High voltage control devices

High voltage control devices are provided comprising a housing, an aprotic liquid contained within said housing, means for effecting heat exchange with said liquid, at least two spaced electrodes contained within said liquid and each of said electrodes extending from said housing. These devices find utility as variable resistors, circuit breakers, thermostats, optical triggering devices, acoustic pulse generators and the like.

This invention relates to a high voltage control device. More particularly, 
this invention relates to the use of certain aprotic liquids in high 
voltage control devices. 
The present invention provides high voltage control devices which utilize 
the unique physical properties of molten sulfur, sulfur monochloride and 
mixtures thereof. For example, molten sulfur exhibits an anomalous 
variation of electrical resistance with temperature under high voltage 
conditions enabling its use as a variable resistance-type control device. 
Molten sulfur, sulfur monochloride and mixtures thereof do not generate 
bubbles or undergo degradation under spark discharge conditions enabling 
the obtainment of highly stable control devices useful over extended 
periods of time. Moreover, molten sulfur and sulfur monochloride are 
photoconductive, thereby providing a convenient trigger mechanism for such 
devices. 
It is an object of this invention to provide devices for current control 
under high voltage conditions. 
It is another object of this invention to provide a high voltage control 
device which does not undergo decomposition when subject to an arc 
discharge. Moreover, said control device does not require purification or 
maintenance to preserve its operability over extended periods. 
It is still another object of this invention to provide a high voltage 
control device which can be used as a thermal switch thereby enabling its 
use as a thermostat, circuit breaker, variable resistor and the like. 
It is a further object of this invention to provide a high voltage control 
device with photoconductive capabilities for control of current and even 
the initiation of electrical breakdown of the medium. Sparking within a 
device containing such aprotic liquids, when coupled to an acoustic 
matching medium, provides an excellent acoustic pulse source. 
These as well as other objects are accomplished by the present invention 
which provide a high voltage control device comprising a housing, an 
aprotic liquid contained within said housing, means for effecting heat 
exchange with said liquid, at least two spaced electrodes contained within 
said liquid and each of said electrodes extending from said housing.

Aprotic liquids are molecular liquids which neither yield protons to a 
solute nor accept protons from a solute. Generally, such liquids are 
characterized by the absence of C--H bonds. The aprotic liquids useful in 
the present invention, unlike liquified noble gases or liquified metals, 
are highly insulating liquids over a wide useful and conveniently obtained 
temperature range, e.g., sulfur: 112.8.degree. C. to 444.6.degree. C.; 
sulfur monochloride (S.sub.2 Cl.sub.2): -80.degree. C. to 135.6.degree. C. 
Mixtures of sulfur and sulfur monochloride can be similarly employed in 
widely varying volumetric ratios, e.g., sulfur: sulfur monochloride 1:99 
to 99:1. 
Molten sulfur is particularly preferred for use as a high voltage 
insulating medium in accordance with the present invention. As indicated 
above, molten sulfur exhibits a wide liquid range and a large viscosity 
increase at 159.degree. C. owing to the opening of molecular S.sub.8 rings 
and polymerization to form long chain molecules at the higher 
temperatures. The low conductivity of molten sulfur (10.sup.-12 ohm.sup.-1 
cm.sup.-1) and the absence of decomposition, even under spark discharge 
conditions, contributes materially to the use of molten sulfur as a high 
voltage insulating medium. The breakdown strength of molten sulfur is 
above 10.sup.6 V/cm. 
Another aprotic molecular liquid useful in the present invention is sulfur 
monochloride, S.sub.2 Cl.sub.2. Although this liquid is less stable 
chemically than pure sulfur and has a higher conductivity (10.sup.-9 
ohm.sup.-1 cm.sup.-1), it provides a wide useful liquid range (-80.degree. 
to 138.degree. C.) about ambient temperature. The dielectric strength of 
S.sub.2 Cl.sub.2 is in excess of 10.sup.6 V/cm. 
Referring now to FIG. 1, there is shown the resistance-temperature 
dependence at high and low fields for molten sulfur which provides an 
effective thermal switching effect at temperatures surrounding the 
viscosity transition at 159.degree. C. At low fields, the resistance 
decreases with increasing temperature according to the usual exponential 
dependence characteristic of semiconductors. Below the viscosity 
transition temperature (159.degree. C.), a high voltage control device of 
the type shown in FIG. 2 (discussed hereinafter) normally operating under 
low field conditions (point A) will, if a high field is impressed, 
decrease markedly in resistance (point B). Typically, the resistance at 
point B is only 2-10% of the value at point A. Raising the temperature of 
the molten sulfur causes the resistance to increase to point (C). 
Typically, this is a 10 to 20-fold increase in resistance over a 5.degree. 
C. range. The use of the viscosity transition as compared to a phase 
transition is advantageous in that there is little expansion or 
contraction of the material upon crossing the viscosity transition 
temperature and the material is liquid in both states, thus conforming to 
the containing vessel and the electrical contacts. The viscosity 
transition is reversible so that many cycles of operation can be effected. 
While the resistivity of molten sulfur is relatively high, doping of the 
molten sulfur with halogens such as chlorine or iodine can provide a means 
for lowering or otherwise altering the resistance-temperature profile. 
The present invention employs this anomalous variation of electrical 
resistance with temperature associated with molten sulfur to provide a 
high voltage control device, one embodiment of which is shown in FIG. 2. 
The molten sulfur 10 is contained within a housing 12. Two spaced 
electrodes 14 and 16 are immersed in the molten sulfur within the housing 
with portions of said electrodes extending outside of the housing to serve 
as leads to and from the device. Means for effecting heat exchange with 
the molten sulfur, such as immersion heater 18, are situated in heat 
exchange contact with the molten sulfur to maintain the molten sulfur 
within the liquid temperature range and to provide a means for selectively 
altering the temperature of the molten sulfur to take advantage of its 
resistance-temperature dependence. Heat exchange also can be effected by 
immersing the device shown in FIG. 2 in a heating bath such as an oil 
bath, fluidized sand bath and the like. 
The high voltage control device of the present invention finds utility as a 
variable resistor which can be employed, for example, in the fault current 
limiting circuit shown in FIG. 3. The voltage source 20 normally feeds the 
load 22 through a power bus 21 containing circuit breaker S.sub.1 which is 
normally closed. The parallel circuit containing the high voltage control 
device 24 of the present invention and normally closed circuit breaker 
S.sub.2 carries only a small fraction of the total current. Thus, in the 
"normal" mode, control device 24 operates as indicated by point A in FIG. 
1. When a fault occurs such as a short circuit across part or all of load 
22, the breaker S.sub.1 will open creating an open circuit in the power 
bus 21. This action is made easier by the fact that the parallel path 
exists containing control device 24 and circuit breaker S.sub.2. Upon the 
sudden increase in voltage across the control device 24, the resistance of 
the device immediately decreases in value (as shown by point B in FIG. 1). 
This sudden decrease in resistance tends to eliminate large voltage 
transients due to inductive effects and further eases the opening of 
breaker S.sub.1. At this point, breaker S.sub.2 carries the fault current 
limited by the resistance of the control device 24. Until the fault has 
been located and cured, it becomes desirable to open breaker S.sub.2 
thereby fully interrupting the circuit between source 20 and load 22. The 
fault current is generally a large current and one which would be 
difficult to interrupt without arcing in conventional circuit breakers. It 
is therefore preferable that the resistance of the control device 24 
undergo a transition to a larger value in advance of the opening of 
breaker S.sub.2 (point C in FIG. 1). This "soft" switching effect can be 
conveniently effected by control device 24 either automatically by the 
temperature change generated by the internal heating effect of the high 
current or by activation of heating means 26. Depending upon the magnitude 
of the currents involved, the internal heating effect of the high current 
may produce this "soft" switching effect even before the fault is cured. 
Once the resistance is raised to the desired level, breaker S.sub.2 can be 
conveniently opened. 
Many materials, metals in particular, possess positive temperature 
coefficients but of such small value that the desired resistance change is 
produced too slowly. To speed the resistance change, some success has been 
achieved with solid-liquid phase transitions of metals such as sodium. 
Certain hazards exist in using this material and it is difficult to 
maintain the integrity of the material for re-use after it has 
re-crystallized. In accordance with the present invention, however, a 
control device employing molten sulfur enables a 10 to 20-fold variation 
in resistance to be effected over a very narrow temperature range, e.g., 
about 5.degree. C. 
Present methods for current interruption involve the rapid separation of 
current contacts in a medium which must rapidly quench the arc discharge 
formed when such contacts are opened and which must rapidly recover 
preparatory to additional interruptions should that be necessary. Present 
methods utilize air, SF.sub.6, hydrocarbon oils and the like as the 
insulating medium. Operation of the circuit breaker seriously decomposes 
these media making it necessary to service the breaker, i.e. to change or 
purify the medium, or requires provision for continuous flow to rid the 
vessel surrounding the contacts of decomposition products. 
Bubble generation has heretofore been an integral part of electrical 
breakdown in known insulating fluids. As gas bubbles containing 
decomposition products persist for long periods of time, the electrical 
strength of the medium is accordingly degraded. Solid decomposition 
products such as free carbon are also formed in typical insulating 
(hydrocarbon) liquids. These materials must be removed to restore the 
integrity of the breaker for further use. 
Gases such as air or SF.sub.6 are commonly used as breaker media. While 
these gases do not form suspended particulates or persistent dielectric 
discontinuities, their decomposition products may be very troublesome 
chemically, e.g., the formation of nitric acid in air discharges with 
water vapor present. The electrical breakdown strength of these gases is 
much less than that of typical liquid insulators. Thus, for breaker 
applications at high voltage such as is used by the power industry, very 
highly compressed gases must be used. The noble gases such as argon are 
not used since they are very poor electrical insulators, and since they 
are not electron attaching, they do not quench electrons from the 
discharge. 
The high voltage control device of the present invention employing molten 
sulfur, sulfur monochloride and mixtures thereof as the insulating medium 
provides an excellent switching medium for circuit breakdown which does 
not decompose when subjected to an arc discharge and which provides a high 
resistance to current flow immediately before and after such discharges. 
Moreover, these switching media do not require purification or maintenance 
to preserve their function over many operations of the breaker. 
Thus, a spark discharge in these aprotic liquids produces no contamination, 
either gaseous or solid. Moreover, these liquids are excellent insulators, 
possessing high electrical resistivities and breakdown strengths. Thus, 
the use of the high voltage control device of the present invention in 
circuit breaking applications provides significant advantages over the 
materials heretofore employed since it would not require frequent service. 
Thermal insulation requirements are minimal owing to the relatively low 
operating temperature (-80.degree. -440.degree. C. range) and the highly 
insulating properties of the liquids themselves. 
Referring now to FIG. 4, a typical circuit utilizing the high voltage 
control device 30 of the present invention as a circuit breaker is shown. 
The load 32 is powered by voltage source 34, both of which are in series 
with control device 30. When an overload occurs and/or sudden current 
interruption is desired, the bridging contact 36 can be withdrawn from 
electrical contact with the spaced electrodes 37 and 38 either manually, 
via a solenoid 39 (as shown), or by other conventional circuit breaking 
techniques thereby creating an open circuit. The temperature of the 
aprotic liquid can be controlled by heating means 31. If desired, the 
required heating to maintain the aprotic liquid in the liquid phase can be 
accomplished by bleeding a small current from the voltage source and 
utilizing an immersion heater technique. 
The high voltage control device of the present invention also provides a 
unique temperature sensing device which can function as a thermostat, a 
heat sensor adapted to activate alarm systems and the like. FIG. 5 
illustrates the use of the high voltage control device of the present 
invention as a thermal switch to activate a system or device. Thus, in the 
"off" mode, the voltage source 40 imposes a high voltage drop across 
control device 42. The temperature of the control device is regulated by 
heat exchange means 44 such that the control device exhibits a high 
resistance (e.g., point C in FIG. 1). When it is desired to activate the 
system 46 or when a temperature is reached which requires the system to be 
activated, the resulting temperature drop either effected by heat exchange 
means 44 or by the temperature conditions of the surrounding environment 
causes a significant drop in resistance (e.g., a drop to point B in FIG. 
1) with a corresponding increase in current thereby activating the system. 
Conversely, an increase in temperature permits a return to point C in FIG. 
1 with corresponding inactivation of the system. 
The high voltage control device of the present invention is also suitable 
for use in microwave transmission devices. 
In microwave power applications, it is of interest to operate switches 
which can insulate against voltages as high as 50 KV but which can close 
upon command to a very low impedance value within about 10.sup.-11 to 
10.sup.-9 seconds, with accordingly small jitter. Solid state devices are 
generally not capable of these high operating voltages, nor of the high 
discharge currents which flow when the switch is closed. High pressure gas 
switches are useful for these purposes but a satisfactory low jitter 
trigger has not, as yet, been found. Liquid dielectric switches have many 
desired features such as high dielectric strength, affording close gap 
spacing at high operating voltages that minimize self-inductance when the 
discharge is formed. A basic difficulty with liquid dielectric spark 
devices has been the formation of bubbles during breakdown thus preventing 
repetitive function. Moreover, such bubbles are the result of 
decomposition of the liquid medium. Suitable triggering means have been 
generally unavailable in liquids which can produce low jitter performance. 
It has now been found that discharges in molten sulfur, sulfur monochloride 
and mixtures thereof produce no bubbles, a behavior previously unknown in 
liquids. It has also been found that these aprotic liquids are 
photoconductive, thus producing free electrons when illuminated with a 
suitable light source. Preconditioning of the liquid is therefore possible 
when a high field is applied that is not quite sufficient for breakdown, 
typically 500 kilovolts/cm. Under these conditions, the appearance of 
photo-induced free electrons will cause rapid electrical breakdown in the 
medium. 
A particularly valuable feature of the photoconduction triggering method is 
associated with the use of an optical path for the triggering signal. This 
permits the triggering of single or multiple switches carrying very high 
potentials where direct connection would prove hazardous. Optical 
triggering also obviates the use of structures such as triggering pins, 
grids and screens. For this reason, reliability and durability of the 
switch are high while cost is relatively low. 
FIG. 6 illustrates another embodiment of the high voltage control device of 
the present invention adapted for photoconductive triggering of the spark 
gap formed by the spaced electrodes. It should be emphasized, however, 
that although photoconductive triggering is illustrated, other triggering 
devices (such as an electron injecting third electrode or most 
conveniently, simply exceeding the breakdown potential by regulation of 
the voltage source) can be similarly employed. In the pretriggered state, 
an electric field from voltage source 50 is applied across the control 
device 52 at a potential below the breakdown field strength of the aprotic 
liquid. Under these conditions, only a small dark current flows. The 
aprotic liquid contained within the control device 52 is maintained in the 
liquid state by heat exchange means 54. It is important when using molten 
sulfur to regulate the temperature of the molten sulfur under such high 
field conditions to maintain the resistance of the molten sulfur at a 
relatively low value, e.g., such as point B in FIG. 1. The housing 56 of 
the control device 52 can contain an optically transparent window 58 
through which the aprotic liquid can be exposed to a light pulse 60. When 
a light pulse is applied which has an appropriate spectral output (e.g., 
about 3000 to 6000 A), photo-induced, relatively mobile charge carriers 
are produced throughout the bulk liquid occupying the high field region. 
The result is the rapid reduction of the dielectric strength and the onset 
of an avalanche breakdown. The removal of the light and the collapse of 
the field across the gap results in the rapid recovery of the 
pre-triggered breakdown value. It has been found that recovery in such 
devices of the present invention is complete in about 10 .mu. sec. The 
resulting current pulse is fed to a microwave transmitting device 62. 
As described above, the high voltage control device of the present 
invention provides a means for accurately controlling the initiation time 
of an electrical discharge between spaced electrodes in a liquid medium. 
The thermal expansion caused by the electrical discharge in the liquid 
medium produces a pressure pulse characterized by a steep shock front 
which decays smoothly due to thermal diffusion in the liquid. As described 
herein, discharges in the aprotic liquids of the present invention produce 
no bubbles, a phenomenon previously unknown in liquids. Owing to this 
absence of bubbles, the pressure pulse is free of undesirable 
oscillations. Also, high repetition rates can be achieved since no bubble 
clearing time is required. 
These factors enable the high voltage control device of the present 
invention to function as an acoustic pulse generator which does not 
produce undesirable oscillations or noise in the post-pulse period while 
affording a high repetition rate, high power pulse generation capability. 
Such a source may be of great usefulness in ranging work such as: sonar, 
geophysical exploration, depth sounding and biological scanning. A quiet 
post-pulse period permits reception of return echoes at short range and 
unambiguous interpretation of return pulse shapes. 
Acoustic pulses for the above purposes are presently generated by explosive 
charges, hammer blows, piezoelectric devices, magnetostrictive devices or 
spark discharges (in water). The electro-mechanical devices find wide use 
in sonar and other communication applications in the sea. In some cases, 
these techniques suffer from the relatively low excess pressure which can 
be generated. Explosive charges and spark discharges in sea water result 
in bubbles which may confuse the acoustic signal or cause it to persist 
for long periods after the pulse is generated. Moreover, these sources are 
generally limited to very low repetition rates. 
FIG. 7 illustrates another embodiment of the high voltage control device of 
the present invention modified to function as an acoustic pulse generator. 
In this instance, the control device 70 comprises an acoustically 
transparent housing 72, at least one wall of which is a flexible membrane 
73 adapted to transmit the generated acoustic pulse from the aprotic 
liquid to another liquid of substantially matching impedance, e.g., water. 
The housing 72 contains a pair of spaced electrodes 74 and 76 immersed in 
the aprotic liquid 78, i.e., molten sulfur, sulfur monochloride or 
mixtures thereof. A heater 80 is employed to maintain the aprotic liquid 
in the liquid phase. A variable voltage source 82 is connected across the 
device to initiate sparking across the electrode gap. Alternatively, an 
optically transparent window (not shown) can be included within the 
housing and sparking can be initiated photoelectrically as discussed above 
in connection with FIG. 6. 
The high voltage control device of the present invention is uniquely suited 
as an acoustic pulse generator. The pressure pulse produced by the thermal 
expansion caused by the electrical discharge in the aprotic liquid medium 
generates a steep shock front which is coupled with a smooth decay brought 
about by thermal diffusion. Oscillations do not exist due to the absence 
of bubbles. Additionally, high repetition rates can be achieved since no 
bubble clearing time is required. The ability to generate an acoustic 
pulse without accompanying bubble generation is of critical importance 
since bubbles confuse the acoustic signal and can cause it to persist long 
after it is generated. 
The high voltage control device of the present invention has thusfar been 
illustrated primarily as a two electrode device. If desired, a third 
electrode can be included within the housing to serve as a cleanup 
electrode which could electrophoretically attract foreign particles in the 
liquid medium. 
Although specific materials, devices, circuits and conditions are set forth 
herein to illustrate the fabrication and use of the high voltage control 
devices of the present invention, these are merely intended as 
illustrations of the present invention. Various other liquids, control 
devices, and circuits may be substituted herein with similar results. 
Other modifications of the present invention enabling other control devices 
and circuits embodying the principles of the present invention will occur 
to those skilled in the art upon a reading of the present disclosure. 
These are intended to be included within the scope of this invention.