



Register today for our EOS Trusted Purity blog:

Subscribe now



Only one question remains: Will you opt for recycled transformer oil OR new transformer oil?

Or will recycled insulating oil not be offered by the service provider at all? Because as a technical manager, you are responsible not only for the process and deadlines, but also for costs and quality in the project. And this increasingly includes sustainable solutions, such as the use of first-class recycled products. 


Our tip - read this blog article:

DOWNLOAD - Practical guide with 25+ questions to ask: 



When insulating oil needs to be replaced, there are many points to consider. This article is intended to support you in asking the right questions in order to be well prepared for maintaining the value of your transformers, also against the background of sustainability. Please feel free to use it as a practical guide!

Electrical Oil Services (EOS®) offers you many products and services in the field of transformer oils. We work in partnership with our customers and are pleased to impart sustainable, comprehensive knowledge about insulating oils.


Learn more about EOS® in these articles: 


We are also happy to answer your questions directly:

Neuer Call-to-Action


Kilian Gerbl

Kilian Gerbl is Head of Sales for Electrical Oil Services GmbH. He is responsible for all commercial-related topics in Europe – this includes customer relationship management as well as new project and business development. His key focus is to expand the EOS activities across Europe and further implement the sustainable EOS Closed-Loop model.
Tel: +49 151 5351 5373