Skip to content


Contact Us!




Haltermann Carless Deutschland GmbH

Phone: +49 6232 134 0

Phone: +49 40 333 180

Haltermann Carless UK Limited

Phone: +44 1372 360000

Phone: +44 0845 602 1003

Phone: +44 1372 360000

Haltermann Carless France S.A.S.

Phone: +33 232 13 14 50

Haltermann Carless Japan G.K.

Phone: +81 45 228 7398

Haltermann Carless US Inc.

Phone: +1 248 422 6548

Phone: +1 281 331 2065

HCS Group Headquarters

Phone: +49 69 695 386 0

Mobile Regeneration Service


Building on over 60 years experience in static insulating oil reclamation, EOS now leads the way in providing on-site regeneration services.

As an additional tool for transformer life extension the EOS regeneration service can be carried out with the transformer on-load, off-load or de-energised leaving your transformer insulation system “deep cleaned”, removing polar contaminants such as acids and sludge and leaving the oil left, in an “as new” condition.

EOS also has considerable experience at treating transformers identified with potential corrosive sulphur which has become a particular concern for operators of power stations where the transformers were filled with unused insulating oil in the 1990s.

Be it life extension or corrosive sulphur concerns, the EOS team of electrical engineers can discuss your precise requirements and guide you carefully through the regeneration service from initial survey to final report and follow up.

The process

Before any on-site regeneration can begin it is usual for a series of detailed consultations to take place, taking into account:

  • Transformer oil test result history

  • Assessment of mechanical strength of the paper insulation

  • Preparation of RAMS taking into account nature of the work – eg on-load, off-load or de-energised

  • Access to the transformer for the mobile processing unit and the mobile regeneration module

Due to the polar nature of water it is imperative that the oil in the transformer is initially reconditioned before being admitted to the regeneration columns. This initial period of reconditioning (heating, drying and degassing) also helps raise the core temperature of the transformer to make the whole regeneration process more effective, especially if being undertaken in a de-energised state.

Once the oil has reached acceptable levels of water content and temperature, it will be admitted into the regeneration module where, through a process of adsorption the products of oil oxidation – acids and sludge are removed from the oil.

Once regeneration beds are “saturated” the module is put into “reactivation” mode in preparation for the next regeneration cycle, which usually takes place on the following day. Whilst the regeneration module is in reactivation mode the transformer oil is continually circulated through the MPU to continue adding heat, removing water from outer layers of paper insulation and degassing the oil.

The regeneration cycles will continue until the oil has reached pre-determined, agreed levels, typically in line with IEC60296:2012.

EOS works closely with electricity companies preparing their responses to OFFGEM and with power stations concerned over the potentially corrosive nature of their oil.



What is the difference between oil regeneration and reclamation?

The terms are often interchangeable but in the UK we generally refer to static, off-site chemical and physical treatment of used insulating oil as “reclamation” – e.g. what we do at the EOS Ellesmere Port operational base, and on site in-situ chemical and physical treatment as “regeneration”.

Why bother with regeneration when I could just change the oil?

An oil change is the traditional remedy to combat rising acidity in a transformer but oil regeneration is a much more effective option (technically and financially) – when you change the oil in a transformer up to 10% of the old, oxidised oil, will remain in the paper insulation. Even with double draining, waiting time and flushing there will likely be as much as 5% remaining. In a large generator transformer in a power station this could be as much as 5,000 litres of old, acidic oil. Once the replacement unused or reclaimed oil is put into the transformer this residual acidic oil begins to “leach” out of the transformer’s paper insulation. This “leachate” acts as a catalyst for further oxidation of the oil.

In-situ regeneration on the other hand has the capacity to reach the parts of the transformer insulation system that a standard oil change just can’t manage. Due to the time on site, constant circulation and heat input the transformer insulation system is “deep cleaned” removing far more products of oxidation (acids and sludge) than a traditional oil change ever will. In addition to being a more effective treatment option, in-situ regeneration reduces the number of vehicles on site (no tankers) and means the transformer’s oil can remain in the transformer the whole time helping to give physical support to the paper insulation that may have a reduced tensile strength due to ageing.

So why the different terms – “reclamation” and “regeneration”?

The reclamation process, as carried out at EOS Ellesmere Port, produces a PRODUCT that conforms to a recognised standard, currently BS148:2009. The the on-site regeneration process however is a SERVICE where the end results will be broadly in line with a recognised standard but end results depend on variables often outside the control of the regeneration service provider.

Isn’t an oil change cheaper?

No, not normally. It depends on the volume of oil in the transformer but once you add in the cost of tankers and additional manpower you would normally expect the regeneration option to be 20-40% cheaper than an oil change.

Someone told me I had to put an additive in the transformer oil after regeneration, is that true?

EOS recommends adding an oxidation inhibitor to the oil at the end of the regeneration process in order to restore the oil’s oxidation stability – the oxidation stability or chemical stability is the key property that differentiates insulating oil from say, vegetable oil. When brand new insulating oil goes into service for the very first time it has to be able to withstand all the stresses and strains that operational life will throw at it. For instance heat from the operation of the transformer or possibly poor cooling and oxygen from the atmosphere in a free breathing transformer. Both of these are catalysts for oxidation – in the same way as metal goes rusty and butter goes rancid so too will mineral insulating oil turn “acidic” in time. It is the oils ability to withstand this oxidation process that marks it out as a good electrical insulating oil.

During an oil’s lifetime (and this can be as little as 10 years or as much as 40 years depending on design and operational constraints) the “natural” oxidation inhibitors – sulphur and aromatics – will gradually get used up and eventually acids and sludge will appear in the oil leading to an increase in the oil’s acid number (a routine test expressed in mgKOH/g carried out on most power transformer oil annually). These acids will attack the transformer paper insulation leading, ultimately to end of life. Modern day asset management of power transformers dictates that this damage to paper insulation is limited so some form of intervention is necessary at around 0.1 – 0.15 mgKOH/g acidity to maximise life extension opportunities.

In-Situ regeneration will deep clean the insulation system and restore the oil to “as good as new” but the oils natural inhibitors – sulphur and sludge remain depleted. It is for this reason that we recommend adding an inhibitor to the oil at the end of the operation.

How much inhibitor should I add?

We recommend fully inhibiting the oil, so that you end up with 0.4% inhibitor content.

Do I have to carry out annual tests to measure the inhibitor content?

Yes, EOS recommends that after 6 months a base line test is carried out then the oil should be tested annually. This could be extended to longer periods depending on your trended results.

When would I have to top up and how?

EOS recommends adding further inhibitor if the measured content falls to 0.1%, contact us for more information.

Is all used oil acceptable for reclamation?

In general, all mineral insulating oil, originally complying with BS148 and produced from naphthenic feedstock, will be reclaimable. We do, however, have strict QA procedures, which are designed to identify any oil unsuitable due to exceptionally severe aging or cross-contamination with other materials.

How does the electrical oil reclamation service work?

The EOS electrical oil reclamation service is a ‘laundering’ service. The customer supplies EOS with his used oil and EOS reclaim it to BS148-standard. The customer is then able to call off, for return, the same volume of reclaimed insulating oil as used oil supplied, less loss in treatment.

Speak to the Experts

Speak to the

Our team has a wealth of accumulated knowledge and experience. To learn more about the way we work and how we could help you, simply get in touch.

Case Studies

Neglected transformers

The customer has around 20, 66kV transformers that are critical to the continual production of steel. These transformers had been…


Corrosive Sulphur

The customer site is a combined heat and power, gas-fired power plant on the east coast of England. Its original generating…


Life extension of transformers

With a greater focus on asset management and making transformers last longer EOS was approached by a leading UK DNO…

Useful Contacts

Electrical Oil Services
Bridges Road, Ellesmere Port 

Cheshire, CH65 4EQ - UK
Tel: +44 0845 602 1003


Our Services

It’s our extensive experience in the (in-situ) treatment, storage and supply of insulating oil that gives our customers peace of mind when it comes to the quality and standard of our services.


Our Products

Not only do we supply unused and regenerated insulating oil, but we also provide a range of robust complimentary products.


Get In Touch

Our people are passionate about what they do, giving honest, pure, expert advice on a subject they know inside and our.