US3714510A

US3714510A - Method and apparatus for ignition of crossed field switching device for use in a hvdc circuit breaker

Info

Publication number: US3714510A
Authority: US
Inventors: G Hofmann
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1971-03-09
Filing date: 1971-03-09
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30
Also published as: DE2208431A1; DE2208431C3; CH534455A; GB1333686A; FR2127846A5; SE377222B; DE2208431B2; CA932783A

Abstract

Ignition method and apparatus for crossed-field switching device for onswitching the device when a high voltage is applied thereacross, comprising applying a plasma puff between the electrodes to initiate arc discharge therebetween, followed by resonantly reducing the current to zero to extinguish the arc and permit initiation of glow mode discharge after the current zero.

Description

United States Patent [191 [111 3,714,510 Hofmann Jan. 30, 1973 [54] METHOD AND APPARATUS FOR [56] References Cited IGNITION OF CROSSED FIELD SWITCHING DEVICE FOR USE IN A STATES PATENTS HVDC CIRCUIT BREAKER 3,290,542 12/1966, Lafferty ..337/15 3,534,226 l0/l970 Lian ...200/l44 R [75] Holman L08 3,356,897 l2/l967 Barr ..313 |s0 geles, Calif. [73] Assignee: Hughes Aircraft Company, Culver r mary inerJ. D- Miller City, Calif. Assistant ExaminerHarvey Fendelman Attorney-W. H. MacAllister, Jr. and Allen A. Dicke, [22] Filed: March 9,1971 Jr.

[2]] Appl. No.: 122,397 [57] ABSTRACT Ignition method and apparatus for crossed-field [52] US switching device for onswitching the device when a Int Cl H05! 7/00 5 17/26 high voltage is applied thereacross, comprising apply- [58] Field iiii 7667144 A 14 6 A- ji37/ 5- g aplasma puff between the electrodes to initiate arc discharge therebetween, followed by resonantly reducing the current to zero to extinguish the arc and permit initiation of glow mode discharge after the current zero.

I l e i l l l l l l l l J L METHOD AND APPARATUS FOR IGNITION OF CROSSED FIELD SWITCHING DEVICE FOR USE IN A I-IVDC CIRCUIT BREAKER BACKGROUND This invention is directed to a method and apparatus for the ignition of a crossed-field switching device for onswitching high voltage DC current.

Crossed-field switching devices have been known for many years as laboratory curiosities, as shown in Penning U.S. Pat. No. 2,182,736; and as simple, undeveloped low power devices, as shown in Boucher U.S. Pat. Nos. 3,215,893 and 3,215,939 and Wasa 3,405,300.

Only recently, however, has the utility of such a switching device in high voltage, high current DC applications been recognized, because careful design is necessary for employment of the crossed-field device concept in high current and high voltage situations. Examples of such use are found in Kenneth T. Lian U.S. Pat. No. 3,534,226, and Gunter A. G. Hofmann and Ronald C. Knechtli U.S. Pat. .No. 3,538,960. As described hereinafter, there is a problem of onswitching a crossed-field switching device when the device is in a non-conductive condition and a high voltage is applied to its electrodes. The method and apparatus of this invention permit the ignition or onswitching of such a crossed-field device under such conditions. Thus, the device can be conveniently employed in high voltage DC circuit breaker situations.

SUMMARY In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to a method and apparatus for ignition of crossed-field switching devices for onswitching high voltage DC current. The method and apparatus comprise a plasma puffer for introducing a puff of plasma between the electrodes of the crossed-field device when the crossed-field device has a high voltage applied across the electrodes and a magnetic field of sufficient strength to permit sustained conduction under some interelectrode potential conditions. The plasma puff causes interelectrode arc discharge to reduce the interelectrode potential. The method and apparatus further include a resonant circuit which causes an interelectrodecurrent zero to extinguish the are after the potential is thus reduced. After the current zero, the interelectrode potential rises comparatively slowly to permit conduction in the glow mode.

Accordingly, it is an object of this invention to provide a method for ignition of crossed-field switching devices for use in high voltage DC circuit breakers, the method including initiating an arc discharge followed by a current zero to extinguish the arc and followed by a sufficiently slow interelectrode potential rise to permit initiation of discharge in a glow mode. It is another object to provide an apparatus for the ignition of a crossed-field tube which includes an interelectrode plasma puffer and a resonant circuit which reduces the interelectrode current to zero subsequent to interelectrode arcing. It'is still another object to provide an economic and convenient means and method which rapidly onswitches a crossed-field switching device having a high potential applied thereto.

Other objects and advantages of this invention will become apparent from a study of the following portion of this specification, the claims, and the attached drawings.

BRIEF DESCRIPTION OF tHE DRAWINGS FIG. 1 is an electrical schematic diagram of a power system which includes a circuit breaker employing the onswitching method and apparatus of this invention.

FIG. 2 is a perspective view, with parts broken away and partly taken in section, of a crossed-field switch device, in accordance with the method and apparatus of this invention.

FIG. 3 is a graph showing the conductivity conditions of a crossed-field switching device relating interelectrode voltage to magnetic field strength.

DESCRIPTION Referring to FIG. 1, the DC power which is to be switched by the circuit breaker employing the crossedfield switch device ignition method and apparatus of this invention is conventionally derived at a power source 10 which delivers power to an AC generator 12. Generator 12 delivers its output to transformer 14 by which the voltage is raised to suitable transmission line voltage. From the transformer, the power is rectified by rectifier l6. Rectifier 16 has positive and negative output lines 18 and 20 respectively. Inductance 22, connected in one of the lines and capacitance 24, connected between the lines, serve as conventional DC filtering and smoothing equipment. They are preferably connected at the output of the rectifier, as shown. In certain circumstances, the reactance of the transmission system may be sufficient to provide adequate smoothing for economic power transmission.

Circuit breaker 26 is serially connected in line 18 between the rectifier l6 and transmission system 30, while an identical circuit breaker 28 is connected in line 20 therebetween. Bipolar circuit breakers are thus provided, because of the high voltages in the exemplary embodiment of employment of the circuit breaker. In

lower voltage systems, only one circuit breaker might be necessary.

In high voltage DC systems, it is customary to have a line potential such that one line is above ground potential, while the other is below. This equalizes the amount of transmission line insulation between the two lines and ground. For this reason, the two

circuit breakers

26 and 28 are required, one in each line. Either one of the lines, through the transmission system or at the load, may fault to each other, as by exemplary fault switch 32, or can fault to ground. Thus, independent line protection is necessary, should a fault to ground occur, rather than the interline fault indicated at 32. However, in either type of fault, a circuit breaker is necessary.

Each of the

circuit breakers

26 and 28 has conventional fault detection equipment associated therewith, as well as conventional programming circuitry to operate the circuit breaker through its breaking cycle. Thus, conventional fault detection and circuit breaker operating equipment is included.

Load 34 is connected at the output of transmission line 30. Switch 32 schematically represents a potential electrical fault across the lines on the load side of the circuit breakers. The fault can occur anywhere therealong, or between a line and ground.

Circuit breaker 26 comprises the line switch 36 and impedance-increasing means 38. Line switch. 36 may have a current transfer circuit in association therewith, as shown in a M. A. Lutz and W. F. Long patent application Ser. No. 122,395, filed Mar. 9, l97l, entitled Current Transfer Circuit as Part of a High Voltage DC Circuit Breaker, filed concurrently herewith, (PD-70229), the entire disclosure of which is incorporated herein by this reference. Alternatively, any suitable line switch 36 which produces a sufficiently high voltage drop thereacross during opening to transfer the current into the impedance-increasing portion 38 can be employed. Thus, a simple switch 36 is illustrated, which switch may be a conventional circuit breaker.

The impedance-increasing means 38 is connected between

breaker buses

40 and 42. Before the impedance is increased, it is necessary that this current be transferred from the line switch 36, in accordance with the concept discussed above. This current transfer is aided by crossed-field switch device 44, which is directly connected between

buses

40 and 42. Thus, during the time of opening of line switch 36 and the current transfer, crossed-field switch device 44 is in conductive condition so that, when the voltage across the

buses

40 and 42 rises to a sufficient potential to permit glow discharge conduction of the crossed-field switch device 44, it begins conducting and the potential between

buses

40 and 42 is clamped at the voltage drop of device 44. In devices of the type under consideration, the voltage drop at high current is about 1 kilovolt. This fairly low potential permits the current to be fully transferred from the line switch 36 so that the line switch can fully open, deionize, and be in condition to hold off the surge potential. After line switch 36 has reached this condition, crossed-field switch device 44 can be turned off. Thereupon, the impedance can be increased by the cyclic switching impedance-increasing means described below. I

Surge capacitor 46 and its energy-absorbing resistor 48 are connected between

breaker buses

40 and 42 so that, upon opening of the various switches, voltage surge peaks are reduced to tolerable limits. Capacitor 24 also enters into this effect.

The impedance-increasing circuit selected as an example for employment of the crossed-field switch device 100, which is equipped for onswitching with a high potential applied thereto, is the cyclic impedanceincreasing circuit illustrated in Wolfgang Knauer patent application, Ser. No. 122,396, filed Mar. 9, 1971, filed concurrently herewith, entitled Impedance-lncreasing Method and Apparatus as Part of a High Voltage DC Circuit Breaker, (PD-69238). Any of the circuits described in that application can be the impedance-increasing circuit.

Other impedance-increasing circuits can employ the method and apparatus of this invention, the teaching of this invention being the onswitching of a crossed-field device having voltage applied thereto. Thus, the impedance-increasing means of K. T. Lian and W. F. Long patent application, Ser. No. 45,147, filed June 10, 1970, entitled Consecutive Crowbar Circuit Breaker and M. A. Lutz patent application, Ser. No. 45,460, filed June ll, 1970, now US. Pat. No. 3,61 1,031, for Series Sequential Circuit Breaker are pertinent. Each of these disclosures is incorporated herein in its entirety by this reference so that the several species of impedance-increasing circuits described therein are within the scope of this application.

These circuits are also useful with the switch and method of this invention. To illustrate a manner of employment of the invention, a cyclic switching circuit is illustrated as the impedance-increasing means 38. Serially-connected with the crossed-filed switch device 100 is energy-absorbing resistor 50. Crossed-field switch device 100 is switched on and off with increasing offperiods so that the time averagec circuit impedance is increased until the switch can remain open.

In order to aid onswitching, as part of the apparatus and in accordance with the method of this invention, sufficient capacitance must be connected across crossed-field switch device 100 and, in association with the capacitance, there must be sufficient inductance to resonantly drive the current to zero. Since the capacitor 46 is connected in series with its resistance 48, it normally is not sufficiently close-coupled to device 100 to accomplish this result. Thus, capacitor 52 is connected in series with inductance 54 and this series connection is paralleled around the device 100. Normally, the inductance required is quite small so that the capacitor leads provide adequate inductance.

To further illustrate the apparatus of this invention, FIG. 2 shows a crossed-field device 100 equipped for ignition, in accordance with the method and apparatus of this invention.

Referring to FIG. 2, the crossed-field switch device comprises housing 102 which is carried upon bottom flange 104. Bottom flange 104 is, in turn, mounted upon base flange 106 and they are secured together to provide a tight seal. Base flange 106 stands upon foot 108 for supporting the switch device structure. Furthermore, foot 108 can act as a vacuum connection for drawing a suitable vacuum on the interior of housing 102 and then letting into the housing the desired gas (e.g., hydrogen, including its isotope deuterium) at the required pressure. Housing 102, together with bottom flange 104, serves as a suitable vacuum tight envelope.

Cathode 110 is in the form of a cylindrical tube. It is spaced inwardly from housing 102. Cathode 110 has a lower cap 112 by which it is supported from base flange 104 by means of standoff 114. Lower cap 112 does not need to effect closure, but simply provides mechanical support for the cathode and reduces plasma end losses. By this construction, the entire cathode can be withdrawn through the large opening in bottom flange 104 when the flanges are separated for inspection and service of the cathode and inspection and service of the interior of housing 102. Cathode 110 is metallic and can be made of stainless steel. The cathode is connected to the foot 108, such as by a metallic strip. Thus, foot 108 provides one of the electrical connections to the switching device 100. Cathode 110 may have an axial slot to prevent the circumferential circulation of current during switching transients, when the axial magnetic field changes with time.

Anode 116 is of cylindrical tubular construction and is positioned concentrically with cathode 110 to provide a radial space therebetween having the dimension d. The radial space d is substantially equal at all facing positions of the anode and cathode. Housing 102 has a top cap 118 upon which anode 116 is positioned. The anode is maintained in position by employing anode cap 120 which is secured to the cylindrical anode 116 and, in turn, carries mounting stud 122. Mounting stud 122 provides both mechanical support by being secured to housing cap 118 and provides electrical continuity through the cap by electrical connector 124. Preferably, anode cap 120 is spaced below top cap 1 l8 and connector 124 passes through insulative mounting stud 122 so that connector 124 and the entire anode are electrically separated from the housing. Alternatively, top cap 118 can be of insulative material.

Anode 116 may be perforated so that the interior space thereof serves as a gas volume to supply gas to the interelectrode space. Furthermore, gas supply means can be provided interiorly of the anode to supply gas as it is consumed by a glow discharge in the interelectrode space. Both of these concepts are taught in Hofmann and Knechtli U.S. Pat. No. 3,558,960. The maintenance of interelectrode space gas pressure. is discussed in more detail in that patent.

Magnet 126 is positioned on the exterior of housing 30 in such a manner as to provide magnetic lines of force in the interelectrode space which are substantially parallel to the axis of the electrodes of switching device 100 over at least part of the electrode length. Magnet 126 is illustrated as being an electromagnet and such is preferred, so that the magnetic field can readily be switched on and off. The power supply to magnet 126 is preferably of such nature as to provide for rapid turnon and off of the field. Its strength is such as to provide a field between 50 and 150 Gauss; 70 Gauss was found to be a preferred value for the dimensions given below used in our experiments to date, considering the turnon and turnoff effects, as well as magnet power consumption.

Once a glow discharge is established and current is flowing,,offswitching is accomplished by reducing the magnetic field strength to a point where cascading ionization cannot continue. Thus, conduction ceases. This is explained in considerably more detail in the l-lofmann and Knechtli U.S. Pat. No. 3,558,960, mentioned above.

However, a problem occurs in onswitching of such a switching device. FIG. 3 illustrates the conductive region of a crossed-field device of a nature discussed, within the hatched area. When the switching device is nonconductive, the circuit voltage is applied across the interelectrode space. The illustrated example is a device which switches l00 kilovolts and, with a 70 gauss magnetic field, the device is in a state indicated at point A. To render this device conductive, without any other ignition means, the magnetic field would have to be increased so that the operating point would finally reach the hatched area and, with the 100 kilovolts applied to the interelectrode space, a field of nearly 500 gauss would be required to initiate cascading ionization.

To overcome this, plasma puffer 128 is secured on device 100 and is positioned to discharge plasma into the interelectrode space. Plasma puffer 128 is described in detail in Lafferty U.S. Pat. No. 3,290,542, the entire disclosure of which is incorporated herein by this reference.

When the switching device 100 is in the state indicated at point A in FIG. 3, operation of the plasma puffer 128 places a conductive plasma in the interelectrode space. This conductive plasma initiates a metallic arc discharge between the electrodes. The electrodes are specifically of such material, such as molybdenum, to permit an appropriate metallic arc discharge. Considering the circuit of FIG. 1, the capacitor discharges through the switch device, and the circuit inductance is sufficient to cause resonance with the capacitor to cause a current zero in the switching device. With such current zero, the arc discharge quickly extinguishes, but now the voltage is reversed. As the circuit brings the voltage towards zero, the rate of voltage rise being limited by the capacitance in the circuit, the operating point passes through the toe of the hatched portion of the curve of FIG. 3, moving upward along the gauss line sufficiently slow that a Penning-type of glow discharge is initiated in the switching device. Once this discharge is established, the device is conducting and the interelectrode voltage is clamped at the voltage drop of the conducting device.

In a particular example of this physical configuration of the switching device of FIG. 2, the interelectrode radial distance is about 15 millimeters, with an anode diameter of millimeters and axial length of 300 millimeters. Normal gas pressure in the interelectrode space is about 0.04 millimeter of mercury. Hydrogen is one possible gas. With such dimensions, the switching device is capable of offswitching DC loads of 1,000 amperes and holdoff 25 kilovolts with recovery time in the order of about 25 microseconds.

Going through the operation, assuming that the crossed-field device is nonconducting and the potential across the

buses

40 and 42 is such that capacitor 54 is charged up to voltage of V,,, which is the 100 kilovolts illustrated at point A in FIG. 3, when the plasma gun 128 injects plasma into the interelectrode space and initiates breakdown into metallic are conduction, the capacitor 52 discharges through this path. Current oscillation starts with a frequency given by the capacitance 52 and inductance 54. In view of the desired high frequency, this inductance should be very small and, as indicated above, can be the inductance of the capacitor leads connecting the capacitor in parallel to the device 100.

At the same time, the main current 40 o/ so starts the flow through the device and is sumperimposed upon the oscillating current I After one-half oscillation, the oscillating current 1,, flows opposite the main current 1 If I is larger than 1 or R 2 VL/C, which is easy to achieve, then the total current through the device goes to zero at time t At this moment, the arc extinguishes and the tube goes into a nonconducting state. The ca acitor 52 is left with the reverse voltage V V, V l L/CR'.

The main current now flows into capacitor 52 and drives the voltage of the capacitor to zero. This rate of voltage change is slow, compared to the capacitor voltage change due to the resonant oscillation. The capacitor voltage V will be zero at time The rate of change of the voltage on the capacitor 52, which is the interelectrode potential of the device 100 near zero, is

These are favorable conditions for ionization breakdown in the interelectrode space in the device 100 into the glow mode. The voltage across the tube rises from zero at a moderate rate, in the order of l kilovolt per microsecond, towards the breakdown voltage of a few hundred volts. The voltage drop during conduction is very close to the breakdown voltage so that the capacitor doesnt feed much current into the tube when breakdown into the glow mode occurs. Additionally, the time to form the glow discharge is short enough to allow breakdown during the time the rising tube voltage is in the breakdown region illustrated in FIG. 3. Now that the conduction is in the glow discharge mode, offswitching can be controlled by the magnetic field.

The method for initiating switching in a crossed-field device thus comprises injecting plasma into the interelectrode space of a crossed-field switching device which has sufficient magnetic field applied to the interelectrode space to permit conduction at a lower voltage when a higher voltage is applied to the interelectrode space to initiate a metallic arc discharge between the electrodes. This initiation of arcing is followed by resonantly reducing the interelectrode current to zero to reduce the interelectrode voltage sub stantially to zero and to extinguish the metallic arc, followed by permitting the voltage to rise sufficiently slowly to permit interelectrode current flow in the glow discharge mode.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art.

What is claimed is:

1. An apparatus 'for ignition of a crossed-field switching device to turn on the crossed-field switching device when a high potential is applied to the crossedfield switching device, said crossed-field switching device comprising:

concentric anode and cathode electrodes having an annular space therebetween, magnetic field means for applying a substantially axial magnetic field to the interelectrode space and a gas within the interelectrode space so that, during conduction of the crossed-field switch device, current can pass between said electrodes in the glow discharge mode, the improvement comprising:

a plasma injector positioned adjacent said electrodes for injecting plasma into the interelectrode space and resonant means connected to said electrode so that the injection of plasma into the interelectrode space causes interelectrode metallic arc discharge to reduce the potential between said electrodes and said resonance means induces a current zero to extinguish the metallic arc mode discharge and permit initiation of glow mode discharge.

2. The apparatus of claim 1 wherein said switching device is part of .a circuit breaker which is connected between an electric current g enerator and a load.

3. The apparatus of clai 2 wherein said resonant means is a serially connected capacitor and inductor connected in parallel to said crossed-field switching device.

4. The apparatus of claim 1 wherein said resonant means is a serially connected capacitor and inductor connected in parallel to said crossed-field switching device. i

5. The method of igniting a crossed-field switching device which has a potential applied thereacross of sufficient magnitude to maintain the crossed-field device in the nonconducting state for the particular magnetic field applied thereto comprising the steps of:

injecting plasma into the interelectrode space to cause metal arc mode conduction between the electrodes to reduce the potential between the electrodes to near zero;

resonantly inducing a current zero in the interelectrode current, followed by:

permitting the interelectrode voltage to rise to pass interelectrode current in a glow discharge regime so that current is transmitted through the interelectrode space of the device in the glow discharge mode.

Claims

1. An apparatus for ignition of a crossed-field switching device to turn on the crossed-field switching device when a high potential is applied to the crossed-field switching device, said crossed-field switching device comprising: concentric anode and cathode electrodes having an annular space therebetween, magnetic field means for applying a substantially axial magnetic field to the interelectrode space and a gas within the interelectrode space so that, during conduction of the crossed-field switch device, current can pass between said electrodes in the glow discharge mode, the improvement comprising: a plasma injector positioned adjacent said electrodes for injecting plasma into the interelectrode space and resonant means connected to said elecTrode so that the injection of plasma into the interelectrode space causes interelectrode metallic arc discharge to reduce the potential between said electrodes and said resonance means induces a current zero to extinguish the metallic arc mode discharge and permit initiation of glow mode discharge.

2. The apparatus of claim 1 wherein said switching device is part of a circuit breaker which is connected between an electric current generator and a load.

3. The apparatus of claim 2 wherein said resonant means is a serially connected capacitor and inductor connected in parallel to said crossed-field switching device.

4. The apparatus of claim 1 wherein said resonant means is a serially connected capacitor and inductor connected in parallel to said crossed-field switching device.