Transformer Oils – 11kV 33kV 66kV Substation Transformer Oil Refilling, Sampling & Analysis

Published 14 Mar 2019

MIDEL 7131 Transformer Oils

Substation Transformer Oil Refilling, Sampling & Analysis MV HV

Transformer Oils

♦ MV HV 11kV 33kV 66kV ♦

  • Guest Post by Neil Denbow (Director - Eden Transformer Oil)

The following Guest Article discusses the “end to end service” provided by Eden Transformer Oil and the benefits of their products to maintain transformer oils including MIDEL 7131 and silicon insulating liquids – Eden test, maintain and purify insulating liquids and electrical transformer oils in MV HV transformers and switchgear at 11kV, 33kV and 66kV voltages.


An important and historical site in central London had been refurbished during which five silicone 11kV transformers were replaced with mineral oil filled units.

Due to the room dimensions, materials used and lack of fire prevention equipment the medium voltage substation no longer complied with BS EN 61936-1:2010.

The customer had a choice of further intrusive construction, prolonged power outages and the associated expense of installing fire prevention or whether to upgrade the transformers to a K Class insulating liquid.

The decision was taken to upgrade the existing transformers to MIDEL 7131 and Eden Transformer Oil were contracted to retrofill each transformer in-situ during one weekend day per transformer. Eden Transformer Oil was the first company in the world to be accredited MIDEL Service Partners and has carried out many MIDEL retro-fills on site.

11kV 33kV 66kV Transformer Oil


The mineral oil was removed US specification drums for removal from site to Eden licensed waste oil storage site and over one week day per transformer the units were professionally upgraded to MIDEL 7131.

A professional upgrade requires dedicated MIDEL 7131 equipment, procedures and staff accredited by the manufacturer of MIDEL. The process includes time to allow the removal of excess transformer oil, heating the liquid to 60-80°C, further cycles of purification via vacuum dehydration plant and new gaskets.

Retrofil with MIDEL Transformer Oil

Retrofilling oil-filled MV HV transformers enables the user to enjoy the benefits of MIDEL without having the capital expense of a new transformer, lift and sift, MV HV cable jointing and prolonged productivity outage.

Replacing mineral oil insulating fluid with MIDEL in an existing medium/high voltage transformer means the MV-HV transformer doesn’t need to be removed and the retrofill procedure can be carried out in-situ and on-site.

MIDEL retrofills can be performed by a MIDEL Service Partner who will advise on the ideal MIDEL fluid depending on transformer design and location.

Transformers up to 66kV can be retrofilled with no design change needed.

MIDEL Retrofilling - Eden Oil

Benefits of MIDEL 7131 Transformer Oils

Fire Safety

  • 100% fire safety record
  • High fire point >300˚C
  • Classified as K-class transformer oil
  • FM Global® approved MV HV transformer fluids for lower insurance premiums (supports Highly Protected Risk status)Midel
  • UL® Classified for lower insurance premiums

Environmental Protection

  • Readily Biodegradable
  • Non-Water Hazardous
  • Non-Toxic
  • Recoverable and dispersible at sea using standard methodsUL & FM Approved
  • Non-Volatile
  • Will not evaporate into the environment

Transformer Life Extension

  • Use of MIDEL can prolong cellulose lifetime
  • This can in turn prolong transformer service
  • Wet transformers can also be returned to service
  • Suitable for 11kV 33kV 66kV transformer applications

Lower Operating Expense

  • MIDEL’s superior moisture tolerance results in lower future maintenance such as purification and / or liquid changes normally associated to mineral oil thereby avoiding maintenance costs and production outages.
  • Many insurance companies give improved premiums for MIDEL filled transformers

Improved Overload Resistance

  • Based on the IEC 60076-14 curve an MIDEL transformer can run 15˚C hotter with the same lifetime as a mineral oil transformer
  • This allows higher normal running temperature
  • Temporary overload temperature can also be higher
  • MIDEL fluids have a much higher flash point than mineral oil, allows safe running at higher temperatures

Suitable For Transformers In High Risk Areas

  • Industrial buildings
  • Residential buildings
  • Public buildings (schools, hospitals etc.)
  • No risk of pool fires
  • Reduced clearances to other equipment or buildings (BS EN 61936-1:2010)
Offshore Transformers 11kV 33kV

Onshore & Offshore Transformers 11kV 33kV 66kV | Medium & High Voltages

MIDEL retrofil checklist

Yes  No Do any these apply to your mineral oil transformer?
Would a transformer fire in this location endanger lives?
Is this transformer supplying power to or located close to equipment which is critical for business continuity?
Is the transformer in a fire-hazardous location?
Would a leak from this transformer harm the environment?
Does your company need to demonstrate positive Corporate Social Responsibility?
Has your insurance company indicated lower insurance premiums by retrofilling this transformer with MIDEL?
Would delaying the purchase of a replacement transformer be advantageous?
Do you need to avoid fire suppression system replacement?
Is this a “wet” transformer that needs recovery?
Is there a benefit in increasing the rating of this transformer?
Do you need to reduce the safety zone/space around this transformer, or install an additional transformer in the same substation?
Transformer Oils
Eden Transformer Oils
About Us
Eden Oil

Eden Transformer Oil Ltd was formed by Alan Denbow and Eddie Gregory in August 1985.

Former employees of Burmah Castrol, they realised that the vast majority of electrical engineers were simply replacing expensive transformer oil without fully appreciating the additional technical benefits of purification to the electrical equipment that Eden could achieve using a more convenient solution at a significantly lower cost.

In more recent years the treatment of transformer oil is the first choice for a cost effective and efficient maintenance program: the increasing cost of new oil, the importance of productivity and equipment downtime, coupled with the environmental benefits of avoiding the transportation of hazardous waste and use of virgin products, are all part of the equation – this applies to medium/high voltage (MV-HV) applications distributing electrical power at 11kV, 33kV and up to 66kV.

As well as providing on-site transformer oil treatment, if needed, Eden can supply and install new or reconditioned insulating liquid and provide all of the on and off-site sampling and analysis to ensure transformers are supervised and maintained according to British Standards (BS EN 60422:2013) and insurance underwriters requirements.

Since August 1985 Eden have been testing and maintaining the insulating liquids in transformers and switchgear in the United Kingdom, Belgium, France and Germany.

Eden provides end-to-end MV HV services and products to maintain transformer oil, MIDEL 7131 and silicon insulating liquid. On their webpage “Why Maintain Oil”, they highlight how Eden can provide a safe, economical and environmentally sound solution to maintaining transformers.

Eden do not just process insulating liquid, but they can also supply new liquids, breathers and consumables used for maintaining transformers.

Whether you are an end user, Electrical Engineering Company, Facilities Manager or Senior Authorised Person, Eden will be able to assist you in the testing and maintenance of your liquid insulated electrical equipment for LV (Low), MV (Medium) and HV (High Voltage) applications up to 66kV.

Transformer Oils

Transformer Oils | Medium & High Voltage MV HV | 11kV 33kV 66kV

LV, MV & HV Jointing, Earthing, Substation & Electrical Eqpt

Thorne & Derrick International are specialist distributors of LV, MV & HV Cable Installation, Jointing, Duct Sealing, Substation & Electrical Equipment – servicing UK and global businesses involved in cable installations, cable jointing, substation, overhead line and electrical construction at LV, 11kV, 33kV and EHV.

THORNE & DERRICK Product Categories: Duct Seals | Cable Cleats | Cable Glands | Electrical Safety | Arc Flash Protection | Cable Jointing Tools | Cable Pulling | Earthing | Feeder Pillars | Cable Joints LV | Joints & Terminations MV HV 

Thorne & Derrick International