Patent Description:
On-load tap changers are known in which two tapped windings are connected in series for obtaining a larger regulating range. Such connections are known from the standard IEEE C57. <NUM>-<NUM>, for example. Also <CIT> discloses a series connection of two tapped windings.

During an operation of a change-over selector, the tapped windings are galvanically disconnected and will become capacitively controlled by their surrounding potentials. This may lead to the change-over selectors being exposed to voltages above their withstand limits.

Embodiments of the disclosure relate to an improved on-load tap changer.

According to a first aspect, an on-load tap changer comprises a first tap selector for connection to a first tapped winding and a second tap selector for connection to a second tapped winding. The first tap selector is connected to a first diverter switch and the second tap selector is connected to a second diverter switch, wherein the diverter switches are connected to each other to connect the tapped windings in series. The on-load tap changer comprises one or more high-ohmic resistances in an electric connection between at least one of the diverter switches and at least one of the tapped windings.

The on-load tap changer further comprises a first change-over selector for connection to the first tapped winding and a second change-over selector for connection to the second tapped winding. The change-over selectors are provided for changing a connection of the tapped windings to a main winding. As examples, the change-over selectors may be in a plus/minus regulation or in a coarse/fine regulation. In a plus/minus regulation, the connection direction of the tapped windings to the main winding can be changed. In a coarse/fine regulation a coarse winding can be connected or disconnected from the tapped winding.

The tap selectors can be formed by two units or by two poles in one unit. Also the diverter switch can be formed by two units or by two poles in one unit. Accordingly, also the on-load tap-changer can be formed by two units or by two poles in one unit.

The on-load tap changer can be provided for accomplishing changes in transformer winding ratios or to change impedances in reactors. By the series connection of the tapped windings, a larger regulation range can be achieved.

By connecting the tapped windings to the diverter switches via the high-ohmic resistances, the tapped windings are connected to a defined potential during a change-over selector operation, i.e. when the change-over selector switches its contacts. Thereby, the voltage level to which the change-over selectors are exposed can be kept below a withstand limit.

The one or more high-ohmic resistances may comprise a first high-ohmic resistance and a second high-ohmic resistance. The first high-ohmic resistance may be located in an electric connection from a connection point between the diverter switches to the first tapped winding and the second high-ohmic resistance may be located in an electric connection from a connection point between the diverter switches to the second tapped winding. The connection point may be a common node.

The first and second high-ohmic resistances may be directly connected to the respective tapped windings. The first and second high-ohmic resistances may also be directly connected to each other. The first and second high-ohmic resistances may also be directly connected to the diverter switches. In this context, "directly connected" means that no further electric component is located in the electric connection line between the respective elements. However, a connection line to a further electric components may be branched off in the connection line between the respective elements.

The tapped windings may be additionally connected to each other via the one or more high-ohmic resistances. As an example, a connection line between the high-ohmic resistances and a connection line between the diverter switches may be connected by a further connection line.

A total resistance provided by one or more of the high-ohmic resistors between one of the tapped windings and one of the diverter switches may have a resistance value in a range from <NUM> kOhm to <NUM> kOhm, for example.

The on-load tap changer may comprise one or more resistance switches for establishing and interrupting the connection of the tapped windings to the diverter switches via the high-ohmic resistances. In normal operation of the electric facility, the resistance switches may be open such that the connection is interrupted and current flow and losses from the resistances are avoided. During a change-over selector operation, the switches are closed so that the tapped windings are at a defined potential.

According to a further aspect, an electrical facility comprises the on-load tap changer as described in the foregoing, the tapped windings and a main winding. The electrical facility is a transformer or a reactor. The first and second tapped windings are directly or indirectly connected to a transformer or a reactor main winding.

According to a further aspect, a method for operating the on-load tap changer and/or the electrical facility comprises opening at least one of the change-over selectors.

When the on-load tap changer comprises one or more resistance switches for establishing and interrupting the connection of the tapped windings to the diverter switches via the high-ohmic resistances, the method may comprise the step of interrupting the connection after the change-over selectors are again closed in a change-over selector operation. A change-over selector operation is a part of a tap change operation when the tap selectors are at a mid-position. Accordingly, the high-ohmic resistances can be connected only for a change-over selector operation and disconnected in normal operation. Thereby, additional losses during normal operation can be avoided.

The present disclosure comprises several aspects. Every feature described with respect to one of the aspects is also disclosed herein with respect to the other aspect, even if the respective feature is not explicitly mentioned in the context of the specific aspect.

Further features, refinements and expediencies become apparent from the following description of the exemplary embodiments in connection with the figures. In the figures, elements of the same structure and/or functionality may be referenced by the same reference signs. It is to be understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

<FIG> shows a schematic diagram of an on-load tap changer <NUM> for regulating voltage in an electric facility. The electric facility is a transformer or a reactor. The transformer may be a power transformer or a phase shifting transformer, for example. The electric facility may be connected to a high voltage transmission line. The depicted circuit may be suitable for a reactor, for example.

The on-load tap changer <NUM> provides a series connection of a first tapped winding <NUM> and a second tapped winding <NUM>. Each of the tapped windings <NUM>, <NUM> comprises a plurality of taps <NUM>, <NUM> for connecting or disconnecting parts of the winding directly or indirectly to or from a main winding of the electrical facility. By connecting two tapped windings <NUM>, <NUM> in series, it is possible to achieve a larger regulation range, because the total voltage across a single regulating winding is limited.

Such an on-load tap changer arrangement can be achieved either by using an on-load tap changer unit with two poles which are connected to each other or by connecting two on-load tap changer units, for example. As an example, a two- or three-pole on-load tap changer can be used, wherein two poles are connected in series.

For each of the tapped windings <NUM>, <NUM> a tap selector <NUM>, <NUM>, each comprising two contact arms, changes the tap in operation. Furthermore, change over selectors <NUM>, <NUM> are provided for changing a connection of a respective one of the tapped windings <NUM>, <NUM> to a main winding.

As an example, the change-over selectors <NUM>, <NUM> may be in the form of change-over selectors for reversing or disconnecting a tapped winding at mid-position operation. To obtain very large regulation ranges, the on-load tap changer <NUM> may have a plus/minus or coarse/fine configuration.

In the tap-changer <NUM> of <FIG>, the change-over selectors <NUM>, <NUM> are in a plus/minus regulation. In plus/minus regulation, either end of the tapped windings <NUM>, <NUM> can be connected to the end of the main winding, so that a current flow in either direction can be selected. Thereby, a double regulating range can be achieved when compared to a regulating winding in linear regulation.

For each of the tapped windings <NUM>, <NUM>, a diverter switch <NUM>, <NUM> is provided for transferring the load current from a first arm of the tap selector <NUM>, <NUM> to a second arm of the tap selector <NUM>, <NUM> in a tap change operation. The diverter switches <NUM>, <NUM> may comprise transistor resistors, which are inserted during operation.

During a change-over selector operation, the tapped winding <NUM>, <NUM> is galvanically disconnected and the potentials of the tapped winding <NUM>, <NUM> will become capacitively controlled by its surrounding potentials. This may lead to that the change-over selector <NUM>, <NUM> are exposed for too high capacitive voltages so that a safe operation cannot be ensured anymore. For this reason, costly winding layouts may have to be used.

In the shown embodiment, high-ohmic resistances <NUM>, <NUM> are connected between a middle one of the taps <NUM>, <NUM> of the tapped windings <NUM>, <NUM> and the diverter switches <NUM>, <NUM>. In the shown embodiment, the high-ohmic resistances <NUM>, <NUM> are connected to a common node <NUM> in the connection line between the diverter switches <NUM>, <NUM>. The high-ohmic resistances <NUM>, <NUM> are directly connected to each other. An electric connection line between the high-ohmic resistances <NUM>, <NUM> is connected to an electric connection line between the diverter switches <NUM>, <NUM>. The connection line between the tapped windings <NUM>, <NUM> via the high-ohmic resistance <NUM>, <NUM> may comprise no further components than the high-ohmic resistances <NUM>, <NUM>. The connection line between the diverter switches <NUM>, <NUM> may comprise only the connection to the high-ohmic resistances <NUM>, <NUM> but no further elements and no connection to further elements.

In other embodiments, further components may be provided in the connection line. It is further possible that only one high-ohmic resistance is present, e.g. in the vertical connection to the diverter switches <NUM>, <NUM>. Is it also possible that the two ohmic resistances <NUM>, <NUM> are not directly connected to each other before being connected to the connection line connecting the diverter switches <NUM>, <NUM> but are separately to the connection line connecting the diverter switches <NUM>, <NUM>.

The high-ohmic resistances <NUM>, <NUM> will connect the tapped windings <NUM>, <NUM> to a defined potential and the tapped windings <NUM>, <NUM> will not be galvanically disconnected during a change-over selector operation. The high-ohmic resistances <NUM>, <NUM> can be also denoted as "tie-in resistors".

By using the high-ohmic resistances <NUM>, <NUM>, a rise of the voltage above the withstand voltage over the change-over selector can be avoided and cost-efficient winding layouts can be used. Cost-efficient winding layouts means that by using high-ohmic resistances <NUM>, <NUM> more options to physically locate the separate windings with respect to each other are available, without that the withstand voltage over the change-over selector <NUM>, <NUM> is exceeded. As an example, two windings may have to be located on two separate concentric layers if high-ohmic resistances are not used. By using high-ohmic resistances the two windings may be located above each other in one and the same concentric layer. The latter layout is significantly more cost-efficient since each separate concentric winding layer adds size, material and labor cost to the transformer or reactor.

<FIG> shows a further embodiment of an on-load tap changer <NUM> in a schematic diagram. The circuit is similar to the embodiment of <FIG> but comprises resistance switches <NUM>, <NUM> for connecting and disconnecting the tapped windings <NUM>, <NUM> to the diverter switches <NUM>, <NUM> via the high-ohmic resistances <NUM>, <NUM>.

When not carrying out a change-over by activating the change-over selectors <NUM>, <NUM>, the resistance switches <NUM>, <NUM> can be open so that the high-ohmic resistances <NUM>, <NUM> do not lead to increased losses during normal operation. During a change-over selector operation, the resistance switches <NUM>, <NUM> are closed to avoid a galvanic disconnection of the tapped windings <NUM>, <NUM>.

<FIG> shows a further embodiment of an on-load tap changer <NUM>. In this embodiment, the change-over selectors <NUM>, <NUM> provide a coarse/fine regulation. In this case, a first coarse winding <NUM> can be selectively added to the first tapped winding <NUM> and a second coarse winding <NUM> can be selectively added to the second tapped winding <NUM>. Also in this embodiment, high-ohmic resistances <NUM>, <NUM> are provided for preventing a high capacitive voltage across the change-over selectors <NUM>, <NUM> during a change-over selector operation.

<FIG> shows a further embodiment of an on-load tap changer <NUM> with change-over selectors <NUM>, <NUM> providing a coarse/fine regulation. As in the embodiment of <FIG>, resistance switches <NUM>, <NUM> are provided for establishing and interrupting the connection of the high-ohmic resistances <NUM>, <NUM> to the diverter switches <NUM>, <NUM>.

<FIG> shows a schematic diagram of steps in operating a change-over selector in a tap change operation. The on-load tap changer can be in accordance with the embodiments shown in the foregoing Figures.

During normal operation of the electric facility contact arms of the tap selectors <NUM>, <NUM> are connected to fixed contacts which are connected to taps <NUM>, <NUM> of the tapped windings <NUM>, <NUM>. The tapped windings <NUM>, <NUM> are connected by the diverter switches <NUM>, <NUM> in series.

The change-over selector is carrying the load current in all positions except the middle position, which is shown in the foregoing figures. In this position, the current flows directly from a main winding or coarse winding <NUM>, <NUM> to the first arms <NUM>, <NUM> of the tap selector <NUM>, <NUM> without passing the change-over selector <NUM>, <NUM> so that the change-over selector <NUM>, <NUM> can be moved without interruptions or arcing.

The change-over selector operation can be divided into three steps for opening and three steps for closing when a resistance switch <NUM>, <NUM> is used, otherwise two steps for opening and two steps for closing.

When a change-over selector operation is initiated in step A, a movable contact on one or both of the second arms <NUM>, <NUM> disconnects from its fixed contact.

After that, in step B, the respective resistance switch <NUM>, <NUM>, if being used, establishes the connection of the tapped windings via the high-ohmic resistances <NUM>, <NUM> to the diverter switches <NUM>, <NUM>. If a resistance switches <NUM>, <NUM> is not used, step B is left out in the operation.

After that, in step C, the respective change-over selector <NUM>, <NUM> opens. Now the respective tapped winding <NUM>, <NUM> is galvanically disconnected and its voltage is controlled by the capacitances and voltages to the surroundings and by the high-ohmic resistances <NUM>, <NUM>. When the change-over selector <NUM>, <NUM> is open, the tapped windings <NUM>, <NUM> are connected via the high-ohmic resistances <NUM>, <NUM> to a defined potential, thereby preventing the change-over selectors <NUM>, <NUM> being exposed to too high voltages.

After that, in step D, the respective change-over selector <NUM>, <NUM> closes. The tapped winding <NUM>, <NUM> is again galvanically connected.

After that, in step E, the respective resistance switch <NUM>, <NUM>, if being used, opens.

After that, in step F, the respective second arm <NUM>, <NUM> closes. Now, the load current can be switched by the respective diverter switches <NUM>, <NUM>. Steps E and F can be in opposite order.

When the on-load tap changer <NUM> comprises the resistance switches <NUM>, <NUM> as shown in <FIG> and <FIG>, the resistance switches <NUM>, <NUM> are in an open position during normal operation. Thereby, current does not flow via the high-ohmic resistances <NUM>, <NUM> during normal operation and additional losses can be avoided.

In some embodiments, both tap selectors <NUM>, <NUM> are activated simultaneously in a change-over operation to disconnect their second arms <NUM>, <NUM> at the same time. Also the change-over selectors <NUM>, <NUM> may be activated simultaneously. As an example, in this case, the tap selectors <NUM>, <NUM> may be configured as two poles of the same tape changer unit, and may be driven by a common drive mechanism. Also the change-over selectors <NUM>, <NUM> may be configured as two poles of the same unit and may be driven by a common drive mechanism. The drive mechanism of the change-over selectors <NUM>, <NUM> may be integrated in the drive mechanism of the respective tap selector <NUM>, <NUM>.

In other embodiments, only one tap selector <NUM>, <NUM> and change-over selector <NUM>, <NUM> is activated at a specific time in a change-over operation. Also only one of the change-over selectors <NUM>, <NUM> may be activated at a specific time. As an example, in this case, the tap selectors <NUM>, <NUM> may be configured as separate units and driven by separate drive mechanisms. Also the change-over selectors <NUM>, <NUM> may be configured as separate units and driven by separate drive mechanisms. Also here, the drive mechanism of the change-over selectors <NUM>, <NUM> may be integrated in the drive mechanism of the respective tap selector <NUM>, <NUM>.

Claim 1:
An on-load tap changer (<NUM>),
comprising a first tap selector (<NUM>) for connection to a first tapped winding (<NUM>) and a second tap selector (<NUM>) for connection to a second tapped winding (<NUM>),
comprising a first diverter switch (<NUM>) and a second diverter switch (<NUM>) being connected to the respective tap selectors (<NUM>, <NUM>), wherein the diverter switches (<NUM>, <NUM>) are connected to each other to connect the tapped windings (<NUM>, <NUM>) in series, characterised by
comprising a first change-over selector (<NUM>) for connection to the first tapped winding (<NUM>) and a second change-over selector (<NUM>) for connection to the second tapped winding (<NUM>) and
comprising one or more high-ohmic resistances (<NUM>, <NUM>) in an electric connection between at least one of the diverter switches (<NUM>, <NUM>) and at least one of the tapped windings (<NUM>, <NUM>).