Patent Description:
Tap changers are used for controlling the output voltage of a transformer by providing the possibility of switching in or switching out additional turns in a transformer winding. A tap changer comprises a set of fixed contacts which are connectable to a number of taps of a regulating winding of a transformer, where the taps are located at different positions in the regulating winding. A tap changer further comprises a moveable contact which is connected to a current collector at one end, and connectable to one of the fixed contacts at the other end, i.e. the so-called connected tap. By switching in or out the different taps, the effective number of turns of the transformer can be increased or decreased, thus regulating the output voltage of the transformer. Tap changers are generally customized for a particular application, especially when the tap changer is intended for higher transformer voltage ratings.

An important design factor for tap changers and transformer arrangements is insulation distance. Insulation distance generally means the distance between components which should be kept electrically insulated from each other. The insulation distance depends on factors such as field strengths, insulation materials, positioning of components and geometric shapes of components. Tap changers are exposed to both external and internal electric potentials/fields. External fields may be electric potentials between phases, such as phases of a multi-phase transformer arrangement, or potentials between phase and ground. Internal fields relate to potentials between components within the tap changer. Since external and internal fields are often be superposed in a conventional tap changer in a transformer arrangement, it is a difficult and time-consuming task to calculate and predict the field strengths in different locations and to design, customize and adapt a tap changer for a specific transformer arrangement. Due mainly to the superposed external and internal fields, but also due to the complex task of predicting the fields, the distances between components are necessarily designed to be relatively large, as a safety measure.

<CIT> discloses a tap changer having a shielding structure which comprises two parts, where one part is formed at least partly by the fixed contacts and the other is arranged to be at the potential of the connected tap. The shielding structure insulates the tap selector such that only internal potentials need to be considered when designing the tap selector, such as for determining insulation materials, insulation distances between components and/or for positioning components in relation to each other. However, non-connected taps are exposed to external potentials between phases. Therefore, the non-connected taps have high insulation requirements to account for potentials between phases and towards ground. Another prior art tap changer is disclosed in <CIT>.

An object of the present disclosure is therefore to provide a tap changer having an improved performance and a simpler customer interface. A transformer arrangement comprising the disclosed tap changer is further provided, which transformer arrangement is more compact and has a smaller footprint and an increased effect per square meter.

The object is achieved according to a first aspect of the present disclosure by a tap changer according to claim <NUM>.

By encapsulating the diverter switch and the tap selector in the shielding structure which is connected to the potential of the connected tap, the inner parts, e.g. the diverter switch and the tap selector, are almost completely protected from external electric fields. Design of the tap changer is thereby simplified since only internal fields need to be considered when determining insulation properties such as materials and distances, which means that the tap changer may be used in a wider range of applications, without extensive customization and adaptation. In addition, insulation distances may be reduced since the external fields are not superposed on internal electric potentials in any significant way, which yields a more compact design.

Having the potential of the shielding structure at the potential of the connected tap provides for smaller and more predictable potential differences, i.e. internal potential differences between components enclosed by the shielding structure, than in a nonshielded scenario where those components are exposed to external fields and potentials, such as between taps of the regulating winding and neighbouring phases or between the taps and ground. This will be explained more in detail below.

The tap changer contacts of the tap changer comprise both selectable contacts, which are connectable to the taps of the regulating winding, and external contacts which are connectable to neighbouring winding/phases, to the shielding structure, to selectable taps, etc. The tap changer contacts are herein sometimes referred to as a customer interface.

According to an aspect of the disclosure, the tap selector is electrically connected to the set of tap changer contacts, comprising at least two tap changer contacts, at least a part of the tap changer contacts being arranged to be connected to a corresponding tap of the regulating winding, wherein the set of tap changer contacts is arranged at an opening of the shielding structure, and wherein the tap changer contacts are arranged on one side of the tap changer.

The opening of the shielding structure is arranged to allow access to the tap changer contacts, such as for attaching connecting cables to the tap changer contacts. The opening of the shielding structure is adapted to be limited to an area required by the tap changer contacts to allow access to the tap changer contacts.

By "one side of the tap changer" is herein meant that the tap changer contacts are arranged on a side of the tap changer which faces one general direction, such as a side facing a regulating winding of a transformer. In other words, the tap changer contacts are arranged in a delimited area of the tap changer and are not spread out around the tap changer. The tap changer contacts are further arranged in an area delimited by the opening of the shielding structure, such as inside the opening of the shielding structure. Connecting cables from the regulating winding may accordingly approach the tap changer contacts in parallel, from one general direction.

According to an aspect of the disclosure, the shielding structure at least partly covers the set of tap changer contacts at the opening of the shielding structure.

As such, a part of the set of tap changer contacts is considered covered by the shielding structure if an edge of the opening overshoots a surface occupied by the tap changer contacts such that a normal drawn from a center point of at least one tap changer contact, from the surface occupied by the tap changer contacts, intersects an inside surface of the shielding structure.

The tap changer contacts at the opening of the shielding structure constitute objects of irregular shapes which may give rise to increased/focused field strengths, such as at pointed ends or sharp corners, which may in turn result in damaging flashovers. The shielding structure may thus cover and shield a part of the tap changer contacts from the external electric field. By arranging the contacts in a limited area and facing a single direction, the non-covered contacts may be dielectrically shielded by means of connecting cables, as described below.

According to an aspect of the disclosure, the shielding structure comprises a first compartment and a second compartment, separated by an electrically insulating barrier, and wherein the first compartment comprises a first insulating medium and the diverter switch, and wherein the second compartment comprises a second insulating medium and the tap selector.

The first and the second insulating medium may be a fluid, such as an oil e.g. a mineral oil, such as silicone oil, a hydrocarbon oil or an ester-based liquid/oil. The second insulating medium may be the same insulating medium as contained in a transformer tank. As such, when the tap changer is assembled with a transformer tank, the second chamber may share the insulating medium with a transformer inside the transformer tank. The first compartment may be fluidly sealed from the second compartment such that the first and the second insulating mediums are not mixed. In this way, contaminations, such as residue resulting from operation of the diverter switch does not contaminate the second insulating medium of the second compartment and of the transformer tank.

According to an aspect of the disclosure, the tap changer further comprises a change-over selector arranged in the first compartment, and wherein the change-over selector is of a plus/minus switching type or of a coarse/fine switching type.

Since the change-over selector may also contaminate the insulating medium, it is preferably arranged together with the diverter switch in the first compartment.

According to an aspect of the disclosure, the second compartment of the shielding structure comprises the opening and wherein the set of tap changer contacts is arranged at the opening.

As such, the opening of the shielding structure may be arranged inside the transformer tank. The tap changer contacts may thus be arranged inside of the transformer tank when the tap changer is assembled with the transformer tank. Connecting cables may thus be conveniently connected between taps of the regulating winding and the tap changer contacts.

The shielding structure thus forms an efficient shield, protecting the internal parts of the tap changer from the external electric field.

According to a further aspect of the disclosure a transformer arrangement comprises a transformer having at least one regulating winding of a rated regulation voltage, which at least one regulating winding has taps. The transformer arrangement also comprises at least one tap changer, having a shielding structure, as described hereinabove. Each of the at least one tap changer is electrically connected to a respective regulating winding such that its shielding structure is electrically connected to a connected tap of the respective regulating winding.

According to an aspect of the disclosure, at least part of the tap changer contacts of the tap changer are electrically connected to a corresponding tap of the regulating winding via electrically insulated connecting cables.

According to an aspect of the disclosure, the connecting cables are arranged in parallel at least in a vicinity of the tap changer contacts.

Arranging the connecting cables in parallel is a convenient way of wiring a transformer arrangement. In addition, it allows for dielectric shielding of the tap changer contacts. That the connecting cables are arranged in parallel at least in a vicinity of the tap changer contacts means that connecting cables are arranged in parallel at least in the delimited area of the opening of the shielding structure, at which opening the tap changer contacts are arranged.

According to an aspect of the disclosure, at least one of the connecting cables is arranged to dielectrically shield at least one of the tap changer contacts before connecting with another tap changer contact.

The connecting cables are arranged to provide dielectric shielding of at least a part of the tap changer contacts. The dielectric shielding is provided by arranging a connecting cable adjacent at least one tap changer contact and connecting the connecting cable to another tap changer contact. Tap changer contacts which cannot be provided with dielectric shielding in this manner are arranged to be covered by the shielding structure, as described above. By arranging the tap changer contacts in a delimited area, such as at the opening of the shielding structure, it is possible to arrange the connecting cables adjacent at least part of the tap changer contacts before connecting to other tap changer contacts.

According to an aspect of the disclosure, the transformer having the at least one regulating winding is housed in a transformer tank containing an electrically insulating medium, and wherein the shielding structure, encapsulating the at least one tap changer, is arranged on a wall of the transformer tank.

The tap changer may be arranged on a wall of the transformer tank such that a part of the tap changer, i.e. a part of the shielding structure, is inside the transformer tank. The tap changer may thus be conveniently interconnected with the transformer inside the tank.

According to an aspect of the disclosure, the transformer arrangement comprises a Y-coupled transformer having three regulating windings and three tap changers, and wherein the three tap changers are encapsulated in one shielding structure electrically connected to a common connected tap of the three regulating windings.

As such, only one shielding structure is required for three tap changers since the phases of the Y-coupled transformer share the potential of the selected tap. Therefore, neighbouring phases do not give rise to superposed potentials. Accordingly, the shielding structure mainly serves to protect the internal components from external potentials between the components and ground.

Further objects and advantages of, and features of the disclosure will be apparent from the following description of one or more embodiments, with reference to the appended drawings, where:.

The present disclosure is developed in more detail below referring to the appended drawings which show examples of embodiments. The disclosure is defined by the appended patent claims.

<FIG> schematically illustrates a tap changer <NUM> which is connected to a regulating winding <NUM> having a set of different taps <NUM>. The tap changer of <FIG> is of diverter switch type and comprises a diverter switch <NUM> and a tap selector <NUM>. The tap selector <NUM> of <FIG> comprises two current collectors <NUM>, two moveable contacts <NUM> and a set of fixed contacts <NUM>, where, each fixed contact <NUM> is arranged to be connected to one of the taps <NUM> of the regulating winding. The tap changer <NUM> of <FIG> has fifteen different fixed contacts <NUM>, and the regulating winding <NUM> has fifteen taps <NUM>. The tap changer <NUM> of <FIG> is mechanically linear in the sense that the current collectors <NUM> are implemented as linear rods, and the fixed contacts <NUM> are arranged in a linear fashion. In the following, the term linear tap changer should be construed as a mechanically linear tap changer, unless stated otherwise. The two current collectors <NUM> together form a current collector part. In a tap changer <NUM> having a single current collector <NUM>, the current collector part is formed by the single current collector <NUM>, etc..

The exemplary diverter switch <NUM> comprises two series connections of a main contact <NUM> and a transition contact <NUM>, with transition resistor <NUM> connected in parallel with transition contact <NUM>. Each of the series connections are, at one end, connected to a respective one of the two current collectors <NUM>, and, at the other end, connected to an external contact <NUM> of the tap changer <NUM>. Other configurations of the diverter switch are possible.

The two moveable contacts <NUM> are, at one end, in electrical contact with a respective one of the current collectors <NUM>. A moveable contact <NUM> can move along the current collector <NUM> to which it is connected, in order to reach different positions, at which the other end of the moveable contact <NUM> is in electrical contact with one of the fixed contacts <NUM>. The moveable contacts <NUM> could for example be sliding contacts arranged to slide along the current collectors <NUM>, to allow for electrical connection between the current collectors <NUM> and the different fixed contacts <NUM>. The driving of the moveable contacts <NUM> of <FIG> is arranged so that if one of the moveable contacts <NUM> is in contact with a fixed contact <NUM>, connected to a first tap, the other moveable contact <NUM> is in contact with a fixed contact <NUM>, connected to a tap <NUM> which is adjacent to the first tap <NUM>.

By switching the main contacts <NUM> and transition contacts <NUM> in a conventional manner, one or the other of the moveable contacts <NUM> will be in electrical contact with the external contact <NUM>, and thus provide an electrical path through the tap changer <NUM>. Similarly, the two current collectors <NUM> will take turns at being part of the electrical path of the tap changer <NUM>. The electrical path through the tap changer <NUM> ends at the external contact <NUM> at one end, and at the fixed contact <NUM> that is currently connected at the other end. An example of a diverter switch <NUM> is described in <CIT>. As mentioned previously, the diverter switch <NUM> of <FIG> is an example only, and any suitable type of diverter switch <NUM> can be used.

As mentioned above, the regulating winding <NUM> has a set of taps <NUM>, which are shown to be connected to the fixed contacts <NUM> of the tap changer <NUM> via connecting cables <NUM>. The connecting cables <NUM> do not form part of the tap changer <NUM> per se, but are provided as part of a transformer arrangement as electric connections between the tap changer and a transformer. The other end of the regulating winding <NUM> is provided with an external contact <NUM>. Depending on which tap <NUM> is currently connected to a fixed contact <NUM>, the electrical path between the external contacts <NUM> and <NUM> will include a different number of the regulating winding turns. The regulating winding <NUM> is often not seen as part of the tap changer <NUM>, and has therefore been surrounded by a solid line in <FIG>.

When the tap changer <NUM> is in use, the different fixed contacts <NUM> will be at different potential levels, corresponding to the different potential levels of the different taps <NUM> of the regulating winding <NUM>. The current collector <NUM>, which is currently connected, will be at the potential of the connected tap <NUM>, while the other current collector <NUM>, which is currently disconnected, will be at the potential of the tap <NUM> which is adjacent to the connected tap <NUM>. Thus, the potential difference between the current collectors <NUM> will correspond to the potential difference between two adjacent taps <NUM>, Uadj. Uadj is typically constant throughout the regulating winding <NUM>. Only one tap <NUM> at a time will be connected to the moveable contact <NUM> which is currently connected to the external connection <NUM> of the tap changer, this tap <NUM> being referred to as the connected tap <NUM>.

The potential difference between a current collector <NUM> and a particular fixed contact <NUM>, on the other hand, varies depending on at which position the moveable contact <NUM> is connected, and could be considerably larger. In a linear tap changer <NUM>, the maximum potential difference between a current collector <NUM> and a fixed contact <NUM> occurs when one of the end fixed contacts <NUM>, denoted 135e in <FIG>, are connected and forms part of the current path through the tap changer <NUM>. In this case, the potential difference between the current collector <NUM> that is connected, and the end fixed contact 135e which is not connected, corresponds to the entire voltage across the regulating winding <NUM>, Ureg. Ureg, also referred to as the regulation voltage, is illustrated in <FIG> by arrow <NUM>. In order to prevent flashover between the current collectors <NUM> and the fixed contacts <NUM>, the distance between the current collectors <NUM> and a fixed contacts <NUM> should reach or exceed the minimum distance over which the medium, in which the tap changer <NUM> is immersed, can withstand the voltage obtained, at a particular regulation voltage Ureg, between the current collector and the fixed contact <NUM> at the position of the moveable contact <NUM> which yields the highest voltage between the current collector <NUM> and the fixed contact (which position of the moveable contact <NUM> yields the highest voltage varies between the different fixed contacts). This distance, denoted dinsul and hereinafter referred to as the rated regulation voltage insulation distance of the tap changer, or insulation distance for short, depends on the medium surrounding the tap selector <NUM>, and increases with increasing rated regulation voltage (which typically depends on the rated voltage of the transformer and the desired number of taps <NUM>). Furthermore, the insulation distance dinsul of the tap changer typically varies along the length of the tap changer <NUM>, so that dinsul=dinsul(y) where y denotes a position along the extension direction of the linear tap changer. The largest possible potential difference between the current collectors <NUM> and the fixed contacts <NUM> can occur at the end fixed contacts 135e, and the nearer the centre of the arrangement of fixed contact(s) <NUM>, the smaller the maximum potential difference between the current collector <NUM> and the fixed contacts <NUM>. The insulation distance at the end fixed contacts 135e is denoted dendinsul. The regulation voltage used for defining the insulation distance is often a test voltage and are one of the parameters for which the tap changer <NUM> is rated.

The actual distance between the current collectors <NUM> and the fixed contacts <NUM> is herein referred to as the contact gap, dgap, and is indicated in <FIG> by arrow <NUM>. The contact cap in <FIG> is shown to be independent of position y along the extension direction. This represents a typical design, where, the contact gap dgap is constant and approximately corresponds to dendinsul However, a contact gap dgap=dgap(y) that varies with position along the extension direction, for example such that dgap(y) is smaller toward the centre of the tap changer <NUM>, could be beneficial in some circumstances.

The contact gap dgap, depends on the insulating medium. In a tap changer <NUM> that is air insulated, the contact gap dgap needs to be considerably larger than in an oil insulated tap changer <NUM>. For example, in an air insulated tap changer <NUM> wherein the insulation distance is <NUM>, the corresponding insulation distance could typically be around <NUM> in an oil insulated tap changer. Thus, an air insulated tap changer <NUM> typically needs to be physically larger than if the tap changer <NUM> were insulated by means of oil. However, in many applications, air insulation is preferred over oil insulation, such as inside buildings, where the risk of fire should be minimized (e.g. in a skyscraper), or in environmentally sensitive areas, where the risk of contamination should be minimized. The term air insulated tap changer <NUM> should here be construed to include tap changers <NUM> which are insulated by air or by air-like gases in a controlled space, such as tap changers <NUM> insulated by nitrogen gas (N<NUM>), tap changers <NUM> insulated by air at a controlled pressure, tap changers <NUM> insulated by SF<NUM>, etc..

The potential difference between the current collectors <NUM> and the fixed contacts <NUM> of the tap changer <NUM> of <FIG> will further be influenced by the surrounding electrical fields. In a three-phase power system, a tap changer <NUM> is typically part of a three-phase tap changer system comprising three different tap changers <NUM> connected to the three phases of a three-phase transformer. Hence, the electrical field at the tap changer <NUM> will be influenced by the electric fields surrounding the other two phases of the tap changer system <NUM>, and the transformer to which the tap changer <NUM> is connected, as well as by other electric fields. For example, the potential difference between the current collectors <NUM> and the fixed contacts <NUM> will be influenced by earth potential. Thus, the contact gap dgap should be large enough to allow for a potential difference caused by internal electrical fields originating from the regulation voltage Ureg, and which is superposed onto the external electric fields. Since the external electric field will vary from application to application, depending on the insulation requirement to ground and between phases, the dimensioning of the contact gap and other parts of the tap changer <NUM> will generally have to be customized to the requirements of each application. This results in costly manufacturing of tap changers <NUM>.

<FIG> illustrates a three-phase transformer <NUM> having three transformer phases <NUM>. The illustrated transformer is a delta-configured transformer, but in the following, a transformer in general, without reference to its configuration, will be referred to by reference numeral <NUM>. Each transformer phase <NUM> has a regulating winding <NUM>. In the configuration shown in <FIG>, the regulating winding <NUM> is located at the end of a (inner or outer) transformer winding - this is given as an illustrative example only, and the regulating winding <NUM> could have an alternative position, for example in the centre of the transformer windings. In <FIG>, various potential differences occurring in a three-phase transformer <NUM> are indicated. Ureg, as presented above, represents the voltage across the entire regulating winding <NUM>. Utransf is the voltage between two phases of the transformer. Uphase is the voltage between two regulating windings serving two different transformer phases <NUM>; and Uearth is the (highest) potential of the regulating winding <NUM>. No tap changers <NUM> are shown in <FIG>. Typically, one tap changer <NUM> will be connected to each regulating winding <NUM> of a transformer <NUM>, although configurations wherein a single tap changer <NUM> can be used for the regulation of three transformer phases <NUM> also exist. The potential of the tap selector <NUM> of a tap changer <NUM> lies within the potential range of the regulating winding <NUM> to which it is connected, i.e. within the range [Dearth, Dearth - Ureg].

Insulation distances in high voltage AC equipment are normally dimensioned in view of rated lightning impulse levels. A rated lightning impulse voltage level for a particular value of the highest voltage for equipment, Um, can be found in standards such as IEC <NUM>-<NUM>. A rated lightning impulse voltage found in the standards is valid for insulation to ground and for insulation between phases. The rated impulse voltage level over the regulating winding <NUM> will to some extent depend on the rating of the transformer <NUM>, but do also depend on the placement and size of the regulating winding <NUM>. During impulse voltages, capacitance from the regulating winding <NUM> to the surrounding (especially from the free end created as the moveable contact <NUM> approaches the external contact <NUM>), as well as capacitance within the regulating winding <NUM> itself, will play a more important role than the transformer magnetic circuit. A tap changer <NUM> is therefore normally rated for a specific impulse voltage level over the regulating winding <NUM>, here referred to as a rated regulation voltage, as well as for a specific Um related to the distance to ground.

According to the present disclosure, a tap changer <NUM>, such as an on-load tap changer (OLTC), is encapsulated in a shielding structure <NUM> which is arranged to shield the tap changer <NUM> from an external electric field. <FIG> schematically shows such a tap changer <NUM> connected to a regulating winding <NUM> of a rated regulation voltage. The tap changer <NUM> comprises a diverter switch <NUM>, a tap selector <NUM> and a set of tap changer contacts <NUM>. The term "tap changer contacts" comprises all connectable contacts of the tap changer <NUM>, which, depending on application, may include fixed contacts <NUM>, connections for external contact <NUM> and external contact <NUM>, a shield contact <NUM> (<FIG>), etc. The exemplary tap changer <NUM> shown in <FIG> is shown as a linear tap changer, such as the prior art tap changer <NUM>.

The tap selector <NUM> and the diverter switch <NUM> are encapsulated in the shielding structure <NUM> arranged to shield the tap selector <NUM> and the diverter switch <NUM> from the external electrical field. The shielding structure <NUM> is arranged to be electrically connected to a connected tap of the regulating winding <NUM>. The shielding structure <NUM> may be electrically connected to the connected tap via a shield connection <NUM>, such as between the shielding structure <NUM> and the external contact <NUM> or between the shielding structure <NUM> and a tap changer contact <NUM>, e.g. the shield contact <NUM> (<FIG>). In the latter example, the shield contact <NUM> would in turn be electrically connected to the connected tap.

As stated above, the external electric field is effectively screened by the shielding structure <NUM>. <FIG> shows a comparison with the prior art three-phase transformer <NUM> of <FIG>. The shielding structure <NUM> removes (or significantly reduces) the external potential between phases and the potential between fixed contacts <NUM> and ground. Ureg, which represents the voltage across the entire regulating winding <NUM>, i.e. the internal maximum potential difference, is essentially the only potential that needs to be considered when determining a contact gap dgap <NUM> (<FIG>). Since superposed external fields are not a significant factor for the presently disclosed tap changer <NUM>, the contact gap dgap can be reduced, resulting in a more compact tap changer <NUM>, as compared to the prior art tap changer <NUM>. A tap changer <NUM> having the disclosed shielding structure <NUM> may further be used in a wider range of applications, i.e. in a wider range of electric field environments, which means that less customization and adaptation is required when manufacturing tap changers <NUM>.

The tap changer <NUM> of the present disclosure may also comprise a change-over selector <NUM>. <FIG> schematically show two types of change-over selectors. In <FIG> a change-over selector 350a for plus/minus switching is shown. The change-over selector 350a extends the regulating range to twice the voltage of the tapped winding, by connecting the main winding to different ends of the regulating winding <NUM>, and thereby reversing a magnetic flux generated by the regulating winding <NUM>. <FIG> shows a change-over selector 350b for coarse/fine switching, which extends the regulating range to twice the voltage of the tapped winding, by connecting or disconnecting the coarse regulating winding <NUM>. The change-over selector <NUM> may be connected to the tap changer <NUM> in a conventional manner and is optionally comprised by the tap changer <NUM>, encapsulated in the shielding structure <NUM>.

<FIG> illustrates a side view of an exemplary, conceptual, design of the tap changer <NUM>. As shown, the shielding structure <NUM> encapsulates the diverter switch <NUM>, the tap selector <NUM> and the tap changer contacts <NUM>. As stated above, the tap changer <NUM> may optionally also comprise a change-over selector <NUM>. The shielding structure is preferably made of an electrically conducting material, such as aluminium. The illustrated shape of the shielding structure <NUM> is only exemplary and is mainly intended to show the extent of the encapsulation.

The tap changer contacts <NUM> may be arranged on a dielectric surface <NUM> and may be electrically connected to the diverter switch <NUM>, the tap selector <NUM> and/or the optional change-over selector <NUM> via lead-throughs (not shown) in the dielectric surface <NUM>, as required.

The number of tap changer contacts <NUM> may vary, depending on application. The tap selector <NUM> is thus electrically connected to the set of tap changer contacts <NUM> which comprises at least two tap changer contacts <NUM>. At least a part of the tap changer contacts <NUM> is arranged to be connected to a corresponding tap of the regulating winding. The part of the tap changer contacts <NUM> which is connected to a corresponding tap <NUM> of the regulating winding <NUM> is the set of fixed contacts <NUM> which are selectable by the tap selector <NUM>.

The set of tap changer contacts <NUM> is arranged at an opening <NUM> of the shielding structure <NUM>, such as inside the opening <NUM> of the shielding structure <NUM>. In <FIG>, the opening <NUM> is shown as a dashed part of the shielding structure <NUM>. The tap changer contacts <NUM> are arranged on one side of the tap changer <NUM>, i.e. such that they are arranged on a side of the tap changer <NUM> which faces one general direction, for instance a side facing a regulating winding <NUM> of a transformer <NUM>.

The opening <NUM> of the shielding structure <NUM> is arranged to allow access to the tap changer contacts, such as for attaching connecting cables to the tap changer contacts <NUM>. The opening <NUM> of the shielding structure <NUM> is further adapted to be limited in size to an area required by the tap changer contacts <NUM>, which allows convenient access to the tap changer contacts <NUM>, while at the same time providing optimal screening of the external electric field.

The tap changer contacts <NUM> at the opening <NUM> of the shielding structure <NUM>, which are exposed to the external electric field, constitute objects of irregular shapes which may give rise to increased/focused field strengths, such as at pointed ends or sharp corners of the contacts, which may in turn result in damaging flashovers. The shielding structure <NUM> at least partly covers the set of tap changer contacts <NUM> at the opening <NUM> of the shielding structure <NUM>. A tap changer contact <NUM> may be defined as covered by the shielding structure <NUM> if a line <NUM> normal to the surface <NUM>, drawn from a center point of said tap changer contact <NUM>, intersects an inside surface of the shielding structure <NUM>. For explanatory purposes, such a line <NUM> is drawn in <FIG> to show how the rightmost contacts in the figure are covered by the shielding structure <NUM>. This is further illustrated in <FIG>, which is a top-side view of the tap changer <NUM> of <FIG>. An edge <NUM>' of the opening <NUM> is shown in <FIG>. The edge <NUM>' overshoots the rightmost line <NUM>' of tap changer contacts <NUM> such that they are covered by the shielding structure <NUM>.

<FIG> further shows how the shielding structure <NUM> may be electrically connected to the connected tap via the shield connection <NUM>. In the exemplary embodiment of <FIG>, the shield connection <NUM> connects the shielding structure <NUM> to a tap changer contact <NUM>, which in the illustrated case is a dedicated shield contact <NUM>. The shield contact <NUM> is in turn electrically connected to the connected tap. <FIG> exemplifies the shield connection <NUM> as a cable connection, but it may be any kind of galvanic connection.

As also shown in <FIG>, the shielding structure <NUM> comprises a first compartment <NUM> and a second compartment <NUM>, separated by an electrically insulating barrier <NUM>. The first compartment <NUM> comprises a first insulating medium and the diverter switch <NUM>. If a change-over selector <NUM> is comprised in the tap changer <NUM>, the change-over selector <NUM> is arranged in the first compartment. The second compartment <NUM> comprises a second insulating medium and the tap selector <NUM>. The first and/or the second insulating medium may be a fluid, such as an oil e.g. a mineral oil, such as silicone oil, a hydrocarbon oil or an ester-based liquid/oil.

The second compartment <NUM> of the shielding structure <NUM> further comprises the opening <NUM>. The set of tap changer contacts <NUM> is thus also arranged in the opening second compartment <NUM>, at the opening <NUM>.

In a further embodiment of the present disclosure, shown in <FIG>, a transformer arrangement <NUM> comprises a transformer <NUM> having at least one regulating winding <NUM> of a rated regulation voltage. Each of the at least one regulating winding <NUM> is comprised in a phase winding <NUM> of the transformer <NUM>. The at least one regulating winding <NUM> has taps <NUM>. The transformer arrangement <NUM> also comprises at least one tap changer <NUM>, having a shielding structure <NUM>, as described hereinabove. Each of the at least one tap changer <NUM> is electrically connected to a respective regulating winding <NUM> such that its shielding structure <NUM> is electrically connected to a connected tap of the respective regulating winding <NUM>.

The transformer <NUM> having the at least one regulating winding <NUM> is housed in a transformer tank <NUM> containing an electrically insulating medium. In the exemplary embodiment of <FIG>, three tap changers <NUM>, but only two phase windings <NUM>/regulating windings <NUM> are shown. A third phase winding <NUM>/regulating winding <NUM> can be imagined outside the figure on the right side.

The shielding structure <NUM>, encapsulating the at least one tap changer <NUM>, may be arranged on a wall <NUM> of the transformer tank <NUM>. By arranging the tap changer <NUM> on the wall <NUM> of the transformer tank <NUM>, a part of the tap changer <NUM>, i.e. a part of the shielding structure <NUM>, may be arranged inside the transformer tank <NUM>. The tap changer <NUM> may thus be conveniently interconnected with the transformer inside the tank via the tap changer contacts <NUM>.

The part of the shielding structure <NUM> arranged inside the transformer tank <NUM> is preferably the second compartment <NUM>, comprising the tap selector <NUM> and the tap changer contacts <NUM>. The second insulating medium of the second compartment <NUM> may be a same insulating medium as contained in a transformer tank <NUM>. As such, when the tap changer <NUM> is assembled with a transformer tank <NUM>, the second chamber may share the insulating medium with the transformer <NUM> inside the transformer tank <NUM>. The first compartment <NUM> may be fluidly sealed from the second compartment <NUM> such that the first and the second insulating mediums are not mixed. In this way, contaminations, such as residue resulting from operation (switching) of the diverter switch <NUM> does not contaminate the second insulating medium of the second compartment <NUM> or insulating medium of the transformer tank <NUM>. Since the change-over selector <NUM> may also contaminate the insulating medium, it is preferably arranged together with the diverter switch <NUM> in the first compartment <NUM>.

A notable special case of a transformer arrangement <NUM> (not shown in the drawings) comprises a Y-coupled transformer <NUM> having three regulating windings <NUM> and three tap changers <NUM>. The three tap changers <NUM> may be encapsulated in a single shielding structure <NUM> electrically connected to a common connected tap of the three regulating windings <NUM> because the phases of the Y-coupled transformer share the potential of the selected tap. Therefore, neighbouring phases do not give rise to superposed potentials. Accordingly, the shielding structure mainly serves to protect the internal components from external potentials between the components and ground.

For the sake of clarity of the illustration of <FIG>, the wiring between the tap changers <NUM> and the regulating windings <NUM> of the transformer <NUM> are not shown in <FIG>. However, at least a part of the tap changer contacts <NUM> of the tap changer <NUM> is electrically connected to a corresponding tap <NUM> of the regulating winding <NUM> via electrically insulated connecting cables <NUM>. Especially, each of the fixed contacts <NUM> of the tap changer contacts <NUM> is connected to a corresponding tap <NUM> of the regulating winding <NUM>. <FIG> shows the top-side view of the tap changer <NUM> of <FIG>, with a number of connecting cables <NUM> added for illustrative purposes. The connecting cables <NUM> are arranged in parallel at least in a vicinity of the tap changer contacts <NUM>, i.e. at least in the delimited area of the opening <NUM> of the shielding structure <NUM>, at which opening <NUM> the tap changer contacts <NUM> are arranged. Arranging the connecting cables in parallel is a convenient way of wiring the transformer arrangement <NUM>. At least one of the connecting cables <NUM> is arranged to dielectrically shield at least one of the tap changer contacts <NUM> before connecting with another tap changer contact <NUM>.

Since the tap changer contacts <NUM> are arranged in the limited area of the opening <NUM>, the connecting cables <NUM> may approach the tap changer contacts <NUM> in parallel from one general direction. This may advantageously be used to provide dielectric shielding for the tap changer contacts <NUM>, which are not covered by the shielding structure <NUM>. The dielectric shielding is provided by arranging an electrically insulated connecting cable <NUM> adjacent at least one tap changer contact <NUM> and connecting the connecting cable <NUM> to another tap changer contact <NUM>. The electric field is reduced at the by-passed tap changer contact <NUM> because the interposed dielectric material of the insulated connecting cable <NUM> acts as a screen for said contact. Tap changer contacts <NUM> which cannot be provided with dielectric shielding in this manner are arranged to be covered by the shielding structure <NUM>, as described above. By arranging the tap changer contacts <NUM> in a delimited area, such as at the opening <NUM> of the shielding structure <NUM>, it is possible to arrange the connecting cables <NUM> adjacent at least part of the tap changer contacts <NUM> before connecting to other tap changer contacts <NUM>. Thereby, all non-covered tap changer contacts <NUM> may be provided with dielectric shielding.

In the example shown if <FIG>, connecting cable 160a dielectrically shields tap changer contact 385a before connecting with tap changer contact 385b. Connecting cable 160b dielectrically shields tap changer contact 385c before connecting with tap changer contact 385d. Connecting cable 160c dielectrically shields tap changer contact 385e and tap changer contact 385f before connecting with tap changer contact <NUM>, which is covered by the shielding structure <NUM>. For the sake of clarity, not all connecting cables <NUM> which are to be connected to the tap changer contacts <NUM> are shown in <FIG>.

The tap changer <NUM> is thus effectively protected from the external field by the shielding structure <NUM> which is electrically connected to the connected tap of the regulating winding <NUM>, and by the connecting cables <NUM> which provide the tap changer contacts <NUM> with dielectric shielding. For illustrative purposes, an external electric field is shown in <FIG>, which is a simulation of tap changer <NUM> according to the invention, arranged in a transformer environment. The shielding structure <NUM> of <FIG> is simplified, having a different shape than in <FIG>, and the opening <NUM> of the shielding structure <NUM> is larger than required. Also, the tap changer contacts <NUM> are not included. In order to further simplify the simulation, the phase winding <NUM>, to which the tap changer <NUM> to the right is connected, is grounded.

Claim 1:
A tap changer (<NUM>) arranged to be connected to a regulating winding (<NUM>) of a rated regulation voltage, the tap changer (<NUM>) comprising a diverter switch (<NUM>), a tap selector (<NUM>) and a set of tap changer contacts (<NUM>);
characterized in that the tap selector (<NUM>) and the diverter switch (<NUM>) are encapsulated in a shielding structure (<NUM>) arranged to shield the tap selector (<NUM>) and the diverter switch (<NUM>) from an external electrical field, the shielding structure (<NUM>) being made of an electrically conducting material and arranged to be electrically connected to a connected tap of the regulating winding (<NUM>), wherein the tap selector (<NUM>) is electrically connected to the set of tap changer contacts (<NUM>), comprising at least two tap changer contacts (<NUM>), at least a part of the tap changer contacts (<NUM>) being arranged to be connected to a corresponding tap (<NUM>) of the regulating winding (<NUM>), wherein the set of tap changer contacts (<NUM>) is arranged at an opening (<NUM>) of the shielding structure (<NUM>), and wherein the tap changer contacts (<NUM>) are arranged on one side of the tap changer (<NUM>), and wherein the shielding structure (<NUM>) comprises a first compartment (<NUM>) and a second compartment (<NUM>), separated by an electrically insulating barrier (<NUM>), and wherein the first compartment (<NUM>) comprises a first insulating medium and the diverter switch (<NUM>), and wherein the second compartment (<NUM>) comprises a second insulating medium and the tap selector (<NUM>).