Technical Field
[0001] The present invention relates generally to tap changers for transformers, and in
particular to a method and a system for a pre-selector of a tap changer of a transformer.
Background
[0002] 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.
[0003] 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.
[0004] 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
[0005] It is an object of the invention to provide a method and a system for a pre-selector
on a tap changer for a transformer without the drawbacks described above.
[0006] This object is achieved by providing a method for operating a system for a pre-selector
with contacts in a tap changer for a transformer. The system comprises a moveable
part being connected to one of the contacts of the pre-selector. The moveable part
is arranged to be pivotally moveable around a pivot point between a first and a second
end position. The system further comprises a first and a second spring arranged to
act on the moveable part. A force is applied to said moveable part overcoming the
forces from the first and the second spring. The springs are arranged to keep the
moveable part in its first end position. The applied force is pivotally moving the
moveable part from the first end position in direction towards the second end position.
The force from the first spring is, when said moveable part is pivotally moved past
a first knee position, acting in the same direction as the started pivotal movement
towards the second end position.
[0007] The force applied by the first pressure spring thus contributes to the movement whereby
quick contact separation of the contacts in the pre-selector is achieved. Quick contact
separation is advantageous since the occurring arc can be extinguished faster which
leads to less contact wear. Further, the faster the arc is extinguished, the less
heat is generated. Still further, with enough quick separation of the contacts, there
is no need for expensive and bulky tie-in-resistors to handle the arcs.
[0008] In a preferred embodiment, the force from the second spring, when the moveable part
is further pivotally moved past a second knee position, is acting in the same direction
as the started pivotal movement towards the second end position.
[0009] In a preferred embodiment, the forces from the first and the second spring are acting
to keep the moveable part in position when positioned in either of the endpoints.
[0010] In a preferred embodiment, the forces from the first and the second spring are equal
and acting to keep said moveable part in position when positioned in an equilibrium
position between the endpoints.
[0011] The invention further relates to a system for operating a pre-selector with contacts
in a tap changer for a transformer comprising a moveable part being connected to one
of the contacts of the pre-selector and being arranged to be pivotally moveable around
a pivot point between a first and a second end position. The system further comprises
a first and a second spring arranged to act on the moveable part. The first and the
second spring is arranged to keep the moveable part in its first end position. The
first spring, when the moveable part is pivotally moved past a first knee position,
is arranged to apply a force on the moveable part acting in the same direction as
the started pivotal movement towards the second end position.
[0012] In a preferred embodiment are the first and the second springs arranged to apply
a force on the moveable part, by which force the moveable part is locked in respective
position when positioned in respective first and second end position.
[0013] In a preferred embodiment can the moveable part be positioned in an equilibrium position
between the first and the second end position.
[0014] In a preferred embodiment is the moveable part kept in the said equilibrium position
by that the forces of the first and the second springs are symmetrically applied on
the moveable part and equal to each other.
[0015] In a preferred embodiment are both the first spring and the second spring arranged
to apply forces keeping the said moveable part locked in respective first and second
end position.
[0016] In a preferred embodiment are the first and the second springs pressure springs.
Brief Description of the Drawings
[0017]
Figure 1 illustrates the system where the driver is positioned in a first locking
position.
Figure 2 illustrates the system where the driver has been moved slightly to a first
knee position.
Figure 3 illustrates the system where the driver has been further moved to an equilibrium
position.
Figure 4 illustrates the system where the driver has been further moved to a second
knee position.
Figure 5 illustrates the system in a second locking position.
Detailed Description
[0018] Figure 1 illustrates a system 1 for locking a pre-selector on a tap changer for a
transformer according to the present invention. A moveable part 2, hereinafter named
as the driver, is connected to the contact of the pre-selector.
[0019] The driver 2 is arranged to be moved by a motor drive (not shown) between a first
and a second position with an intermediate equilibrium position. A first and a second
pressure spring 4, 5 are arranged to act on the said moveable part 2. The first and
second springs are attached to the moveable part 2 in a first and a second attachment
point 6, 7. In Figure 2, the driver 2 is positioned in a first locking position. Both
the first and the second spring 4, 5 keep the driver 2 locked in the position. The
attachment points 6, 7 are chosen to achieve that both the first spring 4 and the
second spring 5 apply spring pressure forces keeping the driver 2 locked in each of
the first and the second position as shown in Figure 1 and 5, respectively.
[0020] During operation, the system 1 works as follows. In a first locked position as shown
in Figure 2, both springs 4, 5 applies pressing forces acting in directions keeping
the driver locked in position as described above. As shown in Figure 3 the moveable
part 2 is moved from its first locked position by a motor drive (not shown) overcoming
the pressure forces of the first and the second spring 4, 5. When the driver is moved
to a first specific position, a first knee-point, the attachment point of the first
spring have been moved to a position where the pressure force from the first spring
instead of locking the driver 2 in its first position acts on the driver helping the
started pivotal movement in the other direction away from the first locked position.
The force applied by the first pressure spring 4 thus contributes to achieve quick
contact separation of the contacts in the pre-selector. Quick contact separation is
preferred since the occurring arc can be extinguished faster which leads to less contact
wear.
[0021] With reference to Figure 4, both pressure springs 4, 5 are arranged to act with equal
forces on the driver 2 when the driver is positioned in an intermediate equilibrium
position, thereby keeping the driver 2 in the intermediate position.
[0022] In the equilibrium position shown in Figure 4, the driver stops from moving to long
in an uncontrolled way. In this intermediate position, the two pressure springs 4,
5 acts with equal forces symmetrically applied to the driver, thereby keeping the
driver in the position.
[0023] The driver is then moved towards the next toggle point by the motor drive as shown
in Figure 5. When the driver is moved to a second specific position, a second knee-point,
the attachment point of the second spring have been moved to a position where the
pressure force from the second spring instead of keeping the driver 2 in its intermediate
position acts on the driver helping the started pivotal movement in the direction
towards the second locked position.
[0024] Both springs 4, 5 applies pressure forces contributing to the movement of the driver
2 to the locked position, resulting in a distinct entering of the contact. In the
second locked position, shown in Figure 5, both springs 4, 5 are applying pressure
forces keeping the driver 2 locked in the second position similar to the locking in
the first position as described above.
[0025] The invention has mainly been described above with reference to a few embodiments.
However, as is readily appreciated by a person skilled in the art, other embodiments
than the ones disclosed above are equally possible within the scope of the invention,
as defined by the appended patent claims.
1. A method for operating a system for a pre-selector with contacts in a tap changer
for a transformer,
said system comprising a moveable part (2) being connected to one of the contacts
of said pre-selector and being arranged to be pivotally moveable around a pivot point
(3) between a first and a second end position,
and at least a first and a second spring (4, 5) arranged to act on the said moveable
part (2),
said method comprising
- applying a force to said moveable part (2) overcoming the forces from said first
and said second spring (4, 5) being arranged to keep the moveable part in its first
end position, and pivotally moving said moveable part (2) from the first end position
in direction towards the second end position,
characterised in that,
the force from the first spring (4), when said moveable part (2) is pivotally moved
past a first knee position, is acting in a direction contributing to the started pivotal
movement towards the second end position.
2. A method according to claim 1, wherein the force from said second spring (5), when
said moveable part (2) is further pivotally moved past a second knee position, is
acting in a direction contributing to the started pivotal movement towards the second
end position.
3. A method according to claim 1 or 2, wherein the forces from said first and said second
spring (4, 5) are acting to keep said moveable part (2) in position when positioned
in either of said endpoints.
4. A method according to claim 1, 2 or 3, wherein the forces from said first and said
second spring (4, 5) are equal and acting to keep said moveable part (2) in position
when positioned in an equilibrium position between said endpoints.
5. A system (1) for operating a pre-selector with contacts in a tap changer for a transformer,
comprising a moveable part (2) being connected to one of the contacts of said pre-selector
and being arranged to be pivotally moveable around a pivot point (3) between a first
and a second end position, and at least a first and a second spring (4, 5) arranged
to act on the said moveable part (2), said first and said second spring (4, 5) being
arranged to keep the moveable part in its first end position,
characterised in that,
said first spring (4), when said moveable part (2) is pivotally moved past a first
knee position, is arranged to apply a force on said moveable part (2) acting in a
direction contributing to the started pivotal movement towards the second end position.
6. A system (1) according to claim 5, wherein
said first and said second springs (4, 5) are arranged to apply a force on the said
moveable part, by which force the said moveable part is locked in respective position
when positioned in respective first and second end position.
7. A system according to claim 6, wherein said moveable part (2) can be positioned in
an equilibrium position between said first and said second end position.
8. A system according to claim 7, wherein said moveable part (2) is kept in the said
equilibrium position by that the forces of the first and the second springs (4, 5)
are symmetrically applied on the said moveable part (2) and equal to each other.
9. A system according to any of claims 5-8, wherein both said first spring (4) and said
second spring (5) is arranged to apply forces keeping the said moveable part locked
in respective first and second end position.
10. A system according to any of claims 5-9, wherein said first and second springs (4,
5) are pressure springs.