WO2014142974A1

WO2014142974A1 - Direct current circuit breaker methods and systems

Info

Publication number: WO2014142974A1
Application number: PCT/US2013/032131
Authority: WO
Inventors: Timothy John Sommerer; James William Bray
Original assignee: General Electric Company
Priority date: 2013-03-15
Filing date: 2013-03-15
Publication date: 2014-09-18

Abstract

Direct current (DC) circuit breaker methods and systems are described. In one example, a DC circuit breaker includes a breaker assembly (316) configured to interrupt a DC current, and a controller (315). The breaker assembly (316) includes an input portion (318), an output portion (320), and a gas tube switch (100) coupled between the input portion (318) and the output portion (320). The gas tube switch (100) includes an anode, a cathode, and a control grid. The controller (315) is configured to selectively operate the breaker assembly (316) to interrupt the DC current between the input portion (318) and the output portion (320).

Description

DIRECT CURRENT CIRCUIT BREAKER

METHODS AND SYSTEMS

BACKGROUND

[0001] The field of the disclosure relates generally to circuit breakers and, more particularly, to direct current circuit breaker methods and systems.

[0002] Many known transmission and distribution systems include mechanical isolation devices, e.g., circuit breakers, to interrupt current flowing between two points of the system. In alternating current (AC) systems, the zero crossings of the current help prevent and/or extinguish an arc generated by opening the contacts of a circuit breaker. Due to the nature of direct current (DC), i.e., no zero-crossing of the amplitudes of DC voltages and currents as a function of time, opening of mechanical isolation devices in a DC distribution system produces a greater risk of arcing and a decrease of service life of the circuit breaker.

[0003] Some DC circuit breakers utilize semiconductor switches to interrupt the DC current flowing through the circuit breaker. The open circuit standoff voltage of each semiconductor switch, however, is relatively low (about 10 kilovolts or less) and many semiconductor switches must be stacked together to handle the high voltages (300-lOOOkV) present in a high voltage DC distribution system. Moreover, multiple stacks of series connected semiconductor switches are often coupled in parallel to create a circuit breaker capable of handling the currents in a high voltage DC distribution system.

BRIEF DESCRIPTION

[0004] In one aspect of the present disclosure, a direct current (DC) circuit breaker includes a breaker assembly configured to interrupt a DC current, and a controller. The breaker assembly includes an input portion, an output portion, and a gas tube switch coupled between the input portion and the output portion. The gas tube switch includes an anode, a cathode, and a control grid. The controller is configured to selectively operate the breaker assembly to interrupt the DC current between the input portion and the output portion.

[0005] In a further aspect, a method for use in interrupting direct current (DC) between an input portion and an output portion of a breaker assembly is provided. The method includes detecting a fault condition, and interrupting, by a controller, a DC current between the input portion and the output portion of the breaker assembly in response to the detected fault condition by selective operation of a gas tube switch coupled between the input portion and the output portion. The gas tube switch includes an anode, a cathode, and a control grid.

[0006] In another aspect, a breaker assembly for use in a direct current (DC) circuit breaker includes an input portion, an output portion, and a gas tube switch coupled between said input portion and said output portion. The gas tube switch includes an anode, a cathode, and a control grid.

DRAWINGS

[0007] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0008] FIG. 1 is a block diagram of an exemplary gas tube switch.

[0009] FIG. 2 is a block diagram of an exemplary direct current (DC) circuit breaker including the gas tube switch shown in FIG. 1.

[0010] FIG. 3 is a block diagram of another exemplary DC circuit breaker including the gas tube switch shown in FIG. 1.

[001 1] FIG. 4 is a flow diagram of an exemplary method for use in interrupting a DC current.

[0012] Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of the disclosure. These features are believed to be applicable in a wide variety of systems comprising one or more embodiments of the disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein.

DETAILED DESCRIPTION

[0013] In the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings. The singular forms "a", "an", and "the" include plural references unless the context clearly dictates otherwise. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about" and "substantially", are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

[0014] Direct current (DC) circuit breaker methods and systems are described herein. The exemplary DC circuit breakers provide an efficient method for providing circuit protection in DC power distribution systems. The example embodiments include one or more gas tube switches, which are high voltage and high power capable gas tube switches. The gas tube switches are capable of opening very quickly and can standoff hundreds of kilovolts (kV). Moreover, the tubes can be combined in series and/or parallel to increase the voltage and/or current carrying capacity of the DC circuit breakers. Some embodiments provide increased efficiency by normally conducting current through a highly efficient switch and diverting the current to a gas tube switch for interruption upon detection of a fault condition.

[0015] FIG. 1 is a diagram of an exemplary gas tube switch 100. Gas tube switch 100 includes a housing 102, an anode 104, cathode 106, and a control grid 108 (also referred to as a switching grid). Anode 104, cathode 106, and control grid 108 are disposed within an interior portion 109 of housing 102. Anode 104, cathode 106, and control grid 108 are sometimes referred to herein as electrodes. Interior portion 109 is filled with a gas. In the exemplary embodiment, the gas is hydrogen at a pressure of 0.1-1 torr (13-133 Pascal) at ambient temperature. Alternatively, the gas may be any other suitable gas, such as helium, or gases that enable operation of gas tube switch 100 as described herein. In some embodiments, gas tube switch includes one or more magnets (not shown) configured to generate a magnetic field (which may be constant or variable) to alter a current carrying capacity of gas tube switch 100. Alternatively, or additionally, gas tube switch 100 may include one or more additional electrodes, including for example one or more grids that maintain a weak ionized gas within gas tube switch 100 to facilitate closing gas tube switch 100 without, for example, use of an ignitor. The exemplary gas tube switch 100 is a plane-parallel gas tube switch and cathode 106 is a planar cathode. Alternatively, gas tube switch 100 and cathode 106 may have any suitable configuration enabling for operation as described herein.

[0016] In the exemplary embodiment, cathode 106 is a liquid cathode. Cathode 106 is liquid at its operating temperature. In the exemplary embodiment, cathode 106 is made of gallium metal, which is liquid above about thirty degrees centigrade (°C). Alternatively, cathode 106 may be made of any other suitable liquid material that enables gas tube switch 100 to operate as described herein. In some embodiments, for example, cathode 106 is made of bismuth, tin, lead, mercury, sodium and/or lithium. In some embodiments, cathode 106 is liquid at room temperature. The exemplary gas tube switch 100 is a cold cathode gas tube switch and the cathode 106 is not heated to permit the gas tube switch 100 to operate. In other embodiments, cathode 106 may be heated to liquefy cathode 106 and/or otherwise permit operation of gas tube switch 100. Anode 104 is a solid metal anode. In other embodiments, anode 104 is a liquid anode.

[0017] In operation, electrical current is conducted is conducted from anode 104 to cathode 106 through the hydrogen gas within interior portion 109. Control grid 108 is an electrode that is used to selectively control gas tube switch 100 through application, removal, and/or variation of a voltage applied control grid 108. Although the exemplary embodiment includes a single control grid 108, other embodiments include more than one control grid 108. When gas tube switch is open (e.g., turned off, not conducting, etc.), the hydrogen gas insulates anode 104 from cathode 106. When gas tube switch 100 is closed (e.g., turned on, conducting, etc.), the hydrogen gas within housing 102 becomes ionized (i.e., some portion of the hydrogen molecules are dissociated into free electrons, hydrogen molecular ions, hydrogen atoms, hydrogen atomic ions, etc.), resulting in an electrically conductive plasma. Electrical continuity is maintained between cathode 106 and the hydrogen gas through secondary electron emission by ion impact. Energetic (e.g., 100-200 electron volts (eV)) ions from the plasma are drawn to the surface of cathode 106 by a strong electric field. The impact of the ions on cathode 106 releases secondary electrons from the surface of cathode 106 into the gas phase. The released secondary electrons aid in sustaining the plasma. In the exemplary embodiment, the material of cathode 106 does not evaporate to an extent that it substantially changes the properties of the hydrogen gas, either in its insulating state, or in its conducting state. Alternatively, there is some interaction between the gas and evaporated material from cathode 106.

[0018] FIG. 2 is a block diagram of an exemplary direct current (DC) circuit breaker 210 including gas tube switch 100. Alternatively, circuit breaker 210 may include any other suitable gas tube switch 100 that permits operation of circuit breaker 210 as described herein. Electrical current flows from power through DC circuit breaker 210, for example from a source to a load. DC circuit breaker 210 includes a controller 215 and a breaker assembly 216. Breaker assembly 216 has an input portion 218 for receiving DC current and an output portion 220 for outputting the received DC current. DC current may flow in either direction between input portion 218 and output portion 220. Gas tube switch 100 is coupled between input portion 218 and output portion 220. Controller 215 is configured to selectively operate breaker assembly 216 to interrupt DC current between input portion 218 and output portion 220. When gas tube switch 100 is open, DC current cannot flow through breaker assembly 216. Conversely, when gas tube switch 100 is closed, DC current can flow through breaker assembly 216. Although a single gas tube switch 100 is illustrated in FIG. 2, breaker assembly 216 may include more than one gas tube switch 100 coupled together in series and/or in parallel to increase the standoff voltage capacity and/or current capacity of breaker assembly 216.

[0019] A sensor 222 is coupled to input portion 218. Sensor 222 is a current sensor, such as a current transformer, a Rogowski coil, a Hall-effect sensor, and/or a shunt that measures a current flowing through breaker assembly 216. Alternatively, sensor 222 may include any other sensor that enables DC circuit breaker 210 to function as described herein. Sensor 222 generates a signal representative of the measured or detected current (hereinafter referred to as "current signal") flowing through breaker assembly 216. In addition, sensor 222 transmits the current signal to controller 215. Controller 215 is configured to control breaker assembly, and more particularly gas tube switch 100, to interrupt a DC current provided upon detection of a fault condition. For example, if controller 215 determines that the current signal, and/or the current represented by the current signal, exceeds a current threshold, a fault condition is determined to exist and controller 215 opens gas tube switch 100 to interrupt the DC current. In other embodiments, DC circuit breaker 210 includes more than one sensor 222. Moreover, in some embodiments, controller 215 is additionally or alternatively configured to interrupt a DC current in response to a signal other than the current signal, e.g., a control signal from another controller, a different sensor signal, etc.

[0020] Controller 215 includes a processor 224 and a memory device 226 coupled to processor 224. Based at least in part on the current data, processor 224 determines when the DC current through breaker assembly 216 exceeds a protection threshold and controls breaker assembly 216 to interrupt the DC current upon determining that the current exceeds the protection threshold. It should be understood that the term "processor" refers generally to any programmable system including systems and microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), programmable logic circuits, and any other circuit or processor capable of executing the functions described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term "processor."

[0021] Memory device 226 stores program code and instructions, executable by processor 224, to control breaker assembly 216. Memory device 226 may include, but is not limited to only include, non-volatile RAM (NVRAM), magnetic RAM (MRAM), ferroelectric RAM (FeRAM), read only memory (ROM), flash memory and/or Electrically Erasable Programmable Read Only Memory (EEPROM). Any other suitable magnetic, optical and/or semiconductor memory, by itself or in combination with other forms of memory, may be included in memory device 226. Memory device 226 may also be, or include, a detachable or removable memory, including, but not limited to, a suitable cartridge, disk, CD ROM, DVD or USB memory.

[0022] FIG. 3 is a block diagram of an exemplary direct current (DC) circuit breaker 310 including gas tube switch 100. DC circuit breaker 310 includes a controller 315 and a breaker assembly 316. Breaker assembly 316 has an input portion 318 and an output portion 320. DC current may flow in either direction between input portion 318 and output portion 320. Gas tube switch 100 is coupled between input portion 318 and output portion 320. Controller 315 is configured to selectively operate breaker assembly 316 to interrupt DC current between input portion 318 and output portion 320. Although a single gas tube switch 100 is illustrated in FIG. 3, breaker assembly 316 may include more than one gas tube switch 100 coupled together in series and/or in parallel to increase the standoff voltage capacity and/or current capacity of breaker assembly 316.

[0023] Sensor 222 is coupled to input portion 318. Sensor 222 generates a signal (not shown) representative of the measured or detected current (hereinafter referred to as "current signal") flowing through breaker assembly 316 and transmits the current signal to controller 315. Controller 315 is configured to control breaker assembly 316 to interrupt the DC current upon detection of a fault condition. For example, if controller 315 determines that the current signal, and/or the current represented by the current signal, exceeds a current threshold, a fault condition is determined to exist and controller 315 operates breaker assembly 316 to interrupt the DC current. Alternatively, DC circuit breaker 310 includes more than one sensor 222. Moreover, controller 315 is additionally or alternatively configured to interrupt the DC current in response to a signal other than the current signal, e.g., a control signal from another controller, a different sensor signal, etc.

[0024] Controller 315 includes a processor 324 and a memory device 326. Based at least in part on the current data, processor 324 determines when the DC current through breaker assembly 316 exceeds a protection threshold and controls breaker assembly 316 to interrupt the DC current upon determining that the current exceeds the protection threshold.

[0025] Memory device 326 stores program code and instructions, executable by processor 324, to control breaker assembly 316. Memory device 326 may include, but is not limited to only include, non-volatile RAM (NVRAM), magnetic RAM (MRAM), ferroelectric RAM (FeRAM), read only memory (ROM), flash memory and/or Electrically Erasable Programmable Read Only Memory (EEPROM). Any other suitable magnetic, optical and/or semiconductor memory, by itself or in combination with other forms of memory, may be included in memory device 326. Memory device 326 may also be, or include, a detachable or removable memory, including, but not limited to, a suitable cartridge, disk, CD ROM, DVD or USB memory.

[0026] Breaker assembly 316 includes a first conductive path 328 and a second conductive path 330 between input portion 318 and output portion 320. Second conductive path 330 extends in parallel with first conductive path 328. First conductive path 328 is configured to carry the DC current flowing through breaker assembly 316 under normal, i.e., non-fault, conditions. In some embodiments, second conductive path 330 carries a relatively small portion of the DC current flowing through breaker assembly 316 under normal conditions.

[0027] First conductive path 328 includes a switch 332 and a variable impedance device 334. Switch 332 is a mechanical switch. In other embodiments, switch 332 is one or more semiconductor switches, or any other suitable switch or switches capable of carrying the DC current with relatively low losses when closed. In other embodiments, switch 332 is a vacuum or gas tube switch, such as gas tube switch 100. In the exemplary embodiment, switch 332 is more efficient (i.e., less lossy) when closed and conducting than gas tube switch 100. Variable impedance device 334 may be any suitable device for providing variable impedance on first conductive path 328, such as a saturable reactor, a fault current limiter, a semiconductor device, etc. Second conductive path 330 includes gas tube switch 100 coupled between input portion 318 and output portion 320.

[0028] Under normal operating conditions, i.e. when there is no fault condition, DC current flows through first conductive path 328. Variable impedance device 334 is set, such as by controller 315, at its lowest impedance, switch 332 is closed, and gas tube switch 100 is open. Thus, first conductive path 328 has a low impedance (e.g., an impedance of about zero ohms) and second conductive path 330 has a very high impedance (e.g., an impedance about equal to an open circuit). DC current entering input portion 318 will travel through the low impedance first conductive path 328 to output portion 320. In the exemplary embodiment, no current travels through second conductive path 330. In other embodiments, a relatively small amount of the DC current travels from input portion 318 to output portion 320 through second conductive path 330 under normal conditions.

[0029] Upon detecting a fault condition, receiving an instruction to trip, or otherwise determining to trip, controller 315 diverts a portion of the DC current flowing between input portion 318 and output portion 320 from first conductive path 328 to second conductive path 330. More specifically, controller 315 increases the impedance of variable impedance device 334 and, at substantially the same time, closes the normally open gas tube switch 100. The impedance of first conductive path 328 is thus increased while the impedance of second conductive path 330 is greatly decreased. A portion of the DC current flowing from input portion 318 to output portion 320 now flows through second conductive path 330. The portion of current diverted to second conductive path 330 is greater than or equal to an amount which will reduce the DC current in first conductive path 328 low enough to permit switch 332 to be opened without damage. In the exemplary embodiment, substantially all of the DC current is diverted to second conductive path 330. In other embodiments, less than substantially all of the DC current is diverted to second conductive path 330. As will be understood by those skilled in the art, the amount of the DC current diverted to second conductive path 330 depends on the amount of time elapsed and the relative impedances of first conductive path 328 and second conductive path 330.

[0030] After the current flowing through first conductive path 328 has decreased sufficiently to permit switch 332 to be opened without damage, controller 315 opens switch 332. The determination by controller 315 of when to open switch 332 may be based on an elapsed time after increasing the impedance of variable impedance device 334 and closing gas tube switch 100, a measurement of the DC current through first conductive path 328, and/or any other suitable method for determining when to open switch 332. In some embodiments, first conductive path 328 and second conductive path 330 each include one or more sensors (not shown) configured to detect the current in their respective first or second conductive paths 328 or 330 and provide the a signal representative of the detected current to controller 315.

[0031] After controller 315 opens switch 332, the DC current flowing through first conductive path 328 will cease and all DC current flowing from input portion 318 to output portion 320 flows through second conductive path 330. Controller 315 next opens gas tube switch 100, thereby completing the interruption of DC current flowing between input portion 318 and output portion 320. The determination by controller 315 of when to open gas tube switch 100 may be based on an elapsed time after opening switch 332, a measurement of the DC current through first conductive path 328, and/or any other suitable method for determining when to open gas tube switch 100. After the DC current interruption is completed, variable impedance device 334 is reset to its lowest impedance state in anticipation of the closing of switch 332, when it is determined to resume normal operation.

[0032] FIG. 4 is a flow diagram of an exemplary method 436 for use in interrupting direct current between an input portion, such as input portion 218 or 318, and an output portion, such as output portion 220 or 320, of a breaker assembly, such as breaker assembly 216 or 316. The method includes detecting 438 a fault condition and interrupting 440 a DC current between the input portion and the output portion of the breaker assembly in response to the detected fault condition by selective operation of a gas tube switch coupled between the input portion and the output portion and including a control grid.

[0033] The above-described DC circuit breakers provide an efficient method for providing circuit protection in DC power distribution systems. The gas tube switch(es) used in the example embodiments are high voltage and high power capable gas tube switches. The gas tube switches are capable of opening very quickly and can standoff hundreds of kilovolts. Moreover, the tubes can be combined in series and/or parallel to increase the voltage and/or current carrying capacity of the DC circuit breakers. Some embodiments provide increased efficiency by normally conducting current through a highly efficient switch and diverting the current to a gas tube switch for interruption upon detection of a fault condition.

[0034] An exemplary technical effect of the methods, systems, and apparatus described herein includes at least one of: (a) detecting a fault condition; and (b) interrupting a DC current between an input portion and an output portion of a breaker assembly in response to a detected fault condition.

[0035] Exemplary embodiments of DC circuit breakers, breaker assemblies, and methods for interrupting direct current, are described above in detail. The DC circuit breakers, breaker assemblies, and methods for interrupting direct current are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the methods may also be used in combination with other systems and are not limited to practice with only the DC circuit breakers, breaker assemblies, and methods as described herein. [0036] Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

[0037] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

WHAT IS CLAIMED IS:

1. A direct current (DC) circuit breaker comprising: a breaker assembly configured to interrupt a DC current, said breaker assembly comprising: an input portion; an output portion; and a gas tube switch coupled between said input portion and said output portion, said gas tube switch comprising an anode, a cathode, and a control grid; and a controller configured to selectively operate said breaker assembly to interrupt the DC current between said input portion and said output portion.

2. A DC circuit breaker in accordance with Claim 1, wherein said controller is configured to cause said gas tube switch to open in response to a detected fault condition to interrupt the DC current between said input portion and said output portion.

3. A DC circuit breaker in accordance with Claim 2 further comprising a sensor coupled to said controller and configured to detect a fault condition.

4. A DC circuit breaker in accordance with Claim 1, wherein said breaker assembly further comprises: a first conductive path between said input portion and said output portion, said first conductive path configured to conduct substantially all of the DC current between said input portion and said output portion; and a second conductive path between said input portion and said output portion in parallel with said first conductive path, wherein said gas tube switch is a normally open gas tube switch coupled between said input portion and said output portion in said second conductive path.

5. A DC circuit breaker in accordance with Claim 4, wherein said breaker assembly further comprises a switch and a variable impedance device coupled between said input portion and said output portion in said first conductive path, said switch coupled in series with said variable impedance device.

6. A DC circuit breaker in accordance with Claim 5, wherein said controller is further configured to substantially simultaneously increase an impedance of said variable impedance device and close said normally open gas tube switch in response to a detected fault condition to divert at least a portion of the DC current between said input portion and said output portion to said second conductive path.

7. A DC circuit breaker in accordance with Claim 6, wherein said controller is further configured to open said switch in said first conductive path after the portion of the DC current has been diverted to said second conductive path.

8. A DC circuit breaker in accordance with Claim 7, wherein said controller is further configured to open said normally open gas tube switch when the DC current ceases to flow through said first conductive path after opening said switch in said first conductive path.

9. A DC circuit breaker in accordance with Claim 1, wherein said cathode comprises a liquid cathode.

10. A method for use in interrupting direct current (DC) between an input portion and an output portion of a breaker assembly, said method comprising: detecting a fault condition; and interrupting, by a controller, a DC current between the input portion and the output portion of the breaker assembly in response to the detected fault condition by selective operation of a gas tube switch coupled between the input portion and the output portion, wherein the gas tube switch includes an anode, a cathode, and a control grid.

1 1. A method in accordance with Claim 10, wherein interrupting a DC current between the input portion and the output portion of the breaker assembly comprises opening the gas tube switch in response to said detected fault condition to interrupt the DC current between the input portion and the output portion of the breaker assembly.

12. A method in accordance with Claim 10, wherein the breaker assembly includes a switch and a variable impedance device coupled in series between the input portion and the output portion in a first conductive path, wherein the gas tube switch is a normally open gas tube switch coupled between the input portion and the output portion in a second conductive path in parallel with the first conductive path, and wherein interrupting a DC current between the input portion and the output portion of the breaker assembly comprises: increasing an impedance of the variable impedance device; and closing the normally open gas tube switch in response to a detected fault condition to divert substantially all of the DC current between the input portion and the output portion to the second conductive path.

13. A method in accordance with Claim 12, wherein interrupting a DC current between the input portion and the output portion of the breaker assembly comprises opening the switch in the first conductive path after said closing the normally open gas tube switch.

14. A method in accordance with Claim 13, wherein interrupting a DC current between the input portion and the output portion of the breaker assembly comprises opening the normally open gas tube switch after said opening the switch in the first conductive path.

15. A breaker assembly for use in a direct current (DC) circuit breaker, said breaker assembly comprising: an input portion; an output portion; and a gas tube switch coupled between said input portion and said output portion, said gas tube switch comprising an anode, a cathode, and a control grid.

16. A breaker assembly in accordance with Claim 15 further comprising a single conductive path between said input portion and said output portion, wherein said gas tube switch is coupled between said input portion and said output portion in said single conductive path and configured to carry a DC current between said input portion and said output portion when closed, and wherein said gas tube switch is selectively operable to open to interrupt the DC current between said input portion and said output portion.

17. A breaker assembly in accordance with Claim 15 further comprising: a first conductive path between said input portion and said output portion, said first conductive path configured to conduct substantially all of the DC current between said input portion and said output portion; and a second conductive path between said input portion and said output portion in parallel with said first conductive path, wherein said gas tube switch is a normally open gas tube switch coupled between said input portion and said output portion in said second conductive path.

18. A breaker assembly in accordance with Claim 17, wherein said breaker assembly further comprises a switch and a variable impedance device coupled between said input portion and said output portion in said first conductive path, said switch coupled in series with said variable impedance device.

19. A breaker assembly in accordance with Claim 18, wherein said switch in said first conductive path comprises a mechanical switch.

20. A breaker assembly in accordance with Claim 14, wherein said cathode comprises a liquid cathode. PAGE LEFT BLANK INTENTIONALLY