|JP05074256||PARTIAL DISCHARGE MEASURING DEVICE|
|WO/2011/141082||LOAD TRANSFER SWITCH FOR A TAP CHANGER|
|JP2004335477||CONTACT SWITCHING DEVICE COMPRISING FLAT-SHAPED NON-CONDUCTIVE COVER|
HEDSTEN, Harald (Ljungbacken 6, Grängesberg, S-772 50, SE)
JOHANSSON, Martin (Konvaljstigen 28, Ludvika, S-771 43, SE)
HEDSTEN, Harald (Ljungbacken 6, Grängesberg, S-772 50, SE)
1. An arrangement for operating a pre-selector of a cylindrical tap changer (10) that includes a cylinder (24) provided with tap contacts and pre-selector contacts (25-27) extending inside the cylinder, the arrangement comprises a contact carrier (9) provided with at least one movable contact (8) for mating the pre-selector contacts of the cylinder (24), and a drive mechanism (21) for providing a rotational movement to the contact carrier (9), which contact carrier (9) during use is rotated by means of the drive mechanism (21) so that the movable contact (8) selectively mate with the pre-selector contacts (25-27) on the inside of the cylinder
c h a r a c t e r i s e d in that the drive mechanism (21) is operatively connected to the contact carrier (9) via a
staggering mechanism (1) arranged to provide a staggered motion to the contact carrier (9) .
2. The arrangement according to claim 1, wherein the
staggering mechanism (1) includes a cam structure (5) and a cam follower (2), which cam follower (2) act on the cam structure (5) during rotation.
3. The arrangement according to claim 2, wherein the cam follower (2) is spring loaded (4) against the cam structure (5) .
4. The arrangement according to any of claims 2 or 3, wherein the cam structure comprises a first and a second cam (5a, 5b) and provides a groove (17) between the first and second cam (5a, 5b) .
5. The arrangement according to any of claims 1-4, wherein the staggering mechanism (1) comprises at least one end groove (16, 18) arranged to provide a first resting position for the mechanism coinciding with the movable contact (8) being in a contact position with one of the cylinder contacts (25-27) . 6. The arrangement according to claim 5 in combination with any of claims 2 to 4, wherein the cam structure (5) comprises a first and a second groove (16, 18) providing a first and second locking position for the cam follower (2), which locking positions provide the first and a second resting position of the mechanism.
7. The arrangement according to any of claims 1-6, wherein movable tap changing contacts of the tap changer are driven at essentially constant speed by the drive mechanism.
8. The arrangement according to claim 7, wherein the tap changing contacts are movable by means of a switching shaft (25) , which is axially and vertically arranged in the centre of the cylinder, and which is driven by the drive mechanism.
9. The arrangement according to any of claims 1-8, wherein the tap changer comprises vacuum interrupters for breaking
currents in the tap changer during switching.
The present invention relates generally to tap changers for transformers, and in particular to an arrangement for
operating a pre-selector of a tap changer of a transformer.
In power transformers, on-load tap changers (OLTC) are used to change tapping connections of transformer windings while the transformer is energised. Since the transformer is in use, arcing will occur when the tapping connections are changed. Pre-selectors are used in on-load tap changers to enable connecting or disconnecting turns of the transformer winding. Pre-selectors may also be used to connect a whole part of a winding. Also in the contacts of the pre-selector, arcing will occur when the contacts are moved. If arcing is not
restrained, increased wear of the contacts occur as well as a risk of damage to the transformer windings with shortened duty operational time for both the tap changer and the transformer as a result. In a worst case scenario, an arc in the pre ¬ selector will turn out to be a short-circuit, causing serious damages to the transformer. To limit arcing in the contacts of the pre-selector, so called tie-in-resistors are used. Instead of causing an arc, the energy stored when breaking the contacts of the pre-selector is released in the form of a controlled current through the tie-in-resistor. Although serving as a good working solution, there are some drawbacks with the tie-in-resistors. They are expensive and bulky and are preferably only installed if there are specific circumstances requiring such installation. A risk is that the conditions at site are not fully known, or are changed, with damages to the transformer being a result if tie-in-resistors are not installed. Further, if tie-in- resistors are installed, the energy will still be present in the form of heat in the resistor.
Further, to secure proper functionality it is of importance that the pre-selector is securely locked when positioned in the appropriate position. There are known systems where the locking function of the pre-selector is achieved by two surfaces being in contact with each other by high friction. However, since the locking function is cased by the two surfaces being in contact, high friction between the surfaces is also caused when it is not wanted, i.e. when the pre ¬ selector contacts are to be separated. If the contacts are not separated quickly, more arcing will occur during separation leading to unnecessary wear of the contacts and risk of damaging the transformer winding.
Summary of invention
It is an object of the invention to provide an arrangement for a pre-selector on a tap changer for a transformer without the drawbacks described above. Especially, it is an object of the invention to provide a fast separation of the pre-selector contacts .
This object is achieved by providing an arrangement for operating a pre-selector of a cylindrical tap changer that includes a cylinder provided with tap contacts and pre ¬ selector contacts extending inside the cylinder, the
arrangement comprises a contact carrier provided with at least one movable contact for mating the pre-selector contacts of the cylinder, and a drive mechanism for providing a rotational movement to the contact carrier, which contact carrier during use is rotated by means of the drive mechanism so that the movable contact selectively mate with the pre-selector contacts on the inside of the cylinder. The pre-selector arrangement is characterised in that the drive mechanism is operatively connected to the contact carrier via a staggering mechanism arranged to provide a rotation with varying angular velocity to the contact carrier. Preferably the geometric positioning of the contacts are provided so that the varying angular velocity is largest upon disconnection of the movable contact from the corresponding cylinder contact.
In a preferred embodiment the staggering mechanism includes a cam structure and a cam follower, which cam follower act on the cam structure during rotation. Preferably, the cam
follower is spring loaded against the cam structure.
In an embodiment, the staggering mechanism comprises at least one end groove arranged to provide a first resting position for the mechanism coinciding with the movable contact being in a contact position with one of the cylinder contacts.
In an embodiment the cam structure and cam follower provides the resting position, wherein the cam follower comprises a first and a second groove providing a first and second locking position for the cam follower, which locking positions provide the first and a second resting position of the mechanism.
In these positions the cam follower can rest, and the
positions corresponds to an in-contact position of the movable contact and the corresponding contact of the cylinder.
Preferably, the cam structure comprises a first and a second cam and provides a groove between the first and second cam. This groove is an intermediate groove for controlling and limiting the rotational speed of the (cam follower and thereby the) movable contact during the movement between two
consecutive fixed contacts of the cylinder.
In an embodiment, the movable tap changing contacts of the tap changer are driven at essentially constant speed by the drive mechanism. The varying speed during operation of the preselector is essentially solely provided by the staggering mechanism, such as by the cam structure and cam follower construction .
It is suitably and preferred embodiment of the tap changer, to let the tap changing contacts be movable by means of a
switching shaft, which shaft is axially and vertically
arranged in the centre of the cylinder, and which shaft is driven by the drive mechanism.
Moreover, in a preferred embodiment the tap changer comprises vacuum interrupters for breaking currents in the tap changer during switching.
Brief Description of the Drawings
Figure 1 illustrates the mechanism with the cam follower positioned in a first locking end position.
Figure 2 illustrates the mechanism with the cam follower moved to a first cam position. Figure 3 illustrates the mechanism in an intermediate
Figure 4 illustrates the cam follower moved to a second cam. Figure 5 illustrates the system where the cam follower is positioned in a second locking and end position. Figure 6 illustrates a perspective view of parts of a pre ¬ selector .
Figure 7 illustrates a transformer with a tap changer. Figure 8 illustrates the main parts of a tap changer and means for switching the tap changer.
Figure 7 illustrates an overview of a transformer 11 having a first 12 and a second 13 set of terminals for transforming electricity between a first voltage level and a second voltage level. The transformer 11 is provided with a tap changer 10, having three sets of tap changing contacts, one for each phase, symbolically illustrated, below a pre-selector having three output contacts 26, 27, 28. Figure 7 illustrates a known type of transformer tap changer which can benefit from being equipped with a pre-selector arrangement of the invention. Figure 8 illustrates main parts of a tap changer, including a pre-selector arrangement in accordance with the invention. The tap changer 10 includes a motor 22 for driving the tap
changer, operatively connected via linking means 23 to a drive mechanism 21 of the tap changer. The tap changer also
comprises an insulated cylinder 24 arranged inside the
transformer housing, the cylinder is provided with annular tap changing contacts, and a switching shaft is arranged inside the cylinder. The switching shaft 25 carries movable contact parts, i.e. moves with the shaft, which contacts is arranged to selectively mate with the corresponding contact parts of the cylinder. The drive mechanism 21 is arranged to provide a driving torque to rotate the switching shaft 25 whereby the contacts of the tap changing part of the tap changer is manoeuvred. The drive mechanism is also operatively connected to the pre-selector and is arranged to drive a contact carrier 9 of the pre-selector that carries the movable pre-selector contacts 8. The movable pre-selector contacts 8 are arranged to selectively mate with the fixed pre-selector contacts 26, 27, 28 of the cylinder, when rotated by the drive mechanism 21. The drive mechanism 21 may include a Malteser wheel gear for providing rotation to both the switching shaft 25 of the tap changer and the contact carrier 9 of the pre-selector. Tap changers having Malteser wheel gears have been provided in several versions by ABB AB (for example the UC tap changer with a BUE or BUL drive mechanism, or of type UZE/UZF with drive BUF 3) , and this gear mechanism will not be described in further detail herein.
The pre-selector also includes a mechanism 1 for varying the speed of the movable contact. This mechanism 1 is provided to transform a uniform rotating motion input from the drive mechanism 21 into a staggered rotating motion applied to the contact carrier 9. The staggering mechanism 1 provides a staggering rotation, wherein the speed varies and the
staggering mechanism 1 is operatively connected the contact carrier 9 so that the greatest speed coincides with the disconnection moment for the movable contact 8, i.e. the moment where the movable contact 8 leaves the fixed cylinder contact 26, 27 or 28. Thus, the staggering mechanism makes a faster separation of the contacts possible.
An example of a staggering mechanism 1 according to the invention will be described with reference to figures 1-5. It should be noted that other mechanisms for providing a
staggered motion can be used without deviating from the invention as defined by the claims. However, the figures describe a preferred mechanism having a reliable construction. The staggering mechanism of the preferred embodiment comprises a cam structure 5 and a cam follower 2 that is arranged to cooperate with the cam structure to provide an alternately halting and accelerating motion. The cam follower 2 is secured to a shaft 3, which is coupled to the drive mechanism 21, and is arranged to impart a rotating motion to the cam follower 2 from the drive mechanism 21. The cam follower 2 is spring loaded by means of a spring 4 into contact with the surfaces of the cam structure. The cam structure 5 comprises a first 16 and a second 18 groove, providing a first and a second end position for the cam follower in the cam structure. Between the end positions (at 16 and 18), are a first 5a and a second 5b cam arranged and between these cams 5a, 5b are a third groove 17 arranged, provided as an intermediate position for the cam follower 2. The staggered motion is created by moving the cam follower 2 between the grooves 16, 17, 18 passing the cams 5a, 5b.
Figure 6 illustrates a perspective view of parts of the pre ¬ selector arrangement with movable contact 8 and staggering mechanism. The tap changer cylinder is provided with external pre-selector tap contacts 7 extending into the cylinder and providing the interior cylinder contacts 26-28 of the pre ¬ selector that are arranged to selectively mate the movable contact 8 carried by the contact carrier 9. Figure 6
illustrate how the staggering mechanism, including the cam structure 2 and follower 5, can be arranged at the top of the cylinder. The functioning of the staggering mechanism is described with reference to figures 1-5. Figures 1-5 illustrates passages for the cam follower 2 running over the cam structure 5 during a switching operation of the pre-selector . In figure 1, the cam follower 2 is situated in the first groove 16 providing the first end position for the staggering mechanism. This end position corresponds to a contact position for the movable contact of the pre-selector, thus, in the end position the movable contact 8 mate with a first one of the fixed cylinder
contacts. Figure 5 shows the opposite end position wherein the movable contact 8 of the pre-selector switch mates a second one of the fixed contacts of the cylinder 2. A typical
switching manoeuvre starts from the position illustrated in figure 1 and end in the position of figure 5. In figure 2 the cam follower 2, rotated by the shaft 3, has initiated its movement from the first end position and encountered the first cam 5a of the cam structure 5. The spring 4 is compressed when the cam follower 2 meets the first cam 5a during the rotation and this compression creates a motion resistance from the contact surface slope between the first cam 5a and the first groove 16. In this way the motion of the cam follower 2 is slowed down. This resistance against motion creates torques in the drive mechanism 21 and motor 22. The resistance keeps the speed of rotation constantly low as the cam follower 2 climbs up the cam 5a. In figure 2 the cam follower has reached the top of the first cam 5a. Passing the cam top releases the torques in the motor 22 and drive mechanism 21 as well as the spring force, and the angular speed increases. In figure 3 the cam follower 2 has entered into the intermediate groove 17 of the cam structure 5 between the grooves 5a, 5b. Between the top of the first cam 5a (figure 2), and the intermediate groove 17 (figure 3) , the cam follower engage the second slope of the first cam 5a wherein the torques of the motor 22 and the drive mechanism 21 as well as the compressed spring pushes the cam follower which moves towards the cam structure 5 and the spring force and the released torques imparts an
acceleration of the cam follower 2 "down the slope". This acceleration phase increase the speed up to a top speed, which suitably is made to coincide substantially with the switching phase where the movable contact 8 loses contact and
disconnects with the fixed contact 26, 27, 28 of the cylinder. Thus, the angular position of the fixed contacts in relation to the movable contact is arranged to provide contact
separation at the end of the acceleration period when the mutual speed of the movable contact and fixed contact is high and close to its top. Preferably, the top speed is reached slightly after the point in time where the contacts separate so that the risks for arcs are minimal.
Figure 3 illustrates the cam follower 2 entering the
intermediate groove 17, which prevents uncontrolled movement of the follower 2, instead the groove 17 provides an
intermediate halting to the cam follower 2 upon meeting the slope of the second cam 5b, so that the follower 2 is slowed down upon moving on the slope between the intermediate groove 17 and the second cam 5b.
Figure 4 illustrates the cam follower 2 reaching the top of the second cam 5b, where the rotational speed has decreased to minimum again. Hereafter, the cam follower 2 will run down the second cam 5b, wherein the compressed spring 4 contributes once again to the rotational speed and the cam follower 2 enters into the second end groove 18. The entering into the second end groove is made to coincide with the entering of the movable contact 8 into contact with a second contact of the fixed contacts on the cylinder. The end position is
illustrated in figure 5.