Ultra pure tetrachloroethylene dielectric fluid

A transformer is disclosed which contains a dielectric fluid of tetrachloroethylene. The dielectric fluid is ultra pure in that it contains less than 100 ppm of chlorohydrocarbons. A diluent, such as mineral oil, may be mixed with the tetrachloroethylene. The fluid can also contain 30 to 100 ppm of an inhibitor.

BACKGROUND OF THE INVENTION 
The prohibition against the use of polychlorinated biphenyls (PCB's) as 
dielectric fluids, because they constitute an environmental hazard, has 
resulted in an extensive search for suitable substitutes. A good 
dielectric fluid should not burn, should be fluid over a wide range of 
temperatures, should be environmentally acceptable, should be inexpensive, 
and, or course, should have good electrical insulating characteristics. 
Fluids which have been used to replace PCB's include silicones, phthalate 
esters, alkylated aromatics, and hydrocarbons. All of these fluids, and 
indeed any fluid, is a compromise of desirable and undesirable properties. 
Fluids which excel in one characteristic may be deficient in another 
desirable characteristic. Generally, there are minimum standards that a 
fluid must meet, however, which are set by the industry and/or government, 
before it will be accepted. 
RELATED APPLICATIONS 
This application is related to application Ser. No. 136,635, titled 
"Electrical Apparatus," filed concurrently herewith by T. W. Dakin, P. 
Voytik and C. L. Moore, which discloses an electrical apparatus containing 
tetrachloroethylene. 
PRIOR ART 
Clark U.S. Pat. No. 2,019,338 discloses tetrachloroethylene in a mixture 
predominantly of petroleum oil for use as a dielectric fluid in 
transformers. 
U.S. Pat. No. 2,752,401 discloses a new process for preparing 
tetrachloroethylene. 
SUMMARY OF THE INVENTION 
We have found that tetrachloroethylene, when it is ultra pure, is an 
excellent dielectric fluid, either alone or mixed with a diluent. 
Tetrachloroethylene has been around a long time, and, as 
"perchloroethylene," is widely used as a dry-cleaning fluid. It has even 
been suggested for use as a dielectric fluid (see U.S. Pat. No. 2,019,338) 
but has not been used commercially because it attacks the metals and 
insulation in the electrical apparatus (e.g., transformers and 
capacitors). 
We have found, however, that it is not the tetrachloroethylene that is 
responsible for the chemical attacks, but rather the damage is due to the 
decomposition of various impurities which are associated with 
tetrachloroethylene. 
We have identified these impurities as chlorohydrocarbons, compounds which 
have both chlorine and hydrogen atoms on the same molecule. While we do 
not wish to be bound by any theories, we believe that these 
chlorohydrocarbons form hydrochloric acid and/or chlorine gas, which 
attack the insulation and metals. Because hydrochloric acid acts as a 
catalyst for the decomposition of cellulose insulation extensively used in 
capacitors and transformers, very small quantities of hydrochloric acid 
can extensively damage a cellulose insulation system. 
The method of manufacturing tetrachloroethylene used until the early 1950's 
inevitably concurrently produced significant quantities of various 
chlorohydrocarbons. Unless the tetrachloroethylene was purified by 
elaborate distillation, which was not commonly done, it would be entirely 
unsuitable for use as a dielectric fluid. 
A current method of producing tetrachloroethylene has been developed (see 
U.S. Pat. No. 2,752,401). This new method can also produce 
chlorohydrocarbons, but the process parameters can be controlled so that 
very pure tetrachloroethylene is produced which can be used as a 
dielectric fluid. 
We have found that ultra pure tetrachloroethylene can be mixed with various 
diluents to produce an excellent dielectric fluid. Alone or mixed in 
proper proportions with a suitable diluent, the fluid is non-flammable in 
that it has no fire point up to its boiling point and it will not sustain 
combustion once an ignition source is removed. Even if the fluid is 
vaporized in a high energy arc the mixture of gases is still 
non-flammable. The low viscosity of the fluid provides improved cooling of 
the electrical apparatus. The fluid is liquid over a wide temperature 
range and is less volatile than many other non-flammable fluids such as 
various fluorinated hydrocarbons. The fluid is relatively inexpensive and 
has good electrical properties, including dielectric strength.

EXAMPLE 1 
In this example, two commercial samples of tetrachloroethylene were used, 
one prepared by the old technique of dehydrochlorination of other 
compounds using caustic or lime, designated "OLD" and the other prepared 
by the new process, designated "NEW" (see U.S. Pat. No. 2,752,401). Both 
samples contained less than 500 ppm of unknown stabilizers provided by the 
manufacturer. 
Each sample was mixed with mineral oil to produce a fluid which was 75% by 
volume C.sub.2 Cl.sub.4 and 25% by volume mineral oil. Gas chromatography 
was performed on each fluid. FIG. 2 is the chromatogram of the fluid 
containing the OLD tetrachloroethylene. Traces of halohydrocarbons can be 
seen as the peaks X, Y, and Z in FIG. 2. Upon aging, these compounds 
decompose by the elimination of chlorine and hydrochloric acid. FIG. 3 is 
the chromatogram of the fluid containing the NEW tetrachloroethylene. 
Each fluid was aged for 60 days at 150.degree. C and was again analyzed in 
a gas chromatograph. FIG. 4 is the chromatogram of the fluid containing 
the OLD tetrachloroethylene and FIG. 5 is the chromatogram of the fluid 
containing the NEW tetrachloroethylene. The chromatograms indicate that 
the NEW fluid was substantially unchanged, but that significant amounts of 
decomposition products (see peaks labelled A, B, and C in FIG. 4) were 
formed in the OLD fluid. These decomposition products are believed to be 
due to the breakdown of chlorohydrocarbons in the OLD tetrachloroethylene. 
This breakdown produces hydrochloric acid and/or chlorine which attack 
metals and insulation, as the following example illustrates. 
EXAMPLE 2 
Samples of the OLD and NEW tetrachloroethylene, both neat (unmixed) and 
mixed with mineral oil as in Example 1, were heated for 20 days at 
150.degree. C. The NEW material yielded less than 1 ppm of chloride ion 
and the OLD material yielded greater than 20 ppm of chloride ion. When 
aged with copper the OLD tetrachloroethylene had greater than 20 ppm of 
soluble metal chlorides. All of the stabilizer was consumed in the OLD 
material during testing. 
EXAMPLE 3 
NEW tetrachloroethylene was mixed in various proportions with mineral oil 
and then tested for pour point and boiling point. The following data shows 
how the mineral oil lowers the pour point and raises the boiling point. 
______________________________________ 
% C.sub.2 Cl.sub.4 
Pour Point (.degree.C.) 
Boiling Point (.degree.C.) 
______________________________________ 
100% -22 121.1 
75% -28 135 
50% -- 145 
______________________________________ 
EXAMPLE 4 
Samples of OLD and NEW tetrachloroethylene, both neat and in a 75%-25% by 
volume mixture with mineral oil were heated at 175.degree. C. for 180 
days. The samples were then tested for power factor, color, clarity, and 
acid number. The following table gives the result. 
______________________________________ 
Power Color Acid 
Sample Factor Scale Clarity Number 
______________________________________ 
OLD-neat 55.88 Black Sediment 
0.412 
OLD-25% Beyond 
oil Limits Black Sediment 
0.936 
NEW-neat 0.40 L-1.5 Clear 0.044 
NEW-25% 
oil 62.7 L-7.0 Sediment 
0.30 
______________________________________ 
The above data show that the NEW tetrachloroethylene produces far less 
decomposition product on aging. 
EXAMPLE 5 
Mixtures of NEW tetrachloroethylene and mineral oil were prepared and 
tested for flammability. The fluids were repeatedly ignited with a torch 
and the time from the removal of the torch to extinguishment of the flame 
was measured. The following table gives the results. 
______________________________________ 
Mixture (by volume) 
Average Time to Extinguish 
______________________________________ 
75% C.sub.2 Cl.sub.4 - 25% oil 
1-2 seconds 
50% C.sub.2 Cl.sub.4 - 50% oil 
1-3 seconds 
40% C.sub.2 Cl.sub.4 - 60% oil 
4-7 seconds 
______________________________________ 
EXAMPLE 6 
Mixtures of NEW tetrachloroethylene and mineral oil were prepared and 
tested for power and dielectric constant. The following table gives the 
results. 
______________________________________ 
Mixture Dielectric 
Power Factor 
Temperature 
(by volume) Constant (100 Tan.delta.) 
______________________________________ 
25.degree. C. 
100% C.sub.2 Cl.sub.4 
2.236 0.025 
75% C.sub.2 Cl.sub.4 - 25% oil 
2.27 0.30 
50% C.sub.2 Cl.sub.4 - 50% oil 
-- -- 
100% oil 2.2 0.01 
100.degree. C. 
100% C.sub.2 Cl.sub.4 0.94 
75% C.sub.2 Cl.sub.4 - 25% oil 
1.00 
50% C.sub.2 Cl.sub.4 - 50% oil 
-- 
100% oil 0.10 
______________________________________ 
EXAMPLE 7 
Mixtures were prepared of silicone oil sold by Dow Corning under the trade 
designation DC561 and ultra pure tetrachloroethylene, and the pour point 
of the mixtures was measured. The following table gives the results: 
______________________________________ 
% C.sub.2 Cl.sub.4 
Silicone Oil Pour Point 
(by volume) (by volume) .degree.C. 
.degree.F. 
______________________________________ 
100 0 -20 -4 
80 20 -22 -8 
75 25 -23 -10 
60 40 -24 -12 
50 50 -26 -15 
40 60 -29 -20 
25 75 -36 -33 
______________________________________ 
EXAMPLE 8 
Nine test transformers containing cellulose insulation were filled with a 
mixture of 75% by volume ultra pure C.sub.2 Cl.sub.4 plus 25% mineral oil 
and three identical monitor transformers were filled with 100% mineral 
oil. Due to the vapor pressure of C.sub.2 Cl.sub.4 it was necessary to 
limit the vacuum to about 18 inches after filling to prevent extracting 
the C.sub.2 Cl.sub.4. The filling procedure was to evacuate the 
transformer then close the exhaust valve and open the input valve 
admitting the liquid and after filling, pull a vacuum to about 18 inches, 
then admit dry nitrogen to atmospheric pressure (0 psig). The three 
control units were filled with oil under vacuum. The hot spot temperatures 
of the monitor units (oil only) were 160.degree. C., 180.degree. C. and 
200.degree. C. 
The electrical ratings of the transformers were 10 kVA, single phase, Type 
S, 7200/12470 y to 120/240 volts, 60 Hertz. 
The original cover was removed from each transformer and replaced with one 
fitted with a pressure gauge, a filling valve, a bottom sampling tube and 
valve and thermocouple gland to measure the liquid temperature. A second 
thermocouple gland was installed on the three control transformers to 
monitor and control the hot spot temperatures during the thermal aging 
cycle. Each transformer was sealed to 15 psig and 30 inches of vacuum 
before processing. 
The processing consisted of connecting a pair of units to a power source 
and circulating a current in the high voltage winding, with the low 
voltage winding shorted, to heat the coil to about 125.degree. C. 
One of the 160.degree. C. hot spot transformers failed at 4200 hours in the 
high voltage winding between turns. The ANSI minimum expected life curve 
for 65.degree. C. rise distribution transformers aged at 160.degree. C. 
hot spot is 2200 hours. 
The units have accumulated the following hours without failures: 
______________________________________ 
Accumulated ANSI Curve 
H.S. Temp. Hours Values 65.degree. C. Rise 
______________________________________ 
160.degree. C. 
4500 2200 
180.degree. C. 
2500 500 
200.degree. C. 
1300 128 
______________________________________ 
These values are considered to be very acceptable. 
The following conclusions were reached: 
1. The transformers filled with 75% C.sub.2 Cl.sub.4 and 25% oil run 
12.degree. C. cooler than the 100% oil-filled unit at 180% load. 
2. The liquid top level temperature was 14.degree. C. cooler than the 
oil-filled unit at 180% load. 
3. The gauge pressure was higher in the C.sub.2 Cl.sub.4 mix units by about 
4.8 psig than the oil units at 180% load. 
4. The design is good for 25 times normal short circuit. 
EXAMPLE 9 
Sample #1 --This sample was 75% by volume ultra pure C.sub.2 Cl.sub.4 -25% 
mineral oil. The container holding the sample was evacuated and backfilled 
with a 1 pound/sq. inch nitrogen atmosphere. The liquid/gas mixture was 
allowed to equilibrate for 30 minutes and then a sample was collected by 
opening a valve and allowing the vapors to expand into a pre-evacuated 
collection volume. The sample consisted of the gases that were trapped in 
the sample chamber after closing suitable valves. All the samples were 
generated in this manner except as noted. 
Sample #2--This sample was generated from #1 by passing an arc just below 
the surface of the solution for 10 seconds and collecting the gases as 
described above. The arc energy was 25 kVAC using a gap of 0.001 inches 
between stainless steel needles at room temperature. 
Sample #3--This sample was generated from sample #2 with a 2-minute arcing 
time. 
Sample #4--This sample was collected from sample #3 by pumping away the 
cover gas and collecting a sample when the solution started to bubble 
(boil under vacuum). 
Sample #5--This sample was collected from sample #4 after a new blanket of 
nitrogen gas was introduced into the system and followed by a 10-minute 
arcing period. 
Sample #6--This sample was collected from sample #5 by pumping away the 
cover gas and collecting a sample when the solution started to boil as in 
#4. 
The samples were all analyzed by mass spectrometric methods. The peaks in 
each sample were scaled so that they would represent the same amount of 
C.sub.2 Cl.sub.4. Peaks due to nitrogen had to be largely ignored since 
they were dependent on the original amount of nitrogen introduced and 
pumping losses that could not be controlled. On a qualitative basis there 
were no peaks detected that were due to a reaction between the C.sub.2 
Cl.sub.4 mixture and the nitrogen blanket. 
Samples #4 and #6 were taken to see if there was anything in the liquid 
phase that was not in the gas phase or vice versa. There were not any 
detectable differences between the liquid phase and gas phase samples. 
In sample #5, the new nitrogen blanket was added to replace the nitrogen 
pumped away to generate sample #4. The arcing time was increased to 10 
minutes but no new peaks were detected. 
Samples #1, #2, #3, and #5 formed a rate-type reaction since they are 
essentially the same reaction sampled at different times. 
No evidence was found to indicate that the C.sub.2 Cl.sub.4 and oil mixture 
produced any unusual products or any explosive gases (such as CH.sub.4, 
C.sub.2 H.sub.6, etc.).