US3662253A

US3662253A - Tap changing system for regulating transformers

Info

Publication number: US3662253A
Application number: US83383A
Authority: US
Inventors: Saburo Yamamoto
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-11-04
Filing date: 1970-10-23
Publication date: 1972-05-09
Anticipated expiration: 1989-05-09
Also published as: JPS507727B1

Abstract

A tap changing system for regulating transformers disclosed here is of one resistor type in which vacuum switches are used for its main arcing contacts and a semiconductor switch or a semiconductor controlled switch bidirectionally conductive on AC is used in series to a current limiting resistor circuit which is switchably connected in parallel to said vacuum switches. Tap changing of one resistor method is effected by on-off operation of the vacuum switches, whereby on-off operation of the semiconductor switch is automatically effected.

Description

United States Patent Yamamoto 1541 TAP CHANGING SYSTEM FOR REGULATING TRANSFORMERS [72] Inventor: Sahuro Yamamoto, 90-2 Kumegawa- Kodam 1-4 Misumi-cho, Higashimurayama-shi. Tokyo. Japan [22] Filed: Oct. 23, 1970 [21 Appl. No.: 83,383

30 Foreign Application Priority Data Nov. 4, 1969 Japan ..44/88266 52 us. Cl. ..323/43.s s, 336/150 [51] Int. Cl. ..G05f 1/14, H02p 13/06 [58] Field oiSearch ..323/43.5 R, 43.5 S;336/150 [56] References Cited UNITED STATES PATENTS 3,544.884 12/1970 Prescott .Q ..323/43.5 R

[151 3,662,253 1 May 9,1972

FOREIGN PATENTS OR APPLICATIONS 1,805,378 5/1970 Germany ..323/43.5R 1,166,772 l0/l969 Great Britain ..323/43.5 S

Primary Examiner-Gerald Goldberg [5 7] ABSTRACT 4 Claims, 10 Drawing Figures BRIEF DESCRIPTION OF THE INVENTION in insulating oil. In this type ,of on-load tap changer, the insulating oil is deteriorated and the contacts, are worn by-arc, and they need frequent check and maintenance for safe operation .of theapparatusnThis is why .the operating efficiency is low in the conventional type of on-loadtap changer.

In view of theforegoing, an object of thisinvention is to'provide a new, compact andlow cost apparatus for tap-changing under load without deteriorating the. insulating.- oil and with very little wearof the contacts.

Another object of the invention isnto provide a.simpler and more reliable apparatus for tap changing underloadwithout causing wear of contacts by arc.

The on-load tap changer of this invention is of one resistor type using vacuum switches for the current switching contacts.

More specifically, the vacuumswitches are usedforthemain current switching contactswhich are disposedbetween the lead on the-load side and two pairs of tap selectors belonging to the odd number of taps and the even number of taps of tap windings respectively, a-semiconductorswitch or a semiconductor controlled switch bidirectionally conductive on AC is used for the current switching contact of a current limitingresistor circuit switchably connected in parallel to the vacuum switches, an d saidsemiconductor switch is made conducting or nonconducting automatically without resorting to contact means, synchronizing with on-off of the two vacuum switches. Thus the switching performance isimproved, the construction of the apparatus-is simplified and thesize and the cost ofthe apparatus is reduced. Namely, the invention provides anovel switching apparatus characterized by a feature that its switching operation, based on one resistor method, is performed safely and securely in such a short period as one cycle or so by utilizing simple, high speed switching operation effected by correlatively operable vacuum switches. The switching apparatus of this invention is further characterized by a feature that another semiconductor controlled switch is connected in parallel to the current limiting resistor circuit comprising serially said semiconductor controlled switch, and the firing control is done by a gate control device using the secondary current of a current transformer inserted into each circuits having a vacuum switch, the tiring control of which is synchronized with the vacuum switch operation, whereby non-arc switching operation is realized.

The invention will be better understood from the following description taken in connection with the accompanying drawings, and its scope will be pointed out in appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are circuit diagrams showing conventional resistor type on-load tap changers using vacuum switches,

FIGS. 3 through 6 are circuit diagrams showing the principles of an on-load tap changer of this invention, and

FIGS. 7 through 10 are circuit diagrams showing switching devices embodying this invention.

DETAILED DESCRIPTION OF THE INVENTION .ofon-load tap changer are shown in FIGS. I and 2. Note that these apparatus are of one resistor type.

Referring to FIG. 1, the reference T denotes a tap winding with a plurality of taps, and S and S are tap selectors belonging to odd number taps and even number taps respectively. V,

and V, are vacuum switches connected between the tap selectors S, and S, and the lead L on the load side. R, denotes a'current limiting resistor switchably connected to the vacuum switches V, and V, by way of a selector S,.F'IG. I shows the state that the vacuum switch V, is in the close position, V, in the open position, selector 5;, in the close position on the side of vacuum switch V,, and current is supplied from the tap on the side of tap selector 8,. Current supply can be changed over from the tap selector S, to S, in such switching sequence that the current limiting resistor R, is bridged across the selector taps by switching the selector 8, from the vacuum switch V, to V,, the vacuum switch V, is opened to transfer the load current to. the circuit of the current limiting resistor R,, and then the vacuum switch V, is closed. In this manner, the tap can be switchedtto-the other side. In this switching apparatus, however, a predischarge for a certain duration takes place ascribable to the voltage across the taps if the switching speed of the selectors, is slow, or an arc is produced due to a large leap at the contact if the switching speed of Sis high. This makes it difficult to preventperfectly wear of the contact and deterioration of the insulating oil. In addition, the vacuum switch must be operated to flow or stop the sum of the load current and the circulating current between taps according to the switching direction in connection with rise and drop of the voltage. This lowers the switching efficiency.

FIG. 2 is to illustrate another switching apparatus in which a current'limiting resistor R as in FIG. I is connected to the side of one of the vacuum switches, and its switching operation is done by a vacuum switch V FIG. 2 a shows the state that the vacuum switch V, is in the close position, V and V in the open position, and current is supplied from the tap on the tap selector 5,. Current supply can be switched from the tap on the tap selector S, to S in such switching sequence that the vacuum switch V, is closed, V, is opened, and then V is closed. To switch back from the tap on S, side to that on S, side, the vacuum switch V, is opened, V, is closed, and then V is opened. In this type of switching apparatus, there is no deterioration of the insulating oil by the reason as in the apparatus of FIG. I. On the other hand, however, increase of the necessary number of vacuum switches used per phase causes both increase of dimension of the apparatus and complication of the driving mechanism of the apparatus. Furthermore the sum of the load current and the circulating current between the taps must be switched according to the switching direction inconnection'with rise and drop of the voltage as in the case of the switching apparatus of FIG. I.

In view of the foregoing, an object of this invention is to provide a new, compact and low cost on-load tap changer operable without deteriorating the insulating oil.

Another object of this invention is to provide a simpler and more reliable on-load tap changer operable without causing wear of the contacts due to are. The invention will be better understood from the following description in connection with the accompanying drawings.

FIG. 3 is a circuit diagram showing the principles of the onload tap changer-of this invention with respect to one phase. Referring to FIG. 3, the reference T denotes a tap winding with a plurality of taps, and S, and S, are tap selectors belonging to the odd number taps and the even number taps respectively. V, and V, are vacuum switches connected between the tap selectors S, and S, and the lead L on the load side. SSS denotes a semiconductor switch displaying bidirectional breakover on AC at a specific voltage. For this semiconductor switch, the silicon symmetrical switch, bidirectional triode thyristor, or reverse-blocking triode thyristors connected reversely parallel to each other may be used. R denotes a current limiting resistor, and S, is a selector for selecting the vacuum switch V, or V, in parallel to the circuit having serially the resistor R and the semiconductor switch SSS. The reference D denotes a high impedance bypass circuit disposed in parallel to the semiconductor switch SSS. For this circuit, a voltage build-up rate reduction circuit which is generally used for semiconductor circuits may be used when the tap voltage is high, or a simple high resistance circuit may be used when the tap voltage is low. FIG. 4 shows an example of voltage build-up rate reduction circuit in which two pairs of circuit comprising a capacitor K connected serially to a circuit having in parallel a silicon diode SR and a discharge resistor Rd are connected in reversely parallel to each other. FIG. 5 shows a circuit having a plurality of serially connected semiconductor switches used when the tap voltage is high. Referring to FIG. 5, voltage build-up rate reduction circuits D, and D are connected in parallel to serially connected semiconductor switches SSS, and SSS respectively, and branching high resistors Rs, and Rs are connected in parallel to D, and D,. This circuit is operated as a serial equalizing circuit of the semiconductor switching circuits during switching operation. FIG. 5 shows an example using a plurality of serially connected bidirectional triode thyristors. When reverse-blocking triode thyristors are used instead of bidirectional triode thyristors, said serial equalizing circuit must be provided for each semiconductor switch for said operation purpose.

The operation of this switching device will be described below.

FIG. 3 shows the state that the vacuum switch V, is in the close position, V in the open position, selector S in the close position on the side of vacuum switch V and current is supplied over the current from the tap on the side of tap selector 5,. To change over the current supply from the tap on S, side to that on S side, the selector S is switched slowly from the side of vacuum switch V to the side of V,, the vacuum switch V, is opened and, after leaving this state for more than half cycle, the vacuum switch V is closed. An arc is drawn when the vacuum switch V, is opened. One of the features of the vacuum switch is that the arc voltage is as low as about 20V almost regardless of the arc current. The capacitor K of the voltage build-up rate reduction circuit D is charged by this low arc voltage. When the load current reaches the next current zero point, the are from the vacuum switch V, is extinguished and the load current is about to be commutated into the voltage build-up rate reduction circuit D. At this moment, the capacitor K which has been charged by the arc voltage is reversely charged. In this process, the capacitor K presents transiently a very low impedance for the period of tens and several microseconds to several tens microseconds. (This period depends on the capacitance of the capacitor K and the value of the load current.) Thus the initial build-up rate of the transient recovery voltage produced at the cutoff of the load current by the vacuum switch can be markedly reduced by the voltage build-up rate reduction circuit D. In this manner, the circuit D operates to let the vacuum switch cut off the load current securely at a high efficiency, and to facilitate the initial commutation of the load current. Assume that this switching apparatus is not provided with the voltage build-up rate reduction D. Then a restriking voltage having a very high build-up rate of the power circuit is applied to both the vacuum switch and the semiconductor switch when an exciting current of no load transformer or a light load current of low power factor is switched off. Without the voltage build-up rate reduction circuit D, therefore, the vacuum switch must have a greater breaking ability, and technical and economical sacrifice must be made for the vacuum switch whose main purpose is to switch the circuit of tap voltage corresponding to only a few percent of the power circuit voltage. Furthermore it is difficult to maintain stable switching operation if the switching apparatus has no circuit D. Since the voltage build-up rate reduction circuit D has avery high impedance against the power source frequency, its terminal voltage is naturally to exceed the voltage between the taps and rapidly increase toward the circuit voltage when the commutated load current flows therein for the period of tens and several microseconds to several tens microseconds from zero value. When this overvoltage exceeds the breakover voltage of the semiconductor switch SSS, the semiconductor switch SSS is directly made conducting and the load current is supplied thereto via the current limiting resistor R. The load current, upon reaching its zero point, is cut off by the semiconductor switch SSS, and again is commutated into the voltage build-up rate reduction circuit D to result in an overvoltage. By this overvoltage, the semiconductor switch SCR is made conducting again, and then the vacuum switch V is closed. As a result, all the load current flows through V the tap circulating current limited by the current limiting resistor R flows in the semiconductor switch SSS and then is cut off at the next current zero point. Thus one switching operation is completed. In the same manner as described above, the tap l3 changed over to the reverse side.

As described above, the first object of this invention is realized by using the circuit arrangement as in FIG. 3. The switching operation can further be stabilized and the reliability can be increased by the arrangement as in FIG. 6 wherein the firing control is done by utilizing a semiconductor controlled switch in place of the semiconductor switch SSS. Referring to FIG. 6, the reference CT denotes a current transformer inserted into a voltage build-up rate reduction circuit D, and C is gate control device for a semiconductor controlled switch SCR. In this switching apparatus, the rise of the load current commutated into the voltage build-up rate reduction circuit D due to the cutoff the load current by the vacuum switch is caught by the current transfonner CT, and the firing control of the semiconductor controlled switch SCR is done by the gate control device C using the rising current of CT. In this switching apparatus, a semiconductor switch having a control electrode is used, and a first means in which the circuit conduction by the load current switched under no-load state to load state is effected by the breakover by the overvoltage produced at the switching operation is provided, and a second means in which the circuit conduction by the load current is made by the firing control of the gate control device is also provided whereby the stability and reliability of the switching operation are increased.

FIG. 7 shows an arrangement to which the switching device as in FIGS. 3 and 6 is applied. This arrangement is such that the circuit of semiconductor controlledswitch SCR is connected in parallel to one of the vacuum switches which is normally closed, and the SCR circuit is changed over to the other vacuum switch which is opened at the switching operation. The semiconductor controlled switch SCR is used reversely with respect to the arrangements as in FIGS. 3 and 6. FIG. 7 shows the state that the vacuum switch V, is in the close position, V in the open position, selector S in the close position on the side of vacuum switch V,, and current is supplied from the tap on the side of tap selector 5,. To change over the current supply from the tap on the side of S, to 8,, the selector S, is switched to the side of vacuum switch V and then the vacuum switch V, is opened and V, is closed. When the vacuum switch V, is opened, the load current is commutated transiently into the voltage build-up rate reduction circuit D, the semiconductor controlled switch SCR is made conducting by firing control of the gate control device C, and the load current is supplied thereto from the tap on the side of tap selector 8,. When the vacuum switch V is closed, all the load current flows through V,. Thus the switching operation is completed.

FIG. 8 shows another example of arrangement to which the invention is applied, wherein the circuit of semiconductor controlled switch is stationarily connected in parallel to one of the vacuum switches. In this arrangement, the tap is changed over iii the manner as in FIG. 6 and that as in FIG. 7 alternately according to the switching direction.

In the switching apparatus as shown in FIG. 3 through 8, the load current is changed over by using the vacuum switches and semiconductor switch and, accordingly, there is no deterioration of the insulating oil. On the other hand, however, wear of the contracts of the vacuum switch is unavoidable. To

\ limiting resistor R inseries is'connected'in parallel to the circuitof another. semiconductor controlled switch, and-the firing control-of the semiconductor-controlled switch is done synchronizing with, the switching of the vacuum switch whereby it is easily made, possible to switch the vacuum switch without causingarc. .FIG. 9 shows an embodiment thereof, wherein .the components common to FIG. 6 are indicatedby theidenticalreferences. Referring to FIG. 9, SCR, denotes a semiconductor controlled switch in which bidirectional triode thyristors similar to SCR .or-reverse-blocking triode thyristors are connected in reversely parallel to each other. CT, and CT are currenttransformers inserted into the circuits of vacuum switches V, and V respectively, and C, and C are gate control devices using the secondary current of'CT, and CT The firing control on the semiconductor controlled switch SCR, is done by the outputsof the gate control devices C, and C through a selector 8,. Thisfiring control of semiconductor controlled switch SCR is done by the gate control device C .using the secondary current of the currenttransformer CT connected to the voltage build-up rate reduction circuit D as in the apparatus of FIG. 6. FIG. 9 shows the-state thatthe vacuum switch V, isin the close position, V in the open position, selector S in the close position on the side of V selector S in the close position on the side of gate control device C and current is supplied from the tap on the side of tap selector 8,. To change overthe current supply frornthe tap on the side of S, to S ,,the selector 8,, is switched to the side'of vacuum switch V,, the selector S is switched to the side of gate control device C,, and the semiconductor controlled switch SCR, is kept under the gate control. Then, by opening the vacuum switch V,, the load current is cut off at the existing phase without causing arc and transferred to the circuit of semiconductor controlled switch SCR,,. However, because the vacuum switch V, has already been opened,'no firing. control is'effected by the gate control device C,, and the load current of the semiconductor controlledswitch SCR, is cut off at the next current zero point and commutated into the voltage build-up rate reduction circuit D. The rise of the load current at this momentis caught by a current transformer CT, and the semiconductor controlled switch SCR is fired by the gate control device C. By this, all the load current flows in the semiconductor controlled switch SCR. Then, when the vacuum switch V is closed, all the load current flows through -V,, the circulating current between the taps flows in the semiconductor controlled switch SCR, and it is cut off at the next current zero point. Thus the switching operation is completed. In the same manner as above, the tap is changed over reversely without causing arc. The breakover characteristic of the semiconductor controlled switch SCR used for this switching apparatus is to be determined so that the semiconductor controlledswitch SCR is made conducting securely prior to the conduction of the semiconductor controlled switch SCR with respect to the external surge overvoltage and the transient overvoltage'produced in the circuit at switching operation. In this switching apparatus, therefore, the semiconductor controlled switch SCR breaks over always prior to SCR, by the transient overvoltage produced at switching operation even if both the semiconductor controlled switches SCR, and SCR fail in firing control under no load or light load condition. Thus switching operation with are can be semiconductor controlledswitches SCR and SCR, in the state of their being connected to the switching circuit at all times.

FIG. 10 shows another embodiment of the invention in which the circuit arrangement using semiconductor controlled ing resistorRare connected in parallel to vacuum switches V,

and V: respectively, whereby the load current is changed over. The circuit devices in FIG. 10 are'indicated'by the references with the appendixesA and a on the side of vacuum switch V,, and B and b on the side of vacuum switch V Further description of these circuit devices is omitted. The references 5, and 8,, denote selectors for switching the output circuits of the gate control deviceaccording to the switching direction. The figure shows thestatethat the vacuum switch V, is in the close position, V in'the open position, the selector S, in the close position on theside of gate control device Cb, the selector S, in the close position on the side of gate control device C and current'is supplied from the tap on the side of selector 8,. To change over the current supply from the tap on the side of selector S, to 3:, the selector S is changed over to the side of gate control device C,, the selector S, to the side of gate control device Ca, thereby placing the semiconductor controlled switch SCR, under the firing control. Then, when the vacuum switch V, is opened, the load current is cut off at the existing phase without arc, and is transferred to the circuit of the semiconductor controlled switch SCR This load current is cutofi at the next current zero point and then is commutated into the voltage'build-up rate reduction circuit Da. Thus the semiconductor controlled switch SCR is made conducting by the firing control of gate control device Ca, and the load current is supplied from the tap on the side of tap selector 8,. Following this operation, when the vacuum switch V, is closed, all the load current flows through the switch V Thus the switching operation is completed. In the same way as above, the tap is changed over reversely without causing arc. The normal switching operation in this switching device can be done without are as in the foregoing manner, without the aid of the semiconductor switches SSS and SSS, comprising serially current limiting resistors Ra and Rb. Now assuming that this switching apparatus is not provided with the circuit of semiconductor switch having said current limiting resistor, there is possibility of causing short-circuit between the taps if both the switches SCR, and SCR should fail in firing control and SCR, and SCR become conducting at random due to the transient overvoltage produced at switching operation. In this switching apparatus, however, the switching operation with arc as in the case with the device of FIG. 3 can be done by the use of semiconductor switches SSS and SSS comprising serially a current limiting resistor R, even in the event that the semiconductor controlled switches SCR, and SCR fail in firing control, therefore, the semiconductor controlled switches 'SCR .and SCR can be used in the state of their being stationarily connected to the switching circuit.

The on-load tap changers embodying this invention as have been illustrated in FlGS. 3 through 8 are advantageous in the followingpoints in comparison with the conventional type of on-load tap changers shown in FIGS. 1 and 2.

l. A semiconductor switch is used for the switching contact of the current limiting resistor circuit and thus the switching operation is done without contact and arc.

2. Unlike the conventional switching method, the semiconductor switch which serves as the current limiting resistor contact is made conducting and non-conducting automatically immediately after the vacuum switches which serve as the main contacts have been operated correlatively.

3. Switching operation is done in a very short period of time by virtue of high speed switching performed by two correlatively operable vacuum switches.

4. Current cutofi by the vacuum switch is done once against only the load current irrespective of the direction of voltage rise or drop.

Also, the load tap changers of this invention as shown in FIGS. 9 and 10 have the following features compared with the conventional type of on-load tap changer.

l. On-off of the vacuum switch can be done without arc by simple circuit arrangement. 1

2. Firing control of each semiconductor controlled switch is performed automatically and at a high speed only depending on the electrical conditions of the switching circuit.

3. It is possible to use the semiconductor controlled switches in the state of their being connected to the switching circuit at all times.

4. The switching period is very short, and the device is protected against overvoltage due to external surge. Therefore, it is hardly probable that the device fails in switching due to misoperation of the semiconductor controlled switches.

As has been described above, the on-load tap changer of this invention makes various advantages available; for example, two vacuum switches are sufficient per phase, circuit construction is simple, and switching performance is excellent. The on-load tap changer of this invention can therefore be used in many ways for any capacity at reduced costs, such as for line voltage regulator of high voltage distribution system and super-high voltage transformer of large capacity.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

Having thus described my invention, 1 claim:

1. A tap changing system for regulating transformers comprising a plurality of vacuum switches interposed between a lead on the load side and two pairs of tap selectors belonging to the odd number taps and the even number taps of tap windings respectively, a semiconductor switch conductive on AC, a high impedance bypass circuit for said semiconductor switch utilizing voltage build-up rate reduction circuit comprising a capacitor, a damping resistor and a silicon diode, and a circuit device having therein a current limiting resistor connected in series to said semicon-ductor switch, said circuit device being connected between a lead on the load side and a selector switch and. arranged to be switchable in parallel to said two vacuum switches, wherein on-load tap changing by one resistor method being performed during correlative switch-on and switch-off operation of said two vacuum switches.

2. A tap changing system for regulating transformers as claimed in claim 1, wherein said circuit device is connected stationarily in parallel to one of the side of said two vacuum switches.

3. A tap changing system for regulating transformers comprising a plurality of vacuum switches interposed between a lead on the load side and two pairs of tap selectors belonging to the odd number taps and the even number taps of tap windings respectively, a semiconductor controlled switch conductive on AC, a high impedance bypass circuit for said semiconductor switch utilizing voltage build-up rate reduction circuit comprising a capacitor, a damping resistor and a silicon diode, a gate control device for said semiconductor controlled switch, said gate control device including control means utilizing the secondary rise current of current transformer inserted in the circuit of said high impedance bypass circuit, and a circuit device having therein a current limiting resistor connected in series to said semiconductor controlled switch, said circuit device being connected between a lead on the load side and a selector switch and. arranged to be switchable in parallel to said two vacuum switches, wherein on-load tap changing by one resistor method being performed during correlative switch-on and switch-off operation of said two vacuum switches.

3. A tap changing system for regulating transformers comprising a plurality of of vacuum switches interposed between a lead on the load side and two pairs of tap selectors belonging to the odd number taps and the even number taps of tap windings respectively, a first semiconductor controlled switch conductive on AC a high impedance bypass circuit for said semiconductor switch utilizing voltage bUlld-U rate reduction circuit comprising a capacitor, a damping resistor and a silicon diode, a gate control device for said first semiconductor controlled switch, said gate control device including control means utilizing the secondary rise current of transformer inserted in the circuit of said high impedance bypass circuit, a second semiconductor controlled switch conductive on AC, gate control device for said second semiconductor controlled switch, said gate control devices including control means utilizing the secondary current of current transformers inserted in the circuit of said vacuum switches, and a circuit .device having therein a current limiting resistor connected in series to said first semiconductor controlled switch and said second semiconductor controlled switch connected in parallel to both of said current limiting resistor and said first semi-conductor controlled switch, said circuit device being connected between a lead on the load side and a selector switch and. arranged to be switchable in parallel to said two vacuum switches switch, wherein non-arc tap changing by one resistor method being performed during correlative switch-on and switch-off operation of said two vacuum switches.

* l it il i

Claims

3. A tap changing system for regulating transformers comprising a plurality of of vacuum switches interposed between a lead on the load side and two pairs of tap selectors belonging to the odd number taps and the even number taps of tap windings respectively, a first semiconductor controlled switch conductive on AC, a high impedance bypass circuit for said semiconductor switch utilizing voltage build-u rate reduction circuit comprising a capacitor, a damping resistor and a silicon diode, a gate control device for said first semiconductor controlled switch, said gate control device including control means utilizing the secondary rise current of transformer inserted in the circuit of said high impedance bypass circuit, a second semiconductor controlled switch conductive on AC, gate control device for said second semiconductor controlled switch, said gate control devices including control means utilizing the secondary current of current transformers inserted in the circuit of said vacuum switches, and a circuit device having therein a current limiting resistor connected in series to said first semiconductor controlled switch and said second semiconductor controlled switch connected in parallel to both of said current limiting resistor and said first semi-conductor controlled switch, said circuit device being connected between a lead on the load side and a selector switch and. arranged to be switchable in parallel to said two vacuum switches switch, wherein non-arc tap changing by one resistor method being performed during correlative switch-on and switch-off operation of said two vacuum switches.