CA1079951A - Electrical devices containing improved dielectric fluids - Google Patents

Abstract

Abstract of the Disclosure Improved electrical devices are disclosed such as transformers, and capacitors containing an improved dielectric fluid consisting essentially of a major amount of a liquid polyorganosiloxane and a minor amount of certain selected nitro substituted organic compounds which are soluble in said siloxane.

Description

; lV7~9Sl In numerous electrical devices, lt i9 necessary to provide a liquid insulating medlum which i8 called a ''dielectrical fluid.'' This liquld has a substantlal higher breakdown strength than alr and by displacing air from spaces between conductors in the electrical s equipment or apparatus, materially raises the breakdown voltage of the electrical device. With the ever increaslng sophistication of electrical equipment, the varlous s electrical devices are operating at hlgher and higher voltages. This means that the dielectric fluids used in such devices are sub~ected to greater and greater stresses. These problems have, of course, necessitated the search for improved dielectric fluids.
With the exception of certain special ~ applications, the polychlorinated biphenyl compounds s (generally known as "PCB's") have been the standard dielectric fluid in electrical devices since the 1930 ~ s when the PCB's replaced mineral oil. Various other liquids including some siloxanes have also been suggested V 20 for use as dielectric fluids. See, for example, U.S.
Patents 2,377,689 and 3,838,056 and British Patents 899,658 and 899,661. Recently, the PCB's have been found to have a negative impact on the environment and efforts 4 are being made worldwide to find suitable replacements for the PCB's.
By way of illustration, corona or partial discharge is a ma~or factor causing deterioration and failure of capacitors or other power factor correction devices. A capacitor operating in corona will have a life of only minutes or hours instead of the expected --1-- ~

iO'~9Sl twenty years. A capacitor properly impregnated with a suitable dielectric fluid will be essentially free of corona discharge to a voltage of at least twice - the rated voltage. During use when a dielectric fluld is placed under increasing stress, a point is reached where breakdown occurs. The voltage at which the capacitor will suddenly flash into corona is known in the art as the corona inception voltage (CIV). This voltage is dependent upon the rate at which the voltage is applied. There is considerable difference between the sensitivity of different fluids to the rate of , rise of voltage. The corona will, however, extinguish with a reduction of voltage. The corona extinctlon voltage (CEV) is not a fixed value for each fluid but is a function of the intensity of corona before ' the voltage is reduced. For best results, both the CIV
.~ and CEV should be as high and as close together as possible.
It has been discovered in accordance with this invention that when certain select nitro substituted organic compounds are incorporated into liquid poly-organosiloxanes that the resulting composition is useful as a dielectric fluid in electrical devices.
~S; It is further believed that these compositions when used as dielectric fluids provide suitable replacements for the PCB's which are currently being employed in the marketplace.
More specifically, this invention relates to an electrical device containing a dielectric fluid wherein the improvement comprises employing as the

-2-~ 1l)795~Sl J

: dielectrlc fluid a composltion conslsting essentlally of a ma~or amount of a liquid polyorganosiloxane and a mlnor amount of a compound soluble in said siloxane ; whlch compound is selected from the group consistlng of nitro substituted aromatic hydrocarbons, nitro substituted allphatic hydrocarbons and nitro substituted cycloaliphatic hydrocarbons.
This invention further relates to a dielectric fluid con~isting essentially of a ma~or amount of a liquid polyorganosiloxane and a minor amount of a compound soluble ln said siloxane which compound is selected from the group consisting of nitro substituted aromatic hydrocarbons, nitro substituted aliphatic hydrocarbons, nitro substituted cycloaliphatic hydrocarbons and mixtures thereof.
The liquid polyorganosiloxanes useful in this invention wlll be composed predominately of ~$~ siloxane units of the formula R2SiO and may al~o contain small amounts of siloxane units of the formulae R3SiO~/2, RSiO3/z, and SiO4/2. Of particular interest are liquid polyorganosiloxanes of the general formula R3SiO(R2SiO)XSiR3. In the foregoing formulae, the R radicals preferably represent hydrocarbon radicals and halogenated hydrocarbon radicals. Illustrative examples of suitable R radlcals are the methyl, ethyl, propyl, butyl, hexyl, decyl, dodecyl, octadecyl, vinyl, allyl, cyclohexyl, phenyl, xenyl, tolyl, xylyl, benzyl, 2-phenylethyl, 3-chloropropyl, 4-bromobutyl, 3,3,3-trifluoropropyl, dichlorophenyl, and alpha,alpha, alpha-trifluorotolyl radicals. Preferably, R contains ; 1079951 from 1 to 6 carbon atoms wlth the methyl, vlnyl and ; phenyl radlcal~ belng the most preferred.
The llquld polyorganosiloxane portlon of the dielectric fluld compositlon of thls lnventlon constltutes a maJor portlon thereof, that ls to say, more than 50 percent of the composltlon and preferably the llquid polyorganoslloxane constltutes from 80 to 99.5 percent by welght of the dlelectric fluld compositlon of thls lnvention. These llquid polyorganosiloxanes are well known materials which are commercially available throughout the world.
The dielectrlc fluld composltlon of this ~i lnvention also contalns a mlnor amount of a compound selected from the group consisting of nitro substituted aromatic hydrocarbons, nltro substituted aliphatic hydrocarbons, nltro substituted cycloaliphatic hydrocarbons and mixtures thereof. It is believed essential that these nitro compounds be soluble in the llquid polyorganosiloxane portion of the composition. Whlle any of the above indicated nitro compounds are useful in accordance with the present invention, the mononitro compounds containing from 1 to 10 carbon atoms are the preferred embodiment at this time. The nitro hydrocarbon compounds used herein constitute a minor portion, that is, less than 50 percent of the composition of this invention. It is generally preferred, however, that these materials be employed in an amount in the range from 0.5 to 20 percent by weight of the composition.

~,, _4_ , ~0~9951 Thus, in accordance with the pres~nt teachings, a dielectric fluid is provided which has a viscosity of 5 to 500 cs. at 25C. and which consists essentially of 50 to 99.5~ by weight of a liquid polyorganosiloxane and 0.5 to 50%
by weight of a compound soluble in said siloxane, which compound is selected from the group consisting of nitro substituted aromatic hydrocarbon, nitro substituted aliphatic hydrocarbons, nitro substituted cycloaliphatic hydrocarbons and mixtures thereof.
. 10 In accordance with a further embodiment, a trans-. former or capacitor is provided which contains as the dielectric ' fluid, a composition which has a viscosity of 5 to 500 cs. at , 25C and contains 80 to 99.5% by weight of a liquid poly-.. organosiloxane and 0.5 to 20% by weight of a nitro substituted aromatic, aliphatic or cycloaliphatic compound or mixtures thereof soluble in the polyo~-ganosiloxane.
: The dielectric fluid compositions of this invention may also contain small amounts of conventional .
., .

-4a-.',"~ ~j 10'7~9Sl addltlves such as HCl scavengers, corrosion lnhibitors and other conventional addltives normally employed - in such compositions so long as they do not have an adverse effect of the performance of the composition9 of this lnventlon.
The two most important electrical devlce~
in which the dielectric fluids of thls lnventlon are ~i useful are ln capacltors and transformers. They are s also very useful dlelectrlc fluids ln other electrlcaldevices such as electrical cables, rectlflers, eletro-magnets, swltches, fuses, circuit breakers and as coolants and insulators for dielectrlc devices such as transmitters, recelvers, fly-back coils, sonar ; bouys and toys. The methods for employing the dielectric ¦ fluids in these various applications (for example, as a reservolr of liquid or as an impregnant) are well known to those skllled in the art. For best reqults, the 4 vlscoslty of the dlelectrlc fluid composition of this invention should be in the range of 5 to 500 centistokes at 25C. If the viscosity exceeds 500 centistokes, they are difficult to use as impregnants and at less than - 5 centistokes their volatility becomes a problem unless they are used in a closed system.
Now in order that those skilled in the art may better understand how the present invention can be practiced, the followlng examples are given by way of illustration and not by way of limitation. All parts and percents referred to herein are by weight and all viscosities measured at 25C. unless otherwise specified.

. .

Example 1 , A screening test for dielectric fluids was developed which it is believed correlates well with results obtained in test capacitors. The main piece of equipment required for this test is a commercial corona detector with a manual variable ; voltage control. The test cell consists of a glass cylindrical container. The base of the cell is a ceramic filled plastic which has a stainless steel metal plate which is connected directly to ground. There is a stainless steel top for the container which has attached thereto a micrometer adjustable high voltage electrode with a steel phonograph needle on the end. The tip of this needle is positioned 0.0635 cm. (25 mils) above the grounded base. In the high voltage line attached to the electrode, there is a 1.67 x 108 ohm resistance. This is a current limiting resistor.
~- During the test, a few cubic centimeters of the test fluid is placed in the container and the top set in place.
As the voltage is increased, partial discharge occurs between ' the tip of the electrode and the ground plate. This draws current which reduces the applied voltage below the discharge ,~ level. When no cuxrent is being drawn, the applied voltage is again at partial discharge potential. Current is drawn by discharges again and the process is repeated. Thus, the current is in effect turned on and off very rapidly and the total breakdown of the fluid can never occur.
In operation, the applied voltage is slowly increased by f adjustment of the variable voltage control. The partial 10~i4~S~

dlscharges are observed on the oscllloscope of the corona ~etector. The polnt at which the ellptical lissa~ous pattern on the screen becomes flooded wlth ; discharges, and there is a constant audlble crackllng .,.
from the cell, ls recorded as the corona inceptlon voltage (CIV). The rate of rise of the applied voltage ls perhap~
a few hundred volts per second. When the CIV has been determined, the voltage ls slowly decreased untll the eliptical lissa~ous pattern on the screen can be seen again due to the partial cessation of discharges.
The polnt at whlch this occurs is recorded as the corona ' extinction voltage (CEV).
A number of dielectric fluid compositions s were prepared which consisted essentially of a liquid trimethylsilyl endblocked polydimethylsiloxane having - :
a vlscoslty of 50 cs. and varlous nitro substltuted hydrocarbons in varylng amounts. These composltlons were tested in the screening test described above. The specific nltro substituted hydrocarbons employed, the amount used (the balance being the siloxane) and the test results are set forth in the table below. A
number with a plus (+) behind it indicates that the test was terminated at that point and the actual value is somewhat greater than the value~reported.

107~

.
: Nitro Amount CIV CEV
ComDound (Wgt ~) (ln KV) (ln KV) A* None None 15.6 14.8 B Nitrobenzene 5 18,0 14.5 2.5 20.0 18.8 1 19.2 17.6 C 2-Nitropropane 10 19.8 17.0 18.4 17.6 2.5 20.0+ 18.0 ~: 10 1 20.0+ 19.0 D o-nitrotoluene 1 19 18 r E** o-nltrodl- <1 18 16.4 ` phenylamlne . , p; ~
. *Included for comparison . **Saturated solution i ' .

; .
r ~ :

i:

,:
r ''

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A dielectric fluid having a viscosity of 5 to 500 cs. at 25°C. and consisting essentially of 50 to 99.5 percent by weight of a liquid polyorganosiloxane and 0.5 to 50 percent by weight of a compound soluble in said siloxane, which compound is selected from the group consisting of nitro substituted aromatic hydrocarbon, nitro substituted aliphatic hydrocarbons, nitro substituted cycloaliphatic hydrocarbons and mixtures thereof.

2. A dielectric fluid as defined in claim 1 consisting essentially of 80 to 99.5 percent by weight of the siloxane and 0.5 to 20 percent by weight of the nitro substituted hydrocarbon.

3. A dielectric fluid as defined in claim 2 wherein the siloxane has the general formula R3Si0(R2Si0)xSiR3 wherein R is a hydrocarbon or a halogenated hydrocarbon radical and x is an integer, and the compound is a mononitro-substituted aliphatic or aromatic hydrocarbon containing from 1 to 10 carbon atoms.

4. A transformer or capacitor containing as the dielectric fluid a composition having a viscosity of 5 to 500 cs. at 25°C. and containing 80 to 99.5 percent by weight of a liquid polyorganosiloxane and 0.5 to 20 percent by weight of a nitro substituted aromatic, aliphatic or cycloaliphatic compound or mixtures thereof soluble in said polyorganosiloxane.

5. The transformer or capacitor of claim 4 wherein the liquid polyorganosiloxane is a trimethylsilyl endblocked dimethylpolysiloxane.