EP0355814A2

EP0355814A2 - A force storage mechanism in an on-load tap changer

Info

Publication number: EP0355814A2
Application number: EP89115577A
Authority: EP
Inventors: Shiro Yokohashi
Original assignee: AICHIDENKI KK; Aichi Electric Co Ltd
Current assignee: AICHIDENKI KK; Aichi Electric Co Ltd
Priority date: 1988-08-26
Filing date: 1989-08-23
Publication date: 1990-02-28
Also published as: EP0355814A3; JPH0821507B2; JPH0260112A

Abstract

An operation frame (26) with two flanges (29, 30) and a drive frame (27) with a single flange (33) disposed between the two flanges (29, 30) are provided for forth and back movement, relative to a base frame (20), guided by a guide bar (24). Two coil spring elements (36, 37) are passed through by the guide bar (24) and are interposed between adjacent two of the flanges (29, 33; 30, 33). When the drive frame (27) is transferred to be deviated relative to the operation frame (26) held at a first operation point, one of the spring elements (36, 37) is compressed and the other (37, 36) tensioned. When the drive frame (27) is transferred to a prescribed position, the holding action on the operation frame (26) is released by a release piece (49) secured on the drive frame (27) and the operation frame (26) is swiftly transferred to a second position by the biasing forces stored in the spring elements (36, 37), operating a switch (5) in an on-load tap changer. If the drive frame (27) is actuated in the reverse direction on the next occasion, the operation frame (26) is transferred from the second to the first operation point similarly by the biasing forces stored in the spring elements (36, 37).

Description

Background of the Invention

Field of the Invention

This invention relates to an on-load tap changer which is used in order to change over swiftly the taps of a transformer, a reactor or the like without interrupting service, and more particularly to a force storage mechanism which is provided in the on-load tap changer for performing the above mentioned swift change-over operation.

Description of the Prior Art

As a force storage mechanism of the above mentioned type is known one having the following structure. Namely, an operation frame for operating a switch is mounted on a base frame for forth and back movement along a guide bar between a first operation point and a second operation point. Furthermore, a drive frame for providing a force storage spring with a stored force is mounted on the guide bar for forth and back movement. One of two flanges which are adapted to get away slidably from each other along the direction of the above mentioned forth and back movement is provided at one end of the operation frame and the other of the flanges is provided at one end of the drive frame at the same side as the one end of the operation frame. Similarly two flanges are provided as well at the other end of the operation frame and the other end of the drive frame, respectively. A force storage spring which is adapted to have a force stored by compression is interposed between the two flanges at the one end and the two flanges at the other end. The guide bar is inserted through this force storage spring. The base frame is provided with two holders for holding the operation frame at the first and the second operation points, respectively. On the other hand, the drive frame is provided with a release member which is adapted to release the holding action on the operation frame by the holders as the result of the deviation of the drive frame, relative to the operation frame, to a prescribed position (DE-A-19 563 690).
In the force storage mechanism of this type, when the operation frame is held at the first operation point, the drive frame is transferred along the guide bar so that the drive frame may deviate relative to the operation frame. Then, the force storage spring is compressed between the flange at one end side of the drive frame and the flange at the other end side of the operation frame and has a stored force. When the drive frame is deviated, relative to the operation frame, to the prescribed position, the holding action on the operation frame is released by the release member. Then the operation frame is swiftly transferred towards the second operation point by the restoring force of the force storage spring. The switch is operated instantly by this swift transfer movement of the operation frame.
In the conventional force storage mechanism mentioned above, the force storage spring to be compressed is of such a large length as ranges from the flange at one end side to the flange at the other end side. Consequently, the force storage spring is liable to bend sideways. If the force storage spring is bent, the bent portion is urged against the guide bar. Moreover, the urged portion is rubbed on the guide bar in the processes of its compression and restoration. While the force storage spring is rubbed in this manner, the spring and the guide bar are worn out to produce metallic powder. The metallic powder deteriorates the electric insulation between the conductive parts in the tap changer. For example, if the medium provided around the force storage mechanism for the purpose of electric insulation is an insulating oil, there is brought about a problem that the metallic powder will float in the oil to deteriorate the electric insulation between the conducting parts. Furthermore, if the medium is an insulating gas such as sulfur hexafluoride gas, the metallic powder scattered will be attached on various insulating parts and deteriorate the electric insulation between the conducting parts. Moreover, if the metallic powder floats in the neighbourhood of the conducting parts, an electric discharge will take place and dissolve the sulfur hexafluoride gas to produce sulphurous gas and hydrogen fluoride. These gases sometimes worsen or erode the metallic parts in the tap changer and thus the use of the tap changer for a long time under stable conditions comes into question. In addition, there appears such a problem that the break off accident of the force storage spring will occur when the wear of the force storage spring proceeded.

Summary of the Invention

One object of the present invention is to provide a force storage mechanism wherein a force storage spring adapted to have a stored force and a switch is swiftly operated by releasing the stored force essentially along a straight line at a stretch.
In the force storage mechanism according to the present invention, the force storage spring has a stored force as the result of the movement of a drive frame. Then the operation frame can be swiftly transferred from a first operation point to a second operation point or vice versa, at one stretch by the stored force. Consequently, there is such an effect that the switch actuated by the operation frame can be swiftly operated.
Another object of the present invention is to provide a force storage mechanism, even with use of a force storage spring, wherein a small length of the spring suffices.
According to the present invention, the mechanism for storing a force in the force storage spring is as follows. A flange on the drive frame is disposed between two flanges on the operation frame. The force storage spring is composed of two coil spring elements. One of the coil spring elements is interposed between one of the flanges on the operation frame and the flange on the drive frame and the other coil spring element between the other flange on the operation frame and the flange on the drive frame. When the drive frame is transferred relative to the operation frame, either one of the coil spring elements is selectively compressed. Due to such a construction, there is a feature that the axial length of the compressed coil spring element is about half the distance between the two flanges on the operation frame. If the axial length of the coil spring element under compression is small to that degree, there is scarcely any possibility that the coil spring element will bend sideways and rub upon a guide bar. Owing to this fact, there appears an effect that the generation of metallic powder resulting from the rubbing action between the coil spring element and the guide bar is prevented and the deterioration of electric insulation in the tap changer can be prevented beforehand. Moreover, there is a further effect that the break off of the coil spring element due to the rubbing action is prevented and the durability of the element is raised. Furthermore, the above mentioned small length of the compressed coil spring element leads to reduction of the amount of the generated metallic powder even if the coil spring element should bend sideways and rub upon the guide bar. Also in this respect, the mechanism according to the present invention has an effect to prevent the above mentioned deterioration of electrical insulation.
One embodiment of a force storage mechanism in an on-load tap changer is described with reference to the drawing as an example.

Brief Description of the Drawings

Fig. 1 is a perspective view showing the main portion of an on-load tap changer;
Fig. 2 is a longitudinal section of the tap changer;
Fig. 3 is a section taken along line III-III in Fig. 2, showing a force storage mechanism;
Fig. 4 is a horizontal section showing a hold mechanism, a buffer mechanism and an output mechanism;
Fig. 5 is a section taken along line V-V in Fig. 4, for explaining the output mechanism;
Fig. 6 is a section taken along line VI-VI in Fig. 3;
Fig. 7 is an elevation of an extended cam groove;
Fig. 8 is a plan view showing the arrangement relationship of a switch drive mechanism;
Fig. 9 is a view showing the state where a biasing force is stored in a force storage spring when a drive frame is transferred from the position shown in Fig. 3;
Fig. 10 is a view showing the state which is established after the operation frame has been swiftly transferred by the biasing force of the force storage spring;
Fig. 11 is an electric circuit diagram showing the connection relationship among the windings of a transformer, a tap selector and the tap changer; and
Fig. 12 is a time chart for explaining the operation of the tap changer.

Description of the Preferred Embodiments

In the following embodiments of the present invention are explained with reference to the drawings: An on-load tap changer 1 as shown in Figs. 1 and 2 comprises a main frame 2, a force storage means 3, a switch drive mechanism 4, a plurality of switches 5 (nine switches 5 in the embodiment referred to) and current-limiting resistors 6, the members 3 through 6 being mounted on the main frame 2, respectively.
The main frame 2 consists of a base plate 10, an upper frame 11 disposed over the base plate, a lower frame 12 disposed under the base plate, and connection members 13 connecting the upper and lower frames 11, 12. A support cylinder 14 is mounted on the central portion of the intermediate base plate 10.
The force storage means 3 includes a force storage mechanism 15, an operation mechanism 16 for operating the force storage mechanism and an output mechanism 17 for outputting the force from the force storage mechanism 15.
First, the force storage mechanism 15 includes a base frame 20. The base frame 20 consists of a circular support plate 21 mounted on support rods 22 secured, in turn, against the base plate 10 and of two support walls 23 secured on the support plate 21. Guide bars 24 are mounted between the two support walls 23 of the base frame 20. On the guide bars 24 an operation frame 26 is mounted so that it may be guided by the guide bars 24 for forth and back movement between a first operation point as shown in Fig. 3 and a second operation point as shown in Fig. 10. On the guide bars 24 also a drive frame 27 is mounted for similar forth and back movement.
The structure for mounting these frames is explained next. The operation frame 26 is provided with two flanges 29 and 30. As shown in Fig. 3, each of the flanges 29 and 30 is provided with a holder 31 with a bearing 32 on the inner surface thereof and the inner surface of each bearing 32 forms a guide portion, through which a guide bar 24 is inserted. The structure for mounting the drive frame 27 is similar. Namely, the drive frame 27 is provided with two flanges 33 each having a bearing 34. The inner surface of each bearing 34 forms a guide portion, through which a guide bar 24 is inserted. As shown in Figs. 1 and 2, force storage springs or biasing springs 28 are interposed between the operation frame 26 and the drive frame 27. The state of the interposition of the springs is now explained. The biasing springs 28 consist each of two coil spring elements 36 and 37. As shown in Fig. 3, each coil spring element 36 is interposed between the flange 29 and one of the flanges 33 and each coil spring element 37 between the flange 30 and one of the flanges 33, respectively. As shown in Fig. 3, each holder 31 and each flange 33 is provided with a spring holder 38 and 40, respectively. On the outer circumferential surfaces of the spring holders 38 and 40 helical grooves 39 and 41, respectively, are formed. The end portions of the coil spring elements 36 and 37 are fitted in the helical grooves 39 and 41, respectively. The length of the coil spring elements 36 and 37 is adopted such that each of the coil spring elements 36, 37 is slightly compressed when they are interposed between the flanges 29, 30 and 33. However, such length of the coil spring elements 36, 37 may be selected that they are neither compressed nor tensioned or be slightly tensioned when they are interposed.
As shown in Figs. 3 and 4, the base frame 20 is provided with a hold mechanism 42 for holding the operation frame 26 at the first and the second operation points, respectively. The hold mechanism 42 is now explained. On the circular support plate 21 pivotal hold members 43 and 44 are mounted by pins 45. Each of the hold members 43 and 44 is L-shaped and is provided with a hold pawl 46 at one end thereof. At the other end thereof each hold member 43, 44 is provided with a driven portion 47. Furthermore, a tension spring 48 is interposed between the aforementioned other end portions of the hold members 43 and 44. As shown in Fig. 6, a release piece 49 is mounted on the drive frame 27 for the purpose of making possible the movement of the operation frame 26 interconnected with the hold members 43 and 44.
Next, as shown in Figs. 1 and 2, buffer mechanisms 51 and 52 are provided between the base frame 20 and the operation frame 26. Each of the buffer mechanisms 51 and 52 comprises a buffer means 53 mounted on the support plate 21 and an abutment piece 54 secured on the operation frame 26. As shown in Fig. 4 each buffer means 53 comprises a case 55 secured on the support plate 21, two buffer pins 56 provided in the case 55 for the forth and back movement in a direction parallel to that of the movement of the operation frame 26 and two compression springs 57 for buffer action interposed between the case 55 and each buffer pin 56. Each buffer pin 56 is provided, at one end thereof, with a contact portion 56a for contacting the abutment piece 54.
Next, the aformentioned operation mechanism 16 is explained. As shown in Fig. 2, a drive shaft 60 is rotatably mounted relative to the upper frame 11 and the base plate 10 by bearings 61 and 62. This drive shaft 60 is connected to an electric drive mechanism not shown. As shown in Figs. 1 and 2, on the drive shaft 60 an eccentric circular disc 63 illustrated as an example of an eccentric member is mounted. On the other hand, sliding members 64 and 65 are mounted on the drive frame 27. These sliding members consist of rollers 66, each of which is rotatably mounted relative to the drive frame 27 by a fastening piece 67. The outer circumferential surface of the roller 66 is in contact with the outer circumferential surface of the eccentric circular disc 63.
The output mechanism 17 is explained in the following. As shown in Fig. 2, a crank member 70 is rotatably mounted relative to the support cylinder 14 by a bearing 71. On the other hand, a transmission member 72 formed with an elongate hole 73 for linking movement is mounted on the operation frame 26, as shown in Figs. 4, 5 and 6. A linking piece 74 secured on the crank member 70 is positioned in the elongate hole 73. As shown in Fig. 4, the crank member 70 is formed with engagement portions 75 and 76 adapted to engage with the hold pawl 46 of the hold members 43 and 44, respectively.
Next, the switch drive mechanism 4 is explained. This mechanism 4 consists of an operation cam 78 and switch operating mechanisms 79, as shown in Fig. 2. The operation cam 78 comprises a circular base member 80 and a cam member 82 mounted on the circumferential surface of the base member 80. The base member 80 is mounted rotatably relative to the support cylinder 14 by a bearing 81 and is integrally connected with the crank member 70. The cam member 82 is made of a material such as ethylene tetrafluoride which is slippery and abrasion proof, and is formed with a cam groove 83 as shown in Fig. 7.
On the other hand, nine switch operating mechanisms 79 are provided in proportion to the number of the switches 5 in the present embodiment and are arranged around the rotation axis of the operation cam 78 as shown in Fig. 8. On the basis of Fig. 2, the structure of one of the switch operating mechanisms 79 built alike is explained. Each switch operating mechanism 79 comprises a casing 85. This casing 85 is engaged in an engage hole 86 made in the base plate 10 in the form of a cut as shown in Fig. 8 and two mounting pieces 87 formed integrally with the casing 85 are secured on the base plate 10. In the casing 85, a movable frame 88 and a drive member 89 are provided for respective vertical movement and a compression spring 90 is interposed between them. A follower member 91 mounted on each movable frame 88 extends outside the casing 85 through a vertically elongated hole 92 formed in the casing 85 and is positioned in the cam groove 83 of the operation cam 78. A guide rod 93 is attached to each drive member 89. A thick upper portion 93a of the guide rod 93 is positioned for vertical movement in a through hole 85a bored in the casing 85. The bottom surface of the thick portion 93a forms an engage portion 94 for engagement with the movable frame 88. Instead of the switch drive mechanism 4, other known mechanisms may be used.
Next, switches 5 are described. For each phase of a three phase alternating current three switcher 5 are provided for such that nine switches are provided for all together. As an example of a switch 5, a vacuum switch is used in the present embodiment, but other known switches may be used also. As is well known, each of the identical vacuum switches 5 consists of an evacuated case member 97, a fixed electrode 98 secured inside the case member 97 and a movable electrode 99 mounted for vertical movement relative to the case member 97. The mounting structure of a switch 5 is explained next. An insulating plate 100 is connected to the base plate 10 by a connection member 101 such as a long bolt in such a manner that the insulating plate 100 is parallel to the base plate 10. A plurality of mounting pieces 102 (their number corresponding to the number of the switches 5) made of electrically conducting material are provided on the insulating plate 100 and the fixed electrode 98 of each switch 5 is secured on the mounting piece 102. An outgoing wire 103 is connected to the mounting piece 102. On the other hand, an electrically neutral ring 104 made of electrically conducting material is mounted on the base plate 10 and the movable electrode 99 of each switch 5 is connected to the neutral ring 104 by a lead wire 105 made of braided wires. One end of a connecting member 106 is connected to the neutral ring 104. The other end of the connecting member 106 is conected to a neutral bushing of a transformer.
The on-load tap changer 1 constructed as mentioned above is put in a container not shown and the container is filled with an insulating medium such as sulfur hexafluoride gas. It is also possible to use insulating oil as the insulating medium.
Next the operation of the on-load tap changer 7 of the structure described is explained. The drive shaft 60 is rotated in the direction shown by an arrow in Fig. 1 from the state shown in Figs. 1 and 2; the eccentric disc 63 is rotated together with the drive shaft 60. When the eccentric disc 63 is rotated, the sliding member 65 is pushed by the eccentric disc 63 and the drive frame 27 is transferred to the left from the state shown in Fig. 3 and is deviated relative to the operation frame 26 as shown in Fig. 9. In Figs. 3 and 9 from the drive shaft 27 only the flange 33 is shown. As the result of the deviation of the drive frame 27, the coil spring elements 37 are compressed as shown in Fig. 9 to have a biasing force stored and at the same time the coil spring element 36 is tensioned to have a biasing force stored. Since in this process, the hold pawl 46 of the hold member 43 holds the engage portion 75 of the crank member 70 as shown in Fig. 4, the operation frame 26 is kept held in the first operation point as shown in Figs. 2 and 3. From the above mentioned state, the eccentric disc 63 is further rotated slightly and the drive frame 27 is slightly transferred in the above mentioned direction. Then, the release piece 49 pushes the driven portion 47 of the hold member 43 and causes the hold member 43 to rotate in the counterclockwise direction in Fig. 4. Then the hold pawl 46 disengages with the engage portion 75. After this disengaging, the operation frame 26 is swiftly transferred by the large biasing forces stored in the coil spring elements 36 and 37 from the first operation point shown in Figs. 2, 3 and 9 to the second operation point shown in Fig. 10. This swift transferring movement is transmitted to the crank member 70 via the transmission member 72 and the linking piece 74 and the crank member 70 is swiftly rotated from the position shown in Fig. 9 to the position shown in Fig. 10. This rotation of the crank member 70 is transmitted to the switch drive mechanisms 4 as the output of the force storage means 3 and the opening and closing action of the switches 5 referred to later is performed.
When the operation frame 26 is swiftly transferred to the second operation point as mentioned above, the transferring movement is stopped, with reduced shock, by the abutment piece 54 in the buffer mechanism 52 abutting on the contact portion 56a in the buffer means 53. Consequently, the operation frame 26, the transmission member 72 and the linking piece 74 in the output mechanism 17 are prevented from being broken. Since the direction of the forth and back movement of the contact portion 56a is parallel to the direction of the forth and back movement of the operation frame 26, the abutment piece 54 abuts along a normal line with the contact portion 56a to reduce shock and they do not rub on each other. Accordingly, a generation of metallic powder resulting from a rubbing movement between the piece 54 and the portion 56a is prevented. Furthermore, when the operation frame 26 reaches the second operation point, the engage pawl 46 in the hold member 44 engages with the engage portion 76 formed on the crank member 70.
Next, the operation is described in the case where the drive shaft 60 is operated to be rotated again in the above mentioned direction. At this time, the drive frame 27 is transferred towards the right from the position shown in Fig. 10 due to the rotation of the eccentric disc 63. In the case of this transferring movement, the transfer of the operation frame 26 is obstructed since the hold pawl 46 of the hold member 44 engages with the engage portion 76. Thus, in accordance with the transfer of the drive frame 27, the coil spring elements 36 are compressed to have a biasing force stored while the coil spring elements 37 are tensioned to have a biasing force stored. Then the release member 49 pushes the driven portion 47 of the hold member 44 and the holding action on the engage portion 76 by the hold pawl 46 is released. As the result, the operation frame 26 is swiftly transferred from the second operation point shown in Fig. 10 to the first operation point shown in Fig. 3. As the result of this transferring movement, the crank member 70 in the output mechanism 17 is swiftly rotated in the direction opposite to that in the above mentioned case and the driving force of the member 70 is transmitted to the switch drive mechanisms 4.
When the drive frame 27 is transferred to the left (or the right) and the biasing force is stored, only the short coil spring elemts 37 (or 36) disposed between the flange 30 (or 29) and the flange 33 is compressed. There is scarcely any possibility that such a short coil spring element as the element 37 or 36 will bend sideways. In addition, since the both ends of the coil spring elements 37 (or 36) are secured to the spring holders 38 and 40, respectively, the possibility of the above mentioned bending is still decreased. Thus, there is a low possibility that the coil spring elements 37 will bend to come into contact with a respective guide bar 24. As the result, the coil spring elements 37 or 36 and the guide bars 24 do not rub on each other and the wear of the spring elements 36, 37 and the guide bars 24 can be savely prevented. This fact eliminates completely the occurence of a trouble due to metallic powder being generated along with the wear of components rubbing against each other as has been the case in the past or due to a breaking of a spring element 37 or 36.
Next, the operation of the switch drive mechanisms 4 and the switches 5 is described in the case where the driving force is output from the crank member 70. When the crank member 70 is rotated, the operation cam 78 is also rotated. This rotation can be visualized from Fig. 7 as a transfer of the cam member 82 to the right or the left. Then, the follower member 91 of each switch operating mechanism 79, positioned in the cam groove 83 is transferred through a tilting portion 83c from a first horizontal portion 83a to a second horizontal portion 83b or vice versa.
The operation of one of the switch operating mechanisms 79 and the corresponding switch 5 in the process where the follower member 91 is transferred from the first horizontal portion 83a to the second horizontal portion 83b is as follows. When the follower member 91 is in the first horizontal portion 83a, the switch operating mechanism 79 is in the state of it shown on the left hand part of Fig. 2. In the process where the follower member 91 is transferred through the tilting portion 83c, the follower member 91 moves upwards. Then, the movable frame 88 moves upwards together with this member. In the first half of this upward movement process, the drive member 89 is kept urged downwards by the compression spring 90 and maintains the state shown in the left of Fig. 2. In the second half of the upward movement process of the movable frame 88, it abuts on the engage portion 94. Then, the movable frame 88 and the drive member 89 move together upwards as a single body. As the result of the upward movement of the drive member 89 as mentioned above, the movable electrode 99 in the switch 5 is separated from the fixed electrode 98. When the follower member 91 reaches the second horizontal portion 83b after the above mentioned operations, the switch operating mechanism 79 is brought in the state as shown on the right hand part of Fig. 2 and the switch 5 is maintained open.
On the other hand, a follower member 91 moves downwards and the corresponding switch 5 becomes closed in the process where the follower member 91 reaches the first horizontal portion 83a through the tilting portion 83c from the second horizontal portion 83b. The operation in this process is as follows. When the follower member 91 moves downwards from its position shown on the right hand part of Fig. 2, the movable frame 88 also moves downwards and the drive member 89 is urged by the compression spring 90 to move downwards also. As the result, the movable electrode 99 in the switch 5 contacts the fixed electrode 98. When the follower member 91 continues to move downwards, the movable frame 88 moves downwards together with it. In this process, however, the drive member 89 moves no longer downwards and the compression spring 90 is compressed. When the follower member 91 reaches the first horizontal portion 83a via these operations, the switch operating mechanism 79 is brought in the state of it as shown on the left hand part of Fig. 2 and the switch 5 is maintained closed. In this closure state, the compressed compression spring 90 exerts a downward biasing force on the drive member 89; the biasing force is exerted on the movable electrode 99 in the switch 5 and the movable electrode 99 is urged on the fixed electrode 98 with a sufficient contact pressure.
Next, the tap changing operation by the above mentioned tap changer 1 is explained in reference with Figs. 11 and 12. In Fig. 11, windings with taps in a three phase transformer are represented by reference numerals 111
113. These windings are provided, for example, with nine taps T1
T9. Since the structure of the winding is similar for each of three phase windings, only the winding 111 is explained in the following. A tap selector and the switch in the tap changer are similarly described in the following. The tap selector 115 has two change-over switches 116 and 117 per one phase. Each change-over switch is provided with a plurality of fixed contacts 118 connected to the taps and two movable contacts 119 adapted to be selectively connected to the fixed contacts as shown in Fig. 11. The movable contacts 119 are changed over in a well known manner by an electric operating mechanism (a mechanism different from the electric operating mechanism for the tap changer). Three switches 5 in the tap changer are used per one phase and are represented by reference numerals 5a, 5b and 5c in this Fig. 11. The connection consisting of the windings of the transformer, the tap selector and the tap changer forms a well known star connection as seen in the Figure.
Next, the operation to change-over the tap T5 to the tap T4 in the above mentioned connection is explained. In the state shown in Fig. 11, the switch 5a is closed and electric current flows through a path including the tap T5 and the switch 5a. In this state, the change-over switch 117 in the tap selector 115 is beforehand changed over to the tap T4. When, under this condition, the driving force is output from the output mechanism 17 of the force storage means 3 and the operation cam 78 of the respective switch drive mechanism 4 is rotated in one direction, a plurality of switches 5a

5c is changed over by a number of switch operating mechanisms 79, respectively, in the following manner. Namely, each of the switches 5a

5c is, at first, in the state at a time A in Fig. 12. The switch 5b becomes first closed at a time shown by B by the rotation of the operation cam 78. The switch 5a becomes next opened at a time shown by C. Then a situation is established where the electric current flows through a path including the tap T5, the current-limiting resistor 6 and the switch 5b. Next, the switch 5c becomes closed at a time shown by D in Fig. 12. Then a situation is established where the electric current flows through a path including the tap T4 and the switch 5c and a path including the tap T5 and the switch 5b. In this case, the taps T5 and T4 are short-circuited but no over current flows since the current-limiting resistor 6 is connected therebetween. Next, at a time shown by E in Fig. 12, the switch 5b becomes opened. As the result, a situation is established where the elctric current flows through a path including only the tap T4 and the switch 5c and continues to a point shown by F in Fig. 12.
By the change-over of the switches 5a

5c performed in this manner, the tap T5 is changed over to the tap T4 without interrupting the electric current in the winding 111 of the transformer. The above mentioned change-over is completed in a very short time, for example, in about 0.1 second. The change-overs for three phases, of course, are simultaneously carried out.
Next, the tap T4 is similarly changed over, for example, to the tap T3. Namely, the change-over switch 116 of the tap selector 115 is beforehand changed over to the tap T3. In this state, the driving force is output from the output mechanism 17 of the force storage means 3; thereby the direction of rotation of the crank member 70 in the output mechanism 17 is opposite to that in the above mentioned case. Then, the operation cam 78 in the switch operating mechanism 4 is rotated in the direction opposite to that in the above mentioned case. As the result, the opening and closing actions of the switches 5a

5c are performed in a sequence reverse to that in the above mentioned case, namely in a time sequence in the order of points F, E, D, C, B and A in Fig. 12.
The form of the cam groove 83 in the operation cam 78 is beforehand determined so that the operation of the switches 5a

5c may be performed in the above mentioned sequences.
Next, an embodiment with a different structure to interpose the coil spring elements 36 and 37 between the flanges 29 and 33 and the flanges 30 and 33 in the force storage mechanism 15 is as follows. Namely, the aforementioned spring holders 38 and/or 40 are not provided for but one end or both ends of each of the concerned coil spring elements are secured directly onto each of the flanges, for example, by welding or soldering. Also in this case, operations can be performed similarly.
In a further different embodiment the spring holders 38 and 40 are similarly avoided and the coil spring elements 36 and 37 are interposed between the flanges freely, without securing the ends of the coil spring element on the flanges. In this case, when one of the coil spring elements is compressed, then the other is not tensioned. Accordingly, the elastic modulus of each coil spring element is selected so that the coil spring element may provide a sufficient biasing force when it is singly compressed. In the case of adopting such a structure, the coil spring element may come into contact with the guide bar 24, when it is compressed. However, as in the aforementioned case, the coil spring element is hard to bend sideways since it is short. Accordingly, the coil spring element does not rub on the guide bar 24 with a large contact pressure. As the result, the wearing out and the generation of metallic powder resulting from the rubbing on the guide bar are very unlikely to occur similarly to the aforementioned cases.

Claims

1. A force storage mechanism (3) in an on-load tap changer (1) comprising:
an operation frame (26) mounted, on a guide bar (24) mounted on a base frame (20) for forth and back movement between a first operation point and a second operation point guided by said guide bar (24),
a drive frame (27) mounted on said guide bar (24) for forth and back movement guided by said guide bar (24),
a force storage spring (36, 37) interposed between said operation frame (26) and said drive frame (27) and adapted to have a force stored therein by the deviation of said drive frame (27) relative to said operation frame (26),
hold members (43, 44) secured on said base frame (20) for holding said operation frame (26) at said first operation point and said second operation point, respectively, and
a release piece (49) mounted on said drive frame (27) and adapted to release the holding action on said operation frame (26) by said hold members (43, 44) when said drive frame (27) is deviated, relative to said operation frame (26) to a prescribed position,
characterized in that,
said operation frame (26) and said drive frame (27) being mounted, relative to at least one of said guide bar (24), in such a manner that at least one flange (33) provided on said drive frame (27) is positioned between two flanges (29, 30) provided, in separated relationship with each other, on said operation frame (26) and said guide bar (24) is inserted through guide portions (32, 34) provided on said flanges (29, 30, 33) so as to guide said two frames (26, 27), and
said force storage spring consists at least of two coil spring elements (36, 37) and each of the coil spring elements (36, 37) being passed through by said guide bar (24) is interposed between one of said flanges (29, 30) on said operation frame (26) and said flange (33) on said drive frame (27) and between the other flange (30, 29) on said operation frame (26) and said flange (33) on said drive frame (27), respectively.

2. A force storage mechanism in an on-load tap changer as set forth in claim 1 wherein the both ends of said coil spring elemtens (36, 37) are secured on said flanges (29, 30, 33).

3. A force storage mechanism in an on-load tap changer as set forth in claim 1 wherein an eccentric circular disc (63) mounted on a rotatable drive shaft (60) is provided in mated relationship with said drive frame (27) and a rotatable sliding member (64, 65) adapted to contact with and slide on the circumferential surface of said eccentric circular disc (63) is provided on said drive frame (27).

4. A force storage mechanism in an on-load tap changer as set forth in claim 1 wherein said base frame (20) is provided with a buffer means (53) with a contact portion (56a) movable forth and back in a direction parallel to the direction of the forth and back movement of said operation frame (26) and said operation frame (26) is provided with an abutment piece (54) for abutting on said contact portion (56a).