US3482070A

US3482070A - Vacuum d.c. breaker with relative contact movement interposing a non-conductor to extinguish arc

Info

Publication number: US3482070A
Application number: US599192A
Authority: US
Inventors: Bo Breitholtz; Fred Carlen
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1965-12-03
Filing date: 1966-12-05
Publication date: 1969-12-02
Anticipated expiration: 1986-12-02
Also published as: CH455912A; DE1590063A1; GB1159017A

Description

Dec. 2. 1969 B. BREITHOLTZ ET AL VACUUM DC BREAKER WITH RELATIVE CONTACT MOVEMENT INTERPOSING A NON- CONDUCTOR TO EXTINGUISH ARC Filed Dec. 5, 1966 4 Sheets-Sheet 2 Fig.3

37 I MID s B a 15L ?/7 Z El Z Z in j W 1' ENT 'B'o fizz r1135;

5 CQRLEN Dec. 2. 1969 a. BREITHOLTZ ET AL 3,482,070

VACUUM DC BREAKER WITH RELATIVE CONTACT MOVEMENT INTERPOSING A NON-CONDUCTOR TO EXTINGUISH ARC Filed Dec. 5, 1966 4 Sheets-Sheet 3 Fig.5

E M VENT OR.

BY Q Sk ph msq -Abw;

Dec. 2. 1969 a. BREITHOLTZ ET AL 3,482,070

VACUUM DC BREAKER WITH RELATIVE CONTACT MOVEMENT INTERPOSING A NON-CONDUCTOR TO EXTINGUISH ARC Filed Dec. 5, 1966 4 Sheets-Sheet 4 .UWEN TOR 'BO BREIDTHQL BY ##8##: Cam? N 73min si ohmvf uew F) TTQRIVEYS United States O U.S. Cl. 200-151 9 Claims ABSTRACT OF THE DISCLOSURE In a DC circuit breaker with an anode and a cathode mounted for separating movement to draw an are between them, the cathode having a plasma spot, an insulating screen is provided so located that, when the electrodes are fully separated, the screen lies between the two electrodes to shield the anode from the plasma jet from the cathode spot.

To'break DC circuits, at least when it is a question of high power, it has been necessary to take special precautions. A- normal breaking process consists of decreasing the current by means of a converter to such a low value that a normal AC breaker or power disconnector is capable of breaking that current. This is one possible solution of the breaking problem with a simple circuit having a converter station at each end of the circuit, but the problem is considerably more complicated if three or more stations are connected in the same circuit. There is a great need for a DC breaker which can alone break a direct current without the help of converters.

The basis of the present invention is the principle that it is possible in a DC circuit with relatively low inductance to generate a vacuum are having at the start low arc voltage drop and then to increase the arc voltage drop thus decreasing the current so much that a normal AC breaker or power disconnector can safely effect breaking. If, however, the circuit has a high inductance, the rapid change in resistance can be utilized to move the current to a parallel branch having a resistor which is able to dissipate the inductive energy. A circuit breaker for DC according to the invention comprises a vacuum container having anode and cathode and means to start a vacuum are between anode and cathode at the beginning of the breaking process said arc extending from a plasma-emitting cathode spot through a current conducting plasma. The circuit breaker is essentially characterized in that the cathode has a non-insulated part on which the cathode spot can be produced and a part surrounded by a screen which at least partly consists of an electrically insulating material and that the circuit breaker is provided with means operable during the breaking process to effect such relative movement between anode and cathode that at the end of the breaking movement the anode is in a position where the screen is situated between the anode and the cathode spot so that the anode is not subjected to the plasma jet direct from the cathode spot.

The construction and function of the invention will be more fully described in the following specification and with reference to the accompanying drawings. In these drawings FIGURES 1-5 show different arrangements of anode and cathode. FIGURES 6 and 7 show how the 3,482,070 Patented Dec. 2, 1969 "Ice anodes in the breaker shown in FIGURE 5 are shaped. FIGURE 8 shows a holding anode, while FIGURES 9 and 10 show a further modification of the invention.

The embodiment of the invention shown in FIGURE 1 has a vacuum container 1 with an anode 2 and a cathode 3. The anode is attached to a rod 4 passing through an opening 5 in the wall of the container. If the walls of the container are of non-insulating material a disc of insulating material 6 is arranged as a bushing for the rod 4 so that the anode is insulated from the container. A bellows 7 is attached to the rod and the container wall so that the vacuum can be maintained. T 0 protect the bellows and insulating disc from vaporized cathode material, a screen 8 is attached to the rod 4. The cathode 3 is made of a material having suitably high vapor pressure. At the sides the cathode is surrounded by a preferably ceramic insulating material 9 so that only the upper surface facing the anode is not insulated. The cathode is attached to a rod 10 which, similarly to the anode, is passed through an opening 11 in an insulating material 12. A bellows 13 seals the Vacuum container outwards. In a certain embodiment of the invention the

rods

4 and 10 may be joined through a resistor 14 as will be described later. The two rods are also connected to operating means, not shown, to effect a longitudinal movement as shown by the

arrows

15, 16 and 17.

The device operates in the following manner. When the current circuit is closed the anode and cathode lie against each other as shown in the figure. At the start of the breaking process the cathode is given a downward movement as the arrow 15 indicates. The anode and cathode are thus separated and a vacuum arc arises. A cathode spot is formed on the cathode which ejects a conducting plasma. Since the cathode surface is smooth the plasma will be above a horizontal plane through the upper surface of the cathode. The plasma jet is strongest perpendicular to this plane and decreases towards the sides, becoming zero along the plane. The next step in the breaking process consists in imparting to the anode a movement to the left according to the arrow 16, whereby it is removed from the most dense part of the plasma jet. At the same time, the distance between anode and cathode increases and both these factors cooperate to produce an increased ignition voltage in the are. When the anode has reached its lefthand limit, the cathode starts to move upwards according to the arrow 17. Thus the anode comes into contact with less dense plasma and the ignition voltage increases. When the anode comes below the above mentioned plane, it is then in a space charge area where the arc is maintained substantially only by the electrons drawn out of the plasma. Thus the ignition voltage increases very rapidly to a high value causing a corresponding decrease of the current in the are. When the anode is drawn under the plane through the cathode spot, there is no neutral gas between the anode and said plane. If a discharge is to be able to exist under these conditions it will be maintained by an electronic current withdrawn from the plasma above the plane down to the anode. The electronic current thus obtained will be space charge limited and follows Childs law that is where d is the distance travelled by the discharge in the space charge. If the arc voltage follows Childs law it will be dependent on the distance d according to the formula U=k.d If the distance travelled by the anode in the negative space charge is great the current may decrease to such a low value that the arc is extinguished and the current circuit is broken. This is for circuits having low inductance.

A long DC line is relatively strongly inductive and the breaking should therefore not take place too rapidly but so slowly that the inductive energy can be dissipated. It may therefore be suitable to join the anode and cathode with an outer resistor 14 which takes over the current when the arc is extinguished. This relatively low remaining current can then be broken with a normal breaker 18.

It is not necessary in closed circuit conditions for the current to flow through anode and cathode as stated here. In another modification of the invention the resistor 14 is short-circuited during operation by a second breaker 19. The anode and cathode are then separated by the cathode being drawn downwards. The current flows through the two closed

breakers

18 and 19. When breaking is to occur, the breaker 19 opens and the voltage across the anode-cathode distance increases so that the arc is ignited and breaking then occurs as described above. The

breakers

18 and 19 may be placed in many different ways and that shown is only one example of the principle.

FIGURE 2 showns a variation of the breaker according to the invention. Here the cathode 3 is rigidly arranged while the anode is turnable and also movable in vertical direction. The anode 2 is attached to a vertical rod 20. On the rod is a pin 21 which runs in a track 22 arranged in a control casing 23 surrounding the rod. This control casing is attached by means of a support 24 to the wall of the container. At the lower end the rod is joined to an operating rod 25 with the help of a ball and socket joint 26 which allows the rod 20 with anode 2 to turn independent of the operating rod 25, which follows movements of the operating rod in vertical direction. The ball and socket joint is bridged by a conductor since the operating rod 25 also acts as current conductor.

The figure shows the breaker in closed position. At the start of the breaking process the operating rod 25, and thus also the rod 20, are pressed upwards. The pin 21 thus also travels upwards in the lefthand vertical groove 27 of the track. When the pin has reached the highest position in the vertical groove, it slips into the upper sloping part 28 of the track thus bringing about a turning of the rod 20 so that the anode swings away from the cathode. When the pin 21 has reached the highest point of the sloping part 28 the upward movement is stopped and replaced by a downward movement whereby the pin follows the track part 29 to its lowest point. The contact surface of the anode is then below the plane running through the plasma-emitting cathode spot and a large part of the are goes through a substantially plasma-free area so that a high are voltage drop and considerably reduced current strength are obtained. As usual with AC breakers for high voltage the breaking path can also be divided into a number of series-connected breaking gaps and FIGURE 3 shows how a breaker of the type shown in FIGURE 2. can be modified.

The circuit breaker according to FIGURE 3 has its breaking path divided into five parts which have been formed by placing two U-shaped

conducting connecting pieces

31 and 32 on the rod 20, each provided with a constituent anode 33 and constituent cathode 34, and two suitably

identical connection pieces

35 and 36 which are attached with the help of insulating

pillars

37 and 38. The last-mentioned connection pieces are also provided with a constituent anode 33 and constituent cathode 34. Between each pair of cooperating constituent anodes and constituent cathodes is inserted a screen 40 of insulating material.

At the start of the breaking process the operating rod 25 is moved downwards thus moving the rod 20 with the electrodes attached to it downwards so that arcs arise between the anode 2 and the upper constituent cathode 34, between the three cooperating constituent anodes 33 and constituent cathodes 34 .and between the cathode 3 and the lowest constituent anode 33. When the pin 21 leaves the left-hand vertical part of the track 22 and enters the sloping part 28, the rod 20 starts to turn and this turning movement continues until the pin has come to the lowest point in the track. The direction of movement of the rod 25 is then reversed and the rod 20 is pushed upwards whereby the anodes will be behind the plane forming the limit for the area where the plasma is. The screens 40 prevent the plasma which is emitted at a breaking point from reaching the electrodes in an adjacent breaking point.

FIGURE 4 shows how the breaker in FIGURE 1 can be modified so that the breaking path is divided into five parts. The rod 10 with cathode 3 is extended upwards with two insulating pillars 47 and two connection pieces 41 and 42, each provided with a constituent anode 43 and constituent cathode 44. Similarly on the rod 4 supporting the anode 2 are arranged two connection-

pieces

45 and 46 separated by an insulating piece 47 and a similar piece between the rod 4 and the connection piece 46.

At the start of the breaking process the rod 10 is drawn downwards, thus igniting the part-arcs. The rod 4 is then moved to the left and finally the rod 10 is pushed upwards, that is the same movement as that described in connection with FIGURE 1. With the help of a magnetic flux indicated by the arrow B a further ignition voltage can be obtained.

FIGURE 5 shows a further variation of the invention. Besides the ignition anode 49 the circuit breaker has a number of

auxiliary anodes

50, 51, 52, 53, 54 and 55 which are attached to the wall of the vacuum container and insulated from each other. The ignition anode and the auxiliary anodes are joined to each other by means of a number of resistors 56, 57, '58, 59, 60 and 61, as is seen from the figure. The auxiliary anodes are surrounded by screens 62 of insulating material, as seen in FIGURES 6 and 7, to prevent arcs arising between them since during the breaking process they will have different potentials. The container wall 1 is suitably cylindrical so that the auxiliary anodes are at the same radial distance from the centre line of the container along which the cathodes move during the breaking process. Further, the auxiliary anodes are suitably arranged helically on the wall.

The breaking process starts by the cathode 3 being lowered and a vacuum arc formed between ignition anode and cathode. The ignition anode then moves to the left and the cathode begins its upward movement. When the cathode has moved so far up that a plane through its upper surface, where the plasma-emitting cathode spot is, lies above the free surface of the ignition anode, the arc voltage increases rapidly. The auxiliary anodes are then situated in the plasma area and the voltage drop between the cathode and each of the auxiliary anodes is of the order of magnitude 20-3O volts. As soon as the resistance in the are between ignition anode and cathode has almost reached the resultant resistance in the are between the cathode and auxiliary anode 50 and in the resistor 56, a current begins to flow in that path and the current in the first are further decreases. Thus the resistor 56 is automatically connected in the DC circuit and a corresponding decrease in direct current is obtained. When the cathode has passed the auxiliary anode 50 the voltage drop in the arc increases and a successive removal of the arc to the auxiliary anode 51 takes place so that the resistor 57 is connected in series with the resistor 56 and causes a further decrease in the current. The process continues until the arc burns between the cathode and auxiliary anode 55 and all six resistors are then connected. The number of auxiliary anodes and the size of the resistors is determined by the network voltage, network current and the desired breaking process.

FIGURE 6 shows that th resistors are placed outside the container 1, but it is also possible to attach the auxiliary anodes to a resistor string 63 helically arranged inside the container as shown in FIGURE 7. It is then necessary to place the protective screen 64 in front of the resistor string to prevent vaporized cathode material from settling on it and causing alterations in the resistance value. In the screens are holes for the anode holders 65 and these holes are covered by screens 66.

As mentioned earlier, part of the purpose of the resistor 14 in FIGURE 1 is to prevent the current in the DC circuit from being uncontrolledly broken when the arc voltage increases and the cathode spot from becoming unstable and dying. FIGURE 8 shows another possibility for preventing such an uncontrolled current breaking. It is based on the principle of generating, with the help of an excitation circuit an auxiliary are between the control anode 80 and cathode 3. The excitation circuit is fed from a current source 81 over a resistor 82 which is preferably adjustable. The auxiliary are 83 maintains a plasma-emitting cathode spot even if the voltage drop in the main discharge should increase to such a high value that the current were broken. The cathode and control anode must be enclosed in insulation 84. The excitation circuit can of course be varied in many Ways and the figure is only one example of how the idea can be realized.

The breaker may also have additional means to generate a magnetic field running substantially perpendicular to the arc or part-arcs. Since the arcs in the embodiments shown run substantially horizontally it is suitable if the magnetic field is vertical.

During tests it has been found that the arc voltage U is dependent on a transverse magnetic field so that U =k.B where k is a proportion constant and B is the induction. In a DC breaker which utilizes this dependence of the arc voltage on the magnetic field, a vacuum arc would first be ignited through contact separation. When the magnetic field then increases with some suitable time constant, U will increase in proportion to B The direction of the magnetic field is marked in the figures with arrows designated Another advantage with the magnetic field directed as indicated is that, besides increasing the arc voltage, it also increases the life of the cathode spot under low currents. In this way the risk of current interruptions is considerably decreased, since another possibility is provided for controlling the discharge and thus the breaking time.

FIGURE 9 shows a somewhat differently shaped variation of the breaker according to the invention. The cathode 3 is formed as a piston which is movable within the insulation 9 forming a cylinder. The rod is surrounded by a pressure spring 90 which presses the cathode towards a seat 91 at the upper edge of the insulation. The cathode is connected to a terminal 92 cast in the wall of the vacuum container, by means of a flexible conductor 93. The anode 2 is are shaped and attached to a rotatably journalled shaft 94 on which a cogwheel 95 is also arranged. The cogwheel cooperates with a number of cogs 96 arranged on an operating rod 97 movable in horizontal direction, so that the shaft with the anode 2 receives a rotary movement when the rod is moved horizontally. A spiral spring 98 is connected to an attachment 99 and to the shaft 94 so that the spring is tensioned when the rod is moved to the right and the anode moves towards the cathode. In the closed position of the breaker the rod 97 is blocked by means of a blocking device, not shown, which is released when the breaker is to start. The spring 98 then effects a rapid movement of the anode away from the cathode to a position where the anode is not subjected to direct plasma-flow from the cathode spot. The anode is connected by means of a flexible conductor 100 to a terminal 101 cast in the vacuum container. A bellows 7 seals the entrance of the rod 97 into the wall 1.

The arrangement shown in FIGURE 9 is mainly intended to break direct current having a definite polarity, but it is, however, possible for it to operate with inverted current direction. If it is desirable for the arrangement to be completely independent of the polarity of the current, however, it may be suitable to modify the movable part as shown in FIGURE 10. The shaft 94 is provided with two similar arms 2 situated diametrically opposite each other and cooperating each with its own electrode 3, the electrodes being made in the same way as the earlier shown cathode so that either of them can act as cathode. The two arc-shaped arms 2 are displaced in axial direction along the shaft 94 so that they are not hampered in their movement by the electrode situated on the other side of the axis.

We claim:

1. A circuit breaker comprising a vacuum container having electrodes including an anode and a cathode and means to start a vacuum are between anode and cathode at the beginning of the breaking process, said are including a plasma-emitting cathode spot and a current conducting plasma, said cathode having a non-insulated first part forming a cathode spot surface on which the cathode spot can be produced and a second part surrounded by a screen, which screen at least partly consists of an electrically insulating material, said second part of the cathode lying below said cathode spot surface, said anode having a contact surface which, when the circuit breaker is in closed position, lies above and in contact with said cathode spot surface, said circuit breaker being provided with means operable during the breaking process to effect such relative movements between anode and cathode that at the end of the breaking movement the anode is in a position remote from the cathode spot surface where said cathode screen is situated between the cathode spot surface and the anode.

2. A circuit breaker according to claim 1 in which the vacuum container is cylindrical about a center line and the cathode is arranged to move along the center line of the cylindrical container, and a number of auxiliary anodes being arranged along a helical line on the cylindrical wall of the container.

3. A circuit breaker according to claim 2 in which the auxiliary anodes are connected to each other by resistors arranged outside the vacuum container.

4. A circuit breaker according to claim 2 in which the auxiliary anodes are connected to a helically arranged resistor string within the vacuum container.

5. Circuit breaker according to cairn 2 in which the auxiliary anodes are provided with screens of insulating material to prevent arcs arising between the auxiliary anodes.

6. A circuit breaker as claimed in claim 1 in which said movement efiecting means comprises means for first moving one of the electrodes away from the other in a first direction substantially perpendicular to the plane of the cathode spot surface, secondly moving one of the electrodes in a second direction substantially perpendicular to said first direction, and thirdly moving one of the electrodes in a third direction opposite to said first direction.

7. A circuit breaker as claimed in claim 6, in which said movement in the second direction is a lineal movement.

8. A circuit breaker as claimed in claim 6, in which the movements in the first and third directions are lineal movements.

9. A circuit breaker as claimed in claim 6 in which the movement in the second direction is a swinging movement.

(References on following page) References Cited UNITED STATES PATENTS Millikan et a1. 200144.2 Milliken.

Lee 200144.2

Kesselring 200151 Porter 200144.2

8 FOREIGN PATENTS 1,282,733 12/1961 France. 1,133,785 7/1962 Germany.

ROBERT K. SCHAEFER, Primary Examiner R. A. VANDERHYE, Assistant Examiner US. Cl. X.R. 20061.19, 144