The present invention relates to an on-line tap changer which employs a vacuum valve. More particularly, the invention relates to an apparatus for protecting a vacuum interrupter type on-line tap changer which uses a vacuum interrupter having an exellent breaking or interrupting performance with long contact lifetime as a current switching device in a change-over switch.
There have heretofore been put to practical use two types of on-line tap changers using a vacuum interrupter, one of which employs a reactor and the other of which employs a resistor as a current limiting impedance. The more recent vacuum interrupter type on-line tap changer of this type is considered to be advantageous in view of breaking performance and production in that it employs a resistor as a current limiting impedance.
Due to recent advances in vacuum interrupter production technology, the reliability of maintaining the degree of vacuum in the vacuum interrupter has been much enhanced compared with the conventional vacuum interrupter. In the case where a vacuum interrupter is, however, used as a current switching element for an on-line tap changer, the interrupter must have a practical lifetime of 1,000,000 switching operations. This imposes very severe mechanical constraints. In addition, if the vacuum is accidentally broken in the vacuum interrupter, a short-circuit occurs between the opened electrodes of the vaccum interrupter, which accordingly causes a short-circuit between winding taps. This further introduces a dangerous state in the transformer with which the interrupter is used. Therefore, it is necessary to proect against an accidental drop in the degree of vacuum in the vacuum interrupter.
For this purpose, a protecting system as indicated in FIGS. 1A through 1F, for example, has been employed with a conventional vacuum interrupter type on-line tap changer which uses a resistor and three vacuum interrupters, hereinafter referred as to "a 1-resistor 3-vacuum interrupter type". More specifically, FIGS. 1A through 1F show a circuit of the conventional vacuum interrupter type on-line tap changer corresponding to one phase segment in which the operation is being switched from a tap no. 1 to a tap no. 2.
In FIG. 1A, reference numeral 10 indicates a tap winding, 12 and 14 taps no. 1 and no. 2 which are connected to a tap winding 10, 16 a tap selector, 18 a disconnecting switch, 20, 22 and 24 vacuum interrupters V.sub.I, V.sub.II and V.sub.III, 26 a current limiting resistor, and 28 a secondary side neutral point of a transformer. Reference character K.sub.1 through K.sub.6 indicate contacts which make contact with the respective contact members S.sub.1 and S.sub.II of the disconnecting switch provided corresponding to taps no. 1 and no. 2.
In such a vacuum interrupter type on-line tap changer thus constructed, there is employed as means for protecting the transformer against a short-circuit between the taps caused by a drop in the degree of vacuum of the vacuum interrupters V.sub.I and V.sub.II a current transformer 30 connected to the circuit at tap no. 2. The transformer 30 serves to detect a short-circuit current between the taps, thereby breaking the transformer from the circuit in a protecting system.
Since in such a protecting system a detecting relay (an overcurrent relay) connected to the current transformer detects only a shorting current between the taps even when the degree of vacuum of the vacuum interrupter V.sub.III drops so that the vacuum interrupter V.sub.III reaches the state where current will flow between the opened contacts thereof, a cross current limited by the resistor 26 (a circulating current between the taps) will flow between the taps for a predetermined time. This occurs during the normal tap changing step even if the detecting current of the detecting relay is set at a small value. Accordingly, it is impossible to distinguish whether the shorting current between the taps is normal or abnormal. Therefore, it is very difficult in the conventional vacuum interrupter type on-line tap changer protecting system to detect an abnormal state due to a short-circuit between taps.
The action of the vacuum interrupter type on-line tap changer shown in FIG. 1 when the degree of vacuum of the vacuum interrupter V.sub.II (22) drops to the extent that a current flows between the opened contacts will be described with reference to the switching sequence diagrams of the disconnecting switch 18 and the vacuum interrupters V.sub.I through V.sub.III shown in FIGS. 2 and 3. FIG. 2 shows the switching sequence where the tap is changed from tap no. 1 to tap no. 2 while FIG. 3 shows the switching sequence where the tap is changed from tap no. 2 to tap no. 3 (or tap no. 1).
If the vacuum interrupter V.sub.II malfunctions in the case where the tap is changed from the tap no. 1 to tap no. 2, a cross current between the taps will flow simultaneously upon closure of the contacts K.sub.2 and K.sub.6 due to a contact S.sub.II of the disconnecting switch 18 (as indicated by a broken line in FIG. 1C and by a current transformer current characteristic in FIG. 2). Accordingly, since the current limiting resistor 26 can operate at its rated heat capacity only for short time, the resistor 26 will overheat resulting in burn-out. Even if the vacuum interrupters V.sub.I, V.sub.II and V.sub.III are switched to a state where the resistor 26 will not be burnt out, the tap cannot be normally changed, eventually resulting in the possibility of a short-circuit between taps.
In FIGS. 1A through 1F, the circuits indicated by thick solid lines show load current paths. If the vacuum interrupter V.sub.III malfunctions in the case where the tap is changed from tap no. 2 to tap no. 3 (or tap no. 1), the vacuum interrupter V.sub.III cannot break a cross current between the taps. In this case, the current limiting resistor 26 will sustain a current for a long time, as indicated by a broken line in the current transformer current characteristic in FIG. 3, that is, until the circuit across the taps is opened between the contacts K.sub.2 and K.sub.6 by a contact member S.sub.II of the disconnecting switch 18. This also causes burn-out of the resistor 26 similar to the above case. If the resistor 26 is not burnt out, the cross current between the taps will be interrupted by the opening of the contacts K.sub.2 and K.sub.6 by the disconnecting switch 18. Since the contact of the switch 18 is not, however, normally provided with an arc resistance property, serious damage will occur due to the interruption of the cross current between the taps.
Further, the above described 1-resistor 3-vacuum interrupter type on-line tap changer is constructed to normally, switch the vacuum interrupters instantaneously by a quick-break mechanism using a spring. In the case where a difficulty arises which causes an increase in the driving resistance at the driven side, of for example, a cam disc, levers, bearings or like of the quick-break mechanism, the energy of the spring is cancelled and consumed, and as a result, the contacts may not normally complete switching operations as are required. More specifically, the switching operation of the vacuum interrupter V.sub.III will stop in the position intermediate from the state shown in FIG. 1C and will be in the state shown in FIG. 1D. Since, even in this case, a cross current flows between the taps or a load current flows through the current limiting resistor for a long time in the same manner as the above case, a serious difficulty such as burn-out of the resistor may occur.
In the conventional vacuum interrupter type on-line tap changer there is thus no protection against incompleted switching of the three vacuum interrupters V.sub.I through V.sub.III and against a decrease or loss of vacuum in the vacuum interrupters V.sub.III.