PROJECT: 400kV substation with heat recovery | UK

ESTER TYPE: MIDEL 7131 synthetic ester

PURPOSE: Ensure operational safety at urban substation


UK grid operator National Grid needed an innovative solution to the challenge of installing three 400kV transformers in Highbury, a densely-populated area of London. The aim was to combine safety and reliability along with a heat exchanging device (directing the captured transformer heat-loss to a nearby school).

Safety, reliability and environmental impact are crucial considerations for grid operators and transformer manufacturers, especially for inner city installations. People, and where they live and work must be protected from the risk of fire or explosion from the installed electrical assets. The environment and sustainability considerations are also critical factors – along with the fact that inner city real estate is extremely expensive and in short supply (and so influencing the substation’s footprint size).

The use of MIDEL ester fluids, as an alternative insulation fluid, provides solutions to many of these obstacles, being far safer in operation due to a higher fire point, and more environment ally friendly than mineral oil due to higher levels of biodegradability.


The operating assets – 400kV transformers – were manufactured by Siemens, and the substation designed so that waste heat from the transformer re-used using heat exchange devices. Planning and construction of this additional feature was done in close collaboration between the transformer factory, the customer, and the supplier of the heat exchange device.

In order to manufacture the transformers according to the required performance levels (dielectric characteristics, insulation and projected lifetime), specialized constructive changes were calculated and planned in the design phase. MIDEL 7131 synthetic ester insulation was chosen for the project as it greatly reduced the risk of fire and explosion. The fire point of synthetic ester fluid (316°C) is very high compared to mineral oil (170°C). Noise also plays a key role alongside reliability and safety. The transformer and cooling devices needed to operate silently regardless of transformer load. The transformer filled with MIDEL 7131 was manufactured with a low-noise-design. Bespoke housing, as well as noise-optimized cooling with
frequency-regulated fans was added to the low-noise concept.


This installation delivers an economical solution that allows waste heat from three power transformers to heat a school located next to the substation. Depending upon the electrical energy used in this area, more than 1 MW of waste heat from the transformers can be recovered to heat local homes, shops and schools in the future.

MIDEL ester insulation greatly reduced the risk of fire and explosion (the firepoint and flashpoint of synthetic ester are very high compared to mineral oil). Esters are fully biodegradable, which adds to the environmental credentials and reduces risks to the environment.

The Highbury project shows that the future of power networks lies in safer, greener, quieter, better performing and more sustainable transformers.

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

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