When and how to change oil in a MV transformer? Costs, procedure and best practices in accordance with EN 60076

How often should you change the oil in a medium voltage transformer?

Imagine having a car that has never had its engine oil changed.

It runs... until it doesn't. Then one day it just stops, and you are left with a repair bill higher than the cost of the car itself. Medium voltage transformers are quite similar.

Oil is their lifeblood, and when it stops functioning properly, it is not just the equipment that shuts down but your entire infrastructure.

In this article, we explain why transformer oil replacement is not just a maintenance issue but a key part of your company's energy strategy. It affects power continuity, compliance with environmental regulations (including the PCB ban), fire safety, and your budget.

If you care about power reliability, compliance with PN-EN 60076, cost control, and a future without PCB, this read is for you. You will gain practical knowledge you can apply immediately.

We also reveal facts rarely found in generic guides:

• Which oil to choose: mineral, regenerated, or synthetic MIDEL ester?

• When and how to replace it without shutting down the entire station?

• What are the costs and savings depending on the type of oil, and how to avoid overspending across the transformer’s life cycle?

• What to do with transformers from the 1980s that may contain PCB? How to act in compliance with the law?

At the end, you will get a BONUS: a ready-to-use action schedule and specific EUR rates for every stage of the process, with no surprises.

Reading time: 12 minutes

Types of transformer oils and maintenance differences

Choosing the right oil for a medium voltage transformer is one of the key elements of long-term operational planning.

Not all oils have the same dielectric properties, thermal resistance, environmental impact, or maintenance requirements.

In technical practice, three main categories of oil are currently used: mineral (new), regenerated, and synthetic esters such as MIDEL (MIDEL is the trade name for biodegradable synthetic esters used as insulating and cooling liquids in transformers, manufactured by M&I Materials Ltd in the United Kingdom.)

Each of them involves different costs and maintenance strategies.

Mineral oil (new, degassed)

This remains the most common choice for medium voltage transformers. Its unit cost is currently around 1.4–1.8 EUR/litre, making it the most accessible solution. It features good dielectric properties but is flammable and non-biodegradable, which limits its use in sensitive environments. It requires more frequent quality inspections (analysis of water content, acidity, and dielectric dissipation factor), and its replacement is recommended every 10–12 years. In aggressive environments, this interval can shorten to as little as 5 years.

MIDEL oil, a synthetic ester, is the premium option

The purchase cost is about 3.2–3.8 EUR/litre, but it offers a range of technical and environmental advantages. MIDEL is biodegradable (>98% within 28 days), fire-resistant (flash point >300°C), and shows high chemical stability even at elevated operating temperatures. This makes it suitable for use in hospitals, public buildings, water protection zones, and Natura 2000 areas. Its lifespan can exceed 20 years without replacement, and maintenance mainly consists of condition monitoring through sensors in next-generation transformers.

Regenerated oil, class II

Often used in the revitalisation of older transformers, this is a cost-effective alternative with a price of about 0.9–1.1 EUR/litre.

It is produced through processes that purify used oil and restore some of its insulating properties. However, its use requires special caution – only in transformers that have been thoroughly cleaned, with no risk of chemical reactions with residues from the previous fluid.

The lifespan of such oil is shorter than that of new oil, so more frequent testing and replacement planning every 3–5 years are advised.

Despite its attractive price, regenerated oil typically does not meet the strictest environmental standards, such as those required for water protection zones or public utility facilities. Therefore, it should not be used in installations with elevated safety requirements where both reliability and risk minimisation for people and the environment are crucial.

Summary comparison of transformer oil types:

• Mineral oil (new, degassed)
  Price: 1.4–1.8 EUR per litre
  Flash point: approx. 160°C
  Biodegradability: none
  Replacement frequency: every 10–12 years (as often as every 5 years in aggressive environments)
  Applications: heavy industry, transmission networks, standard MV substations

• MIDEL (synthetic ester)
  Price: 3.2–3.8 EUR per litre
  Flash point: above 300°C
  Biodegradability: very high (>98% in 28 days)
  Replacement frequency: every 15–20 years (or longer with proper use)
  Applications: water protection zones, hospitals, public buildings, data centres, critical infrastructure

• Regenerated oil (class II)
  Price: 0.9–1.1 EUR per litre
  Flash point: from 140°C to 160°C
  Biodegradability: none
  Replacement frequency: every 3–5 years
  Applications: revitalised transformer stations, older equipment after cleaning, facilities without heightened environmental standards

PCB in transformer oil – what you need to know and how to comply with regulations

When discussing the regeneration of transformer oil, there are certain substances that completely rule out this option. One compound – now banned – still causes sleepless nights for engineers and infrastructure managers. We're talking about PCB.

Polychlorinated biphenyls (PCB) are a group of chemical compounds that for decades were considered nearly ideal insulating media: stable, non-flammable, and resistant to high temperatures. Sounds great, right? Unfortunately, it is exactly this durability that makes PCB persist in the environment for decades. Today we know they are highly toxic, carcinogenic, and accumulate in living organisms.

In the European Union, the use of PCB has been completely banned since the 1990s. Under Regulation (EU) 2019/1021 (also known as the POPs Regulation), all equipment containing PCB must be decommissioned no later than the end of 2025.

How to check if your transformer contains PCB

If the equipment was manufactured before 1986, an analysis of PCB content in the oil is mandatory.

A sample must be taken in accordance with standard PN-EN 61619, and the result is expressed in mg/kg (ppm):

• below 50 ppm – the oil is considered PCB-free and may continue to be used

• 50 ppm or more – the oil contains PCB and must be disposed of as hazardous waste

Oil containing PCB may not be regenerated or mixed with other fluids. There are no half-measures here – full disposal is required via specialised facilities with the appropriate permits.

In practice, this means:

• Commission an oil analysis if your transformer dates back to the communist era

• Register the result in the BDO system and label the equipment in accordance with regulations

• If the result exceeds 50 ppm – ensure safe decommissioning of the device by 31 December 2025

At the same time, consider not just replacing the oil but also undertaking a broader modernisation – such as purchasing a transformer filled with biodegradable MIDEL oil, which meets current environmental standards and offers peace of mind for years to come.

Because PCB is not just an environmental problem. It is also a legal, financial, and reputational risk. Mishandling equipment containing PCB may result in administrative fines, criminal liability, and difficulties in obtaining ESG certificates or qualifying for public tenders.

In the next section, we will show you what a complete oil replacement procedure looks like – in compliance with PN-EN 60076 – and what “properly executed maintenance” truly means.

Insulating oil replacement procedure in a transformer in accordance with PN-EN 60076

Replacing oil in a medium voltage transformer is not a “quick fix.” It is a precise process, where every step impacts safety, insulation quality, and continued operation of the equipment. Standard PN-EN 60076 (especially parts 22-1 and 7) outlines not only technical parameters but also the testing requirements and service procedures.

In practice, the procedure includes several main steps:

Assessing the condition of the oil
Before anything is drained or filled, a sample of the oil must be taken and sent for laboratory analysis. Parameters tested include breakdown voltage, acid number, water content, dissolved gases, and furan levels (indicating paper insulation degradation).

Preparing the transformer
The transformer must be disconnected from the power supply, secured against accidental re-energisation, and cooled down. This requires proper coordination with the grid operator or internal infrastructure manager.

Draining the old oil
Oil is drained through the bottom valve, and the entire system – including the conservator, cooling pipes, and chambers – should be emptied into an appropriate tank. The old oil can be regenerated, disposed of, or sent for recycling in line with Directive 2008/98/EC.

Cleaning the system
This step is often overlooked but is crucial – more on this in the next section. The system is flushed with a special service oil or a clear, dry oil of the same type.

Filling with new oil
The new oil must be properly dried, degassed, and filled in a way that prevents air from entering the system. It is usually introduced from the bottom through a vacuum filter.

Pressure testing and breakdown voltage testing
After filling, a leak test and on-site breakdown voltage testing are carried out. If there are any doubts, additional tests are performed in accordance with PN-EN 60296.

Startup and monitoring
After the oil change, the transformer should operate in supervised mode for at least 24 hours – with monitoring of temperatures, load, and electrical parameters.

It is important to remember that according to Ecodesign requirements and occupational health and safety regulations, all operations must be carried out by trained personnel, with proper documentation and an emergency plan.

Cost of oil replacement in an MV oil transformer

How much does it cost to replace oil in a transformer? This question comes up regularly – and rightfully so. Although the cost of the oil itself is just one part of the overall bill, it often determines whether the investment will be carried out.

Estimated costs (based on the European market in 2025):

• Mineral oil (new, degassed): 1.4–1.8 EUR/litre

• MIDEL 7131 (synthetic ester): 3.2–3.8 EUR/litre

• Regenerated oil (class II): 0.9–1.1 EUR/litre

Cost of the replacement service (including system cleaning, vacuuming, testing):

• For a 630 kVA transformer: 1800 to 3200 EUR

• For units over 1600 kVA: even 5000–7000 EUR, especially in container stations

Additionally, there are:

• Disposal or recycling costs for old oil: 0.4–0.8 EUR/litre

• Laboratory analysis: 100–200 EUR for a full set of tests

• Transport and logistics: depending on location

Interestingly, a transformer filled with MIDEL oil requires less frequent replacement and can operate longer under boundary conditions. This translates to lower total cost of ownership (TCO), despite the higher initial price.

From a project budgeting perspective, it is therefore wise to consider not only the price per litre but also the overall usage costs, failure risks, and potential maintenance savings.

Replacing mineral oil with MIDEL in a transformer – step by step

More and more infrastructure owners are choosing to replace traditional mineral oil with advanced insulating fluids such as MIDEL. Why? MIDEL is a synthetic ester with a very high flash point, low toxicity, and full compliance with European Union environmental requirements. But the decision is just the beginning.

The process of replacing mineral oil with MIDEL in an MV transformer involves several key stages:

Verification of design compatibility
Not every transformer can immediately accept MIDEL oil. Before starting, the compatibility of sealing materials, rubbers, varnishes, and cellulose components must be verified. MIDEL operates at higher temperatures and has different dielectric properties, so some older materials may not be suitable.

Manufacturer and design documentation
Consultation with the transformer manufacturer is required, or an independent material assessment must be carried out. Additionally, some transformer stations require this modification to be registered as a technological change.

Removal of old oil and system cleaning
The transformer must be completely drained of mineral oil. Next, the system must be flushed with a neutral oil (typically a technical version of MIDEL) or a dried cleaning mixture to remove any residue from the previous fluid.

Filling with MIDEL oil
MIDEL must be introduced under vacuum conditions, with temperature and moisture level control. The fluid must be properly dried (<10 ppm water content) and filtered before use.

Post-replacement testing
Breakdown voltage tests are performed, as well as dielectric dissipation factor measurements and leak testing. It is also necessary to monitor winding temperature during the first weeks of operation, as MIDEL has a higher heat accumulation capacity.

The benefit of such a replacement is not only improved fire safety and compliance with environmental standards (REACH, RoHS) but also the possibility of installing the transformer in areas with strict requirements – such as near water sources, in national parks, or in hospitals.

How to check transformer oil quality and when to replace it

The condition of transformer oil is one of the best indicators of the overall health of the equipment. A properly selected schedule of oil analyses not only allows for failure prediction but also helps optimise the timing of oil replacement, avoiding unnecessary costs.

Key oil quality tests:

• Breakdown voltage (BDV): shows the oil’s ability to withstand voltage without dielectric failure. A result below 30 kV indicates the need for regeneration or replacement.

• Water content: as little as 40–60 ppm of moisture can significantly reduce dielectric properties. MIDEL tolerates higher values, but monitoring remains essential.

• Acid number (TAN): a rise above 0.3 mg KOH/g indicates oil oxidation.

• Dissolved gas analysis (DGA): especially the presence of hydrogen, acetylene, and methane may point to internal faults or overheating.

• Furan indicators: furans signal paper insulation degradation. Above 250 µg/l, an inspection or partial insulation replacement should be considered.

When should the oil be replaced?

• When BDV drops below 25–30 kV

• When water content exceeds the permissible limit for the specific oil type

• When chemical parameters (TAN, DGA) indicate degradation

• After 10–15 years of use or sooner if operating conditions were extreme

Important: even MIDEL, though more resistant to aging, is not an eternal medium. Regular analysis is essential to maintain high transformer performance.

Does oil replacement in a transformer require shutting down the entire station?

For many operators, the key question is not whether to replace the oil, but whether it can be done without shutting down the entire infrastructure. The answer is: it depends on the type of station, connection configuration, and safety requirements.

In medium voltage transformers installed in container or prefabricated stations, the typical setup involves a transformer that plays an integral role – without it, power cannot be supplied to the loads. In such systems, a complete power shutdown is necessary, the transformer must be isolated, and the rest of the installation properly secured.

However, there are exceptions where oil replacement can be carried out with minimal disruption to operations:

• Stations with redundancy, where two or more transformers operate in a backup configuration

• Facilities supplied from multiple sources, where loads can be temporarily switched over

• Large industrial sites with their own main power substations (GPZ), allowing for partial reconfiguration of supply paths

From a health and safety perspective and under PN-EN 60076-1 standards, any oil replacement – regardless of type – must be conducted under controlled conditions: with the unit de-energised, grounded, and with the work site properly prepared.

In practice, even if partial power continuity is possible, the transformer itself must always be switched off and cooled before starting the procedure.

It is important to note:

• A well-planned oil replacement (e.g. during annual maintenance) can reduce downtime to as little as 6–8 hours

• In transformers filled with MIDEL, the downtime may be longer due to the need for more thorough system drying

• For larger transformers (>2500 kVA), it is worth arranging service from a specialised company with a mobile lab

Is it worth replacing the oil and how to plan it properly?

Replacing the oil in a medium voltage transformer is not just a maintenance procedure. It is a strategic decision that impacts operational safety, regulatory compliance, service costs, and equipment longevity.

Using fire-resistant, biodegradable fluids such as MIDEL brings additional benefits: the ability to install in environmentally sensitive areas, reduced fire risk, and less environmental impact.

But most importantly – it’s all about planning. Regular oil analyses, anticipating critical thresholds, consulting the manufacturer, and a well-thought-out schedule are key. The cost of oil replacement (on average 1200–1800 PLN per 100 litres plus labour) is a small expense compared to the cost of a failure or production downtime.

A well-maintained transformer is an investment that pays back over decades.

And if you're looking for transformers ready for demanding conditions, with enhanced environmental resistance and full compliance with PN-EN 60076 – we’re here to help.

BONUS: Ready-to-use action schedule for oil replacement in an MV transformer

Below is a complete action schedule for oil replacement in a medium voltage transformer, tailored to the realities of 2025, compliant with PN-EN 60076 and BDO obligations:

Stage 1: Diagnosis and planning (Week 1–2)

Goal: assess whether oil replacement is necessary and determine which insulating fluid should be used.

• Commission oil sample analysis: BDV, TAN, DGA, water content, furans

• Check transformer production date (PCB testing required if before 1986)

• Consult transformer manufacturer or carry out an independent material assessment, especially if switching to MIDEL

• Prepare a preliminary service schedule including planned power outages

Estimated cost:

• Oil analysis: 100–200 EUR

• Material assessment: 300–600 EUR (optional)

Stage 2: Formalities and logistics (Week 2–3)

Goal: prepare documentation and organise technical resources.

• Register in the BDO system and report hazardous waste (if applicable)

• Order new oil (mineral, MIDEL, or regenerated)

• Select and book a certified service provider with relevant experience

• Arrange tanks for used oil and secure transportation for disposal, regeneration, or recycling

• Confirm availability of vacuum and filtration equipment

Estimated cost:

• Transport and logistics: 300–800 EUR

• New oil: depending on type (e.g. MIDEL: 3.2–3.8 EUR/litre)

Stage 3: Oil replacement (1 working day)

Goal: carry out replacement in accordance with PN-EN 60076 procedure.

• Disconnect transformer from power and secure the work area

• Drain the system, including conservator and piping

• Clean the system – flush with neutral oil or dry service mixture

• Introduce new oil (degassed and dried) using vacuum protocol

• Perform breakdown voltage test and leak test

Estimated cost of service (including testing and cleaning):

• 630 kVA transformer: 1800–3200 EUR

• Over 1600 kVA: 5000–7000 EUR

Stage 4: Commissioning and monitoring (Week 4)

Goal: verify proper operation after replacement.

• Start transformer and operate in monitored mode (24–48 hours)

• Record winding temperatures, load, and electrical parameters

• Archive service documentation and update device record in the BDO system

• Schedule next oil analysis in the maintenance cycle

Optional:

• Install online sensors (e.g. humidity, temperature, DGA) for critical transformers.

Thank you for your time and interest.

Check our current Energeks transformer portfolio – available in versions with routine tests and special testing options. You can also ask about transformers designed for use with MIDEL oil, hermetically sealed metal enclosures, or prefabricated stations with quick-exchange functionality.

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Sources:

IEC: 100 years of standards for insulation fluids!

C57.147-2018 - IEEE Guide for Acceptance and Maintenance of Natural Ester Insulating Liquid in Transformers

Regulation (EU) 2019/1021 of the European Parliament and of the Council of 20 June 2019 on persistent organic pollutants (recast) (Text with EEA relevance.)