BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to load tap changer apparatus, and more specifically to load tap changer apparatus having protective circuitry for monitoring its operation.
2. Description of the Prior Art
Tap changer apparatus for changing taps on a transformer winding without interrupting load current, commonly utilizes a no-load type tap selector switch having first and second movable contact arms, a divided reactor having first and second winding portions for reducing the magnitude of circulating 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 in a 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, 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 includes 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 connected 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, allowing the 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 assignee as the present application.
It is important not to move a no-load tap selector switch contact arm to a new contact position when load current is flowing therethrough. For example, this might 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 arrangements have been used to monitor tap changer systems, to prevent damaging the contact arms of the selector switch, but all have had disadvantages, such as providing incomplete protection, or being relatively costly, or both. For example, protective circuits that monitor the vacuum of the 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 art protective arrangements for load tap changer systems are costly to manufacture, due to the necessity of insulating the protective circuitry, 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 and improved protective apparatus for load tap changer systems which protects the tap selector switch from damage due to a malfunction of the tap changer apparatus, and which has a relatively low manufacturing cost regardless of the voltage level of the transformer apparatus and the tap changer system that the protective circuitry is associated with.
SUMMARY OF THE INVENTION
Briefly, the present invention is a loadtap changer system 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 used to 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
The invention may be more readily understood when considered in view of the following detailed description of exemplary embodiments thereof, taken with the accompanying drawings, in which:
FIG. 1 is a schematic diagram of load tap changer apparatus and protective circuitry constructed according to a first embodiment of the invention; and,
FIG. 2 is a schematic diagram of load tap changer apparatus and protective circuitry constructed according to another embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
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 illustrated in FIG. 1, tap changer system 10 may be connected to the windings 12, 14 and 16 of an electrical transformer. The transformer may be single or polyphase, 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 stationary contacts C1 through C8 connected to taps T1 through T8, respectively, on winding 14, 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 sequentially moving between the spaced stationary contacts C1 through C9. The ends of tapped winding 14 are connected to the stationary contacts 26 and 28 of a reversing switch 30, which has a movable contact arm 32 connected to winding 16, and to stationary contact C9 of tap selector switch 20. 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 C9, and the other contact arm is in transition to or from contact C9, to add the tap voltage to, or subtract it from, the voltages of windings 12 and 16, depending upon the position of the reversing switch 30.
In order to enable the movable contact arms 22 and 24 to be connected to adjacent taps, and thus bridge a portion of winding 14, and 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 10 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 12. 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 windings of 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.
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 U.S. 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 C1 and C2. In the absence of a magnetic field, the emitter current flow divides equally between the two collector electrodes C1 and C2. 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 C1 and C2 are connected to a source of 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 terminal 84. Typical power supply voltage magnitudes for transducer 72 are shown in FIG. 1.
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 electrodes C1 and C2 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 C5 and it is desired to operate on both contacts C4 and C5, 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 C4. 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 57 and the output of transducer 72 will be balanced. Thus, there will be no 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 arm 22 will then move to contact C4, 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 10' 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 switch 50. 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 70 shown 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 beyond the 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. In other 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 arm 24, associated with the first branch circuit, is to be moved during the tap change cycle, the drive means 100 will close switch 96' 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 is provided for tap selector switching means 20, as it protects the tap selector switch 20 against malfunction of either the bypass switch 52 or the transfer switch 50.
In summary, there has been disclosed new and improved tap changer apparatus and associated protective 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, while being spaced from the conductors beyond the voltage jump distance. Therefore, the protective circuitry need only be insulated for the low control voltage to ground, substantially reducing the manufacturing cost of the apparatus.