US3602807A

US3602807A - Load tap changer apparatus with magnetic transducer protective circuitry

Info

Publication number: US3602807A
Application number: US29157A
Authority: US
Inventors: Herbert L Prescott
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1970-04-16
Filing date: 1970-04-16
Publication date: 1971-08-31
Anticipated expiration: 1988-08-31
Also published as: CA926938A; ZM7471A1; FR2086115A1; JPS465023A; FR2086115B1

Abstract

Load tap changer apparatus, including a tap selector switch, a load transfer switch, and a protective circuit having a magnetic transducer which magnetically checks for current flow through a predetermined conductor of the apparatus, while being spaced from the predetermined conductor beyond the voltage jump distance to ground. The protective circuit utilizes an electrical signal from the magnetic transducer to prevent the operation of the tap selector switch when current is flowing through the predetermined conductor.

Description

United States Patent [72] inventor Herbert L. Prescott Brookfield, Ohio [2 l Appl. No. 29,157

[22] Filed Apr. 16, 1970 [45] Patented Aug. 31, 1971 [73] Assignee Westinghouse Electric Corporation Gateway Center, Pa.

[54] LOAD TAP CHANGER APPARATUS WITH MAGNETIC TRANSDUCER PROTECTIVE CIRCUITRY 8 Claims, 2 Drawing Figs.

[52] US. Cl. 323/435 R, 317/11, 317/14 [5 1] Int. C H02p 13/06, H02p 13/04, 02h 7/00 [50] Field of Search 323/43.5 R,

43.5 S; ZOO/144; 317/11, 14

[56] References Cited UNITED STATES PATENTS 3,206,580 9/1965 McCarty 200/ l 44 3,395,327 7/1968 Kaiser et al 323/435 RX 3,436,646 4/1969 Prescott 323/43.5 R

Primary Examiner--Gerald Goldberg Attorneys-A. T. Stratton, F. E. Browder and D. R. Lackey ABSTRACT: Load tap changer apparatus, including a tap selector switch, a load transfer switch, and a protective circuit having a magnetic transducer which magnetically checks for current flow through a predetermined conductor of the apparatus, while being spaced from the predetermined conductor beyond the voltage jump distance to ground. The protective circuit utilizes an electrical signal from the magnetic transducer to prevent the operation of the tap selector switch when current is flowing through the predetermined conduc- I01.

PATENTEU M1831 Ian 3,602,807

FIG.2.

INVENTOR Herbert L. Prescott 2 ATTORNEY Low m cusses]: APPARATUS wmr MAGNETIC TRANSDUCER PROTECTIVE CIRCUITRY BACKGROUND OF THE INVENTION lating currents when the tap changer apparatus is bridging two taps, a load transfer switch, such as a vacuum switch, and a bypass switch which shorts the vacuum switch when the tap changer is not ina-tap change cycle, to reduce the heating of the vacuum switch contacts. The bypass switch has first and second contacts connected in first and second branch circuits,

i and a third contact connected to the main power circuit, and is actuable to connect the main power circuit to either or both of the first and second branch circuits. The first branch. circuit includes. the first contact arm of the tap selector switch, the

first winding portion of the divided reactor, and the first contact of the bypass switch, and the second branch circuit ineludes the second contact arm of the tap selector switch, the second winding portion of the divided reactor, and the second terminal of the bypass switch. The vacuum switch is conv ncctcd between the first and second terminals of the bypass switch. During a tap change cycle, the bypass switch opens a preselected branch circuit, without substantial arcing, as the vacuum switch is closed at this point, with. the current flow transferring from the opening side of the bypass switch to the vacuum switch. The vacuum switch then opens its contacts to isolate the selected branch circuit, allowingthe tap selector contact arm connected in the isolated branch circuit to move to a new tap position without arcing. The vacuum switch and bypass switch then sequentially reclose to complete the tap change operation. A tap changer system of this type, and an operating mechanism for operating the vacuum switch, is described in. copending application Ser. No. 792,349, filed Jan. 21, 1969, which application is assigned to the same as- I signee as the present applications It is important not to move a no-load tap selector switch contact arm to a new contactposition when load current is flowing therethrough.. For example, this mightv occur due to the failure of the. bypass switch to open the branch circuit of the tap selector switch contact arm, or failure of the vacuum switch to interrupt load. current, such as due'to loss of vacuum, or failure of the operating mechanism. to actuate the vacuum switch to its open. position. Many prior art. protective arrange ments. have been. used to monitor tap changer systems, to prevent damaging. the contactv arms of the selector switcm but all have. had disadvantages, such as providingincompletc protectiomor being relatively costly, or both. For example, protective circuits that monitor the vacuum ofthe vacuum switch do not indicate whether the bypass switch has operated. properly, or whether the vacuum switch. operating mechanism has opened the contacts. Circuits which monitor current flow through the vacuum switch do not indicate whether the bypass switch has. operated properly. Also, prior'artprotective arrangements. for load tap changer systems are costly to manu facturc, due. to the necessity of insulating the protective cir cuitry, such as current transformers, and the like, for the full voltage of the power circuit to ground.

Thus, it would be desirable to provide new andiruproved protective apparatus for load tap changer systems which protects the tapselector switch from damage due to a malfunction of the tap changer apparatus, and which has a relatively low manufacturing cost regardless or thevoltage level. at? the SUMMARY or The INVENTION Briefly, the present invention is a loadtap changersy'stem which includes new and improved protective apparatus for monitoring the operation of the tap changer and preventing the operation of its no-load tap selector switch when current is flowing through the contact arm of the transfer switch scheduled to move to a different tap position in a tap change cycle. The protective apparatus includes a magnetic transducer disposed in the magnetic field produced by current flow through a predetermined conductor of the tap changer apparatus, which conductor should not have current flow therein when the tap selector switch is operated. An output signal from the magnetic transducer, immediately prior to the scheduled operation of the tap selector switch contact arm, is usedto either deenergize the transformer the tap changer system is "operated with, or to disable the tap changer and prevent the operation of the tap selector switch. Or, it may be used to initiate any type of desired protective function. The magnetic transducer, while being within, and sensitive to, the magnetic field provided by current flow through the predetermined conductor, is spaced therefrom beyond the voltage jump distance to ground, enabling the protective apparatus to be operated near ground potential without being insulated for the voltage of the tap changer system to ground BRIEF DESCRIPTION OF THE DRAWING and protective circuitry constructed according to a first em-' bodiment of the invention; and,

FIG. 2 is a schematic diagram of load tap changer apparatus and protective circuitry constructed according to another em bodiment of the invention.

DESCRIPTION or rm: PREFERRED EMBODlMENTS I Referring now to the drawings, and FIG. 1 in particular, there is shown a schematic diagram of tap changer apparatus 10 which may utilize the teachings of the invention. As illus trated in FIG. 1, tap changer system 10 may be connected to the windings 12, I4 and i an electrical transformer. The transformer may be single or polyphas'e, and either of the autotransformer or isolated winding type, with only a portion of a single phase being illustrated in FIG. 1, as other phases would. be similarly arranged.

The tap changer system 10 is of the type which includes a no-load type tap selector switch 20, having a plurality of sta tionary contacts Ct through C8- connecmd to taps Tl through T8, respectively, on winding l4, and a stationary contact C9 connected to winding 16. Tap selector switch 20 has a pair of movable contact arms 22- and 24 for selectively and sequen tially' moving between the spaced stationary contacts CI through C9. The ends oi? tapped winding 14 are connected to the stationary contacts-'26 and 28 of a reversing switch 30,- wltich hasa movable contact arm 32 connected to winding. i6,

and to stationary contact C9 of tap selector switch 2th The reversing switch 30 may be actuated to change its movable contact arm 32 from one stationary contact to the other, when one of the movable contact arms 22 and 24 of the tap selector switch 20 is in engagement with the stationary contact (11am! the other contact arm is in transition to orfr'om contact C9, to add the tap voltage to, or subtract it from, the voltages of windings l1 and I6, depending upon .the position of the reversing switch 30'.

lo order to enable the movable contact arms 27; and 24 tobe I connected to adjacent taps, and thus bridge a portion of wind ing N also enable a: tap changer system to operate continuously in the bridging position and obtain a voltage half way between the voltage of the two adjacent taps, the contact arms are connected to winding 12 through a split or divided reactor 40 having winding

portions

42 and 44 disposed on a common magnetic core 46. The windings are wound to present a high impedance to circulating currents, while providing very little impedance to power current flow in the same direction through the two windings.

A single arcing duty load transfer switch, such as the normally closed vacuum switch 50 shown in FIG. 1, and a bypass switch 52, complete the tap changer system 10, with the bypass switch 52 having first and second

stationary contacts

54 and 56, and a movable contact 58. The movable contact 58 is connected to winding 12, and the

stationary contacts

54 and 56 are connected to winding

portions

42 and 44 of reactor 40. The movable contact 58 is arranged to engage both

stationary contacts

54 and 56, or to individually select either of the stationary contacts. The vacuum switch 50 has contacts 62 and 64 disposed within an evacuated envelope, with one of the contacts being movable relative to the other via a bellows, which maintains the vacuum seal. The vacuum switch 50 is connected across the

contacts

54 and 56 of the bypass switch 52, via

conductors

55 and 57.

When the tap changer system is in a steady state position, the power circuit of the transformer includes winding 16, the portion of winding 14 between the selected position of reversing switch 30 and the tap or taps selected by the contact arms 22 and 24, through the two branch circuits of the contact arms, and winding l2. The first branch circuit includes contact arm 24, winding section 42, and the position of the bypass switch 52 which includes stationary contact 54; and, the second branch circuit includes contact arm 22, winding section 44, and the position of bypass switch 52 which includes stationary contact 56. The branch circuits combine in the movable contact 58 of bypass switch 52, and the power circuit continues to winding 12. Instead of having tapped winding 14 connected between two windingsof the transformer, it may also be disposed at either end of a main transformer winding.

The vacuum switch 50 has its contacts normally closed, but since it is normally shorted by the bypass switch 50, there is negligible current flow therethrough. Therefore, the contacts of the vacuum switch are not heated by the current flowing in the transformer windings. I

Tap changer apparatus 10 includes protective apparatus 70 for monitoring the operation of the tap changer, and for protecting the tap selector switch 20 against operation while load current is flowing through the contact arm to be moved. Protective apparatus 70 includes a magnetic transducer 72 of the type which provides an electrical output signal when it is subjected to a magnetic field produced by current flowing in a conductor, and which has a sensitivity which enables it to be responsive to the magnetic field produced by the current flow, while being spaced from the conductor beyond the voltage jump distance. A magnetic transducer which may be used is disclosed in US. Pat. No. 3,389,230, which describes a magnetically sensitive transistor having a base electrode b, an emitter electrode e, and first and second collector electrodes C, and C In the absence of a magnetic field, the emitter current flow divides equally between the two collector electrodes C and (3,. A bias circuit may be added to insure balanced flow in the absence of current in the conductor to be monitored. The presence of a magnetic field due to current flow through the conductor to be monitored results in more current flowing to one of the collector electrodes than'to the other. The collector electrodes C and C, are connected to a sourceof electrical potential at terminal 76, via

resistors

78 and 80 respectively. The base electrode b is connected to a source of electrical potential at terminal 82, and the emitter electrode e is connected to a source of electrical potential at terminal84. Typical power supply voltage magnitudes for transducer 72 are sbowninFiGB.

In this embodiment of the invention, transducer 72 is disposed in the magnetic filed 86 produced by current flowing through the vacuum switch 50, such as adjacent conductor 57, but the transducer 72 is spaced from conductor 57 beyond the voltage jump distance therefrom for the particular voltage magnitude applied to conductor 57. Typical clearances are 6.5 inches for 69 kv. and 14.5 inches for 115 kv.

Other magnetic transducers having the required sensitivity may be used, such as transducers of the Hall generator type.

The unbalanced current flow to the two collector electrodes of transducer 72, when transducer 72 is subjected to a magnetic field, may be detected and amplified by'differential amplifier 74, which is connected to collector ele'ctrodesC and C via

conductors

88 and 90 respectively, and the'output of the differential amplifier 74 is applied to the electromagnetic coil 92 of a solenoid 94, via a cam operated switch 96. Solenoid 94 mechanically actuates its contacts 98 to their closed position, when the electromagnetic coil 92 is energized, with the contacts 98, in this embodiment, being connected in an electrical circuit associated with the tap changer drive means 100. The tap changer drive means 100 may include a reversible electric motor and control shaft, with suitable mechanical linkages and cams being associated with the control shaft for operating the bypass switch 52, vacuum switch 50, tap selector switch 20, and switch 96, in a predetermined sequence, when it is desired to change tap connections. The mechanical linkages and cams between the drive means 100 and these devices are shown generally by the dotted lines 102.

The protective apparatus 70 magnetically monitors the current flow through the transfer switch 50, and it checks for current flow immediately prior to the operation of the tap selector switch 20 during a tap change cycle, via the cam operated switch 96, which is mechanically closed-by the tap changer drive 100. An output signal from differential amplifier 74 indicates current flow through the transfer switch 50, and the impending operation of the tap selector switch should be prevented. Thus, the contacts 98 of solenoid 94 are shown connected in a circuit associated with the drive means 100', to disable the drive means 100 and prevent the operation of the tap selector switch 20. However, the output signal from differential amplifier 74 may by used in other suitable circuits, such as a circuit which deenergizes the transformer, if the application is such that deenergizing the transformer is not detrimental to the load.

While the contacts 98 of solenoid 94 are shown normally open, they may also be normally closed and used in a circuit designed to effect the desired protective function with normally closed contacts. Also, the switch 96 is illustrated as being normally open, but it could be normally closed and opened by the drive means only when it is desired to check for current flow through the transfer switch. For example, the switch 96 could be normally closed and connected across the output terminals of differential amplifier 74, opening in response to drive means 100 just prior to the operation of the tap selector switch 20.

In the operation of the tap changer apparatus 10, assume that both tap selector arms 22 and 24 are in contact with contact C and it is desired to operate on both contacts C and C such as signalled by a voltage sensing circuit (not shown). Tap changer drive means 100 will sequentially operate bypass switch 52 to open the second series branch, since tap selector arm 22 must eventually move to contact C When bypass switch 52 opens the second series branch, the circuit through winding section 44 and tap selector switch contact arm 22 is maintained through the vacuum switch 50, thus creating very little arcing at the contacts of the bypass switch 52. Then, the vacuum switch 50 is actuated to open its contacts and completely isolate the second series branch. Arcing occurs at the contacts of the vacuum switch, but the arc is quickly extinguished due to the vacuum environment surrounding its contacts 62 and 64. Then, switch 96 is closed by drive means 100, to initiate the protective function of protective apparatus 70. If there has been no malfunction in the prior steps of the tap change cycle, i.e., the vacuum switch operating mechanism opened the contacts of the vacuum switch 50, and there has been no loss of vacuum in the vacuum switch there will be no current flow in conductor 5.7 and the output of transducer 72 will be balanced. Thus, there will beno output from differential amplifier 74 and the solenoid 94 will not be energized when switch 96 is closed. The tap change cycle is, therefore, allowed to continue. If current is flowing in conductor 57 when switch 96 is closed by drive means 100, the output of transducer 72 will be unbalanced, resulting in an output voltage from differential amplifier 74 which energizes the electromagnetic coil 92 of solenoid 94, causing it to close its contacts 98 and disable the tap changer drive means 100.

Assuming that the tap changer had operated normally and there was no current flowing through conductor 57, contact am 22 will then move to contact C vacuum switch 50 will close its contacts to reestablish the circuit through winding portion 44 of reactor 40 and through the contact arm 22, and bypass switch 52 will reestablish the second branch circuit. A tap change involving movement of contact arm 24 would be similar to that described for movement of contact arm 22, except the bypass switch would open the first series branch, instead of the second series branch.

Protective apparatus 70 shown in FIG. 1 will protect the tap selector switch 20 from malfunction of the vacuum switch 50, or of the vacuum switch operating mechanism. However, it will not'protect it against malfunction of the bypass switch 52. FIG. 2 is a schematic diagram of tap changer apparatus having protective apparatus 70 and 70' which provides complete protection for the tap selector switch 20, protecting it against malfunction of both the bypass switch 52 and of the vacuum switch50. The

tap changer systems

10 and 10 shown in FIGS. 1 and 2, respectively, are similar in construction, with like reference numerals being used to indicate like components.

The protective apparatus 70wshown in FIG. 2, is the same as the protective apparatus 70-, shown in FIG. 1, except transducer 72 is disposed to be responsive to a magnetic field 110 produced by current flow through the second branch circuit, including the contact arm 22,..and a similar protective circuit 70' includes a magnetic transducer 72"disposed to be responsive to a magnetic field 112 produced by current flow through the first branch circuit, including contact arm 24. Similar to the first embodiment of the invention it is important that the magnetic transducers 72 and 72' be spaced from their associated branch circuits beyondthe voltage jump distance.

' Switches 96 and 96 are responsive to drive means 100, with the switches closing just prior to, the operation of the tap selector switch associated with the series branch which they are monitoring. lnother words, if contact arm 22, associated with the second series branch,is to be moved, the drive means 100 will close switch 96 just prior to the operation of contact arm 22. if contact arm24, associated with the first branch circuit, is to be moved during the tap change cycle, the drive means 100 will close switch 6' just prior to the scheduled operation of contact arm 24. By monitoring the'current flow in the first and second branch circuits associated with contact arms 24 and 22, respectively, complete protection isprovided for tap selector switching means 20, as it protects the tap selector switch 'againstmalfunction of eitherthe bypass switch 52 or the transfer switch 50.

in summary, there has beendisclosed new and improved tap changer apparatus and associatedprotective and monitoring circuitry, which protects the no-load tap selector switch of the apparatus against operation while current is flowing therethrough. Further the protective function is provided without requiring that the protective apparatus be insulated for the full transformer voltage to ground. The magnetic transducers utilized in the protective circuitry are responsive to the magnetic field produced by current flow through predetermined conductors, whilebeing spaced from the conductors beyond the voltage .jumpdistance. Therefore, the. protective circuitry need only be insulatedfor the low control voltage to ground, substantially reducing the manufacturing cost of the apparatus.

[claim as my invention:

l. A load tap changer system comprising:

an electrical circuit including a winding having a plurality of taps, I

tap selector switching means having first and second contact arms selectively movable between a plurality of electrical contacts connected to taps on said winding, reactor means having first andsecond winding portions; bypass switching means having first, second and third terminals, actuable to connect the third terminal to either or both of the first and second terminals, said third terminal being connected in said electrical circuit, I i

means serially connecting said first contact arm and the first winding portion of said reactor means to the first terminal of said bypass switching means, providing a first series branch,

means serially connecting said second contact arm and the second winding portion of said reactor means to the second terminal of said bypass switching means, providing a second series branch,

a load transfer switch connected between the first and second terminals of said bypass switching means, drive means for operating said bypass switching means, said transfer switch, and said tap selector switching means in a predetermined sequence, when it is desired to change the operating position of said tap selector switching means,

magnetic transducer means disposed in the magnetic field produced by current flowing through said transfer switch, but spaced therefrom beyond the jump distance to ground for the voltage magnitude applied to said transfer switch, said magnetic transducer means providing an electrical signal only when current is flowing through said transfer switch, and protective means connected to said magnetic transducer means, said protective means being responsive to the electrical signal from said magnetic transducer means, preventing the operation of said tap selector switching means while current is flowing through said transfer switch. 2. The load tap changer system of claim 1 wherein the load transfer switch is a vacuum switch.

3. The load tap changer system of claim 1 wherein the protective means includes a camoperated switch actuatedby the drive means prior to the operation of the tap selector switching means, to check for current flow through the transfer switch, said cam operated switch being connected to render the protective means operative to prevent the operation of the tap selector switching means while current is flowing through the transfer switch.

4. The load tap changer system of claim 1 wherein the drive means is mechanically linked to the bypass switching means, the transfer switch, and the tap selector switching means, such that when a tap change is desired the drive means sequentially actuates the'bypass switching means to open one of the series branches, opens the transfer switch, moves the contact arm of the tap selector switching means associated with the open series branch, to a different electrical contact, closes the vacuum switch, and actuates the bypass switching means to close the open series branch.

5. A load tap changer system comprising: an electrical circuit including a windinghaving a plurality of taps,

tap'selector switch means having first and second contact arms selectively movable between a plurality of electrical contacts'connected to taps on said windin g,

reactor means having first and second winding portions,

bypass switching means having first, second and-third terminals, actuable to connect the third terminal to either or both of the first and second terminals, said third terminal being connected in said electrical circuit,

means serially connecting said first contact arm and'the'first winding portion of said reactormeans to the first terminal of said bypass switching means, providinga first'series branch,

a load transfer switch connected between the first and second terminals of said bypass switching means,

drive means for operating said bypass switching means, said transfer switch, and said tap selector switching means in a predetermined sequence when it is desired to change the operating position of said tap selector switching means,

first and second magnetic transducer means disposed in the magnetic field produced by current flowing through said first and second series branches, respectively, but spaced from said first and second series branches beyond the voltage jump distance to ground, said first and second magnetic transducer means providing electrical output signals in response to current flow through said first and second series branches, respectively,

and protective means connected to said first and second magnetic transducer means, said protective means being responsive to the electrical signals from said first and second magnetic transducer means, preventing the operation of said tap selector switching means when current is flowing through the series branch associated with the contact arm of the tap selector switching means which is to be actuated.

6. The load tap changer system of claim 5 wherein the transfer switch is a vacuum switch.

7. The bad tap changer system of claim 5 wherein the protective means includes first and second cam operated switches, operated by the drive means prior to the movement of the first and second contact arms, respectively, of the tap selector switching means, to check for current flow through the first and second series branches, respectively, and render the protective means operative to prevent operation of a contact arm when current is flowing therethrough.

8. The load tap changer system of claim 5 wherein the drive means is mechanically linked to the bypass switching means, the transfer switch, and the tap selector switching means such that when a tap change is desired the drive means sequentially actuates the bypass switching means to open one of the series branches, opens the transfer switch, moves the contact arm of the tap selector switching means associated with the open series branch to a different electrical contact, closes the vacuum switch, and actuates the bypass switching means to close the open series branch.

Claims

1. A load tap changer system comprising: an electrical circuit including a winding having a plurality of taps, tap selector switching means having first and second contact arms selectively movable between a plurality of electrical contacts connected to taps on said winding, reactor means having first and second winding portions, bypass switching means having first, second and third terminals, actuable to connect the third terminal to either or both of the first and second terminals, said third terminal being connected in said electrical circuit, means serially connecting said first contact arm and the first winding portion of said reactor means to the first terminal of said bypass switching means, providing a first series branch, means serially connecting said second contact arm and the second winding portion of said reactor means to the second terminal of said bypass switching means, providing a second series branch, a load transfer switch connected between the first and second terminals of said bypass switching means, drive means for operating said bypass switching means, said transfer switch, and said tap selector switching means in a predetermined sequence, when it is desired to change the operating position of said tap selector switching means, magnetic transducer means disposed in the magnetic field produced by current flowing through said transfer switch, but spaced therefrom beyond the jump distance to ground for the voltage magnitude applied to said transfer switch, said magnetic transducer means providing an electrical signal only when current is flowing through said transfer switch, and protective means connected to said magnetic transducer means, said protective means being responsive to the electrical signal from said magnetic transducer means, preventing the operation of said tap selector switching means while current is flowing through said transfer switch.

2. The load tap changer system of claim 1 wherein the load transfer switch is a vacuum switch.

3. The load tap changer system of claim 1 wherein the protective means includes a cam operated switch actuated by the drive means prior to the operation of the tap selector switching means, to check for current flow through the transfer switch, said cam operated switch being connected to render the protective means operative to prevent the operation of the tap selector switching means while current is flowing through the transfer switch.

4. The load tap changer system of claim 1 wherein the drive means is mechanically linked to the bypass switching means, the transfer switch, and the tap selector switching means, such that when a tap change is desired the drive means sequentially actuates the bypass switching means to open one of the series branches, opens the transfer switch, moves the contact arm of the tap selector switching means associated with the open series branch, to a different electrical contact, closes the vacuum switch, and actuates the bypass switching means to close the open series branch.

5. A load tap changer system comprising: an electrical circuit including a winding having a plurality of taps, tap selector switch means having first and second contact arms selectively movable between a plurality of electrical contacts connected to taps on said winding, reactor means having first and second winding portions, bypass switching means having first, second and third terminals, actuable to connect the third terminal to either or both of the first and second terminals, said third terminal being connected in said electrical circuit, means serially connecting said first contact arm and the first winding portion of said reactor means to the first terminal of said bypass switching means, providing a first series branch, means serially connecting said second contact arm and the second winding portion of said reactor means to the second terminal of said bypass switching means, providing a second series branch, a load transfer switch connected between the first and second terminals of said bypass switching means, drive means for operating said bypass switching means, said transfer switch, and said tap selector switching means in a predetermined sequence when it is desired to change the operating position of said tap selector switching means, first and second magnetic transducer means disposed in the magnetic field produced by current flowing through said first and second series branches, respectively, but spaced from said first and second series branches beyond the voltage jump distance to ground, said first and second magnetic transducer means providing electrical output signals in response to current flow through said first and second series branches, respectively, and protective means connected to said first and second magnetic transducer means, said protective means being responsive to the electrical signals from said first and second magnetic transducer means, preventing the operation of said tap selector switching means when current is flowing through the series branch associated with the contact arm of the tap selector switching means which is to be actuated.