DIRECT CURRENT CIRCUIT BREAKER

BACKGROUND

[0001] The field of the disclosure relates generally to circuit breakers and, more particularly, to fast-switching direct current circuit breakers.

[0002] Many known transmission and distribution systems include mechanical isolation devices, e.g., circuit breakers, to interrupt current flowing between two points in the system. In alternating current (AC) systems, 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 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 failure to open and a reduced service life of the circuit breaker.

[0003] At least some DC circuit breakers utilize mechanical and/or electromechanical switches to interrupt the DC current flowing through the circuit breaker. Although many mechanical and electromechanical switches present relatively low transmission losses, the switching speed of many mechanical and electromechanical switches is slower than desired in high power distribution systems, where delays more than one millisecond may result in undesirable transient behavior within the distribution system.

BRIEF DESCRIPTION

[0004] In one aspect, a direct current (DC) circuit breaker is provided. The DC circuit breaker includes a breaker assembly having an input portion and an output portion. The breaker assembly also includes a first conductive path extending between the input portion and the output portion, where the first conductive path is configured to conduct substantially all of the DC current between the input portion and the output portion, and where the first conductive path includes at least one variable impedance device and excludes a mechanical switch. The breaker assembly also includes a second conductive path extending between the input portion and the output portion, where the second conductive path is configured to conduct at least a portion of the DC current between the input portion and the output portion in response to a detected fault condition. The DC circuit breaker also includes a controller configured to selectively operate the breaker assembly to interrupt the DC current between the input portion and the output portion.

[0005] In another aspect, a breaker assembly is provided. The breaker assembly includes an input portion and an output portion. The breaker assembly also includes a first conductive path extending between the input portion and the output portion, where the first conductive path is configured to conduct substantially all of the DC current between the input portion and the output portion, and where the first conductive path includes at least one variable impedance device and excludes a mechanical switch. The breaker assembly also includes a second conductive path extending between the input portion and the output portion, where the second conductive path is configured to conduct at least a portion of the DC current between the input portion and the output portion in response to a detected fault condition.

[0006] In yet another aspect, a method for interrupting direct current (DC) between an input portion of a breaker assembly and an output portion of the breaker assembly is provided. The method includes detecting a fault condition, and diverting, by a controller and in response to the detected fault condition, at least some of a DC current flowing in a first conductive path extending between the input portion and the output portion onto a second conductive path extending between the input portion and the output portion, where the second conductive path includes a gas switch. The method also includes opening, by the controller, at least one semiconductor device in the first conductive path to block the first conductive path, and opening, by the controller, the gas switch in the second conductive path to i) block the second conductive path and ii) divert the DC current from the first conductive path and the second conductive path to a surge arrester in a third conductive path extending between the input portion and the output portion.

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 switch;

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

[0010] FIG. 3 is a block diagram of an exemplary bi-directional fast switching DC circuit breaker including two of the gas switches shown in FIG. 1 ; and

[0011] FIG. 4 is a flow diagram illustrating an exemplary method for interrupting a DC current using either of the fast-switching DC circuit breakers shown in FIG. 2 and FIG. 3.

[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] Several fast-switching DC circuit breakers are described for providing circuit protection in DC power distribution systems. The DC circuit breakers incorporate one or more high voltage high power gas switches as well as one or more semiconductor switches (e.g., power semiconductors). The switches are also capable of opening very quickly and can standoff (i.e., prevent current flow) hundreds of kilovolts or more. Moreover, the gas switches can be combined in series or parallel to increase the voltage and current carrying capacity of the DC circuit breakers. In addition, gas switches may be variously combined, as described herein, to accomplish a fast-switching bi directional DC circuit breaker. Some embodiments provide increased switching speed by conducting DC current through a highly efficient semiconductor switch or series of semiconductor switches, such as a plurality of series connected thyristors, and diverting the current to a gas switch for interruption upon detection of a fault condition.

[0015] FIG. 1 is a high-level schematic diagram of an exemplary gas switch 100. Although specific details of gas switch 100 are not central to an understanding of the present disclosure, some basic description is provided below for convenience. Additional detail related to gas switch 100 may be found with reference to U S. Patent Application No. 14/776,758, filed May 21, 2013, COLD CATHODE SWITCHING DEVICE AND CONVERTER, which is hereby incorporated by reference in its entirety.

[0016] Accordingly, and as shown, gas switch 100 includes a housing 102, an anode 104, a cathode 106, and at least one control grid, such as a control grid 108. 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 some embodiments, gas switch is an oil insulated gas switch, which may facilitate a more compact gas switch architecture in at least some cases.

[0017] 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 any other gases that enable operation of gas 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 switch 100. Alternatively, or additionally, gas switch 100 may include one or more additional electrodes, including for example one or more grids that maintain a weak ionized gas within gas switch 100 to facilitate closing gas switch 100. The exemplary gas switch 100 is a plane-parallel gas tube switch and cathode 106 is a planar cathode. Alternatively, gas switch 100 and cathode 106 may have any suitable configuration enabling operation as described herein.

[0018] In operation, electrical current 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 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.

[0019] When gas switch 100 is open (e.g., turned off, not conducting, etc ), the hydrogen gas insulates anode 104 from cathode 106. When gas 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. In some cases, electrons are released by field emission (in response to strong electric field at the surface) and thermionic emission (locally high surface temperatures). 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.

[0020] FIG. 2 is a block diagram of an exemplary fast-switching direct current (DC) circuit breaker 200 (or“DC circuit breaker” for brevity). DC circuit breaker 200 includes a controller 202 and a breaker assembly 204. [0021] Controller 202 includes a processor 206 and a memory device 208. Based at least in part on current data provided, as described herein, by a sensor 228, processor 206 determines when the DC current through breaker assembly 204 exceeds a protection threshold and controls breaker assembly 204 to interrupt the DC current upon determining that the current exceeds the protection threshold.

[0022] Memory device 208 stores program code and instructions, executable by processor 206, to control breaker assembly 204. Memory device 208 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 208. Memory device 208 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.

[0023] In the exemplary embodiment, breaker assembly 204 includes an input portion 210 and an output portion 212. As described herein, DC current may flow in either direction between input portion 210 and output portion 212.

[0024] Breaker assembly 204 also includes a first conductive path 214, a second conductive path 216, and a third conductive path 218. First conductive path 214 extends between and connects input portion 210 and output portion 212. Second conductive path 216 is electrically connected in parallel with first conductive path 214 and also extends between and connects input portion 210 and output portion 212. Third conductive path 218 is electrically connected in parallel with first conductive path 214 and second conductive path 216 and also extends between and connects input portion 210 and output portion 212.

[0025] First conductive path 214 includes at least one variable impedance device 220 and at least one semiconductor switch 222. In the exemplary embodiment, variable impedance device 220 may include any suitable device for providing a variable impedance on first conductive path 214, such as a fault current limiter, a semiconductor device, such as an insulated-gate bipolar transistor (IGBT), or a power metal oxide semiconductor field effect transistor (power MOSFET), and the like. [0026] In some embodiments, a plurality of variable impedance devices 220 are electrically connected in series on first conductive path 214 to create a desired impedance on first conductive path 214. For example, in at least some embodiments, a plurality of variable impedance devices 220 in a range of two or more variable impedance devices 220 may be electrically connected in series on first conductive path 214. In other embodiments, a single variable impedance device 220 per polarity is sufficient (e.g., two variable impedance devices 220 and 302 in a bi-directional system, such as DC circuit breaker 300, and a single variable impedance device 220 in a unidirectional system, such as DC circuit breaker 200).

[0027] In the exemplary embodiment, semiconductor switch 222 includes any suitable semiconductor switch, such as any suitable thyristor. In addition, semiconductor switch 222 excludes mechanical and electromechanical switches. In at least some embodiments, a plurality of semiconductor switches 222 may be connected electrically in series on first conduction path 214 to standoff a desired DC current (e.g., a larger number of semiconductor switches 222 may be connected in series to stand off a larger DC current, and vice versa). In some embodiments, a number of thyristors connected in series may depend upon a system voltage For example, in a 150kV system, approximately twenty thyristors may be series connected, where each thrysitor is capable of blocking approximately 7,500 V (7.5 kV) to 10,000 V (lOkV).

[0028] Moreover, as described herein, semiconductor switch 222 may be embodied as a semiconductor, such as a thyristor, to achieve a faster switching DC circuit breaker 200. As described in greater detail below, the use of one or more semiconductors in semiconductor switch 222 may, in addition to enabling faster or more efficient switching speed, result in a slight increase in electrical power losses on first conductive path 214 during normal (e.g., non-fault) operation; however, semiconductor switch 222 may nonetheless be implemented over a slower switching mechanical or electromechanical switch to achieve a faster switching DC circuit breaker 200. In some embodiments, DC circuit breaker 200 may, using semiconductor switch 222, be capable of opening (e.g., in response to detection of a fault) in one millisecond or less. [0029] Second conductive path 216 includes a gas switch 224, such as gas switch 100 (described above) coupled between input portion 210 and output portion 212. The physical operation of gas switch 224 is not central to an understanding of the present disclosure. However, details of gas switch 224 are described above with reference to FIG. 1 and exemplary gas switch 100 for convenience. Further, as described below with reference to FIG. 3, another gas switch 304 may be electrically connected in antiparallel with gas switch 224 to achieve a fast-switching bi-directional DC circuit breaker 300.

[0030] Moreover, in at least some embodiments, second conductive path 216 may, like first conductive path 214, include one or more variable impedance devices 220 (e.g., IGBTs). Specifically, in at least some embodiments, second conductive path 216 may be arranged to include a plurality of variable impedance devices 220 that perform substantially the same function as gas switch 224. As a result, in at least some embodiments, second conductive path 216 may exclude gas switch 304 in favor of a plurality of variable impedance devices 220.

[0031] Third conductive path 218 includes at least one surge protection device, such as a surge arrester 226. As described herein, surge arrester 226 is configured to absorb substantially all of the DC current flowing between input portion 210 and output portion 212 during an electrical fault condition in DC circuit breaker 200. In various embodiments, surge arrester 226 may include any electrical conductor capable of safely dissipating as heat energy substantially all of the DC current.

[0032] Sensor 228 is coupled to input portion 210 and controller 202. In the exemplary embodiment, sensor 228 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 204. Alternatively, sensor 228 may include any other sensor that enables DC circuit breaker 200 to function as described herein. Sensor 228 generates a signal representative of the measured or detected current (hereinafter referred to as a“current signal”) flowing through breaker assembly 204. In addition, sensor 228 transmits the current signal to controller 202. In other embodiments, DC circuit breaker 200 may include more than one sensor 228. [0033] Under normal operating conditions (e g., when there is no fault condition), DC current flows through first conductive path 214 between input portion 210 and output portion 212. Specifically, approximately 1-5 kA of DC current flows through first conductive path 214 during normal operation; however, in other embodiments, greater or lesser current may flow through first conductive path 214 during normal operation. More particularly, DC current flows on first conductive path 214, through variable impedance device 220 and semiconductor switch 222, from a DC source connected to input portion 210 to a load connected to output portion 212.

[0034] In addition, during normal operation, variable impedance device 220 is set, such as by controller 202, at its lowest impedance, semiconductor switch 222 is closed (i.e., conducting), and gas switch 100 is open (i.e., non-conducting). In addition, the inclusion of surge arrestor 226 in third conductive path 218 makes third conductive path 218 a high impedance path.

[0035] Thus, first conductive path 214 has a low impedance (e.g., an impedance of about zero ohms), while second conductive path 216 and third conductive path 218 have a very high impedances (e.g., impedances about equal to an open circuit impedance). DC current entering input portion 210 will therefore selectively travel through the low impedance first conductive path 214 to output portion 212. In the exemplary embodiment, no current travels through second conductive path 216 or third conductive path 218 during normal operation (e.g., while DC current is flowing on first conductive path 214). In other embodiments, a relatively small amount of the DC current travels from input portion 210 to output portion 212 through second conductive path 216 under normal (i.e., non-fault) conditions.

[0036] In response to detecting a fault condition, receiving an instruction to trip, or otherwise determining to trip, controller 202 diverts a portion of the DC current flowing between input portion 210 and output portion 212 from first conductive path 214 to second conductive path 216. More specifically, controller 202 increases the impedance of variable impedance device 220 and, at substantially the same time, closes gas switch 224.

[0037] As a result, the impedance of first conductive path 214 is increased while the impedance of second conductive path 216 is greatly decreased. Accordingly, most of the DC current flowing from input portion 210 to output portion 212 is diverted and now flows through second conductive path 216.

[0038] The portion of current diverted to second conductive path 216 is greater than or equal to an amount which will reduce the DC current in first conductive path 216 low enough to permit semiconductor switch 222 to be opened. In the exemplary embodiment, substantially all of the DC current is diverted to second conductive path 216 and through gas switch 224. As will be understood by those skilled in the art, the amount of the DC current diverted to second conductive path 216 depends on the amount of time elapsed and the relative impedances of first conductive path 214 and second conductive path 216.

[0039] As described herein, the use of semiconductor switch 222 rather than any mechanical or electromechanical switch in first conductive path 214 enables a rapid opening of first conductive path 214. For example, where semiconductor switch 222 includes one or more thyristors, first conductive path 214 may transition from conducting very large DC current to a non-conductive or open circuit in less than or equal to approximately two milliseconds. In contrast, a circuit breaker employing an mechanical or electromechanical switch may require as much as three or more milliseconds to accomplish a open circuit within first conductive path 214.

[0040] A difference of one or two milliseconds in the time required to close first conductive path 214 to current flow (and so trip DC circuit breaker 200) represents, however, a non-trivial improvement. To illustrate, in one example, assume 3 kA of nominal current flow through first conductive path 214 during normal operation, and a rate of current rise during a fault of approximately 3 kA/millisecond. Assume also that a fault is capable of detection by sensor 228 in approximately 150 microseconds. In such a case, if it takes 2 milliseconds to open DC circuit breaker 200, the current will climb to approximately 9 kA, and all components must withstand this current transiently without damage. Likewise, if it takes even a single millisecond longer to detect the fault, the current would, under these conditions, climb to approximately 12 kA. Thus, variations (of even a single millisecond) in the time required to trip DC circuit breaker 200 result in substantial increases in the amount of current that components must tolerate without damage. In addition, variations in the time required to trip DC circuit breaker 200 may result in additional energy being provided to the fault and consequently a variety of related hazards.

[0041] Further, this improvement in breaker speed constitutes a significant advantage, even in the presence of a tradeoff of slightly higher losses through semiconductor switch 222 during normal operation. In other words, although semiconductor switch 222 may dissipate a small amount of electrical power during normal operation (e.g., more than a mechanical or electromechanical switch), the advantage in breaker speed achieved through the use of semiconductor switch 222 over a mechanical or electromechanical switch is sufficient to warrant the use of semiconductor switch 222 in place of a mechanical or electromechanical switch.

[0042] Therefore, after the current flowing through first conductive path 214 has decreased sufficiently to permit semiconductor switch 222 to be opened, controller 202 opens switch 222 (e.g., by reverse biasing the one or more thyristors in semiconductor switch 222). The determination by controller 202 of when to open semiconductor switch 222 may be based on an elapsed time after increasing the impedance of variable impedance device 220 and closing gas switch 224, a measurement of the DC current through first conductive path 214, and/or any other suitable parameter for determining when to open semiconductor switch 222. In some embodiments, first conductive path 214 and second conductive path 216 each include one or more sensors (not shown) configured to provide a signal representative of the detected current to controller 202.

[0043] After controller 202 opens semiconductor switch 222, the DC current flowing through first conductive path 214 will cease and all DC current flowing from input portion 210 to output portion 212 flows through gas switch 224 on second conductive path 216. It will be appreciated that a single gas switch 224 is capable of handling a very high current (e.g., all of the DC current), representing a significant improvement over a variety of other solutions, such as, for example, a stacked or series connected plurality of power semiconductor switches.

[0044] Once the DC current is diverted to second conductive path 216, controller 202 next opens gas switch 224 to interrupt the flow of DC current on second conductive path 216. The determination by controller 202 of when to open gas switch 224 may be based on an elapsed time after opening semiconductor switch 222, a measurement of the DC current through first conductive path 214, and/or any other suitable method for determining when to open gas switch 224. Finally, in response to opening of gas switch 224, the stored energy is diverted to third conductive path 218, where the stored energy flows into surge arrester 226 and is dissipated, thereby completing the interruption of DC current flowing between input portion 210 and output portion 212.

[0045] After the DC current interruption is completed by DC circuit breaker 200, variable impedance device 220 is reset to its lowest impedance state prior resuming normal operation, and remains at low impedance during normal operation 2. Specifically, variable impedance device 220 is reset to its lowest impedance prior to determining, by controller 202, to resume normal operation. Similarly, controller 202 may close semiconductor switch 222 in advance of resuming normal operation of DC circuit breaker 200. In some embodiments, a final mechanical disconnect (not shown) may be opened to ensure the safety of workers after the stored energy is dissipated in surge arrester 226. Specifically, a final mechanical disconnect for safety is kept opened after DC circuit breaker 200 has interrupted the current. It is turned on prior resuming normal operation.

[0046] FIG. 3 is a block diagram of a fast-switching bi-directional DC circuit breaker 300 (or“DC circuit breaker” for brevity). DC circuit breaker 300 is substantially similar to DC circuit breaker 200 (described above with reference to FIG. 2), except that DC circuit breaker 300 is configured to conduct DC current in either direction (e g., from input portion 210 to output portion 212 or from output portion 212 to input portion 210).

[0047] To this end, DC circuit breaker 300 includes (in addition to all of the components of DC circuit breaker 200), a second variable impedance device 302 disposed between output portion 212 and semiconductor switch 222 and a second gas switch 304 included in a fourth conductive path 306.

[0048] In the exemplary embodiment, fourth conductive path 306 is arranged in parallel with first conductive path 214, second conductive path 216, and third conductive path 218. Physically, in at least some embodiments, fourth conductive path 306 is disposed between second conductive path 216 and third conductive path 218. However, it will be appreciated that fourth conductive path 306 may be physically disposed elsewhere or in a different orientation or position within breaker assembly 202, provided fourth conductive path 306 remains electrically connected in parallel with the other conductive paths 214, 216, and 218.

[0049] Further, second gas switch 304 is configured on fourth conductive path 306 to conduct DC current in a direction opposite gas switch 224. In this respect, the orientation of second gas switch 304 with respect to gas switch 224 may regarded as an “antiparallel” orientation. Further, in at least some embodiments, gas switch 224 may be designed to operate in both directions (obviating the need for second gas switch 304 in this case). Therefore, as described above, gas switch 224 is arranged to conduct DC current from input portion 210 to output portion 212, while second gas switch 304 is configured to conduct DC current (when closed) from output portion 212 to input portion 210. For clarity, when DC circuit breaker 300 is configured to conduct from output portion 212 to input portion 210, output portion 212 may be regarded as an“input portion” and input portion 210 may be regarded as an“output portion.” However, these notations are merely indicative of the direction of DC current flow.

[0050] In like manner, second variable impedance device 302 is arranged between output portion 212 and semiconductor switch 222, such that, when output portion 212 functions as a DC input or“input portion” (e.g., when DC current flow is reversed or flowing from output portion 212 towards input portion 210), controller 202 may increase an impedance of second variable impedance device 302 (e.g., as described above with reference to variable impedance device 220) to divert or transition DC current flow from first conductive path 214 to second gas switch 304 on fourth conductive path 306. Once DC current is shifted, as described herein, to fourth conductive path 306, DC circuit breaker 300 functions as described above with reference to DC circuit breaker 200. For example, controller 202 may open or switch off semiconductor switch 222 and open second gas switch 304 to divert current into surge arrestor 226 on third conductive path 218, etc.

[0051] Thus, DC circuit breaker 300 may function as a fast-switching bi directional DC circuit breaker, in that DC current may flow in either direction (e g , from input portion 210 to output portion 212 or vice versa). In either instance, DC circuit breaker may, as described herein, be capable of detecting and interrupting a large DC current flowing in either direction within one millisecond or less. [0052] FIG. 4 is a flow diagram illustrating (and summarizing) an exemplary method 400 for interrupting a DC current using either of the fast-switching DC circuit breakers 200 and 300 (shown in FIG. 2 or FIG. 3). The exemplary method (or algorithm) is also described in detail above with reference to FIG. 2 and FIG. 3. Accordingly, in at least one embodiment, controller 202 detects a fault condition, such as using sensor 228 (step 402).

[0053] In response to detection of a fault condition, controller 202 diverts at least some of a DC current flowing in first conductive path 214 between input portion 210 and output portion 212 (and/or vice versa in the case of bi-directional DC circuit breaker 300) onto second conductive path 214 (and/or fourth conductive path 306 in the case of bi-directional DC circuit breaker 300) between input portion 210 and output portion 212 (and/or the reverse as described above) (step 404). For example, controller 202 increases an impedance of variable impedance device 220 or second variable impedance device 302 to divert at least some of the DC current onto second conductive path 216 or fourth conductive path 306.

[0054] Once at least some of the DC current is diverted to second conductive path 216 and/or fourth conductive path 306, controller 202 opens semiconductor switch 222 in first conductive path 214 to block first conductive path 214 entirely, whereupon all (or substantially all) of the DC current flows through a respective gas switch 224 or 304 between input portion 210 and output portion 212 (or vice versa, as described herein) (step 406). Concurrently, in at least some embodiments, a small voltage sufficient to turn on gas switch 224 and/or 304 is established prior to blocking first conductive path 214. Lastly, controller 202 controls the gas switch 224 or 304 through which the DC current flows to open thereby blocking the second conductive path 216 or fourth conductive path 306 as well as diverting the DC current flowing in the DC circuit breaker 200 or 300 into surge arrester 226, where remaining stored energy it is finally dissipated (step 408). Controller 202 may, in addition, reset variable impedance device 220 or second variable impedance device 302 to a lowest or desired lower impedance and/or reset close semiconductor switch 222 prior to resuming normal operation (step 410). [0055] The above-described fast-switching DC circuit breakers provide an efficient method for providing circuit protection in DC power distribution systems. The gas switch(es) used in the example embodiments are high voltage and high power capable gas switches. The switches are also capable of opening very quickly and can standoff hundreds of kilovolts or more. Moreover, the gas switches can be combined in series and/or parallel to increase the voltage and/or current carrying capacity of the DC circuit breakers and/or, as described herein, to accomplish a fast-switching bi-directional DC circuit breaker. Some embodiments provide increased switching speed by normally conducting DC current through a highly efficient semiconductor switch or series of semiconductor switches, such as a plurality of series connected thyristors, and diverting the current to a gas switch for interruption upon detection of a fault condition.

[0056] Exemplary technical effects of the methods, systems, and apparatus described herein include, for example: (a) detecting a fault condition; (b) rapidly interrupting a DC current between an input portion and an output portion of a breaker assembly in response to a detected fault condition; (c) use of one or more semiconductor switches to increase the switching speed of the circuit breakers described herein; and (d) bi directional switching capability by implementing a plurality of gas switches on parallel conductive paths in conjunction with a plurality of variable impedance devices, some disposed between a semiconductor switch and an input portion and others disposed between the semiconductor switch and an output portion.

[0057] 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.

[0058] 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.

[0059] 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.