PROJECT: Underground transformer in hotel | UK

ESTER TYPE: MIDEL 7131 synthetic ester

PURPOSE: Improve fire safety for on-site transformer


Canary Wharf is a critical central business district in London, located amid some of the most expensive real estate in the world. It is one of the main financial services centres of the United Kingdom and the world and has a daily working population of more than 120,000 people.

Canary Wharf also boasts a flagship, blue-chip brand hotel in the heart of the area. Featuring more than 500 guest rooms and multi-level parking, the hotel places a premium on its guests’ comfort and safety.

The hotel’s electricity supply is provided by an 11kV/1250kVa mineral oil-filled unit, last refurbished in 1991, and located along with its switchgear in the hotel’s underground car park three floors below street level.


With no fire suppression system in place, the hotel owner’s insurance company recommended that the transformer be retrofilled with a less flammable insulating liquid.

In March 2020, the hotel group’s Estate Management team examined the site’s transformer and decided the choice was between:

• Installing a fire suppression system
• Installing a brand-new ester transformer (without fire suppression) and retiring the existing unit in situ
• Replacing the mineral oil in the unit with an ester fluid (without fire suppression)


The decision was option 3 – to retrofill the transformer with MIDEL 7131 synthetic ester, because it provided much increased fire safety (its fire point is >300°C – a K class fluid) and improved environmental credentials, as MIDEL 7131 is readily biodegradable. Retrofilling the existing transformer also avoided the substantial cost of buying a new unit.

Due to the transformer’s location, a bespoke process for retrofilling was developed by all the project stakeholders and approved by the MIDEL technical team. The local Distribution Network Operator visited the site and provided power isolation.

The retrofill operation went ahead using more than 1500 litres of MIDEL 7131, and was completed in one day with the minimum of power disruption.

The use of MIDEL ester fluids in these sectors supports the following UN Sustainable Development Goals:

Get in touch