US3641358A

US3641358A - Consecutive crowbar circuit breaker

Info

Publication number: US3641358A
Application number: US45147A
Authority: US
Inventors: Kenneth T Lian; Willis F Long
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1970-06-10
Filing date: 1970-06-10
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08
Also published as: GB1334171A; FR2094171A1; DE2127770B2; SE381366B; DE2127770A1; CA921602A; FR2094171B1; JPS5228991B1; DE2127770C3

Abstract

A circuit breaker is inserted in a high-current, high-voltage DC power line between the source and the load. The circuit breaker comprises a parallel connection of a transfer switch, an electronic switch, a first consecutive interrupter having a preferably nonlinear resistor in series therewith, and a second consecutive interrupter having a preferably nonlinear resistor in series therewith. Furthermore, a surge capacitor and its suppression resistance are serially connected in parallel around the second consecutive interrupter. When a fault occurs, the transfer switch is opened and is deionized during conduction of the electronic switch. Offswitching of the electronic switch causes current flow through the two parallel consecutive interrupters with their series resistances to decrease circuit current. During the period that the consecutive interrupters are sequentially opened, the electronic switch is conductive so that the consecutive interrupters can be deionized.

Description

United States Patent [151 3,641,358 Lian et a1. 1 1 Feb. 8, 1972 [54] CONSECUTIVE CROWBAR CIRCUIT 3,476,978 11/1969 Greenwood ..307/ 176 X BREAKER 3,515,940 6/1970 Hobson ..307/ 136 X 3,522,472 8 1970 B tholtz ..3l7 11 C X 72 Inventors: Kenneth T. Lian; Willis F. Long, both of I m I Thousand oaks Cahf' Primary Examiner--Robert K. Schaefer [73] Assignee: Hughes Aircraft Company, Culver City, Assistant Examiner-William J. Smith Calif. Attomey-James K. Haskell and Allen A. Dicke, Jr.

[22] Filed: June 10, 1970 [57] ABSTRACT [21] APPl' A circuit breaker is inserted in a high-current, high-voltage DC power line between the source and the load. The circuit [52] US. Cl. ..307/ 136, 317/11 C breaker comprises a parallel connection of a transfer switch, [51] Int. Cl.. ..H0lh 9/30 an electronic switch, a first consecutive interrupter having a 1 1 Fwd Search 1 1 11 11 preferably nonlinear resistor in series therewith, and a second 307/136 consecutive interrupter having a preferably nonlinear resistor in series therewith. Furthermore, a surge capacitor and its sup- [56] Ream cued pression resistance are serially connected in parallel around the second consecutive interrupter. When a fault occurs, the UNITED STATES PATENTS transfer switch is opened and is deionized during conduction 3,534,226 10/ 1970 Lien ..317/1 1 C of the electronic switch. Offswitching of the electronic switch 3,475,620 10/ 1969 Murray et a1. ..307/136 causes current flow through the two parallel consecutive inter- 02 1/ 1964 Cable 317/11 X rupters with their series resistances to decrease circuit cur 3,237,030 2/1966 Cobum 307/136 X rent. During the period that the consecutive interrupters are 3,249,810 5/1966 Strom et .....317/l1 A sequentially opened, the electronic switch is conductive so 3,401,303 9/1968 walker 1 that the consecutive interrupters can be deionized. 3,430,063 2/1969 Webb 307/136 3,448,287 6/1969 Giammona ..307/ l 36 12 Claims, 4 Drawing Figures CONSECUTIVE CROWBAR CIRCUIT BREAKER BACKGROUND OF THE INVENTION This invention is directed to a consecutive crowbar circuit breaker of such nature as to permit the stopping of current flow in high-current, high-voltage DC powerlines.

The multiphase transmission of electric power at a frequency corresponding to the generating source and the load equipment is widely used at present. The employment of alternating current is desirable because it permits the use of transformers to change voltages from a value suitable for generation, to a value suitable for transmission, to a value suitable for distribution, and finally a value suitable for use.

Increasing power demands by the technologically advancing community has resulted in a transmission at higher voltages and for longer distances. The transmission line reactance is such that further increase of transmission line length or voltage becomes uneconomic. The user must pay for even greater power losses as distances and voltages are increased.

As a result of this, efforts have been made to transmit electric power by direct current links. Direct current is much more satisfactory from a reactance viewpoint for subsea or subterranean installations. Thus, modern interisland ties have been DC in nature. The same considerations apply to longer overground installations, and to underground installations. With increasing size of urban centers, and with the aesthetic demands that lines be placed underground wherever possible, it is expected that future urban transmission lines will be subterranean. This requirement points to the need for employing direct current transmission.

Of course, the resistant loss of a DC line is decreased by increasing the voltage and decreasing the line current. However, switching and interrupting devices for such higher voltage, and especially high-current DC transmission lines have previously been unavailable.

SUMMARY In order to aid in the understanding of this invention it can be stated in essentially summary form that it is directed to a consecutive crowbar circuit breaker for the breaking of a highwoltage, high-current DC circuit. The circuit breaker comprises a transfer switch in the DC line which normally carries the DC current. Parallel to the transfer switch are an electronic switch, a first consecutive interrupter having a resistor in series therewith and a second consecutive interrupter having a resistance in series therewith. The transfer switch, the electronic switch and the consecutive interrupters are controlled to operate in proper sequence upon fault detection.

Accordingly, it is an object of this invention to provide a consecutive crowbar circuit breaker which is capable of breaking a high-voltage, high-current DC circuit. It is a further object to provide a circuit breaker which will permit the employment of DC transmission lines with circuit breakers therein to control circuit faults. It is another object to provide a circuit breaker having a single electronic switch therein, which electronic switch is closed during the opening steps of contacts in parallel thereto so that arcing is limited by the voltage drop on the electronic switch. It is still another object to provide a consecutive crowbar circuit breaker which consecutively opens contacts, with the contacts opening after the first one having resistance in series therewith for the absorption of circuit energy. It is a further object to employ nonlinear resistances in series with consecutively opening interrupters so that a maximum amount of circuit energy can be absorbed per interrupter to limit the number of consecutive interrupters to two. It is another object to provide a circuit which employs an electronic switch which can be opened or interrupted to interrupt current flowing therethrough so that arcing from opening mechanical contacts is minimized.

Other objects and advantages of this invention will become apparent from the study of the following parts of the specification, the claims and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic drawing of a high-voltage, high-current DC circuit having the consecutive crowbar circuit breaker of this invention connected therein.

FIG. 2 is a graph of circuit current vs time during the opening sequence of the circuit breaker of this invention.

FIG. 3 is a graph showing the voltage across the DC buses, during the sequence shown in FIG. 2.

FIG. 4 is a diagrammatic showing of the control equipment which controls the switches and interrupters.

DESCRIPTION FIG. 1 illustrates a high-voltage DC circuit with the consecutive crowbar circuit breaker of this invention incorporated therein. The circuit is generally indicated at 10 and the circuit breaker is generally indicated at 12.

The circuit 10 comprises positive bus 14 and return bus 16. Connected therebetween is a high-voltage, high-current DC power source 18 which is conveniently illustrated as being a battery. However, as is well known to those in the art, the power source usually comprises an engine or turbine driven multiphase AC generator which supplies power to transformers. The transformers increase the voltage and supply the rectifiers which are connected between positive bus 14 and return bus 16. The preferred example given in this specification is for a 400 megawatt system, because that power level appears to be appropriate for future use in power generation for adjacent urban environments. It is a level which might be used in underground transmission of power from nearby generating plants to urban areas. In such an example, the normal current is 1,000 amperes, as illustrated by the ordinate in FIG. 2 where each of the numerals indicates 1,000 amps. Furthermore, the normal voltage level between the positive bus 14 andreturn bus 16 is 200 kilovolts, as illustrated by the ordinate in FIG. 3 where the numbers illustrate thousand volts. Furthermore, return bus 16 is preferably at ground potential and a duplicate circuit 10 is provided with a negative bus at 200 kv.and a duplicate of the circuit breaker 12. In other words, FIG. 1 illustrates half of a system which, for purposes of example and illustration throughout this specifica tion, is a 400 mw. system.

Inductance 20 is serially connected with power source 18. Inductance 20 represents the inductance of the entire circuit. The circuit inductance limits the change in current with respect to time, and should the normal circuit inductance be too low, an additional inductor can be installed for smoothing and for limiting the rate of current increase in fault conditions. In the specific example of this specification, the circuit inductance is one-half henry so that at the 200 kv. power source voltage the rate of change of current with respect to time upon occurrence of a fault is 400 amps per millisecond.

Serially connected in the line is fault sensor 22, transfer switch contacts 24 and load 26. Load 26 can be any conventional commercial load or any special load which employs the power produced by the power source. Thus, load 26 can include inverters, transformers and distribution equipment to the ultimate load. Lines 28 and 30 represent transmission portions of the positive bus 14 and return bus 16, respectively, which transmit the power from the source to the load. Thus, the circuit breaker 12 is preferably adjacent the source 18 and transmission over a distance occurs in lines 28 and 30.

Connection 32, with its switch 34, between lines 28and 30, represents a short circuit such as might occur at the input to load 26 or in the lines 28 and 30 leading thereto. Closure of the switch 34 represents an inadvertent short circuit and thus, connection 32 with its switch is schematically illustrative of other types of highly conductive electrical connections between lines 28 and 30.

Buses 36 and 38 are part of the circuit breaker l2 and are connected to positive bus 14 on opposite sides of transfer switch contacts 24. Connections between these buses 36 and 38 thus present parallel connections across transfer switch contacts 24. Electronic switch 40 is connected between these buses. The series combination of first consecutive interrupter 42' and its series resistance 44 are connected therebetween. Similarly, second consecutive interrupter 46 and its series resistor 48 are connected therebetween. If required, additional series combinations of consecutive interrupters and resistances can be connected therebetween for successive operations. Finally, suppression resistance 50 and capacitor 52 are serially connected together and are connected in parallel around second consecutive interrupter contacts 46.

Referring to FIG. 4, fault sensor 22 senses a fault condition and is connected to control unit 54 which contains control circuitry to function as is hereinafter described. The output of control unit 54 goes to transfer switch contact operator 56, electronic switch operator 58, first consecutive interrupter contact operator 60, and second consecutive interrupter contact operator 62. I

Fault sensor 22 is any convenient and conventional fault sensor which is responsive to voltage between

buses

14 and 16, is responsive to rate of change of the voltage between the buses, is responsive to current in bus 14 or is responsive to the change in current with respect to time in bus 14, or a combination of these signals. Suitable fault sensors are shown in the following U. S. Pat. Nos: 3,353,17l; 3,419,791; 3,463,998; 3,471,784; 3,473,106; 3,475,653; 3,478,352; and 3,489,920. Any one or more of these can be employed as fault sensor 22. The particular fault sensor is not critical to the invention and any conventional fault sensing means can be employed.

Transfer switch contact operator 56 and its contacts 24 are in the nature of those found in a conventional circuit breaker such as shown in U. S. Pat. No. 3,268,687 operating in SF to generate sufficient arc voltage. The requirements are that the transfer switch contacts 24 be able to carry 1,000 amps when closed (the maximum current in the exemplified DC circuit of this specification), and to withstand without conduction the surge voltage of the circuit. For the purpose of the example, the surge voltage is selected to be 1.7 times the normal circuit voltage. With the normal circuit voltage at 200 kv., the surge voltage is 340 kv. in accordance with this example. Thus, when opened and deionized, the transfer switch contacts 24 must be able to withstand an applied DC voltage of 340 kv.

The electronic switch 40 can be either a crossed field switching device or a liquid metal cathode-switching device, both of which are described in detail in patent application Ser. No. 681,632, filed Nov. 9, 1967, now U. S. Pat. No. 3,5 34,226, granted Oct. 13, I970. The requirement of the electronic switch 40 is that it be able to turn on with voltage applied thereacross from a fairly small voltage value to a voltage value above the normal rated circuit voltage. In the present instance, on switching should be able to be accomplished for any value of voltage applied thereacross from its own minimum voltage drop to at least 220 kv., as seen in FIG. 3.

With respect to conduction, it must be able to conduct up to four times the normal circuit current. In accordance with the example of the specification, the maximum current has been chosen to be limited to four times normal current, which is consistent with surge voltages of 1.7 times normal voltage, and a iz-henry system inductance. Thus, electronic switch 40 must be capable of conducting up to 4,000 amps.

Furthermore, the electronic switch 40 must be capableof offswitching against this current. In order to be satisfactory for operation in the circuit of this example, the increase in voltage withstood by the switch 40 with respect to time should be at least 1.0 kv. per microsecond. The crossed field switch and the liquid metal cathode switch of the above-identified patent are satisfactory for this purpose. Of course, the electronic switch 40 may represent one or more serially connected electronic switches as described in the patent, to provide the desired standoff voltage for offswitching capability should the characteristics of electronic switch devices of commercial configuration so indicate.

Electronic switch operator 58 is connected to the electronic switch 40 in order to control its on and off switching. As is described in U. S. Pat. No. 3,534,226 the electronic switching devices can be controlled for on and off switching.

First consecutive interrupter contacts 42 and its operator 60 are. again in the nature of a circuit breaker such as is shown in U. S. Pat. No. 3,268,687, but for this use, a circuit breaker having that fast an operating speed is not necessary. The contacts must be capable of withstanding 340 kv. when open and must be capable of conducting not more than 3,200 amps when closed. The second consecutive interrupter with contact 46 and operator 62 is identical to the first consecutive interrupter. These interrupters share the 4,000 amp maximum currentand divide them in accordance with their

series resistors

44 and 48.

Series resistors

44 and 48 are shown as being nonlinear re-- sistors. Such are preferable, for with nonlinear resistors the circuit breaker 12 of this invention is able to accomplish the circuit breaking function of the example with only the first and second

consecutive interrupters

42 and 46. If linear resistors were employed instead of

nonlinear resistors

44 and 48, at least three consecutive interrupters would be required.

Series resistors

44 and 48 are silicon carbide devices. These resistors divide the maximum current between

contacts

42 and 46 on the basis of 2,700 and 1,300 amps.

Surge capacitor 52 is of conventional oil filled character and has a value of about 2.0 microfarads in the example of the specification. It is capable of withstanding the 340 kv. voltage to arrest the final voltage surge. Its surge suppression resistor 50 has a value of ohms and is capable of carrying 700 amps in surge suppression duty.

In normal operation of circuit 10, power source 18 is supplying l,000 amps of current through inductance 20, fault sensor 22, closed contacts 24 of the transfer switch and through load 26. The voltage drop across the load is the nominal circuit value of 200 kv. Under these circumstances, the

contacts

42 and 46 of the first and second consecutive interrupters are closed. Additionally, the electronic switch 40 is in standby condition so that when a voltage is applied thereacross, it will be conductive. This is the state of affairs illustrated in FIGS. 2 and 3 along the abscissa from the intersection to time point a.

At this point in time, a fault appears short-circuiting lines 28 and 30, as represented by the closing of switch 34. This fault causes a drop in voltage to near zero, and an increase in current as limited by the value of inductance 20. Current increases at the rate of 400 amps per ms., as previously described. Sensor 22 senses the increase or the rate of increase of current, or the decrease or the rate of decrease of voltage between buses, or the combination of these signals to determine that a fault has occurred. Such determination occurs at point b along the abscissa of the graphs of FIGS. 2 and 3. This sensing by sensor 22 causes control unit 54 to signal operator 56 to open transfer switch contacts 24. These switch contacts open, and as they open an arc is drawn and voltage drop occurs thereacross. The electronic switch 40 is in its standby condition wherein it is ready to conduct as soon as there is sufficient voltage thereacross. The opening of contacts 24 and the voltage drop in the are thereacross provides this sufficient conduction voltage for electronic switch 40. In the case of a single crossed field tube conducting current of the magnitude indicated, an appropriate voltage drop is 500 v. The voltage drop across the electronic switch is indicated along the abscissa in FIG. 3 between b and c. The voltage drop of a liquid metal are switching device would be somewhat lower.

Since the current is increasing from a to c, this interval must be kept to an absolute minimum. Thus, a fast reacting sensor operating contacts having a minimum opening time is employed. The electronic switch 40 remains conducting a sufficient length of time from b to c to permit the transfer switch contacts 24 to fully open and deionize so that they can withstand the peak 340 kv. which will be applied thereto. For the transfer switch 24 above, the time interval from b to c is 1-5 ms.

At the time point 0, control unit 54 operates electronic switch operator 58 to turn off the electric switch 40. This causes the voltage surge at point c since all current must pass through the paralleled first and second

consecutive interrupter contacts

42 and 46, and their respective resistances. The value of these resistances is such as to cause a reduction in current from time point c to point d.

During this time period, the voltage drop does not decay in proportion to current because of the nonlinear characteristics of

resistances

44 and 48. Ideally, the maximum energy absorption would be obtained in the shortest time if the top of the voltage curve was level. However, the curve shape illustrated is the most satisfactory shape available with present devices. When the voltage and current have reduced to such an amount that first consecutive interrupter contacts 42 can be opened without surging the voltage above the permissible 340 kv., control unit 54 causes the first consecutive interrupter contacts 42 to be opened and again causes conduction of electronic switch 40 for a short time period. Since the contacts are deionized only against the small voltage drop across the conducting electronic switch 40, arcing quickly stops. As soon as the electronic switch starts conducting, current stops flowing in interrupter 42. During this time, the circuit is again effectively almost short circuited so that the current rises a small amount at time d, as indicated in Fig. 2. The amount of time switch 40 is conductive is short, in the order of 1 ms., so that little current rise occurs.

After the first consecutive interrupter contacts 42 are fully open to withstand the surge voltage, control unit 54 again causes the tumofi of electronic switch 40 to bring the voltage back up to its peak point, as illustrated at time d in FIG. 3. The current is forced to flow through resistor 46 and second consecutive interrupter contacts 46. During the time period from d to e, the current is reduced to a point where it can be accommodated by surge capacitor 52 with its surge suppression resistor 50. This capability occurs at point e whereupon control unit 54 causes opening of second consecutive interrupter contact 46 and the short period of conduction of electronic switch 40. When the contacts 46 are deionized, electronic switch 40 is again turned off by the control unit 54 and the balance of the surge current in the system is directed into surge capacitor 52 and resistor 50. The current has been brought to zero and the applied voltage across the buses is at the nominal value of source 18, as illustrated in FIGS. 2 and 3. The circuit breaker 12 can be employed as a main switch for opening the bus, either at the source and/or load end thereof. Furthermore, it can be employed as a switch for a branch line on a transmission line. Thus, the circuit breaker I2 is a special purpose application of a generic switch.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

We claim:

1. A DC circuit interrupter comprising a DC electric line and transfer switch, contacts connected in said DC electric line to open said DC electric line; i

an electronic switch capable of interrupting DC current initially flowing therethrough connected in parallel to said transfer switch contacts;

a series combination of first consecutive interrupter contacts and first resistor connected in parallel to said transfer switch contacts;

a series combination of second consecutive interrupter contacts and a resistor connected in parallel to said transfer switch contacts, a capacitor connected across said second consecutive interrupter contacts; and

control means connected to operate said transfer switch contacts, said electronic switch, said first consecutive interrupter contacts and said second consecutive interrupter contacts for successive opening of said transfer tronic switch to be conductive during each of the successive openings and nonconductive intermediate the openings so that said electronic switch is conductive during opening of said contacts to stop current flow through said contacts and permit them to deionize.

2. The interrupter of claim 1 wherein said first and second resistors respectively in series with said first and second consecutive interrupter contacts are nonlinear resistors.

3. The interrupter of claim 2 wherein a series combination of a surge capacitor and a surge suppression resistor is connected in parallel to said transfer switch contacts to absorb current upon electronic switch opening.

4. The interrupter of claim 3 wherein said series connected surge capacitor and surge suppression resistor are connected in parallel across said second consecutive interrupter contacts.

5. The interrupter of claim 4 wherein fault sensing means is connected to said DC line to detect a fault therein, said fault sensing means being connected to said control means so that upon the sensing of a fault in said DC line, said control means causes said interrupter to open.

6. The interrupter of claim 5 wherein said electronic switch is a crossed field switch tube.

7. The interrupter of claim 5 wherein said electronic switch is a liquid metal cathode switch tube.

8. The interrupter of claim 1 wherein fault sensing means is connected to said DC line to detect a fault therein, said fault sensing means being connected to said control means so that upon the sensing of a fault in said DC line, said control means causes said interrupter to open.

9. The interrupter of claim 1 wherein said electronic switch is a crossed field switch tube.

10. The interrupter of claim 1 wherein said electronic switch is a liquid metal cathode switch tube.

11. The method of interrupting a DC circuit having direct current flowing through transfer switch contacts serially connected in the circuit and having connected in parallel to the transfer switch contacts and in parallel to each other an electronic switch, a first consecutive interrupter having serially connected contacts and resistance, and a second consecutive interrupter having serially connected contacts and resistance comprising the consecutive steps of:

opening the transfer switch contacts while the electronic switch is conductive to permit deionization and discontinuance of current flow through the transfer switch contacts; making the electronic switch nonconductive while said contacts of the first consecutive interrupter are closed to cause DC current flow through said the consecutive interrupter contacts and its respective series resistance to increase circuit impedance and reduce current flow therein; opening the contacts of the first consecutive interrupter while the electronic switch is conductive to permit deionization and discontinuance of current flow through the contacts of the first consecutive interrupter; and terminating conduction through the electronic switch while the contacts of said second consecutive interrupter are closed to cause current fiow through the consecutive interrupter contacts and respective resistance to further increase circuit impedance. 12. The process of claim 1 including the further additional steps of:

' opening the contacts of the second consecutive interrupter while said electronic switch is conductive to permit deionization and discontinuance of conduction through the contacts of said second consecutive interrupter; and

causing discontinuance of conduction through the electronic switch and absorbing the off-switching voltage pulse in a capacitor connected in parallel to said transfer switch contacts.

* k k i 253 3 UNITED S ATES PATENT OFFICE CERTIFICATE OE CORRECTION Patent 3,641,358 Dated v February 8, 1972 l Kenneth T. Lian et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 64, cancel "1" and substitute ll. (claim 12, line 1) Signed and sealed this 8th dayofMay 1973.

(SEAL) Attest:

EDWARD I-LFLETgIHERJR. B T GOTTSCHALK Attesting Officer -v Commissioner of Patents

Claims

1. A DC circuit interrupter comprising a DC electric line and transfer switch, contacts connected in said DC electric line to open said DC electric line; an electronic switch capable of interrupting DC current initially flowing therethrough connected in parallel to said transfer switch contacts; a series combination of first consecutive interrupter contacts and first resistor connected in parallel to said transfer switch contacts; a series combination of second consecutive interrupter contacts and a resistor connected in parallel to said transfer switch contacts, a capacitor connected across said second consecutive interrupter contacts; and control means connected to operate said transfer switch contacts, said electronic switch, said first consecutive interrupter contacts and said second consecutive interrupter contacts for successive opening of said transfer switch contacts, said first consecutive interrupter and said second consecutive interrupter and for causing said electronic switch to be conductive during each of the successive openings and nonconductive intermediate the openings so that said electronic switch is conductive during opening of said contacts to stop current flow through said contacts and permit them to deionize.

10. The interrupter of claim 1 wherein said electronic switch is a liquid metal cathode switch tube. 11. The method of interrupting a DC circuit having direct current flowing through transfer switch contacts serially connected in the circuit and having connected in parallel to the transfer switch contacts and in parallel to each other an electronic switch, a first consecutive interrupter having serially connected contacts and resistance, and a second consecutive interrupter having serially connected contacts and resistance comprising the consecutive steps of: opening the transfer switch contacts while the electronic switch is conductive to permit deionization and discontinuance of current flow through the transfer switch contacts; making the electronic switch nonconductive while said contacts of the first consecutive interrupter are closed to cause DC current flow through said the consecutive interrupter contacts and its respective series resistance to increase circuit impedance and reduce current flow therein; opening the contacts of the first consecutive interrupter while the electronic switch is conductive to permit deionization and discontinuance of current flow through the contacts of the first consecutive interrupter; and terminating conduction through the electronic switch while the contacts of said second consecutive interrupter are closed to cause current flow through the consecutive interrupter contacts and respective resistance to further increase circuit impedance.

12. The process of claim 1 including the further additional steps of: opening the contacts of the second consecutive interrupter while said electronic switch is conductive to permit deionization and discontinuance of conduction through the contacts of said second consecutive interrupter; and causing discontinuance of conduction through the electronic switch and absorbing the off-switching voltage pulse in a capacitor connected in parallel to said transfer switch contacts.