CA1157117A - High power current limiter with a current commutating resistive element - Google Patents
High power current limiter with a current commutating resistive elementInfo
- Publication number
- CA1157117A CA1157117A CA000373964A CA373964A CA1157117A CA 1157117 A CA1157117 A CA 1157117A CA 000373964 A CA000373964 A CA 000373964A CA 373964 A CA373964 A CA 373964A CA 1157117 A CA1157117 A CA 1157117A
- Authority
- CA
- Canada
- Prior art keywords
- resistor
- current
- cell
- commutation device
- electrolytic cell
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01H—ELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
- H01H33/00—High-tension or heavy-current switches with arc-extinguishing or arc-preventing means
- H01H33/002—Very heavy-current switches
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01C—RESISTORS
- H01C1/00—Details
- H01C1/14—Terminals or tapping points or electrodes specially adapted for resistors; Arrangements of terminals or tapping points or electrodes on resistors
- H01C1/1406—Terminals or electrodes formed on resistive elements having positive temperature coefficient
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- Engineering & Computer Science (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Emergency Protection Circuit Devices (AREA)
- Details Of Resistors (AREA)
- Conductive Materials (AREA)
- Electrolytic Production Of Metals (AREA)
- Driving Mechanisms And Operating Circuits Of Arc-Extinguishing High-Tension Switches (AREA)
Abstract
13 48,637 ABSTRACT OF THE DISCLOSURE
A high power current commutation device for use with electrolytic cells to permit the system current to be shunted from the cell, and to thereafter be commutated back through the cell. The commutation device employs a plurality of parallel path vacuum switches, with at least one vacuum switch being serially connected to a resistor which increases in resistance with temperature. This resistor has a specific heat characteristic and thermal time constant such that current through the resistor will result in a potential drop across the resistor exceeding the electrolytic cell back potential to thereby commutate current back through the cell. A specific resistor design for use in the current commutation device is detailed.
A high power current commutation device for use with electrolytic cells to permit the system current to be shunted from the cell, and to thereafter be commutated back through the cell. The commutation device employs a plurality of parallel path vacuum switches, with at least one vacuum switch being serially connected to a resistor which increases in resistance with temperature. This resistor has a specific heat characteristic and thermal time constant such that current through the resistor will result in a potential drop across the resistor exceeding the electrolytic cell back potential to thereby commutate current back through the cell. A specific resistor design for use in the current commutation device is detailed.
Description
~7 ~ 17 1 48,637 HIGH POWER C ~ENT LIMITER WITH A CURRENT
COI~MUTATING RESISTIVE ELEMENT
BACKGROUND OF THE INVENTION
m e present invention relates to devices for limiting or commutating high power, high current electri-cal systems and a resistor element for use in such de-vices. Many present day AC and DC electrical distributionsystems have need for high power and high current limita-tion or commutation devices for use alone or with circuit breaker devices. Such current limitation and commutat~on de~ices provide added n exibility for the electrlcal distribution network permitting greater circuit protection and disconnect capability.
In the electrochemical industry, a basic compon-ent is an electrolytic cell in which an electrolyte solu-tion can be decomposed when electric~l current is passed through the cell. The decomposed and separated chemlcal constituents are collected ~or use as the raw stock mater-ial in the Ghemical industry. Such electrolytic cells and the electrical æystems associated therewlth typically operate at low voltage of ~rom about 5 to 50 volts with a ~ery hlgh current of tens of klloamperes to in excess of 100 kiloamperes for the system. It i8 sometimes necessary to electrically bypass or remove one of a plurallty of serially connected electrolytic cells from service, for purposes of maintenance and/or component replacement. It is the practlce to utilize an electrical shunting or bypass device which employs a circuit breaker element such as a ~acuum switch whlch ca~ries the electrical system ", ~' 7i ~7 4~ 6~ 7 ~urrent while the cell is bypassed, and which upon th~
opening of the vacuum circuit element or switch diverts the current back to the electrolytic cell. It is the practice to utilize a plurality of parallel connected vacuum switches for such electrolytic cell bypass devices.
In ~iverting the system current back to the electrolytic cell, the vacuum switch devices are open circuited and a significant amount of inductively stored energy in the circuit paths must be dissipated. This is largely a-l~ chieved by the arc that forms between the parted switchcontacts. This arcing within the vacuum chamber of the switch can cause erosion of the contact surfaces with an eventual end to the useful life of the switch. It is virtually impossible to insure perfect synchronization for the open.ng of these parallel vacuum switch devices and one of the plurality of parallel path switches will ulti-mately l)e the last to open switch. It is this last to open switch which may be subject to the greatest require-ment for energy dissipation and the shortest operating lifetime. This is more particularly a problem for a certain characteristic type of electrolytic cell, that is a diaphragm type cell where a generally portable cell bypass or switch unit is moved from cell to cell and con-nected in place when it is desired to bypass the particu-lar cell. In such portable bypass switch units th~ leadsor the electricaI conductors from the shorting switches to the cell contribute a relatively high value of distributed inductance. In this situation the last to open switch must dissipate the relatively high inductive energy stored in the leads for all of the switches. This energy must be dissipated by the last to open switch, and since this last to open switch must carry nearly the full electrical line or system current, the contact opening of the last to open switch can cause overvoltages generated by the large value of the inductance generated voltage.
It is a general objective to be able to minimize the energy which must be dissipated in the last to open switch of such cell bypass switch devices.
~ ~ ~7 1 ~7 3 48,637 In U.S. Patent No. 4,302,642 issued August 3, 1982, entitled "Vacuum Switch Assembly," a vacuum switch cell bypass de~ice is shown having plural parallel vacuum switch paths and pointing out how the resistance and in-ductance of such parallel paths impacts upon the last to open switch problem. U.S. Patent No. 4,172,268 discusses DC circu~t commutation systems which utilize a shunt path containing a non-linear resistor in series with a circuit breaker as part of the system. The use of vacuum switches as cell bypass switches for electrochemical cell systems is taught by U.S. Patent 4,075,448.
SUMMARY OF THE INVENTION
The present invention is a high power current commutation device adapted to be electrically connected in parallel across an electrical system. The current commu-tation device comprises at least two circuit interrupters in electrical parallel path relationship to each other, with one interrupter serially connected with a resistor to form a current commutation path. The resistor is ~ormed of a conductive material which has a resistance which increases with temperature of the conductor material.
This conductor materlal has a speci~ic heat characteristic and a thermal time constant such that current passing through the conductor heats the conductor and ~ncreases its resistance as a ~unction of time. The voltage which develops across the heated resistor exceeds the electrical system voltage to efiectuate commutation of the current back through the electrolytic cell.
m e electrlcal resistor of the present invention is designed for high power, high current commutation, or limitation and employs a conductor material which has a relatively high melting point to permit operation o~er a wide range o~ operating temperatures.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of an electrolytic cell with a cell bypass assembly which in-cludes plural parallel path vacuum switch elements.
4 48,637 Figure 2 is ar, electrical schematic representa-tion which electrically represents the system of Figure 1.
Figure 3 is a plot of the functional relation-ship of the parameters of opera~ion of the system of Figures 1 and 2 as a function o~ time.
Figure 4 is an elevational view in section of an embodiment of the resistor employed in the present inven-tion.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention can be best ~nderstood by refer-ence to the embodiment seen in Figure 1 in which electro-lytic cell 10 is schematically shown electrically con-nected to a DC electrical system with operating current lo normally flowing through the electrolytic cell. One el~ctrical terminal of the cell 10 is more positive than the other with current flowing in the appropriate direc-tion from the more positive terminal to the more negative terminal. A high power current commutation device 12 is connected via electrical leads 14 and 16 to the terminals of the electrolytic cell 10. An inductive impedance 14a and 16a is shown in each of the electrical leads for connectors 14 and 16. The commutation device 12 comprises three electrically parallel vacuum switch assemblies 18a, 18b and 18c. An inductive impedance 20a, 20b and 20c is shown represented in each of the paths from leads 1'~ and 16 to the vacuum switches 18a, 18b and 18c. An electrical resistor 22 is serially connected with vacuum swtich 18c in one of the parallel electrical paths across the elec-trolytic cell. In the commutation device 12 the imped-ances 20a, 20b and 20c associated with the vacuum switchesand tne conductors connected to the switches as part of the compact commutation device 12 are relatively low impedances because of ~he close physical proximity o~ the switches and conductors. In contrast, the impedances 14a and l~a associated with the leads 14 and 16 running from the electrolytic cell terminals to the plural switch paths of device 12 have a relatively high value of distributed impedance. During normal electrolytic cell operation with 7 ~ 1 7 S 48,637 switches 18a, 18b, and 18c open circuited the commutation device 12 passes no current from the electrolytic cell terminals~ and current Io is the normal cell drive cur-rent, and is entirely directed through cell 10 which produces dissociation of the chemical constituents in the cell and production of the desired chemicals. The commu-tation device 12 and electrical leads 14 and 16 may be readily disconnected from the cell terminals and moved to another cell in the system. W~hen the electrolytic cell is ~o be taken out of service for mainterance and/or inspec-tion purposes, the commutation device 12 is electrically connected acrcss the cell as seen in Figure 1. The commu-tation device 12 is connected with the vacuum switches 18a, 18b and 18c in an open position and these switches are thereafter substantially simultaneously closed to effectuate electrical shunting of the electrolytic cell relative to the electrical drive system. The diversion of current from cell 10 to the commutation device 12 will occur so long as the shunt resistance associated with commutation device 12 is less than the internal character-istic resistance of the cell. The drive system current Io then passes through the commutation device 12 dividing itself among the three parallel paths through the closed vacuum switch devices. For a typical diaphragm type electrolytic cell for producing chlorine and sodium hydroxide, a current of 50 to 100 kiloamperes would norm-ally be flowing through the electrolytic cell, or the shunting commutation device 12 when it is brought into play.
In prior art commutation devices which utilized parallel path vacuum switch devices the vacuum switches were designed to be open~d approximately simultaneously.
Since perfect simultaneous physical opening is not pos-sible, the relatively high energy, that is one-half of the product of the inductance times the current squared, which is stored in the conductc,r leads, must be dissipated in the last to open switch. Because of the relatively high value of distributed inductance in the bus conductors from 7 ~ ~'7 6 4~,637 the cell to the switches, and the relatively low induct-ances associated with the switches, very little of the current would normally be diverted back through the cell while the last to open switch is closed or has an arc 5 burning across the switch contacts.
In the system of Figure 1, the parallel current path through vacuum switch 18c, impedance 20c and resistor 22 is determined to carry the system current Io~ and vacuum switch 18c is kept in a closed position after vacuum switches 18a and 18b are opened. Thus, vacuum switch 18c is made to be the last to open switch of this commuta~ion device. A system for determining the last to open switch of a plurality of parallel path switches is shown in U.S. Patent No. 4,121,268.
A schematic representation of the system of Figure 1 after vacuum switches 18a and 18b are opened and while vacuum switch 18c is s~ill in the closed contact current carrying position is shown in Figure 2. The resistor 22 is shown as a variable resistor in Figure 2.
The variable resistor 22 has a non-linear resistance value which increases with current carrying time. The electro-lytic cell 10 is represented as a circuit having a resis-tance RC and an inherent back potential Ec. The curren~
flowing in the electrical system is Io~ the current flow-ing through the resistor 22 and the vacuum switch 18c isIR, and the current flowing through the electrolytic cell is Ic. It is apparent that until the voltage drop (IR x R) across the resistor 22 exceeds the back voltage of the cell, Ec, the entire electrical system line current Io flows through the resistor 22 and IR equals Io~ and IC
equals 0. The current IR passing through the variable resistor 22 causes the resistor to heat up, and with the passage of time the potential across the resistor (VR) will exceed the cell back voltage Ec, and thereafter the following equations represent the system:
A~ L7 7 4~, 6:3 IR IC
IR X R = I~ RC + EC
= (Io - IR) RC + EC
herefore IR R + RC
The functional relationships of the various parameters of this system are seen as a function of time in ~igure 3. Note that as the resistor 22 is heated by t~e current Io~ its resistance increases. At time tl the voltage across the resistor 22 equals the back voltage of the cell, that is, VR = Vc, and at that time a current IC
starts to flow through the cell while the current through the resistor and the switch contacts IR starts to decay.
This condition affords the opportunity to delay the open-ing of the vacuum switch 18c until the current passing through the switch has decayed to a low enough value that the arc between the opening contacts does not cause severe contact erosion. The opening of the vacuum switch 18c is indicated in Figure 3 at a time when the current IR has substantially decayed, and upon opening of the switch 22 the current IR is 0, and the total electrical system cur-rent Io is equal to IC with the entire current flowing or being commutated back through the electrolytic cell.
A specific embodiment of the resistor 22 is seen in Figure 4. The resistor 22 comprises a hermetically sealed envelope 26 comprised of a generally cylindrical envelope portion 28, which is end sealed to a pair of annular insulating members 30 and 32 at opposed ends of the cylinder 28 with flexible diaphragm end seal members 34 and 36 at opposed ends. A conductive lead-in 38a, 38b is sealed through each end diaphragm seal means and is connected via a cylindrical molybdenum sleeve member 40a, 40b to a tun~sten rod 42which has the desired temperature coefficient of resistivity, and heat capacity necessary to 7 i ~7 8 48,637 serve as the variable resistor. A generally cylindric~l heat shield 44 is coaxially disposed about the tungsten rod, and supported from the cylindrical envelope portion 28 by support means 46. The resistor envelope 26 in addition to being evacuated, may be filled with chemically non-reactive fill gas such as the inert gases ar~on, krypton.
In this resistor embodiment the variation of re-sistance va~ue with current carrying time is had from the resistor design and selection o~ a current carrying con-ductive material with an appropriate specific heat charac-teristic and thermal time constant. In general, the resistor conductor is desirably a refractory metal such as tungsten or molybdenum which have high melting points in excess of 3000C to permit operation over a wide tempera-ture range. It is desirable that the conductor resistor have a high temperature coefficient of resistance, and thus for tungsten the rat,io of resistivity in ohm centi-meters between 3500K and 293K is about 20. The conduct-or material should exhibit an increase in resistivity of at least a factor of four as the conductor is heated from room temperature to near its melting point. The resistor conductor is selected from those materials the electrical resistivity of which increases with temperature, and which possess sufficient thermal mass to provide the desired specific resistance characteristic as a function of time.
A high melting point is not essential, but permits opera-tion over a wider range.
A typical resistor embodiment has a tungsten rod about 6 inches long and about 0.375 inch in diameter; or 8 inches long copper cylindrical fast shield with an inside diameter o~ 1.8 inches and an outside diameter of 2 inch-es. A system such as seen in Figures 1 and 2 may have the following values: system current is 72 kiloamps, and the cell back potential is 23 volts with a cell interval resistance of 170 microhms. For a resistor at 293K at the time current starts to flow through the resistor, the resistivity is 6.74 ohm-centimeters and the initial resis-7i ~'~
9 48,637 tor current of 72 kiloamps will be reduced to about 41 kiloamps in 16 milliseconds, with the potential across the resistor increasing to about 28 volts and about 31 kilo-amps commutated through the cell. At lO0 milliseconds the current through the resistor is down to bout 14 kiloamps with about 58 kiloamps through the cell. The switch l8c can now be opened at this much lower current level, and since most of the current has been commutated back through the cell, without having to dissipate significant energy within this last to open switch.
The physical dimensions of the resistor can of course be widely varied, and while in the embodiment described above a short commutation time of the order of 100 milliseconds is described, much longer commutation times can be employed. The shorter the commutation time as determined by the resistor design, the quicker the reset capability for the commutation device. For longer commutation times the resistor must be structured so a~
not to be excessively heated.
A plurality vf vacuum switching may be disposed in parallel electrical path relationship for the current commutator, and current dissipating resistors may be included in series with more than one such vacuum switch to effect commutation of higher current systems.
In Figure 2 and the discussion regarding this figure, the inductance 20c which would be naturally asso-ciated with the bus conductor has not been discussed because it is not a significant factor and to simplify the discussion. Also since at the time of switch opening the switch current will be reduced, this reduces the effect of this inductance.
The resistor of the present invention can be used in series with one or more circuit interrupter means for use as a current commutation device or current limiter or diverter when connected in parallel with an electrical line.
COI~MUTATING RESISTIVE ELEMENT
BACKGROUND OF THE INVENTION
m e present invention relates to devices for limiting or commutating high power, high current electri-cal systems and a resistor element for use in such de-vices. Many present day AC and DC electrical distributionsystems have need for high power and high current limita-tion or commutation devices for use alone or with circuit breaker devices. Such current limitation and commutat~on de~ices provide added n exibility for the electrlcal distribution network permitting greater circuit protection and disconnect capability.
In the electrochemical industry, a basic compon-ent is an electrolytic cell in which an electrolyte solu-tion can be decomposed when electric~l current is passed through the cell. The decomposed and separated chemlcal constituents are collected ~or use as the raw stock mater-ial in the Ghemical industry. Such electrolytic cells and the electrical æystems associated therewlth typically operate at low voltage of ~rom about 5 to 50 volts with a ~ery hlgh current of tens of klloamperes to in excess of 100 kiloamperes for the system. It i8 sometimes necessary to electrically bypass or remove one of a plurallty of serially connected electrolytic cells from service, for purposes of maintenance and/or component replacement. It is the practlce to utilize an electrical shunting or bypass device which employs a circuit breaker element such as a ~acuum switch whlch ca~ries the electrical system ", ~' 7i ~7 4~ 6~ 7 ~urrent while the cell is bypassed, and which upon th~
opening of the vacuum circuit element or switch diverts the current back to the electrolytic cell. It is the practice to utilize a plurality of parallel connected vacuum switches for such electrolytic cell bypass devices.
In ~iverting the system current back to the electrolytic cell, the vacuum switch devices are open circuited and a significant amount of inductively stored energy in the circuit paths must be dissipated. This is largely a-l~ chieved by the arc that forms between the parted switchcontacts. This arcing within the vacuum chamber of the switch can cause erosion of the contact surfaces with an eventual end to the useful life of the switch. It is virtually impossible to insure perfect synchronization for the open.ng of these parallel vacuum switch devices and one of the plurality of parallel path switches will ulti-mately l)e the last to open switch. It is this last to open switch which may be subject to the greatest require-ment for energy dissipation and the shortest operating lifetime. This is more particularly a problem for a certain characteristic type of electrolytic cell, that is a diaphragm type cell where a generally portable cell bypass or switch unit is moved from cell to cell and con-nected in place when it is desired to bypass the particu-lar cell. In such portable bypass switch units th~ leadsor the electricaI conductors from the shorting switches to the cell contribute a relatively high value of distributed inductance. In this situation the last to open switch must dissipate the relatively high inductive energy stored in the leads for all of the switches. This energy must be dissipated by the last to open switch, and since this last to open switch must carry nearly the full electrical line or system current, the contact opening of the last to open switch can cause overvoltages generated by the large value of the inductance generated voltage.
It is a general objective to be able to minimize the energy which must be dissipated in the last to open switch of such cell bypass switch devices.
~ ~ ~7 1 ~7 3 48,637 In U.S. Patent No. 4,302,642 issued August 3, 1982, entitled "Vacuum Switch Assembly," a vacuum switch cell bypass de~ice is shown having plural parallel vacuum switch paths and pointing out how the resistance and in-ductance of such parallel paths impacts upon the last to open switch problem. U.S. Patent No. 4,172,268 discusses DC circu~t commutation systems which utilize a shunt path containing a non-linear resistor in series with a circuit breaker as part of the system. The use of vacuum switches as cell bypass switches for electrochemical cell systems is taught by U.S. Patent 4,075,448.
SUMMARY OF THE INVENTION
The present invention is a high power current commutation device adapted to be electrically connected in parallel across an electrical system. The current commu-tation device comprises at least two circuit interrupters in electrical parallel path relationship to each other, with one interrupter serially connected with a resistor to form a current commutation path. The resistor is ~ormed of a conductive material which has a resistance which increases with temperature of the conductor material.
This conductor materlal has a speci~ic heat characteristic and a thermal time constant such that current passing through the conductor heats the conductor and ~ncreases its resistance as a ~unction of time. The voltage which develops across the heated resistor exceeds the electrical system voltage to efiectuate commutation of the current back through the electrolytic cell.
m e electrlcal resistor of the present invention is designed for high power, high current commutation, or limitation and employs a conductor material which has a relatively high melting point to permit operation o~er a wide range o~ operating temperatures.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of an electrolytic cell with a cell bypass assembly which in-cludes plural parallel path vacuum switch elements.
4 48,637 Figure 2 is ar, electrical schematic representa-tion which electrically represents the system of Figure 1.
Figure 3 is a plot of the functional relation-ship of the parameters of opera~ion of the system of Figures 1 and 2 as a function o~ time.
Figure 4 is an elevational view in section of an embodiment of the resistor employed in the present inven-tion.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention can be best ~nderstood by refer-ence to the embodiment seen in Figure 1 in which electro-lytic cell 10 is schematically shown electrically con-nected to a DC electrical system with operating current lo normally flowing through the electrolytic cell. One el~ctrical terminal of the cell 10 is more positive than the other with current flowing in the appropriate direc-tion from the more positive terminal to the more negative terminal. A high power current commutation device 12 is connected via electrical leads 14 and 16 to the terminals of the electrolytic cell 10. An inductive impedance 14a and 16a is shown in each of the electrical leads for connectors 14 and 16. The commutation device 12 comprises three electrically parallel vacuum switch assemblies 18a, 18b and 18c. An inductive impedance 20a, 20b and 20c is shown represented in each of the paths from leads 1'~ and 16 to the vacuum switches 18a, 18b and 18c. An electrical resistor 22 is serially connected with vacuum swtich 18c in one of the parallel electrical paths across the elec-trolytic cell. In the commutation device 12 the imped-ances 20a, 20b and 20c associated with the vacuum switchesand tne conductors connected to the switches as part of the compact commutation device 12 are relatively low impedances because of ~he close physical proximity o~ the switches and conductors. In contrast, the impedances 14a and l~a associated with the leads 14 and 16 running from the electrolytic cell terminals to the plural switch paths of device 12 have a relatively high value of distributed impedance. During normal electrolytic cell operation with 7 ~ 1 7 S 48,637 switches 18a, 18b, and 18c open circuited the commutation device 12 passes no current from the electrolytic cell terminals~ and current Io is the normal cell drive cur-rent, and is entirely directed through cell 10 which produces dissociation of the chemical constituents in the cell and production of the desired chemicals. The commu-tation device 12 and electrical leads 14 and 16 may be readily disconnected from the cell terminals and moved to another cell in the system. W~hen the electrolytic cell is ~o be taken out of service for mainterance and/or inspec-tion purposes, the commutation device 12 is electrically connected acrcss the cell as seen in Figure 1. The commu-tation device 12 is connected with the vacuum switches 18a, 18b and 18c in an open position and these switches are thereafter substantially simultaneously closed to effectuate electrical shunting of the electrolytic cell relative to the electrical drive system. The diversion of current from cell 10 to the commutation device 12 will occur so long as the shunt resistance associated with commutation device 12 is less than the internal character-istic resistance of the cell. The drive system current Io then passes through the commutation device 12 dividing itself among the three parallel paths through the closed vacuum switch devices. For a typical diaphragm type electrolytic cell for producing chlorine and sodium hydroxide, a current of 50 to 100 kiloamperes would norm-ally be flowing through the electrolytic cell, or the shunting commutation device 12 when it is brought into play.
In prior art commutation devices which utilized parallel path vacuum switch devices the vacuum switches were designed to be open~d approximately simultaneously.
Since perfect simultaneous physical opening is not pos-sible, the relatively high energy, that is one-half of the product of the inductance times the current squared, which is stored in the conductc,r leads, must be dissipated in the last to open switch. Because of the relatively high value of distributed inductance in the bus conductors from 7 ~ ~'7 6 4~,637 the cell to the switches, and the relatively low induct-ances associated with the switches, very little of the current would normally be diverted back through the cell while the last to open switch is closed or has an arc 5 burning across the switch contacts.
In the system of Figure 1, the parallel current path through vacuum switch 18c, impedance 20c and resistor 22 is determined to carry the system current Io~ and vacuum switch 18c is kept in a closed position after vacuum switches 18a and 18b are opened. Thus, vacuum switch 18c is made to be the last to open switch of this commuta~ion device. A system for determining the last to open switch of a plurality of parallel path switches is shown in U.S. Patent No. 4,121,268.
A schematic representation of the system of Figure 1 after vacuum switches 18a and 18b are opened and while vacuum switch 18c is s~ill in the closed contact current carrying position is shown in Figure 2. The resistor 22 is shown as a variable resistor in Figure 2.
The variable resistor 22 has a non-linear resistance value which increases with current carrying time. The electro-lytic cell 10 is represented as a circuit having a resis-tance RC and an inherent back potential Ec. The curren~
flowing in the electrical system is Io~ the current flow-ing through the resistor 22 and the vacuum switch 18c isIR, and the current flowing through the electrolytic cell is Ic. It is apparent that until the voltage drop (IR x R) across the resistor 22 exceeds the back voltage of the cell, Ec, the entire electrical system line current Io flows through the resistor 22 and IR equals Io~ and IC
equals 0. The current IR passing through the variable resistor 22 causes the resistor to heat up, and with the passage of time the potential across the resistor (VR) will exceed the cell back voltage Ec, and thereafter the following equations represent the system:
A~ L7 7 4~, 6:3 IR IC
IR X R = I~ RC + EC
= (Io - IR) RC + EC
herefore IR R + RC
The functional relationships of the various parameters of this system are seen as a function of time in ~igure 3. Note that as the resistor 22 is heated by t~e current Io~ its resistance increases. At time tl the voltage across the resistor 22 equals the back voltage of the cell, that is, VR = Vc, and at that time a current IC
starts to flow through the cell while the current through the resistor and the switch contacts IR starts to decay.
This condition affords the opportunity to delay the open-ing of the vacuum switch 18c until the current passing through the switch has decayed to a low enough value that the arc between the opening contacts does not cause severe contact erosion. The opening of the vacuum switch 18c is indicated in Figure 3 at a time when the current IR has substantially decayed, and upon opening of the switch 22 the current IR is 0, and the total electrical system cur-rent Io is equal to IC with the entire current flowing or being commutated back through the electrolytic cell.
A specific embodiment of the resistor 22 is seen in Figure 4. The resistor 22 comprises a hermetically sealed envelope 26 comprised of a generally cylindrical envelope portion 28, which is end sealed to a pair of annular insulating members 30 and 32 at opposed ends of the cylinder 28 with flexible diaphragm end seal members 34 and 36 at opposed ends. A conductive lead-in 38a, 38b is sealed through each end diaphragm seal means and is connected via a cylindrical molybdenum sleeve member 40a, 40b to a tun~sten rod 42which has the desired temperature coefficient of resistivity, and heat capacity necessary to 7 i ~7 8 48,637 serve as the variable resistor. A generally cylindric~l heat shield 44 is coaxially disposed about the tungsten rod, and supported from the cylindrical envelope portion 28 by support means 46. The resistor envelope 26 in addition to being evacuated, may be filled with chemically non-reactive fill gas such as the inert gases ar~on, krypton.
In this resistor embodiment the variation of re-sistance va~ue with current carrying time is had from the resistor design and selection o~ a current carrying con-ductive material with an appropriate specific heat charac-teristic and thermal time constant. In general, the resistor conductor is desirably a refractory metal such as tungsten or molybdenum which have high melting points in excess of 3000C to permit operation over a wide tempera-ture range. It is desirable that the conductor resistor have a high temperature coefficient of resistance, and thus for tungsten the rat,io of resistivity in ohm centi-meters between 3500K and 293K is about 20. The conduct-or material should exhibit an increase in resistivity of at least a factor of four as the conductor is heated from room temperature to near its melting point. The resistor conductor is selected from those materials the electrical resistivity of which increases with temperature, and which possess sufficient thermal mass to provide the desired specific resistance characteristic as a function of time.
A high melting point is not essential, but permits opera-tion over a wider range.
A typical resistor embodiment has a tungsten rod about 6 inches long and about 0.375 inch in diameter; or 8 inches long copper cylindrical fast shield with an inside diameter o~ 1.8 inches and an outside diameter of 2 inch-es. A system such as seen in Figures 1 and 2 may have the following values: system current is 72 kiloamps, and the cell back potential is 23 volts with a cell interval resistance of 170 microhms. For a resistor at 293K at the time current starts to flow through the resistor, the resistivity is 6.74 ohm-centimeters and the initial resis-7i ~'~
9 48,637 tor current of 72 kiloamps will be reduced to about 41 kiloamps in 16 milliseconds, with the potential across the resistor increasing to about 28 volts and about 31 kilo-amps commutated through the cell. At lO0 milliseconds the current through the resistor is down to bout 14 kiloamps with about 58 kiloamps through the cell. The switch l8c can now be opened at this much lower current level, and since most of the current has been commutated back through the cell, without having to dissipate significant energy within this last to open switch.
The physical dimensions of the resistor can of course be widely varied, and while in the embodiment described above a short commutation time of the order of 100 milliseconds is described, much longer commutation times can be employed. The shorter the commutation time as determined by the resistor design, the quicker the reset capability for the commutation device. For longer commutation times the resistor must be structured so a~
not to be excessively heated.
A plurality vf vacuum switching may be disposed in parallel electrical path relationship for the current commutator, and current dissipating resistors may be included in series with more than one such vacuum switch to effect commutation of higher current systems.
In Figure 2 and the discussion regarding this figure, the inductance 20c which would be naturally asso-ciated with the bus conductor has not been discussed because it is not a significant factor and to simplify the discussion. Also since at the time of switch opening the switch current will be reduced, this reduces the effect of this inductance.
The resistor of the present invention can be used in series with one or more circuit interrupter means for use as a current commutation device or current limiter or diverter when connected in parallel with an electrical line.
Claims (6)
1. A low DC voltage high current commutation device adapted to be electrically connected in parallel across the terminals of an electrolytic cell which exhibits an electro-lytic potential, and which terminals are connected to a poten-tial source for driving the electrolytic cell, which current commutation device comprises: at least two circuit interrupt-ers in electrical parallel path relationship to each other, with one interrupter serially connected with a resistor to form a current commutation path, with the resistor having a resistance which increases with the temperature of the resistor conductor, which resistor has a specific heat characteristic and a thermal time constant such that when the circuit inter-rupter not in series with the resistor is opened, the current which flows through the still closed interrupter and series connected resistor heats the resistor and increases the re-sistor resistance, with the potential drop across the resistor increasing until it equals and exceeds the electrolytic cell back potential to thereby commutate current from the resistor and interrupter path to the electrolytic cell, until the resistor and interrupter path current is reduced to a pre-determined value at which time the interrupter in series with the resistor is opened to commutate the full current through the electrolytic cell.
2. The current commutation device set forth in claim 1, wherein the resistor conductor is a refractor metal rod disposed within a hermetically sealed envelope with electrical lead-ins connected to each end of the rod and sealed through a hermetically sealed envelope, and wherein a tubular heat shield is disposed within the envelope coaxial about the resistor conductor.
11 48,637
11 48,637
3. The current commutation device set forth in claim 2, wherein the envelope includes insulating body portions which permit electrical isolation of the lead-ins from each other.
4. The current commutation device set forth in claim 1, wherein the resistor conductor material is selected from one of the refractory metals tungsten and molybdenum.
5. The current commutation device set forth in claim 1, wherein the resistor conductor material has a melting point which exceeds 3000°C.
6. The current commutation device set forth in claim 1, wherein the resistor conductor material exhibits an increase in resistivity of at least a factor of four as the conductor is heated from room temperature to about the melting point of the conductor material.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US13937480A | 1980-04-08 | 1980-04-08 | |
| US139,374 | 1980-04-08 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1157117A true CA1157117A (en) | 1983-11-15 |
Family
ID=22486316
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000373964A Expired CA1157117A (en) | 1980-04-08 | 1981-03-26 | High power current limiter with a current commutating resistive element |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPS56158406A (en) |
| CA (1) | CA1157117A (en) |
| DE (1) | DE3113395A1 (en) |
| GB (1) | GB2073953A (en) |
-
1981
- 1981-03-26 CA CA000373964A patent/CA1157117A/en not_active Expired
- 1981-04-03 DE DE19813113395 patent/DE3113395A1/en not_active Withdrawn
- 1981-04-07 JP JP5131381A patent/JPS56158406A/en active Pending
- 1981-04-07 GB GB8110807A patent/GB2073953A/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| DE3113395A1 (en) | 1982-05-06 |
| GB2073953A (en) | 1981-10-21 |
| JPS56158406A (en) | 1981-12-07 |
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| MKEX | Expiry |