Note: Descriptions are shown in the official language in which they were submitted.
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Description
Method for removing corrosive sulfur compounds from a
transformer oil
The invention relates to a method for removing corrosive sulfur
compounds from a transformer oil.
Transformers frequently use as insulation and cooling media
transformer oils which, due to their long-term chemical
characteristics, have for many years been used for operating
transformers. One problem associated with using transformer
oils is, however, the presence of natural or added sulfur
compounds which contribute to the oxidation stability of the
oil itself, in particular in the case of uninhibited
transformer oils. Conductive copper sulfide compounds are
consequently formed which are preferentially deposited in the
paper insulation and impair its insulating properties. This
phenomenon is promoted in particular at elevated operating and
ambient temperatures.
When unlacquered, paper-insulated copper conductors are used
within a transformer and under conditions of limited oxygen
content, for example when a transformer is operated with
exclusion of air, transformer oils comprising corrosive sulfur-
containing constituents form layers of-copper sulfate on the
paper insulation. Starting from the copper conductor, copper
sulfide layers form within the paper layers surrounding the
copper conductor. As a result, the insulation properties of the
paper insulation are sometimes durably impaired, such that
partial discharges and voltage flashovers may occur between the
live copper conductors due to the
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reduced insulation properties of the paper insulation.
These corrosive sulfur compounds, in particular mercaptans and
disulfides, form above all in transformers, chokes or passages
under specific 'operating and temperature conditions and reduce
the insulation properties of the paper insulation to a
considerable extent; sometimes down to just 20 percent of the
original electric strength of the paper insulation.
The attempt has accordingly been made in the prior art to
suppress the reaction of the corrosive sulfur compounds within
the transformer oils with the copper conductor and
simultaneously to improve oxidation resistance by "passivating"
the transformer oils, in particular by means of metal
passivators comprising benzotriazole-based compounds. A problem
in this case is in particular that the metal passivator may be
consumed during the ongoing operation of the transformer and
the quantity of passivator available must thus be permanently
monitored. Moreover, the extent to which long-term passivation
modifies the properties of the transformer oils is as yet
unknown.
WO 2005/117031 A2, for example, accordingly describes a method
and a device for adding a passivator to a conductor. The above-
stated patent application proposes winding the passivator
directly around the conductor and then sheathing it with a
further layer of an electrical insulator and so providing
overall electrical insulation for the conductor with the
passivator layer.
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WO 2007/096709 A2 moreover describes a method for permanently
removing corrosive components from a transformer oil. The
above-stated patent application proposes removing the
transformer oil from a transformer tank and, after heating and
addition of an acid-containing liquid, bringing it into contact
by means of a sulfide free-radical scavenger and then filtering
it. After filtration, the transformer oil purified in this
manner is reintroduced into the transformer tank.
The same applies to WO 2007/144696 A2 as a method for
deactivating corrosive sulfur in transformer oils. According to
the invention, the above-stated patent application proposes
adding a sulfide-forming chemical component to the transformer
oil comprising corrosive sulfur compounds, such that said
chemical component reacts with the sulfur compound and the
corrosive sulfur compounds are thus removed from the
transformer oil.
DE 10 2005 006 271 Al moreover describes a method for purifying
transformer oil, the transformer oil initially being subjected
to a pretreatment by filtration, before it is passed through a
packing of an inert inorganic support coated with a reactive
metal. The transformer oil is then filtered through a bleaching
earth bed and then returned to the transformer.
The object of the present invention is accordingly to avoid the
disadvantages in the prior art and to provide a method for
removing corrosive sulfur compounds from a transformer oil
which easy to handle and ensures virtually complete removal of
corrosive sulfur compounds from the transformer oil.
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Said object is achieved by the features of the method as
claimed in claim 1. According to the invention, a method is
proposed for removing corrosive sulfur compounds from a
transformer oil, in which, with addition of a mixture of -rare
earths containing aluminum oxide/aluminum silicate to the
transformer oil, said transformer oil enriched in this manner
is heated to up to 300 degrees Celsius and then, with
enrichment with an aqueous solution of soluble metal salts, is
cooled.
The transformer oil enriched with the rare earth mixture
containing aluminum oxide/aluminum silicate is then once more
heated to up to 200 degrees Celsius for at least two hours and
then cooled to room temperature.
Heating of the mixture of rare earths containing aluminum
oxide/aluminum silicate activates the adsorption centers of the
matrix by removing water fractions. The heavy metal salts
present in the mixture of the rare earths containing aluminum
oxide/aluminum silicate are thereafter dispersed in a little
water and the mixture is slowly heated. This gives rise to
heavy metal oxides which are insoluble and firmly bound to the
fuller's earth matrix of the mixture of rare earths containing
aluminum oxide/aluminum silicate.
In this manner, the adsorbent is prepared. The solution
presented here is based on removing the reactive corrosive
sulfur compounds present in the transformer oil by using a
mixture of inorganic adsorbents with a wide range of
applications. It mainly comprises a mixture of rare earths
containing aluminum oxide/aluminum silicate and
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are optionally enriched with silver, copper, zinc and/or iron
in metallic or oxide form. In particular, the metal oxides
formed by means of the rare earth mixture containing aluminum
oxide/aluminum silicate bind the corrosive sulfur compounds and
may be collected at a suitable point and removed from the
transformer oil, optionally together with simultaneous removal
of oil ageing products.
The advantage of this method is that no additional foreign
substances, such as for example passivators, are added to the
transformer oil. Ageing products and corrosive sulfur compounds
are simultaneously eliminated from the transformer oil.
Oxidation capacity is consequently increased and the fraction
of corrosive sulfur compounds within the transformer oil is
greatly reduced, so durably increasing the service life of the
transformer.
It is considered advantageous according to the present method
for the ratio between the fraction comprising aluminum oxide
and the fraction of aluminum silicate in the rare earth mixture
containing aluminum oxide/aluminum silicate to be in a ratio of
20:80 to 80:20, preferably of 50:50. The catalytic action of
the rare earth mixture containing aluminum oxide/aluminum
silicate is best ensured within the preferred ratio range of
the fractions.
A bulk density of 50 to 80 g/l is advantageously used in order
to provide the greatest possible surface area of the rare earth
mixture containing aluminum oxide/aluminum silicate. In this
bulk density range, an in particular granular rare earth
mixture containing aluminum oxide/aluminum silicate has an
effective surface area for binding the corrosive sulfur
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compounds present in the transformer oil. The aqueous solution
advantageously has a solution fraction of up to 40% of in
particular soluble metal salts. Adding copper and silver salts
in particular leads to improved binding of the corrosive sulfur
compounds onto the metal salts present in the aqueous solution.
This precisely prevents the corrosive sulfur compounds present
in the transformer oil from reacting chemically with the copper
conductor. The pH value of the rare earth mixture containing
aluminum oxide/aluminum silicate is advantageously 6.5 to 9Ø
The metal oxides formed react with the corrosive sulfur
compounds at the highest possible rate of reaction i-n the
above-stated pH range.
An advantageous development of the method provides that the
ratio of the rare earth mixture containing aluminum
oxide/aluminum silicate to transformer oil, relative to their
respective weights, is in a ratio of 0.01:100 to 40:100,
preferably of 10:100. The highest possible rate of reaction is
ensured in particular at the preferred weight ratio of 10:100
of rare earth mixture containing aluminum oxide/aluminum
silicate to transformer oil due to their respective
concentrations. Advantageously, the rare earth metals of group
3 of the periodic table of elements including the lanthanoids
are a constituent of the rare earth mixture containing aluminum
oxide/aluminum silicate. In an advantageous development of the
method, silver, copper, zinc and/or iron are admixed with the
rare earth mixture containing aluminum oxide/aluminum silicate.
Moreover, silver nitrate to form silver oxides and/or copper
salts to form copper oxides and/or iron oxides is/are admixed
with the rare earth mixture containing aluminum oxide/aluminum
silicate. The metal oxides present
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in this manner within the rare earth mixture containing
aluminum oxide/aluminum silicate are highly reactive and
combine with the corrosive sulfur compounds within the
transformer oils and neutralize the corrosive sulfur compounds.
An advantageous development of the method provides that the
rare earth mixture containing aluminum oxide/aluminum silicate
is arranged in a container, in which the container may be
fitted on a transformer housing and the transformer oil is
passed into the container and purified, and the sulfides bound
in the rare earth mixture containing aluminum oxide/aluminum
silicate as reaction products of the corrosive sulfur compounds
remain in the container. Thanks to the reaction of the
corrosive sulfur compounds of the transformer oils within the
container and the accumulation of the bound sulfides in the
container, these waste products may be disposed of on removal
of the container. At the same time, any further contamination
of the transformer oils with the bound sulfides in the
container is ruled out, such that corrosive sulfur compounds
may virtually completely be removed from the transformer oil by
the above-stated method.
In the event of complete consumption of the rare earth mixture
containing aluminum oxide/aluminum silicate, the container is
advantageously removed from the transformer housing. In an
advantageous development of the method, the container comprises
an indication of the reactive rare earth mixture containing
aluminum oxide/aluminum silicate which is present. In the
context of servicing, this indication may be used to establish
whether sufficient reactive rare earth mixture fractions
containing aluminum oxide/aluminum silicate
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are present and proper performance of the method is ensured.
A filter system is advantageously introduced within the
container, the filter system comprising the rare earth mixture
containing aluminum oxide/aluminum silicate, and the
transformer oil is introduced into the filter system. By means
of the filter system, the bound sulfides and the transformer
oil end-of-life products may in particular more readily be
retained within the filter system and so collected within the
container.
An advantageous development of the method provides that the
container may be connected with a purifying device, in which
the purifying device may be connected with the transformer
housing and the transformer oil may be transferred out of the
transformer housing for purification in the purifying device
and thus the corrosive sulfur compounds are removed in the
container outside the transformer housing.
Further advantageous developments are revealed by the
subclaims.
Example:
A rare earth mixture containing aluminum oxide/aluminum
silicate has a bulk density of 600 g/l with a ratio of aluminum
oxide to aluminum silicate of 50:50. The pH value is 7Ø One
kilogram of the rare earth mixture containing aluminum
oxide/aluminum silicate is activated at 150 C and, after
cooling, treated in portions with 400 ml of a 20% aqueous
solution of soluble salts of silver, copper, zinc or iron. The
mixture is homogenized and heated stepwise to 120 C within five
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hours. This temperature is maintained for 15 to 20 hours. After
cooling, the mixture is kept in a closed vessel. The ratio
relating to the weights of the active rare earth mixture
containing aluminum oxide/aluminum silicate to treated
transformer oil is 0.5:100 to 10:100, depending on the state of
ageing and corrosiveness of the transformer oil.
