1.  Field of the Invention
This invention relates to a secondary arc extinction device for an electrical power system whereby the arcing current (hereinbelow termed secondary arcing) that flows through the distributed electrostatic capacity which exists between the uninterrupted phase or uninterrupted circuit and the breakdown phase when there is accidental grounding of a high-voltage transmission line, even though the circuit breakers at both ends of the line are open, can be extinguished in a short time, thereby shortening the period for which the reclosing circuit is without voltage. 
2.  Description of the Prior Art
Usually for high-voltage, high-capacity transmission lines, use is made of multi-conductor transmission lines, which have a large distributed electrostatic capacity between the phases or between the circuits.  In such transmission lines, when flash-over due to lightning damage etc.,  occurs in the region of the insulators, even though the circuit breakers at both ends of the transmission line are open, induced current and induced voltage are supplied through the distributed electrostatic capacity which exists between the uninterrupted phase or uninterrupted circuit and the breakdown phase.  The result of this is that, for some time, arcing current continues to flow in the region of the insulators, and the breakdown condition is not removed. 
FIG.  1 is a circuit diagram of a transmission system showing such a condition.  Reference designation 1 denotes the transmission line, 2A and 2B designate the electric station or substation buses, whereby the two ends of the transmission line 1 are connected through circuit breakers CB, C designates the distributed electrostatic capacity that exists between the breakdown phase and the unaffected phases of the transmission line 1, and IF designates the secondary arcing that occurs due to supply of induced current (arrow in the drawing) at the point F where the breakdown occurs, from the uninterrupted phase, through this distributed electrostatic capacity.  Consequently, when such secondary arcing occurs in the region of the insulators etc.,  of the transmission line, as mentioned above, this arcing is not extinguished for some time, so when a single-phase or multi-phase reclosing circuit is made, the reclosing circuit does not have sufficient time for zero voltage, leading to problems in stability of the system.  This tendency appears to a marked extent in high-voltage, large-capacity transmission lines, which have a large distributed electrostatic capacity between the lines.  For this reason, to realize the ultra-high voltage (UHV) transmission systems of the future, some positive means of secondary arc extinction will be necessary. 
A means which has recently been proposed for secondary arc extinction is the use of a fixed reactor device with zero-phase compensation, as shown in FIG.  2.  In FIG.  2, reference designation 3 designates the line impedance consisting of the line inductance 6 and the distributed electrostatic capacity 5 which exists between the lines of each phase and the distributed electrostatic capacity 4 which exists between ground and the lines of each phase of the transmission line 1; and 7 designates an arc extinction reactor device which is connected in the service line of the substation.  This arc extinction reactor 7 is constructed by connecting one end of each of reactors L1, L2 and L3 to respective phases of the transmission line 1 and connecting their other ends in common, and connecting a neutral point reactor Lg between ground and the common star connection point. 
The arc extinction function of this arc extinction device may be explained as follows.  The admittance matrix determined by the distributed electrostatic capacity of the transmission line 1 is expressed by formula (1). ## EQU1## The admittance matrix of the arc extinguishing circuit elements is expressed by formula (2). ## EQU2## The total admittance of the transmission line 1 is therefore expressed by formula (3).  EQU Y=Y.sub. C +Y.sub. L ( 3)
Since the characteristics of the admittance matrices Y.sub. C and Y.sub. L represented by the above formulas (1) and (2) are different, by suitably choosing the value of Y.sub. L, the mutual admittance can be made zero, i.e., the distributed electrostatic capacity between the lines can be made zero.  In other words, by making the line impedance to indefinite as described above, induced current can be made zero.  Secondary arc extinction is thereby made possible. 
However, an extinction reactor construction as above cannot make zero the electrostatic induction from the unterrupted lines in the case of jointly used transmission lines.  To deal with this case, investigations of a reactive extinction device of the construction shown in FIG.  3 have been made.  Specifically, as shown in FIG.  3, an extinction device 8 is constructed in which the ends of reactors L1, L2 and L3 are connected to respective phases of the transmission line 1 of one circuit, while ends of reactors L4, L5 and L6 are connected to respective phases of the transmission line 1 of the other circuit, the other ends of these reactors L1-L6 being connected in common, and a neutral point reactor Lg being connected between ground and this common connection point. 
The arc extinction function of an extinction device constructed as above is the same as described earlier.  Specifically, the admittance matrix determined by the distributed electrostatic capacity of the transmission lines 1, 1 of the two circuits is expressed by formula (4), as follows: ##EQU3##
The admittance matrix of the extinction reactor device 8 is expressed by formula (5), as follows: ##EQU4## The total admittance of the transmission lines 1, 1 of the two circuits is therefore expressed in the same way as formula (3) given above, and, by suitably choosing the value of YL, the extinction of secondary arcing can be achieved. 
In choosing the value of the admittance Y.sub. L in an extinction reactor device as described above, either the value of Y.sub. L may be made a constant value found by means of the admittance due to the distributed electrostatic capacity, or may be set to a calculated value at which a breakdown phase is extinguished. 
However, the admittance due to the distributed electrostatic capacity varies depending on the breakdown phase, and appreciable errors are unavoidable in the calculation of the line constant which forms the reference value for the setting of the optimum calculated admittance.  Furthermore, due to the effects of weather conditions, a particular value of Y.sub. L is not necessarily always the optimum value.  It is therefore difficult to obtain extinction of the secondary arc in a short time, and the value of Y.sub. L may vary above and below the optimum value.  For these reasons secondary arc extinction is often difficult.