Note: Descriptions are shown in the official language in which they were submitted.
CA 02263046 1999-02-25
Title: TRANSFORMER OIL
FIELD OF THE INVENTION
This invention relates to oils for use in transformers.
BACKGROUND OF THE INVENTION
Conventional transformer oils are typically
manufactured from vacuum gas oil fractions derived from
naphthenic erodes and in particular light naphthenic distillates.
(~60N). Although transformer oils made from naphthenic erodes
perform adequately, they suffer from poor biodegradability and eco-
toxicity. Paraffinic erodes, hydroprocessed oils and synthetic fluids
such as poly-alpha-olefins and others are more environmentally
friendly and less toxic. Paraffinic erodes and synthetics also exhibit
enhanced oxidative and electrical properties. Therefore, they would
be preferred basis for the manufacture of transformer oils.
Unfortunately, it has not been possible to formulate
transformer oils using such base stocks because they do not perform
adequately in the ASTM D2300b hydrogen gassing test.
Commercially available naphthenic transformer oils exhibit
negative hydrogen gassing tendencies whereas paraffinic,
hydroprocessed and synthetic based transformer oils exhibit positive
hydrogen gassing values. Consequently naphthenic based
transformer oils are currently preferred because in the event that
hydrogen is evolved due to electrical stress they would tend to
absorb the evolved hydrogen thus reducing the chances of an
explosion.
DESCRIPTION OF THE INVENTION
It has now been discovered that is possible to
manufacture transformer oils from base stocks comprising
paraffinic erodes, hydrocracked oils and from synthetics with
enhanced hydrogen gassing properties through the addition of an
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additive. In terms of its chemical structure, the general class of
additives that are effective at improving (i.e. lowering) the
hydrogen gassing value are hydrogen donor molecules, that is,
compounds which incorporate within them labile hydrogen atoms.
Surprisingly, purely aromatic compounds such as naphthalene do
not effect the hydrogen gassing value. Accordingly, the additive may
be any compound which is a hydrogen donor other than a pure
aromatic compound , i.e. a non-unsaturated, unsubstituted
compound.
Examples of such compounds include
dihydrophenanthrene, phenyl ortho xylyl ethane, Agent 791TM
(alkylated benzene), Dowtherm RPTM (tetrahydro-5-(1-phenylethyl)-
naphthalene, Mobil MCP 917TM (alkylated naphthalene),
acenaphthene, tetrahydronaphthalene and, tetrahydroquinoline.
Preferably, such compounds are selected from the group consisting
of acenaphthene, tetrahydronaphthalene and tetrahydroquinoline.
Without being limited by theory, it is believed that in
use transformer oils are subject to high electrical stresses which
cause bonds to break in the transformer oil base stock. Without the
hydrogen donor additive of the invention, hydrogen is evolved
from the transformer oil. In the presence of the additive, the
availability of hydrogen from the additive causes alternate
compounds to form and thus reduce the amount of hydrogen
which is evolved from the transformer oil.
These hydrogen donor molecules may be added to the
base oil in amounts from 0.1 to 10 wt. % based on the weight of the
transformer oil, preferably from about 1 to 2 wt. %.
Since only a small amount of the additive is required,
the additive itself does not have a significant negative effect on the
human and eco-toxicological properties of the inherently good
human and eco-toxicological properties of paraffinic, hydrocracked
and synthetic fluids.
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Additionally, the addition of the hydrogen gas
suppressing additive to paraffinic, hydroprocessed and synthetic
base stocks does not have a negative impact on other physical,
chemical and electrical attributes of the electrical fluid.
Fluids used in the preparation of transformer oils can
be made using a variety of hydroprocessing technologies that are
described in the literature. Hydroprocessing or hydrotreating
involves the contacting of a hydrocarbon feedstock. Paraffinic
feedstocks (i.e., those possessing a predominant amount of normal
and iso-paraffins) are preferred since they respond well to
hydroconversion. Thus paraffinic feedstocks possessing the
appropriate physical attributes such as density, aromatics content,
viscosity and volatility are contacted with a catalyst at elevated
temperatures and pressures in a hydrogen gas atmosphere. The
hydroprocessing conditions are adjusted to effectuate the
conversion of polynuclear aromatics to smaller hydrogenated
species, as well as the elimination of sulphur and nitrogen
molecules. Typical hydroprocessing conditions are temperatures of
300 - 425°C, pressures of 600 - 4000 psig and liquid hourly space
velocities of 0.1 to 5.0 hr-1. Catalyst used in the hydrotreating step
include those based on sulfided group VIB and VIII metals.
Additionally, the hydrotreated product can be treated
over a hydroisomerization rare earth catalyst for the purpose of
converting the normal paraffins to iso-paraffins which has the
benefit of lowering the pour point of the oil.
As a last step the hydrotreated or the sequentially
hydrotreated - hydroisomerized oil can be finished by passing the oil
over a hydrogenation catalyst at thermodynamically favorable
conditions. Typically these conditions include high pressures (600
to 4000 psig) and temperatures lower than those used during the
hydrotreating or hydroconversion step (220 - 330°C). Catalyst used
in the hydrogenation step include those based on noble metals as
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well as those based on sulfided group VIB and VIII metals.
Example 1
A series of hydrogen donor additives were added to
phoenix N65DWTn~t to form a transformer oil. Phoenix N65DWT"s is
a base stock prepared by the sequential hydrotreatment,
hydroisomerization and hydrogenation and subsequent
atmospheric and vacuum distillation of a paraffinic vacuum
bottoms feedstock. The test employed for the initial screening was
the ASTM D2300B gassing test. The amount of the additive and the
gassing level are set in Table 1.
TABLE 1
List of additives and Gassing Values
Feedstock: N65DW (657-0812)
D23008
Additive Gassing Value, Additive Concentration,
microUmin wt%
None 5t.7 0
Voltesso 35 (657-1207) -17.7 -
Aoenaphthene -6 1
.4cenaphthene 3.7 0.5
Acenaphthalene (contains several-2 1
% Acenap
Tetrahydronaphthalene -36.8 1
Tetrahydroquinoline -13.8 1
Dihydrophenanthrene 27.8 1
Phenyl ortho xylyl ethane 35.2 2
Agent 791 (aklylated Benzene)46.7 1
Dowtherm RP (Tetrahydro-5-(1-phenylethyl)38.9 1
-naphthalen
Alkylated Naphthalene Mobil 39.2 1
MCP 9t7
Alkylated Naphthalene Mobil 40.2 1
MCP 917
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The gassing tendency of Voltesso 35T"' (a commercially
available transformer oil) was tested to be -17.7 ~L/min. As it is
evident from Table 1 some of hydrogen donor type molecules, (i.e.
the partially hydrogenated molecules) had a significant effect on
suppressing gassing while the purely aromatic molecules did not
have an impact. This is a very interesting result since it was always
assumed that good gassing performance was purely a function of the
aromatics level. The compounds which performed best were
acenaphthene, and tetralin. Addition of 1 to 2% of these compounds
produced level gassing performance which approached or actually
depressed below that observed with Voltesso 35TM.
Example 2
Two fully formulated oils were prepared and tested to
determine whether they met the Ontario Hydro and the US ASTM
3239 requirements. One electrical oil was made with 2% tetralin in
N65DWTM and the other was composed of a 1% acenaphthene in
50/50 mixture of N65DWTM and HT60TM. HT60 is a paraffinic high
saturates white oil prepared by the sequential hydrotreating -
solvent dewaxing - hydrogenation of a narrow cut paraffinic gas oil
fraction. Solvent dewaxing is used to reduce the level of normal
paraffins and as a consequence to lower the pour point of the oil.
The only other ingredients added to the oils were 0.08 wt.
DBPCTM and 0.07 wt. % Agent 27TM (Pearsol 100) pour point
depressant. Results of the study are shown in Table 2.
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CA 02263046 1999-02-25
_7_
As it can be seen, both formulations easily met all of
the important physical property requirements. In addition 24 hour,
64 hour and 164 hour D2440 oxidation test results were either
similar to or better than the results reported for Voltesso 35TM
There also appears to be significant performance benefits over
Voltesso 35T"'' in many of the dielectric tests.
The gassing performance was -26.6 ~L/min for the
tetralin case and -7.6 ~L/min for the acenaphthene case.
Example 3
To simulate a plant trial, two 4.lkV pad transformers
were chosen. One unit was drained of Voltesso 35TH'', flushed and re-
filled with 135 gallons of test fluid, while the other unit was drained
and re-filled with 135 gallons of fresh Voltesso 35TM.
The test fluid was prepared using VHV 12 as a base
stock. VHV12 is a severely hydrogenated and hydroisomerized oil,
prepared by the sequential hydrotreatment, hydroisomerization and
hydrogenation and subsequent atmospheric and vacuum
distillation of a paraffinic vacuum bottoms feedstock. 0.08 wt.
DBPC antioxidant and 2 wt. % of tetrahydronaphthalene the anti-
gassing additive) were added to the base stock. Prior to its use the
test fluid was 'dried' by means of sparging with nitrogen gas for a
period of about 1-hour. This had the effect of reducing the water
content from > 50 ppm to < 35 ppm (which is standard for most
electrical oils), while at the same time increasing the dielectric
power factor to >40kV. The physical properties of the finished
product are shown in Table 3.
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TABLE 3
Syaeclfications
658.0598 SSA Class B
Ink
Anti-Gassing Additive, % 1.94
DBPC, % 0.074
VHVI 2 (970510) 97.986
Density, 15C, kglt. D4052 0.8718 0.906 0.91
Viscosity@100C, cSt D445 2.35
Viscosity@40C, D445 8.186 12 max 12 max
cSt
Viscosity@-40C, cSt D445 1627 6000 max
Pour, C D97 -45 -40 max -40
max
Flash, C, COC D92 166 145 min 145
min
Colour D1500 <0 5 0.5 max 0.5
max
The condition of the transformer unit was evaluated
every few weeks. The evaluation included routine gas analysis and
electrical test, D2300 hydrogen gassing tendency, and D2440
oxidation stability (Q76h & 164h).
The electrical loading and performance of two
transformers was continuously monitored. Analysis of the data
indicates that the two transformers operated 'normally' over the
period of the test run.
The gassing results are shown in Table 4. The results
show that gassing tendency of the test fluid is significantly lower
than Voltesso 35TM and that, the gassing values remained relatively
constant over time. Additionally, the results suggest that the
concentration of the anti-gassing additive can be reduced from 2 wt.
to a lower value.
Results of the 76 hour and 164 hour D2440 tests are
shown in Table 5. As expected, the oxidation stability of the test
fluid is exceptionally good. In fact it is so good that it passes the
CA 02263046 1999-02-25
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CSA/ASTM oxidation requirements for uninhibited and inhibited
electrical oils which can contain up to 0.4% DBPC. Electrical oils
containing 0.08 wt. % and less DBPC are considered to be
uninhibited. Conversely, Voltesso 35 only meets the oxidation
requirements for uninhibited oils.
The biodegradability of the fully formulated test
electrical oil was evaluated using the standard OECD 301B test. The
result of the 28-day test was 60%, which signifies that it can be
classified as readily biodegradable.
TABLE 4
Gassing Tendenc~r of Test Fluid
date 08-Jun-98 30-Jun-98 05-Aug-98 02-Sep-98
08-Jun-98 30-Jun-98 05-Aua-98 02-See-9~
Sample # 658-0764 658-0840 658-0949 658-1044
Gassing Tendency, microUmin -41.2 -46 -37.3 -51.6
Gassing Tendency of Voltesso 35 Electrical Fluid
Oate Q8-Jun-98 30-Jun-98 O5-Aua-98 Q$
Sample # 658-0765 658-0839 658-0950 658-1045
Gassing Tendency, microUmin -» '»~7 -12
CA 02263046 1999-02-25
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TABLE 5
D2440 Of Test Fluid
Oxidation
Stabilit;~
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