Process for removing undesirable substances from electrical devices

The removal of undesirable substances such as water, PCB's and air from the internal components of electrical devices such as transformers and power capacitors is efficiently and effectively achieved by contacting, in the absence of air, the internal components of these various electrical devices with a heat-stable fluid material. The heat-stable fluid materials are preferably halogenated aliphatic hydrocarbons and most preferably perchloroethylene or FREON 113.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for removing undesirable substances, 
e.g., water, oil and air, entrained within various insulating materials as 
well as other components of non-operating electrical devices. 
More specifically, the present invention relates to removing substances 
that degrade and otherwise adversely affect the functioning of the 
components, e.g., transformer cores, of electrical devices, particularly 
of the "oil-filled" type. The present invention further relates to 
retrofilling oil-filled electrical devices, particularly transformers. 
2. State of the Art 
Among the more commonly used commercial and industrial electrical devices 
are the "oil-filled" type. The oil acts as an insulator and at the same 
time as a cooling fluid when used in such devices as transformers, power 
capacitors, various cables, switches, circuit breakers and the like. In 
the recent past, the liquids which have found the widest use in these 
various "oil-filled" electrical devices are silicone oils, which have been 
limited in their use due to their expense, and polychlorinated biphenyls 
and diphenyls (PCB's). These and similar materials have been disclosed in 
the art, for example, in U.S. Pat. Nos. 1,931,373; 2,139,948 and 
4,177,156. While PCB's and the like have been found to be functionally 
advantageous, the industry has moved away from using these materials due 
to their toxicity and environmental impact. Substitute oils such as 
halogenated aliphatic hydrocarbons are now coming into use replacing PCB's 
and like materials. Such halogenated aliphatic hydrocarbons are disclosed 
in U.S. Pat. Nos. 1,953,216 to Elsey; 1,966,901 to McMahon; and 2,019,338 
to Clark. Furthermore, various fluorinated and chlorinated aliphatic 
hydrocarbons have found use in a number of applications for electrical 
devices. For example, U.S. Pat. No. 3,733,218 discloses azeotropic solvent 
mixtures of trichloroethylene/isobutanol, perchloroethylene/cyclopentanol, 
perchloroethylene/ethylene glycol monoethyl ether and 
perchloroethylene/N-amyl alcohol used to clean rosin-based solder circuit 
boards. U.S. Pat. No. 3,957,531 teaches similar azeotropic solvent 
mixtures for cleaning circuit boards by immersion in a tank containing 
such a solvent mixture. It is disclosed in U.S. Pat. No. 4,054,036 that a 
constant boiling mixture of 1,1,2-trichlorotrifluoroethane and 
cis-1,1,2,2-tetrafluorocyclobutane are useful as refrigerants, heat 
transfer media, gaseous dielectrics and the like. It is also disclosed in 
U.S. Pat. No. 4,276,530 that a fluorocarbon liquid and perchloroethylene 
are useful as a water collector for a vapor-cooled electrical apparatus 
which is hermetically sealed to the atmosphere where the disclosed process 
is applicable solely for an operating electrical apparatus. 
The replacement of PCB's as the oils used in the previously mentioned 
electrical devices has resulted in either the need for new "oil-filled" 
electrical devices coming on the market replacing those in existence or 
the retrofilling of those presently in use. The term "retrofilling," as 
used herein, is to be understood to mean the removal of the dielectric 
fluid material contained in an electrical device which generally has been 
in use, optionally purging the emptied electrical device followed by the 
refilling of the electrical device with the same or different dielectric 
fluid material. Problems, however, have been encountered with the 
installation of these new electrical devices, in particular transformers. 
During the preparation of transformer cores as well as the internal 
components of the other mentioned electrical devices, atmospheric water is 
absorbed by the insulator material which is conventionally kraft paper. 
Also air is entrapped in the core during the winding procedure. As is 
readily recognized by one skilled in the art, the presence of even small 
amounts of water and oxygen will create problems in transformer cores as 
well as the internal components of capacitors or switches. These 
substances will degrade the materials of the particular component 
potentially creating "shorts" which in turn will cause more heat to be 
generated by the component which further accelerates and aggravates the 
problem. Furthermore, materials finding use as substitute oils (such as 
halogenated aliphatic hydrocarbons including perchloroethylene and 
trichloroethylene), while quite stable in the absence of oxygen and light, 
will autooxidize quite rapidly when exposed to oxygen particularly in the 
presence of ultraviolet radiation to degrade to various undesirable 
corrosive products as acetyl chlorides, carboxylic acids, hydrogen 
chloride and phosgene. Therefore, it has been quite critical during the 
installation, refilling or maintenance of the electrical device to insure 
that substantially all degradative or corrosive substances have been 
removed from the internal components prior to putting the device into 
operation. In the past, in the case with transformers, this drying or 
purging procedure has been accomplished by allowing the transformer core 
to heat up to relatively high temperatures (250.degree. C. or more) under 
greatly reduced pressure (1 mmHg). This method, however, is not all that 
satisfactory since it requires 5 to 6 hours or more and it is obviously 
energy intensive. Also, this method has not always been as effective as 
desired since residual atmospheric water and air is reabsorbed by the core 
material. 
To retrofill an electrical device, particularly a transformer which 
contained PCB's, it is necessary to reduce the PCB concentration level to 
below 500 ppm and most preferably below 50 ppm. There is no known method 
which can achieve these PCB levels economically and, thus, it has been 
necessary to dispose of the electrical device. Disposing of an electrical 
device can represent a major economic loss particularly when the device is 
a transformer. 
Therefore, there is a need for a method for removing undesirable and 
degradative substances from the internal components of "oil-filled" 
electrical devices, particularly transformers which method is relatively 
fast, economical, less energy intensive and most importantly highly 
effective in removing substantially all of the undesirable substances 
while not allowing them to re-enter the system. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, it has been discovered that 
substantially all the undesirable substances can effectively be removed 
from the internal components of an oil-filled, non-operating electrical 
device while overcoming or minimizing the problems of the methods 
presently in use by contacting, in the absence of air, the internal 
components of the particular oil-filled electrical device with a 
heat-stable fluid material which preferably forms an azeotrope with the 
undesirable substance contained within said internal component.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The method of the present invention greatly improves upon the method now in 
use for drying and removing substances which degrade and affect the 
functioning of transformer cores. It also provides an effective and 
economical method for retrofilling transformers which are or have been in 
use. Clearly, it is necessary to effectively remove substantially all of 
such undesirable substances during the installation or maintenance of 
transformer cores. Otherwise, the efficiency of the transformer is greatly 
reduced as well as the lifetime of the transformer. 
It will be readily recognized by one skilled in the art that one of the 
problems involved with the method presently in use, previously described, 
is that a partial vacuum is an inefficient heat transfer media. Therefore, 
while it is highly desirable for the contacting procedure be conducted in 
the absence of air, the material to which the transformer core is to be 
contacted should be an effective heat transfer media which requires that 
the material be heat-stable. It would also be desirable that the material 
have a relatively high density, i.e., greater than air, as well as being 
insoluble in water but capable of forming a heterogeneous azeotrope with 
water. In addition, it should also be recognized that the material 
preferably be nonflammable, be readily and rapidly absorbed by insulating 
materials such as kraft paper and the like and compatible with other 
materials and components of the particular electrical device such as 
aluminum and copper metals. Such material also will preferably have 
dielectric properties, low toxicity and be environmentally safe. While the 
above-listed properties are considered to be desirable and/or preferable, 
it is pointed out that it is not critical to the present invention that 
the particular material selected have each and every one of these 
properties. It should be clear that the selection of the specific material 
will be dependent upon a variety of factors including the particular 
electrical device, its function, its parameters, material availability, 
expense and the like. 
While a number of different materials can be employed in the process of the 
invention, as is evident from the above, it would be desirable to use 
materials which also may be used as the "oil" to fill the particular 
electrical device, specifically a transformer. Such materials as 
halogenated aliphatic hydrocarbons have been found to be particularly 
useful in the process of the present invention. These halogenated 
aliphatic hydrocarbons include methylene chloride, carbon tetrachloride, 
chloroform, trichloroethylene, perchloroethylene, bromochloromethylene, 
FREON 111, FREON 113 and the like. The most preferred materials being 
perchloroethylene and FREON 113 (1,1,2-trichloro-1,2,2-trifluoroethane). 
One embodiment of the present invention is illustrated in the diagram of 
the drawing. While the invention is to be further described in connection 
with the attached drawing diagram, it is intended that the drawing 
included as a part of this specification be illustrative of a preferred 
embodiment of the invention and should in no way be considered as a 
limitation on the scope of the invention. Referring now to the drawing, 
the diagram illustrates the process for drying and removing undesirable 
substances, in accordance with the present invention, from transformer 
core 12 installed in transformer 10. For the purposes of this 
illustration, the contacting material selected is perchloroethylene which 
is contained within the reservoir of the thermo siphon reboiler 23. The 
pressure of the system is first reduced to approximately 1 mmHg by vacuum 
system 16. The transformer 10 is then closed off to the line of vacuum 
system 16 and perchloroethylene vapor is introduced to the top of 
transformer 10 through flexible interconnect 13 and line 20. 
Perchloroethylene vapor is continuously generated by thermo siphon 
reboiler 23 from which the vapor is passed into a superheat exchanger 25, 
if required, prior to its introduction to the top of transformer 10. The 
perchloroethylene vapor not absorbed and/or condensed by transformer core 
12 is passed out of the transformer through flexible interconnect 14 along 
with removed water, air and other undesirable substances. The outlet vapor 
temperature is measured at 22. This outlet vapor containing removed 
substances is passed into a primary condenser 27 and double effect 
decanter 24. Removed water is taken off through 36 while the inert purge 
is taken off through inert chiller 28 and purge line 29. The separated 
perchloroethylene is simultaneously recycled back into the reservoir of 
reboiler 23. Once the outlet vapor temperature, measured at 22, equals the 
inlet vapor temperature, measured at 21, transformer 10 has become 
saturated with perchloroethylene vapor. At this point, the valves of the 
flexible interconnects 13 and 14 are closed and vapor bypass valve 26 is 
opened. The valves to flexible interconnect 15 are now opened and the 
pressure is reduced to approximately 20 mmHg by solvent vacuum system 30 
and vacuum pump 31 where perchloroethylene vapor plus residual air and 
water are passed through inert chiller 32, and the inert purge is removed 
at 35. Separated and condensed perchloroethylene is then recycled from 
reservoir 33 by centrifugal pump 34 to the reservoir of reboiler 23. When 
the pressure of the system reaches approximately 20 mmHg, the system is 
closed from solvent vacuum system 30. Transformer 10 can then be filled 
with perchloroethylene or another appropriate "oil" and the pressure 
equalized with nitrogen. 
Optionally, repeated cycles of vapor flushes can be carried out in 
accordance with the procedures set out hereinabove. Additionally, the 
above-described vapor flush(es) may be followed by or alternated with hot 
liquid flush(es). Obviously, the hot liquid flush need not be conducted 
under reduced pressure where the hot liquid, e.g., perchloroethylene or 
FREON 113, is pumped through the same system as diagrammed in FIG. 1. The 
necessity of repeated vapor flushes or the inclusion of hot liquid flushes 
will be determined by such factors as the substance being removed from the 
electrical device, the particular electrical device, the size or volume of 
the electrical device, the particular fluid material used as the flushing 
medium, the desired degree of decontamination and the like. The process of 
the present invention has achieved PCB concentration levels down to 10 ppm 
for retrofilled transformers. Maximum PCB removal was achieved by 
installing a distillation column between the thermo siphoned reboiler 23 
and the super heat exchanger 25. The installation of the distillation 
column was found effective for the specifical removal of PCB's. However, 
it is not required for theremoval as such substances as water. A 1500 KVA 
Uptegraff transformer retrofilled in accordance with the process of the 
present invention is presently back in service and operating within the 
expected efficiency range. 
The process of the present invention, as described above, may require only 
2 hours or less to substantially remove all undesirable substances from 
the transformer core and does not allow for air to be reintroduced into 
the transformer or for further exposure to the atmosphere. The advantages 
of the process of the present invention are clearly seen when compared to 
the method conventionally used, and previously described, wherein the 
conventional process requires at least 5 to 6 hours to achieve the desired 
results, and the core is then temporarily exposed to the atmosphere 
allowing for the reabsorption of atmospheric moisture. Among the other 
advantages of the process of the present invention, there may be mentioned 
that perchloroethylene and the like is clearly a much more efficient heat 
transfer media than air or a partial vacuum while at the same time the 
perchloroethylene is rapidly and quite effectively absorbed by the 
insulating material of the core and thus wicking away impurities such as 
water from the insulating material as well as not allowing for its 
reabsorption. Therefore, the process of the present invention requires 
substantially less energy and time, and at the same time is much more 
efficient and effective than the conventional method now in use. 
The method of the present invention has been illustrated above by removal 
of undesirable substances from a transformer core. It is not intended for 
the scope of the present invention to be limited solely to transformers. 
It will be recognized by those of ordinary skill in the art that the 
process of the present invention can readily be adapted for use with 
various electrical devices, particularly the "oil-filled" type previously 
mentioned, as well as in other technologies. Other features and aspects of 
this invention will be appreciated by those skilled in the art upon 
reading and comprehending this disclosure. Such features, aspects and 
expected variations and modifications of the described method are clearly 
within the scope of this invention where the invention is limited solely 
by the scope of the following claims.