Power transformer for on-load tap changer applications

Abstract

A power transformer (1) for on-load tap changer applications comprises a winding arrangement (2) having an iron core (5), a number of windings (6, 7, 8) wound around the iron core (5), and a shield (9) located outside the outermost one of the windings (6, 7, 8), where the shield (9) comprises or consists of an electrically conductive or semi-conductive material.

Description

Technical Field

[0001] The present disclosure relates to a power transformer for on-load tap changer applications. Background Art

[0002] During the switching operation of an on-load tap changer (OLTC), the main winding of the power transformer is connected and disconnected from the regulating winding. As a result, the tap changer may be subjected to high stress due to the high recovery voltage. In order to keep the recovery voltage below the maximum level allowed by the specific OLTC design, so-called tie-in resistors can be provided. Tie-in resistors are external additional devices for resistance control of the recovery voltage and are also described in the international standard IEC / IEEE60214-2 for tap changers. In addition, during continuous operation, a tie-in switching device can be used to disconnect the resistor and avoid additional losses during operation.

[0003] However, when a strapping resistor is used, the size and cost of the power transformer increases. The strapping resistor and the strapping switch require additional space, which is usually only available in the selector of a larger tap changer, so the impact of the strapping resistor is usually greater for smaller units and smaller tap changer models. Increasing the size of the tap changer also means that a larger tank and a larger oil volume are required to accommodate the tap changer. When a strapping resistor is used without a switch, the losses (especially no-load losses) increase. In addition, the strapping in the resistor may affect the performance of the connected transformer, such as the peak efficiency index (PEI).

[0004] US2021 / 057147 A1 discloses a tap changer assembly in which a semiconductive coating and a conductive shield are applied to the cylindrical outer portion of the coil. DE 3534843 A1 discloses a winding arrangement for a transformer in which a regulating winding is covered by a shielding ring at the top and bottom of the end face. JP 59-126615A discloses a winding arrangement for an on-load tap-changing transformer in which an insulated shielded conductor is wound on an insulating cylinder, wherein both ends of the shielded conductor are connected to the end static shield of the tap winding.

[0005] Dieter Dohnal, September 1, 2013, "On-load tap changer for power transformer" (retrieved from the Internet on December 7, 2017; https: / / www.reinhausen.com / XparoDownload.ashx?raid=58092) discloses a winding arrangement for on-load tap changer applications, in which a shield is installed between the windings to reduce the recovery voltage. AT 260352B discloses a winding arrangement for a transformer, in which a capacitor formed by a partial cylinder is provided for surge protection. Summary of the invention

[0006] Embodiments of the present disclosure relate to an improved power transformer for on-load tap changer applications.

[0007] According to a first aspect, a power transformer for on-load tap changer applications comprises a winding arrangement having a core, a plurality of windings wound on the core, and a shield comprising a conductive or semi-conductive material, wherein the shield is located outside an outermost winding of the windings. The shield covers an angular range of up to 270° around a winding axis of the winding arrangement.

[0008] By means of the shield, the level of the recovery voltage can be reduced without the need for a strapping resistor. The shield requires less space and reduces no-load losses when compared to when a strapping resistor is used.

[0009] As an example, the outermost winding may be a regulating winding, which comprises a plurality of lead outlets for changing the output voltage of the transformer. In addition, the winding arrangement may comprise a primary winding and a secondary winding. The primary winding may be a high voltage winding, and the secondary winding may be a low voltage winding. The secondary winding, the primary winding and the regulating winding may be wound one above the other. The transformer may be a three-phase transformer. As an example, the core may comprise three winding core legs, wherein each core leg corresponds to one phase.

[0010] The shield may be in the form of a layer of conductive or semi-conductive material. As an example, the shield may be in the form of a metal plate. As another example, the shield may be in the form of a layer of insulating material with conductive or semi-conductive particles added to obtain sufficient conductivity for electrical shielding.

[0011] The shield may also have openings. As an example, the shield may have a mesh structure. The geometry of the shield may be adapted to the geometry of the outermost winding. As an example, the shield may have a curved shape. The shield may have a cylindrical shape. The cylindrical surface may have openings.

[0012] The shield may be connected to ground potential, or to a regulated neutral or center point potential.

[0013] The shield may circumferentially enclose the outermost winding but leave a gap to prevent circulating currents from flowing.The shield may cover an angular range of at least 45°.

[0014] In an embodiment, the shield may be located only on one side of the winding arrangement. In this case, the shield may cover an angular range of up to 180° or less than 180°. The shield may not extend into the space between adjacent winding core legs. In this case, the size of the core and the distance between the core legs do not have to be increased.

[0015] The shielding member may have an opening. The opening may be provided for a lead wire outlet. The lead wire outlet may be led out to the tap changer contact through the opening.

[0016] The shield may completely or almost completely cover the outermost winding in the direction along the winding axis. As an example, the shield may extend along at least 90% of the extension of the outermost winding along the winding axis.

[0017] The power transformer may comprise a tank in which the winding arrangement is located. The power transformer may further comprise an on-load tap changer electrically connected to the winding arrangement. The on-load tap changer may also be located in the tank.

[0018] Further features, improvements and variations will become clear from the following description of exemplary embodiments in conjunction with the accompanying drawings. In the accompanying drawings, elements of the same structure and / or function may be represented by the same reference numerals. It should be understood that the embodiments shown in the accompanying drawings are illustrative representations and are not necessarily drawn to scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The schematic diagram shows the power transformer winding arrangement.

[0020] Figure 2 A cross-sectional view showing the power transformer winding arrangement,

[0021] Figure 3 The power transformer winding arrangement is shown in a perspective view.

[0022] Figure 4 A possible structure that can be used for the shield is shown in a top view.

[0023] Figure 5 The arrangement of power transformer windings in an oil-filled tank is shown schematically. DETAILED DESCRIPTION

[0024] Figure 1A schematic diagram of a power transformer 1 is shown, comprising a winding arrangement 2 for on-load tap changing. The winding arrangement 2 is connected to an on-load tap changer 3 and is located in an oil-filled tank comprising a tank wall 4.

[0025] The winding arrangement 2 comprises an iron core 5 on which several windings 6, 7, 8 are wound one above the other. The outermost winding 6 is a regulating winding for changing the output voltage of the transformer. The regulating winding is connected to the tap changer 3. The regulating winding is arranged on an inner winding 7, which may be a high voltage winding. The innermost winding 8 may be a low voltage winding. Different arrangements of the high voltage winding, the low voltage winding and the regulating winding are possible.

[0026] During switching operation, the regulating winding is disconnected from the high voltage winding and connected to it again via the tap changer 3. When the contacts are opened during switching operation, the tap changer may be subject to high stress due to the high recovery voltage. For example, the regulating member can be connected via a coarse-fine or positive-negative changeover selector.

[0027] The main factor of the level of the recovery voltage is the ratio between the internal capacitance C1 generated between the outermost winding 6 and the closest innermost winding 7 and the internal capacitance C2 generated between the outermost winding 6 and the tank wall 4. In general, the smaller the ratio C2 / C1, the higher the recovery voltage generated on the change-over selector.

[0028] exist Figure 1 In the embodiment, V1 is the potential to which the geometric midpoint of the nearest inner winding 7 rises in use, or is zero in the case of an iron core leg. V3 is the potential to which the geometric midpoint of the nearest outer winding rises in use, or is zero in the case of a tank wall 4.

[0029] Figure 2 A schematic cross-sectional view of an embodiment of a winding arrangement 1 is shown. The winding arrangement 2 is as follows Figure 1 , but with an additional shielding 9 arranged at the outside of the outermost winding 6. The shielding 9 may consist of a conductive or semi-conductive material, or may be mainly made of this material except for, for example, edge protection. The shielding 9 may also include an insulating material to which one or more conductive or semi-conductive materials are added to obtain conductive or semi-conductive properties, thereby obtaining a shielding effect.

[0030] For example, the shield 9 may be a conductive material, such as aluminum. The shield 9 may also be a semiconductive material. As an example, carbon may be used as the semiconductive material. The shield 9 may be made of insulating paper to which conductive or semiconductive particles are added. The shield 9 may be carbonized paper.

[0031] The shield 9 can consist of a conductive or semi-conductive material or can be made mainly of this material except for, for example, edge protection. The shield 9 is outside the windings 6, 7, 8 wound around the core 5, i.e. the shield is not enclosed by another winding wound around the corresponding core part. The shield 9 is a component in addition to the windings 6, 7, 8, in particular in addition to the electrodes of the windings 6, 7, 8. The shield 9 can be in the form of a thin layer of conductive material. The geometry of the shield 9 is adapted to the outer surface of the outermost winding 6. The shield 9 can be in the form of an open cylinder. According to an example not consistent with the claims, the cylinder can also be almost closed, leaving only a small gap to prevent the flow of circulating current.

[0032] The shield 9 is connected to ground potential or to the regulating neutral point or center point potential. The center point potential can be, for example, the potential in a three-phase voltage system arranged in an equivalent star connection. When the shield has the same potential as the regulating element, is placed at the neutral end or is directly grounded, this will result in a very low potential difference between the shield and the regulating element, allowing the shield to be closer to the winding. As a result, voltage reflections or oscillations on the regulating element body during pulse distribution can be reduced, resulting in a more compact and safe overall solution. The shield 9 is used as Figure 1 The "tank wall" shown in the schematic diagram of FIG. 9 is outside. By means of the shield 9, the capacitance C2 can be increased significantly, resulting in a reduced recovery voltage value. When the shield 9 is used, no additional jumper resistor is required in order to reduce the recovery voltage on the change-over selector. The shield 9 provides a cost-effective and space-saving alternative to jumper resistors.

[0033] The shield 9 may cover only a portion of the outermost winding 6. The shield 9 may be arranged only on one side of the winding arrangement 1. As an example, the shield 9 may cover an angular range α of less than 180° of the circumference of the winding arrangement 1. In other embodiments, the shield may cover 180° or more than 180° of the circumference. The shield 9 may cover an angular range of at least 45°. The geometry of the shield 9 may avoid approaching the regulating lead outlet.

[0034] The shield 9 may be covered from both sides by an insulating material, such as a pressboard or paper layer. In addition, the shield may have additional edge protection at the top and bottom near the winding ends.

[0035] Figure 3 A winding arrangement 2 is shown comprising a core 5 having three winding limbs 10, 11, 12. Each of the winding limbs 10, 11, 12 is associated with a different phase. Each of the winding limbs 10, 11, 12 may have a winding arrangement 2 such as Figure 2. In each case, the shield 9 is located on the outermost winding. The shield 9 covers an angular range of less than 180°, so that the shield 9 does not extend into the gaps between the winding limbs 10, 11, 12. This has the advantage that no additional space for the shield 9 between the winding limbs 10, 11, 12 is required and the size of the core 5 does not have to be increased. As a result, an increase in the core limb pitch, which would lead to increased no-load losses in the transformer, can be avoided.

[0036] Figure 4 Another embodiment of a shielding member 9 for a winding arrangement 2 is shown. In this example, the shielding member 9 is in the form of a conductive mesh. The shielding member 9 may be wrapped around the outermost winding 6, such as Figure 2 As shown. The shield 9 comprises reinforcements 13 at the edges and corners. The shield 9 can be fixed to the outer surface of the winding arrangement 2, for example, by mechanical fasteners or by gluing. The mechanical fasteners can be in the form of insulating supports. As an example, the support of the winding can be extended, thereby also achieving the fixing of the shield 9.

[0037] Figure 5 A transformer 1 is shown, which comprises a winding arrangement 2 and an on-load tap changer 3. The winding arrangement 2 and the on-load tap changer 3 are located in an oil-filled tank 15.

[0038] Winding arrangement 2 with Figure 3 The winding arrangement 2 of FIG. 1 is identical, but is shown from the opposite side. The position of the shield 9 is indicated with a dashed line. However, the shield 9 is positioned on the side of the winding core that faces away from the observer.

[0039] The on-load tap changer 3 is connected to the lead outlets 14 of the regulating winding (only some connections are depicted). Due to the limited angular range of the shielding 9, the connection of the lead outlets 14 is not affected.

[0040] The shielding 9 may also have an opening for a lead outlet 14. According to an example not consistent with the claims, in this case, the shielding 9 may extend around almost the entire circumference of the winding core 10, 11, 12, leaving only a small gap to prevent the circulation flow. The gap may extend along the entire length of the shielding 9 in the direction of the winding axis. According to an example not consistent with the claims, the shielding 9 may cover an angular range of almost 360° (e.g., 340° or more). In addition to the gap, the shielding 9 may also have an opening for a lead outlet 14. The gap may also provide an opening for the lead outlet 14.

[0041] In the following, characteristic values ​​of a transformer with a strapping resistor and a transformer with a shielding design are compared with each other.

[0042] In both cases, the tap changer has positive and negative regulation and a graded neutral level. The connection is a three-phase star point connection.

[0043] For the transformer design without strapping resistors and without shielding, the maximum AC recovery voltage is 57.1 kV, which is higher than the maximum allowed level of 35 kV.

[0044] When using the strapping resistor, an additional no-load loss of about 3.1% is added. For the capacitors, the following values ​​are obtained:

[0045] C1=1.776nF

[0046] C2=0.995nF.

[0047] By connecting the resistor, the maximum AC recovery voltage is reduced to 16.8 kV and is therefore below the permitted level.

[0048] For comparison, an external shield was used instead of the strapping resistor. The external shield was located on the neutral adjustment and connected to the neutral terminal.

[0049] In this case, the following capacitance values ​​are obtained:

[0050] C1=1.776nF

[0051] C2=3.126nF.

[0052] Therefore, C2 is greatly increased by using an external shield. Due to the increase in C2, the maximum AC recovery voltage is reduced. In this example, the maximum AC recovery voltage is calculated to be 32.5 kV and is therefore below the maximum level allowed.

[0053] Overall, when using an external shield design instead of a strapping resistor, the AC recovery voltage can be kept below the permitted level while the additional cost and losses of the strapping resistor can be avoided. Thus, a power transformer with improved environmental and efficiency indices is obtained. Furthermore, the shield can be easily retrofitted on the winding arrangement without requiring significant additional space.

[0054] Reference numerals

[0055] 1 Power Transformer

[0056] 2 Winding arrangement

[0057] 3 On-load tap-changer

[0058] 4 Tank wall

[0059] 5 Core

[0060] 6 Outermost winding

[0061] 7 Inner winding

[0062] 8 Innermost winding

[0063] 9 Shielding part

[0064] 10 Core column

[0065] 11 Core column

[0066] 12 Core column

[0067] 13 Reinforcing part

[0068] 14 Lead outlet

[0069] 15 Storage tank

Claims

1. A power transformer (1) for on-load tap changer applications, The power transformer comprises a winding arrangement (2), the winding arrangement having an iron core (5), a plurality of windings (6, 7, 8) wound around the iron core (5), and a shield (9) comprising a conductive or semi-conductive material, the shield (9) being located outside the outermost winding (6) of the windings (6, 7, 8), in, The shield (9) covers an angular range of up to 270° around the winding axis.

2. The power transformer (1) according to claim 1, in, The outermost winding (6) is an adjusting winding, and the adjusting winding comprises a plurality of lead outlets (14) for changing the output voltage of the transformer.

3. A power transformer (1) according to any one of the preceding claims, in, The shielding member (9) is in the form of a material layer.

4. A power transformer (1) according to any one of the preceding claims, in, The shield (9) is in the form of a partial cylinder.

5. A power transformer (1) according to any one of the preceding claims, in, The shield (9) is connected to ground potential or to the regulated neutral or center point potential.

6. A power transformer (1) according to any one of the preceding claims, in, The shield (9) covers an angular range of at least 45° around the winding axis.

7. A power transformer (1) according to any one of the preceding claims, in, The shielding member (9) covers an angular range of less than 180° around the winding axis.

8. A power transformer (1) according to any one of the preceding claims, in, The shield (9) is formed by a layer of insulating material to which conductive or semi-conductive particles are added.

9. A power transformer (1) according to any one of the preceding claims, in, The shielding member (9) is formed of carbonized paper.

10. A power transformer (1) according to any one of claims 1 to 8, in, The shield (9) comprises a conductive material, wherein the conductive material is a metal.

11. A power transformer (1) according to any one of the preceding claims, in, The iron core (5) comprises a plurality of winding core columns (10, 11, 12), each winding core column comprising a plurality of windings (6, 7, 8) and a shielding member (9) located outside the outermost winding (6) among the windings (6, 7, 8).

12. The power transformer (1) according to claim 11, The power transformer is a three-phase power transformer, in, Each of the winding core legs (10, 11, 12) is associated with a different phase.

13. A power transformer (1) according to any one of the preceding claims, The power transformer comprises an on-load tap changer (3) electrically connected to the winding arrangement (2).

14. A power transformer (1) according to any one of the preceding claims, The power transformer comprises an oil-filled tank (15) in which the winding arrangement (2) is located.

15. A power transformer (1) according to any one of the preceding claims, comprising a plurality of winding core legs (10, 11, 12), in, The shield (9) does not extend into the space between adjacent winding core legs (10, 11, 12).

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