Vacuum Switch Assembly

Abstract

A polyphase vacuum switch assembly having a plurality of vacuum switching valves, a star-connected surge-absorbing unit connected in parallel with a load circuit, and a star-connected unbalance-detecting unit connected substantially in parallel with the surge-absorbing unit. The surge-absorbing unit repeatedly absorbs frequently occuring switching surge voltages while effecting emergency backup absorption of lightning surges with follow-current interruption by melting fuses incorporated therein. The unbalance-detecting unit detects the melting of the fuse and other unbalance of the load circuit.

Description

This invention relates to a vacuum switch assembly, and more particularly to a polyphase vacuum switch assembly for high-voltage circuits, which ensures reliable switching operation while protecting its loads against any dangerous surge voltages and against deterioration of vacuum in the switching assembly itself. The vacuum switch assembly of the invention protects its loads against surge voltages caused by current chopping, as well as surge voltages generated by reclosures, discharge streamers, and uneven closing of polyphase circuits. The vacuum switch assembly also detects any deterioration in the vacuum of switching valves thereof, so as to provide indicating or alarm signals to operators or control means in response to such detection.

In a known vacuum switch, an electrode material consisting, for instance of Cu-W alloy or Cu-Bi-Sb alloy, is provided on the surface of the switching electrodes for effecting easy vaporization to prevent an occurrence of surge voltage caused by chopping effect, when the switch is opened at a comparatively low load current. Such known vacuum switch has several disadvantages. Firstly, the electrode material for preventing the occurrence of chopping tends to deteriorate in a comparatively short period so that the life of the switch is limited. Secondly, in case if a switch having a large capacity is wanted, such electrode material, which tends to fuse, limits the switching capacity. Thirdly such fusible alloy may melt together under a heavy current so that a switching accident may happen. Moreover, such known-type switch is costly and very delicate in the design and handling.

The need of the delicate and costly surge-absorbing means has been one of the reasons for limiting the use of vacuum switches for certain applications.

In order to mitigate such difficulties of known surge-absorbing means for vacuum switches, the inventors noticed the fact that the magnitude of the discharge currents, or the peak value thereof, caused by the switching surge voltages is at most several amperes, which can be repeatedly interrupted by conventional discharge gaps of simple construction, without causing any detrimental effects to the gap per se. If the protection against lightning surges is left to separately installed conventional lightning arresters, which are specifically designed for absorbing very rare lightning surges, then surge absorbing means for vacuum switch can greatly be simplified.

According to the present invention, there is provided a vacuum switch assembly equipped with a simple surge-absorbing unit consisting of discharge gaps, resistors in series with the discharge gaps, and fuses. The surge-absorbing unit is so simplified that it can easily be united with the vacuum switch assembly in a very compact form for providing an economical and efficient vacuum switch assembly.

Conventional vacuum switches have another shortcoming in that if the degree of vacuum in the vacuum switching valve should be deteriorated, due to mishandling during the transportation or any other reasons, it is very difficult to detect such vacuum deterioration. More particularly, an insufficient degree of vacuum has been detected only by complicated processes, such as careful inspection of getter metal of the vacuum valve or insulation withstand tests. As a result, the deterioration of the degree of vacuum has not been detected in the early stage of such trouble, and the vacuum switches are often operated with an insufficient degree of vacuum.

Therefore, another object of the present invention is to provide an efficient unit for quickly detecting such insufficient degree of vacuum in the switching valve of vacuum switches, which unit is very simple in construction.

The detecting unit of the invention is also effective in finding any lack of normal voltage due to faulty conditions, such as a molten fuse or line breakage. The inventors found that the desired detecting unit can be constructed by using simple capacitors, which are connected in such a manner that it will quickly detect any shift of neutral point.

It is one of the important features of the present invention that the incorporation of the aforesaid surge-absorbing unit and the detecting unit in one vacuum switch assembly results in synergistic effects. When a load current to a highly reactive circuit is interrupted by a conventional vacuum switch, a surge voltage with a very steep wave front is apt to be generated. With the vacuum switching assembly of the invention inclusive of the resistors of the surge absorbing unit and the capacitors of the detecting unit, the steepness of the voltage oscillation in response to the interruption of the load current to a highly inductive load can be lowered to a certain extent.

For a better understanding of the present invention, reference is made to the accompanying drawings, in which:

FIG. 1 is a schematic diagram, showing a surge absorber, usable in the present invention;

FIG. 2 is a schematic diagram illustrating an embodiment of the present invention;

FIG. 3 is a diagrammatic illustration of a modified surge absorber, which can be used in a vacuum switch assembly of the present invention;

FIGS. 4 and 5 are oscillographic diagrams, showing the relation between the interruption of load currents and the generation of quick voltage oscillation immediately after the interruption; and

FIG. 6 is an oscillographic diagram illustrating the manner in which an insufficient degree of vacuum in a switching valve is detected.

Like parts are designated by like numerals throughout the drawings.

Referring to FIG. 1, a vacuum switch assembly of the invention, which is generally designated by a reference numeral 20, is inserted between a power source 1, such as a generator or a transformer, and a load 3. The vacuum switch assembly 20 comprises a switching valve unit 2, which selectively completes and interrupts the circuit from the power source 1 to the load 3, and discharge gaps 4 connected in parallel with the load 3, through fuses 6 and resistors 5, respectively. The illustrated embodiment is adapted for the three-phase application, but the present invention is not restricted to three-phase alone, but the vacuum switch assembly of the invention can be suitably modified for any balanced polyphase system. Each of the discharge gaps 4 is so adjusted as to flash over at a preselected voltage higher than the operation voltage of the circuit including the power source 1 and the load 3, but lower than the insulating strength of the circuit. The resistance value of the resistors 5, in a preferred embodiment, increases when the discharge currents flow therethrough, so as to ensure the interruption of the follow currents through the gaps.

With the vacuum switch assembly of the aforesaid construction, when the magnitude of a surge voltage, which is generated in response to opening of the switching unit 2, increases in excess of the aforesaid preselected flashover voltage, one or more gaps 4 flashover depending on whether such high switching surge voltage is generated in one or more phases. Thus, the voltage of the circuit is kept below the flashover voltage. The energy of the switching surge is mostly dissipated as heat in the resistors. The magnitude of the flashover current is comparatively low, i.e., a few amperes at most. The maximum flashover current through the discharge gaps 4 is restricted within a certain limit by the current-chopping characteristics of the switching unit 2. When such flashover current flows through the circuit with a certain surge impedance, there will be generated a surge voltage across the surge impedance. Thus, the magnitude of the surge voltage also depends on the chopping characteristics of the switching unit 2.

According to the present invention, the resistors 5 are so designed as to quickly absorb the discharge energy or surge energy, whenever the discharge gaps 4 flashover. Whereby, the interruption of the follow current through the discharge gaps is ensured. As a result, the discharge current does not give any adverse effects to the discharge gaps, so that the discharge gaps can withstand a number of repeated flashovers, without any substantial melting or deterioration.

For lightning surges, which only very rarely come from the atmosphere, the conventional lightning arresters (not shown) are expected to operate without waiting for the flashover of the discharge gaps 4 of the vacuum switching assembly 20 of the invention. If, however, the conventional lightning arresters should fail to properly operate, and if the surge voltage in the system surpasses the predetermined flashover voltage of the discharge gaps 4, the discharge gaps 4 may flashover and a follow current corresponding to the short circuit current of power source system will flow through the discharge gaps 4. In this case, all of the fuses 6 which carry such follow current melt away to interrupt the short circuit current.

Although it is not essential in the present invention, the resistors 5 may be provided with the aforesaid nonlinear resistance characteristics, so that the resistance value of the resistors 5 increases in response to such surge current, for facilitating the interruption of the follow current therethrough.

In the embodiment of FIG. 1, the fuses 6, resistors 5, and the discharge gaps 4 are connected in series in the aforesaid order, starting from the load side. However, the arrangement of such elements of the vacuum switch assembly 20 is not restricted to that of FIG. 1. For instance, such elements can be arranged in the order of discharge gaps 4, fuses 6, and resistors 5, as illustrated in FIG. 3.

FIG. 2 illustrates another embodiment of the present invention. In this embodiment, a surge-absorbing unit consisting of fuses 6, resistors 5, and discharge gaps 4 is connected to a circuit between a switching unit 2 and a load 3, in parallel with the load 3. In addition to such switching unit 2 and the surge-absorbing unit, a vacuum switch assembly 20 of this embodiment includes an unbalanced-detecting unit. The unbalance-detecting unit includes three high-impedance elements, e.g., capacitors 7, which are connected in star, so that the neutral point of the capacitors 7 thus connected is grounded through a rectifier 8 and a DC capacitor 9. The opposite ends of the star-connected capacitors 7 are connected to the joints between corresponding resistors 5 and the discharge gaps 4, respectively.

With the unbalance-detecting unit of such construction, as long as the load circuit is balanced, no potential will appear at the neutral point of the star-connected capacitors 7, relative to the ground potential. Thus, the DC capacitor 9 is not charged. On the other hand, there will be generated a considerable shift of the neutral point of the load circuit under any one of the following conditions.

1. One of the fuses 6 is melted away, for instance by follow currents subsequent to the flashover of the corresponding discharge gap 4.

2. Any one phase of the polyphase load 3 is interrupted by some reasons or other, while leaving other phases as closed.

3. The degree of vacuum in any one or two of the switching valves in the switching unit 2 is deteriorated to a level lower than that of the remaining switching valves of the unit 2.

In response to such shift of the neutral point, the potential at the neutral point of the star-connected capacitors 7 becomes to have a finite value relative to the ground potential. As a result, the DC capacitor 9 is charged by such finite neutral voltage through the rectifier 8. When the voltage across the DC capacitor 9 increases to a certain level, a discharge element 10 is actuated, so as to operate an indicator 11 or the like.

The voltage applied to each capacitor 7 is essentially a line to ground voltage, and if this voltage should exceeds a certain predetermined level, the discharge gap 4 flashes over. Accordingly, there is no need for extra insulation for the capacitors 7 of the unbalance-detecting unit.

The operation of the surge absorbing unit of this embodiment is identical with that of the preceding embodiment, as described in detail hereinbefore, referring to FIG. 1. The capacitors 7 of the embodiment of FIG. 2 act to reduce the steepness of surge voltage generated in response to the chopping of the load current. Thus, the embodiment of FIG. 2 is not a mere combination of the surge-absorbing unit of FIG. 1 and an unbalance-detecting unit, but it has synergistic effects of reducing the steepness of surge voltages in the circuit.

In order to demonstrate the effects of the vacuum switch assembly of the invention, tests were made by interrupting a load current of 5.25 a. in a 3 kv. three-phase circuit from an inductive load of 2 H with an iron core, by using a vacuum switch assembly of the invention, as shown in FIG. 2. The results are given in FIG. 5 in the form of oscillograms. For the sake of comparison, a load current of 5.25 a. was interrupted from an inductive load of 2 H with an iron core in a 3 kv. circuit by using a conventional vacuum switch having neither surge-absorbing unit nor unbalance-detecting unit. The results are shown in FIG. 4 in the form of oscillograms.

FIG. 6 shows the result of a test of the vacuum switch assembly of the invention for detecting the deterioration in the degree of vacuum in a switching valve, the T-phase valve in this case.

It is apparent from the comparison of the aforesaid test results that the surge voltages generated by current chopping, as shown in FIG. 4, are completely eliminated by the use of the vacuum switch assembly of the invention, as shown in FIG. 5. FIG. 6 shows that the insufficient degree of vacuum in the T-phase switching valve was clearly detected, and a backup circuit breaker was actuated in 3.22 seconds in the illustrated test.

As described in the foregoing disclosure, with the vacuum switch assembly of the invention, surge voltage of frequently occurring magnitude can be absorbed by a surge absorbing unit including discharge gaps, resistors, and fuses, while rerely occurring extra high surges, e.g., lightning surges, are left to conventional lightning arresters, but failure of the operation of such lightning arresters can be backed up by the flashover of the discharge gaps with follow currents being interrupted by the melting of the fuse. Furthermore, with the vacuum switch assembly of the invention, the melting of the fuse, or any other unbalance in the voltage of the load circuit, can be detected by an unbalance-detecting unit consisting of star-connected detecting impedance elements, e.g., capacitors, whose neutral point is grounded through a rectifier and a DC capacitor. The DC capacitor is charged in response to the occurrence of an unbalance of the load circuit, and a voltage across the DC capacitor is monitored for the detection of the unbalance, by a suitable monitoring means, such as a discharge valve and a relay.

In practice it is preferred to compose the resistors 5 by resistive material having high temperature coefficient so that to limit the discharge current as far as possible. On other words the unit of this circuit can have an effect of interrupting the follow current.

Thus the invention as substantially defined in the following claims contributes greatly to the industry.

Claims

What is claimed is:

1. A polyphase vacuum switch assembly, comprising vacuum switching valves which are in series with the load and the same in number with the phases of a loadcircuit, a star-connected unbalance-detecting unit connected to the load circuit in parallel therewith, a rectifier connected to neutral point of the unbalance-detecting circuit, a DC capacitor connected between the rectifier and the ground so as to be charged by the neutral voltage through the rectifier, and a DC voltage monitor detecting a voltage across the DC impedance element.

2. A polyphase vacuum switch assembly according to claim 1, wherein said polyphase is three-phase.

3. A polyphase vacuum switch assembly according to claim 1, wherein said high impedance of each arm of said star-connected unbalance-detecting unit is a capacitor.

4. A polyphase vacuum switch assembly, comprising vacuum switching valves which are in series with the load and the same in number with the phases of a load circuit;

a star-connected surge-absorbing unit connected to the load circuit in parallel therewith, each arm of the star-connected surge-absorbing unit consisting of a series circuit including a discharge gap, a resistor, and a fuse; and

a star-connected unbalance-detecting unit connected to the surge-absorbing unit in parallel therewith, a rectifier connected to neutral point of the unbalance-detecting circuit, a DC capacitor connected between the rectifier and the ground so as to be charged by the neutral voltage through the rectifier, and a DC voltage monitor detecting a voltage across the DC capacitor in excess of a certain level, each arm of said star-connected unbalance-detecting unit comprises a high capacitive impedance element whose free end is connected to a point on a corresponding arm of the surge-absorbing unit, the high capacitive impedance elements of the unbalance-detecting unit simultaneously functioning as absorbing elements in the surge-absorbing unit.

5. In a polyphase vacuum switch assembly, which includes vacuum switching valves which are in series with the load and the same in number as the phases of a load circuit;

a star-connected surge-absorbing unit connected to the load circuit in parallel therewith, each arm of the star-connected surge-absorbing unit consisting of a series circuit including a discharge gap, a resistor, and a fuse successively connected from the neutral point of the star connection; and

a star-connected unbalance-detecting unit each arm of which is connected respectively to a corresponding arm of the surge-absorbing unit at a point between the resistor and air gap on each arm, a rectifier connected to the neutral point of the unbalance-detecting circuit, a DC capacitor connected between the rectifier and the ground so as to be charged by the neutral voltage through the rectifier, and a DC voltage monitor for detecting a voltage across the DC capacitor in excess of a predetermined level in a recoverable manner, each arm of said star-connected unbalance-detecting unit comprises a high-impedance capacitor element whose free end is connected respectively to a point between the resistor and the air gap of the corresponding arm of the surge-absorbing unit, whereby the high impedance capacitor element in the unbalance-detecting unit functions as an absorbing element in the surge-absorbing unit.