Abstract:
A circuit including arcing contacts is connected in parallel with main contacts of a disconnector to facilitate interruption of a loop current caused through the disconnector when switching main bus bars in a power station. A switching surge suppression resistor is adapted to be inserted in and removed from the circuit of the arcing contacts. Plural pairs of arcing contacts are provided in the circuit of arcing contacts.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a gas insulated connector capable of interrupting a loop current. 
     Disconnectors in power station premises are usually installed adjacent to circuit breakers. They are used typically for isolating a circuit after interruption of current therein by a circuit breaker or for switching power transmission systems. The former function is the isolation of a no-load circuit by the disconnector. At the time of the isolation of the no-load circuit by the disconnector, re-arcing repeatedly takes place between the disconnecting switch contacts giving rise to a commonly termed switching surge having high sharpness. This occurs because of the fact that the switching speed of the disconnector is slow compared to the circuit breaker. It has been the practice to add a resistor to the disconnector in order to suppress such a switching surge. 
     An example of the latter function of the disconnector is the switching of main bus bars in a power station by disconnectors. FIG. 1 shows the case of switching the connection of bus bars A and B in (a) to that in (b) by making use of a satisfactory interrupting capacity of SF 6  gas. In this operation, a current which is close to the rated current in the circuit including the disconnectors A1 and B1 (which is referred to as loop current) is interrupted. 
     The function of isolating a no-load circuit and the function of switching a loop current, as noted above, are often required for one disconnector in a power station. 
     A prior art disconnector provided with a resistor for suppressing the switching surge as described above is shown in FIG. 2. The disconnector comprises a tank 1, which is filled with gas capable of extinguishing arc. The tank 1 is sealed by insulating spacers 2 and 3. Conductors 4 and 5 are secured to the respective insulating spacers 2 and 3. A shield member 6 made of a conductor is secured to the conductor 4. The shield member 6a has an opening. A main contact 7 is disposed as a fixed contact in the shield member 6 such that it faces the opening 6a thereof. The main contact 7 is secured to and electrically connected to the shield member 6. A resistor 8 is secured at one end to the shield body 6. The resistor 8 includes an insulating rod 8a and a resisting element 8b as shown in FIG. 6 and as will be described later in detail. A cylindrical spring case 9 is secured to the other end of the resistor 8. A conductive support rod 10 is slidably supported in the spring case 9 and extends in the fixed contact 7. The support rod 10 is provided at one end with a fixed arcing contact 10a which is capable of withstanding arcing. It can be moved up to a position, at which the end of the arcing contact 10a projects from the shield member 6. The arcing contact 10a is connected to the other end of the resistor 8 through the spring case 9. A spring 11 biases the support rod 10 in such a direction that it projects from the shield member 6. Another shield member 12 made of a conductor is secured to the conductor 5. The shield member 12 has an opening 12a. A movable main contact 13 is movably supported in the shield member 12. The main contact 13 is provided at one end with a movable arcing contact 13a which is capable of withstanding arcing. As the main contact 13 is moved to the right from the state of FIG. 2, it is broken apart from the fixed main contact 7 before the arcing contacts 10a and 13a are broken apart. A contact 14 which can be in sliding contact with the movable main contact 13 is connected to the shield member 12. An insulating operating rod 15 is coupled to the main contact 13. A link mechanism 16 transmits a driving force of a driving source (not shown) to the main contact 13 via the insulating operationg rod 15. 
     In operation, with the rightward movement of the insulating operating rod 15 in FIG. 2, the movable main contact 13 is broken apart from the fixed main contact 7. The arcing contact 10a is caused to follow the movable arcing contact 13a by the action of the spring 11. The arcing contact 10a can be moved up to a position, at which the electric field intensity at the end of the arcing contact 10a is higher than the electric field intensity at all parts of the shield member 6, i.e., at a position at which the end of the arcing contact 10a projects from the shield member 6. The main contact 13 can be moved beyond this position so that arcing is eventually produced between the arcing contacts 10a and 13a. The arcing contact 10a is stopped at a position, at which the electric field intensity at its end is higher than the electric field intensity at all parts of the shield member 6. Thus, a number of arc discharges (re-arcing) all occur between the arcing contacts 10a and 13a during the switching operation. In consequence, an abnormal voltage (i.e., switching surge) generated at the time of isolating the no-load circuit can be suppressed. 
     FIGS. 3 and 4 are equivalent circuit diagrams showing a succession of states that occur when a loop current is interrupted using a disconnector provided with the surge suppression resistor shown in FIG. 2. In the state of FIG. 3, the disconnector is perfectly closed, and in the stage of FIG. 4 only the main contacts have been broken apart. In FIGS. 3 and 4, the same reference numerals and symbols as in FIG. 2 designate corresponding parts. Indicated at Zm is the impedance of the main current path of the disconnector, indicated at Zl is the impedance of the loop established at the time of the switching of the system, and indicated at R is the resistance of the surge suppression resistor. These impedance values are generally related as 
     
         Zm&lt;&lt;Zl&lt;&lt;R                                                  (1) 
    
     The equivalent circuit of FIG. 4 shows the state that results immediately after the start of operation of the disconnector with the surge suppression resistor shown in FIG. 2 to interrupt a loop current. At this time, R and Zm are related as Zm&lt;&lt;R from the inequality 1. If the resistance R is set to be considerably greater than the resistance offered to the arc produced between the main contacts 7 and 13, the loop current scarely flows through the path including the resistor 8, fixed arcing contact 10a and movable arcing contact 13a, that is, it substantially flows through the path including the main contacts 7 and 13. That is, most of the loop current i, for instance about 10 killoamperes, must be interrupted between the main contacts 7 and 13. 
     In a disconnector capable of interrupting a large loop current, the arcing contacts 10a and 13a usually are capable of withstanding arcing so that an arc due to the loop current can be interrupted between the arcing contacts 10a and 13a for ensuring a current-carrying performance after the interruption of the current. In other words, the main contacts 7 and 13 usually are incapable of withstanding arcing. 
     If the resistance R of the resistor 8 is set to be comparatively low with respect to the resistance offered to the arcing generated between the main contacts 7 and 13 when these contacts 7 and 13 are broken apart, a large current is caused to flow through the path including the resistor 8 and arcing contacts 10a and 13a with arcing generated between the main contacts 7 and 13 when the contacts 7 and 13 are broken apart. A voltage drop across the resistor 8 is thus applied between the main contacts 7 and 13. In this case, the interruption of the loop current is extremely difficult compared to the case of absence of the resistor 8, i.e., the case where the voltage drop across the resistor 8 is zero. If the resistance R of the resistor 8 is set to be very low, even the function of surge suppression substantially cannot be obtained. 
     For the above reasons, it has been thought difficult to add a function of interrupting loop current to the conventional disconnector with a surge suppression resistor. 
     SUMMARY OF THE INVENTION 
     The invention has been intended in the light of the foregoing, and its object is to provide a disconnector, which can interrupt loop current with bypass contacts provided to shunt a surge suppression resistor when interrupting the loop current. 
     Another object of the invention is to facilitate the interruption of loop current and prevent wear of the main contacts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a and 1b show a bus bar switching circuit in different states; 
     FIG. 2 is a longitudinal sectional view showing a prior art disconnector; 
     FIG. 3 is an equivalent circuit diagram showing the disconnector of FIG. 2 used for interrupting a loop current; 
     FIG. 4 is an equivalent circuit diagram showing the disconnector of FIG. 2 with main contacts broken apart; 
     FIG. 5 is an equivalent circuit diagram showing an embodiment of the disconnector according to the invention; 
     FIG. 6 is a longitudinal sectional view showing the construction of the embodiment of the disconnector shown in FIG. 5; 
     FIG. 7 is a fragmentary enlarged-scale longitudinal sectional view showing part of the embodiment of FIG. 6; 
     FIG. 8 is an equivalent circuit diagram showing the embodiment of FIG. 6 with main contacts broken apart; 
     FIG. 9 is an equivalent circuit diagram showing the embodiment of FIG. 6 with arcing contacts broken apart after the state of FIG. 8; 
     FIG. 10 is an equivalent circuit diagram showing the embodiment of FIG. 6 with bypass contacts broken apart after the state of FIG. 9; 
     FIG. 11 is a view similar to FIG. 7 but with main contacts broken apart; 
     FIG. 12 is a view similar to FIG. 7 but with arcing contacts broken apart after the state of FIG. 11; 
     FIG. 13 is a view similar to FIG. 7 but with bypass contacts broken apart after the state of FIG. 12; and 
     FIG. 14 is a view similar to FIG. 7 but showing a state of the disconnector when the isolating operation is completed. 
     In the Figures, like parts are designated by like reference symbols. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 5 is an equivalent circuit diagram of the disconnector according to the invention, FIG. 6 is a longitudinal sectional view showing the construction of the disconnector of FIG. 5, and FIG. 7 is a fragmentary enlarged-scale sectional view showing part of the construction of FIG. 7. Referring to thse Figures, a pair of bypass contacts 10b and 20 are provided in series with the resistor 8. The bypass contact 20 is a fixed bypass contact. The bypass contact 10b is a movable bypass contact 10b which can be brought into contact with and broken apart from the fixed bypass contact 13. The movable bypass contact 10b is integral with the arcing contact 10a via the support rod 10. The bypass contacts 10b and 20 are adapted to be broken apart after the arcing contacts 10a and 13a have been broken apart and be brought into contact again with each other after the arcing contacts 10a and 13a have been brought into contact. Designated at 22 and 23 are nozzles for controlling the operating speed of the arcing contact 10a. As a piston 10c which is moved by a biasing force of the spring 11 compresses gas in the spring case 9, the gas is discharged through the nozzles 22 and 23, thereby changing the speed of the arcing contact 10a. In FIG. 5, Zm is the impedance of the main current path of the disconnector, Zb is the impedance of the circuit including the arcing contact 10a, support rod 10 and bypass contact 10b, Zl is the impedance of the loop established at the time of the switching of the system, and indicated at R is the resistance of the resistor 8. 
     Generally, Zm, Zb, Z and R are related as 
     
         Zb≃Zm&lt;&lt;Z1&lt;&lt;R                                 (2) 
    
     The function and operation of the disconnector according to the invention will now be described. 
     FIGS. 8 to 11 show the state of the disconnector immediately after the start of the isolating operation when interrupting the loop current. As the main contact 13 is moved, the main contacts 7 and 13 are broken apart. At this time, the arcing contacts 10a and 13a and bypass contacts 10b and 20 are in their closed state so that the resistor 8 is shunted by the bypass contacts 10b and 20. 
     In this stage, Zm and Zb are related as Zb≃Zm from the inequality 2. Thus, when the main contacts 7 and 13 are broken apart, the loop current i turns to flow entirely through the path including the bypass contacts 10b and 20 and arcing contacts 10a and 13a. For this reason, no substantial damage is caused to the main contacts 7 and 13 due to the arcing. 
     With further movement of the main contact 13, a state as shown in FIGS. 9 and 12 results. In this state, the piston 10c which is integral with the arcing contact 10a has cleared the nozzle 22 formed in the spring case 9. Thus, as soon as this state is reached, the speed of the arcing contact 10a is suddenly reduced. In consequence, the arcing contact 10a commences to be broken apart from the movable arcing contact 13a, producing an arc 24 between both the arcing contacts 10a and 13a. 
     The arc produced when interrupting the loop current as mentioned may be extinguished by making use of a flow of gas produced in a flow guide 28 by a puffer cylinder 28 and a piston 27 as shown in FIG. 6. 
     With further movement of the main contact 13, a state as shown in FIGS. 10 and 13 results. More particularly, after the arc produced between the arcing contacts 10a and 13a when interrupting the loop current has been extinguished, the movable bypass contact 10b integral with the arcing contact 10a commences to be broken apart from the fixed bypass contact 20. At this time, the main contacts 7 and 13 and arcing contacts 10a and 13a have already been broken apart, that is, it is not that a circuit carrying current is broken when the bypass contacts 10a and 20 are broken apart. 
     When isolating a non-load bus bar, it is necessary to suppress the switching surge by inserting a resistor in series with the circuit including the disconnector as noted before. The switching surge is generally proportional to the potential difference between two contacts of the disconnector at the time of the re-arcing. The intercontact potential difference is proportional to the intercontact distance at the time of the discharge. Therefore, the high surge which is so high that it must be suppressed by inserting a resistor, occurs in case when an arc is produced with the main contact 13 at a certain distance from the arcing contact 10 (the distance being specifically 1/4 of 1/3 of the full inter-contact distance although it depends on the apparatus). When the main contact 13 is in a state in the initial stage of operation as shown in FIGS. 11 and 12, the bypass contacts 10b and 20 are not broken apart yet, so that the surge suppression resistor 8 is in the shunted state. 
     FIG. 13 shows the state in which the main contact 13 is broken apart from the arcing contact 10a. In this state, the resistor 8 is connected in series with the arcing contacts 10a and 13a since the bypass contacts 10b and 20 are broken apart. If re-arcing 25 is produced between the contacts at this time, the resultant switching surge can be effectively disposed with by the resistor 8. 
     FIG. 14 shows the disconnector when the isolating operation is completed. When the re-closure of the no-load bus bar by the disconnector is started from the state of FIG. 14, re-arcing is produced before the arcing contact 13a is brought into contact with the fixed arcing contact 10a. At this time, the surge suppression resistor provides effective action for all re-arcing. 
     As has been described in the foregoing, the disconnector having bypass contacts according to the invention can interrupt a loop current in one case and suppress the switching surge generated at the time of the switching of the no-load bus bar with the resistor in the other case. 
     The bypass contacts serve to insert the surge suppression resistor in the circuit of the arcing contacts or shunt the resistor. This means that with a disconnector which has the purpose of interrupting loop current and is not provided with any resistor, the bypass contacts may be used as the arcing contacts. That is, by providing two pairs of arcing contacts and causing these two pairs of contacts to be broken apart substantially simultaneously, it is possible to ensure reliable interruption of the loop current and prevent wear of the main contacts.