CIGRE 2012
http : //www.cigre.org
D1.1
SCD1 Materials and emerging test techniques
PS1 Dielectric performance and aging of insulation materials
M/DBT, new alternative dielectric fluids for transformers
J. WALKER and N. BERGER
Arkéma France
France
jeremie.walker@arkema.com
Z.D. Wang and Q. Liu
University of Manchester
UK
zhongdong.wang@manchester.ac.uk
Mono and dibenzyltoluene (M/DBT) are based on synthetic aromatic hydrocarbons.
They are insulating liquids with specifications meeting IEC 867 standard where they
are defined as methylpolyarylmethane derivatives.
M/DBT have been used in high voltage capacitors for over 25 years. They have also
been approved for other electrical equipments such as Capacitive Voltage
Transformers and bushings.
Today, mineral oils are highly refined to meet health and environment requirements
(carcinogenic poly-aromatic hydrocarbons content must be less than 0.1%) and more
stringent requirements for corrosive sulfur. Highly refined oils contain less aromatic
compounds due to the hydrogenation process. As a consequence the gassing tendency
of the mineral oils becomes more positive. The gassing tendency refers to the
behaviour of an insulating liquid when gasses have been produced due to high
electrical stresses. When the gassing tendency is negative, the gases are absorbed;
when the gassing tendency is positive, more gasses are produced and partial
discharges occur.
The level of aromatic content has a strong effect on gassing tendency of the oil,
consequently on the electrical behaviour of the oil and also on the technical
performances of the transformer. In theory using M/DBT as complement to mineral
oils will restore the original gassing performances of the mineral oil. The experimental
studies confirm that by adding 5-10% of M/DBT, a gas-evolving mineral oil can
become gas absorbing. A transformer developing partial discharges could be cured by
adding M/DBT. In addition, M/DBT will increase the dielectric strength of the mineral
oil.
This paper reports the comparative experimental studies made on mineral oil,
synthetic ester, natural ester and M/DBT. AC breakdown voltages and the resulting
1% withstand voltage through statistic analysis, gassing tendencies and partial
discharge behaviors are discussed. Besides, the above characteristics of individual
insulating liquids with a certain percentages of M/DBT are also studied and reported.
Methylpolyarylmethane derivatives have already used as heat transfer fluids, up to
330°C (service temperature), thanks to their excellent thermal stability. As a synthetic
liquid, M/DBT have a very well defined composition, as a consequence their
chemical, physical and electrical properties are well-defined and constant. Extensive
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studies have been carried out to compare electrical, thermal, toxicological and
environmental performances of M/DBT and mineral oils. These studies show that
technical advantages could be reached in using such synthetic fluids in transformers in
place of mineral oils.
As the final conclusion, the experimental results show that in some
specific transformer applications when negative gassing tendency is strongly required,
the addition of a few percents of M/DBT will be the technical solution. The
consequence will be a better electrical behaviour of the oil and a stronger resistance to
partial discharges. The high dielectric performances of M/DBT will allow a strong
technological improvement of the electrical equipment in which they are used.
M/DBT could also be used in transformers as new alternative fluids to mineral oils, or
natural and synthetic esters. The use of M/DBT would bring the potential to allow
either the increase of the power, or the reduction of the transformer size.
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