TRANSMISSION AND DISTRIBUTION
Vegetable or mineral? – transforming choices for DSOs1 March 2009
EdF has introduced an edible oil made from soya beans on to its grid reinforcement menu, and is currently trialling its qualities in comparative tests on two 132 kV distribution transformers in the UK.
The first purpose-built 132 kV 90 MVA vegetable oil-filled transformer, designed, manufactured and tested by Areva T&D, has been officially switched on by EdF Energy Networks in Luton, UK. EdF hopes that this will enable it to reduce slightly its impact on the environment while still meeting the needs of its customers. Vegetable oil has been used before in electricity distribution at a lower voltage, and other companies have retro-filled existing 161 000 volt transformers with it, but this is the first time it has been used at such a high voltage in the UK, or anywhere in a transformer specially built for the purpose.
Filled with Envirotemp FR3 oil, an innovative insulation fluid manufactured by US company Cooper Power Systems, the unit has been purpose built in Aeva T&D’s factory in Stafford, UK, as part of an EdF Energy Networks programme to reinforce the electricity network. Made from soya beans, the fluid is fully biodegradable and has greater fire-resistance than mineral oil. These properties help to achieve higher levels of safety, together with major environmental benefits compared to the use of mineral oil.
The project required Areva T&D to manufacture two transformers to the same specification, the only difference being that one is filled with FR3 fluid while the other contains conventional mineral oil. About 30 000 litres of oil are needed to act as a coolant and insulator between the metal tank and the electrical equipment inside.
Utilising its MS 2000 online monitoring system, Areva T&D will continue to work with EdF to monitor the performance of each of the transformers. This will provide EdF with a better understanding of the characteristics of such transformers in various load/environmental conditions in order to optimise its assets and further improve the design, overload capacities and increase the lifetime of its vegetable oil transformers currently on offer.
Paul Dyer, an EdF Energy Networks transformer specialist, said: ‘We have installed the transformer in Luton on a trial basis and will be monitoring its performance in association with the company which built it. If successful, we hope to roll out the technology to other suitable sites across our electricity distribution areas in the East of England, London and the South East.
‘Energising the vegetable oil transformer is a huge breakthrough for the UK electricity industry and the environment. We are extremely happy with the work that Areva T&D put in to make this transformer a reality and with the commitment and expertise shown throughout the project. It has demonstrated its expertise in the thermal, and dielectric issues and in the special manufacturing process requested by the new oil’s characteristics.’
Ghislain Lescuyer, executive vp for the Business Unit Products of Areva T&D added: ‘We are proud of our role in introducing this innovative transformer to the world, paving the way to making the electricity industry as eco-friendly as possible for future generations. This transformer has properties that can result in an increase of overload ability and/or longer transformer insulation life, resulting in both lower life cycle costs and longer overall asset life.’
Existing vegetable oil transformers
Using vegetable oil has some immediate benefits for operators. It exhibits low-flammability, very high biodegradability (99%) and is claimed to increase the unit lifetime by 30%-50% when applied as a retro-fill. In fact Areva already has such a transformer on the market under the name Melody, using natural ester-based biodegradable vegetable oil as the dielectric medium, but only for voltages up to 82.5 kV.
This kind of transformer is well suited to ecologically sensitive and fire sensitive areas such as infrastructural projects, private and public buildings or offshore platforms, although it is equally adaptable to all common transformer applications. To add to its attraction specifically in the USA, these kinds of fluids are not subject to the Federal Regulation of Used Oils, which are concerned with environmental risk, but by the Edible Oil Regulatory Reform Act, which are not.
Why use vegetable oil?
Specialty transformer fluid provides the ability to slow down the rate of transformer ageing in a key utility asset while dramatically reducing the risk of a fire or spill. The process that resulted in a soybean based transformer oil was kicked off by a company called Waverly Light and Power and began in 1994 when a truck knocked down a bank of transformers in Waverly. The resulting twenty gallon spill of PCB contaminated mineral oil cost the utility $30 000 in clean up costs. To make matters worse, the transformer was only one of four PCB units left on the system at the time.
Cooper Power Systems developed Envirotemp FR3 an edible-seed-oil based transformer fluid, in the 1990s. It can be used as a direct replacement for conventional, petroleum-based mineral oil. FR3 fluid can extend transformer life, while mitigating the safety and environmental risks associated with transformer failures.
Insulating paper is a major component of a transformer’s internal construction. A significant percentage of transformer failures are due to breakdown of the insulating paper. Extensive testing has shown that when immersed in FR3 fluid the paper ages at a much slower rate than in a transformer filled with mineral oil. One of the key reasons is that FR3 fluid actually reduces the moisture content of the paper. Laboratory testing has shown that the fluid’s asset management impact is even more pronounced when the fluid is used in new transformers. An FR3 fluid retro-filled transformer can typically be loaded 12 to 17% more than a mineral oil transformer and have the same expected insulation life. An additional advantage is the mitigation of oil fires associated with failed transformers due to the fluid’s very high ignition temperature. Being able to install electrical equipment closer to the loads also reduces the length of expensive bus work.