Mineral oil replacement with MIDEL avoids fire suppression costs for important historic building

The Customer

A landmark building in central London next to the River Thames.

The Problem

In earlier decades the site had been fitted with five 800kVA transformers with mineral oil as the insulating liquid. Because of changes over the years, the installation no longer complied with BS EN 61936-1:2010, both in terms of fire protection and the need for containment.

There was no space within the existing structure to install fire suppression systems and due to the age of the building it was not possible to demolish walls to increase the footprint area to gain more clearance. Therefore the customer was faced with a schedule of invasive construction, prolonged outages and the expense of installing fire prevention systems.

The MIDEL Solution

After reviewing a range of options to meet the BS EN 61936-1:2010 requirements, the decision was made to upgrade the mineral oil transformers by retrofilling them with an ester-based K-Class insulating fluid – MIDEL 7131.

MIDEL Service Partner Eden Transformer Oil, a company specialising in on-site oil purification and replacement, was brought in to perform the retrofilling process. Each transformer was retrofilled in-situ during one weekend day per transformer. The mineral oil was removed from the site to Eden Transformer Oil’s licensed waste oil storage site and the units refilled with MIDEL 7131.

A professional transformer retrofill requires dedicated MIDEL 7131 equipment, procedures and trained staff. The process includes allowing time to remove excess oil, heating the ester fluid to 60-80°C, further cycles of purification via vacuum dehydration plant and the addition of new gaskets.


“Dealing with such an iconic landmark, extensive
building alteration was never an option. Using
MIDEL 7131, with its high fire point that makes
transformer pool fires virtually impossible,
it meant we could satisfy the requirements of
BS EN 61936-1:2010 without inflicting costly
and intrusive building works.”


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