Power transformer for on-load tap changer applications

Abstract

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

Description

Technical Field

[0001] This disclosure relates to a power transformer for on-load tap changer applications. Background Technology

[0002] During the switching operation of an on-load tap changer (OLTC), the main winding and regulating winding of the power transformer are connected and disconnected. As a result, the tap changer may be subjected to high stress due to high recovery voltage. To keep the recovery voltage below the maximum level allowed by a particular OLTC design, so-called tie-in resistors can be provided. Tie-in resistors are external auxiliary devices for resistive control of the recovery voltage and are also described in the international standard IEC / IEEE 60214-2 for tap changers. Furthermore, during continuous operation, tie-in devices can be used to disconnect the resistors and avoid additional losses during operation.

[0003] However, the use of lap resistors increases the size and cost of power transformers. Lap resistors and lap switches require additional space, which is typically only available in selectors for larger tap changers; therefore, the impact of lap resistors is usually greater for smaller units and smaller tap changer models. Increasing the size of the tap changer also means requiring a larger tank and a larger oil volume to house it. When lap resistors are used without the switch in use, losses (especially no-load losses) increase. Furthermore, the lap in the resistor can affect the performance of the connected transformer, such as its 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 the regulating winding is covered by shielding rings at the top and bottom of its end faces. JP 59-126615A discloses a winding arrangement for an on-load tap-changing transformer in which an insulating shielding conductor is wound around an insulating cylinder, wherein the two ends of the shielding conductor are connected to the end static shielding of the tap winding.

[0005] Dieter Dohnal's paper, "On-Load Tap Changer for Power Transformers," published on September 1, 2013 (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 shielding is installed between the windings to reduce recovery voltage. AT 260352B discloses a transformer winding arrangement in which capacitors formed by partially cylindrical sections are provided for surge protection. Summary of the Invention

[0006] Embodiments of this 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 includes a winding arrangement having a core, a plurality of windings wound around the core, and a shielding member comprising a conductive or semiconductive material, wherein the shielding member is located outside the outermost winding of the windings. The shielding member covers an angular range of up to 270° around the winding axis of the winding arrangement.

[0008] By using shielding, the recovery voltage level can be reduced without the need for a bridging resistor. Shielding requires less space and reduces no-load losses compared to using a bridging resistor.

[0009] As an example, the outermost winding may be a regulating winding, which includes several lead outlets for changing the transformer's output voltage. Furthermore, the winding arrangement may include 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, primary winding, and regulating winding may be stacked and wound around each other. The transformer may be a three-phase transformer. As an example, the core may include three wound cores, where each core corresponds to one phase.

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

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

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

[0013] The shielding element can circumferentially enclose the outermost winding but leave gaps to prevent circulating current. The shielding element can cover an angle range of at least 45°.

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

[0015] The shield may have openings. These openings can be provided for lead wire exits. The lead wires can be led out to the tap changer contacts through these openings.

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

[0017] A power transformer may include a tank in which the windings are arranged. The power transformer may further include an on-load tap changer electrically connected to the winding arrangement. The on-load tap changer may also be located within the tank.

[0018] Further features, improvements, and variations will become clear from the accompanying drawings and the following description of exemplary embodiments. In the drawings, elements of the same structure and / or function may be designated by the same reference numerals. It should be understood that the embodiments shown in the drawings are illustrative and not necessarily drawn to scale. Attached Figure Description

[0019] Figure 1 The schematic diagram illustrates the winding arrangement of a power transformer.

[0020] Figure 2 The arrangement of the power transformer windings is shown in a cross-sectional view.

[0021] Figure 3 The arrangement of the power transformer windings is shown in a 3D diagram.

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

[0023] Figure 5 The schematic diagram illustrates the arrangement of the power transformer windings in an oil-filled tank. Detailed Implementation

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

[0025] Winding arrangement 2 includes an iron core 5, with several windings 6, 7, and 8 stacked and wound around this iron core. The outermost winding 6 is a regulating winding used to change the transformer's output voltage. The regulating winding is connected to a tap changer 3. The regulating winding is arranged on an inner winding 7, which can be a high-voltage winding. The innermost winding 8 can be a low-voltage winding. Different arrangements of the high-voltage winding, low-voltage winding, and regulating winding are possible.

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

[0027] The main factor affecting the recovery voltage level is the ratio between the internal capacitance C1 generated between the outermost winding 6 and the nearest innermost winding 7, and the internal capacitance C2 generated between the outermost winding 6 and the tank wall 4. Generally, the smaller the ratio C2 / C1, the higher the recovery voltage generated on the selector.

[0028] exist Figure 1 In the above, V1 is the potential that rises during use at the geometric midpoint of the inner winding 7, or zero in the case of the iron core column. V3 is the potential that rises during use at the geometric midpoint of the outer winding, or zero in the case of the tank wall 4.

[0029] Figure 2 A schematic cross-sectional view of one embodiment of winding arrangement 1 is shown. Winding arrangement 2 is as follows: Figure 1 As shown, but with an additional shielding element 9 disposed outside the outermost winding 6. The shielding element 9 may be made of a conductive or semi-conductive material, or may be made primarily of such material except, for example, edge protection. The shielding element 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 achieving the shielding effect.

[0030] For example, shielding element 9 can be a conductive material, such as aluminum. Shielding element 9 can also be a semi-conductive material. As an example, carbon can be used as a semi-conductive material. Shielding element 9 can be made of insulating paper with added conductive or semi-conductive particles. Shielding element 9 can be carbonized paper.

[0031] The shield 9 can be made of a conductive or semi-conductive material, or primarily of that material except for, for example, edge protection. The shield 9 is outside the windings 6, 7, 8 that are wound around the core 5; that is, the shield is not enclosed by another winding wound around the corresponding core portion. The shield 9 is a component other than the windings 6, 7, 8, and particularly other than 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 circulating flow.

[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 regulator, is placed at the neutral terminal, or is directly grounded, the potential difference between the shield and the regulator is very low, allowing for a closer distance between the shield and the winding. This reduces voltage reflections or oscillations on the regulator body during pulse distribution, resulting in a more compact and safer overall solution. The shield 9 is used as... Figure 1 The diagram shows the outer "tank wall". Through shielding 9, capacitance C2 can be significantly increased, resulting in a reduced recovery voltage. When using shielding 9, there is no need to add a bridging resistor to reduce the recovery voltage on the selector. Shielding 9 provides a cost-effective and space-saving alternative to bridging resistors.

[0033] The shield 9 may cover only a portion of the outermost winding 6. The shield 9 may be disposed 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 greater than 180° of the circumference. The shield 9 may cover an angular range of at least 45°. The geometry of the shield 9 may be designed to avoid proximity to the adjustment lead outlet.

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

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

[0036] Figure 4 Another embodiment of the shield 9 for the winding arrangement 2 is shown. In this example, the shield 9 is in the form of a conductive mesh. The shield 9 may be wrapped around the outermost winding 6, such as... Figure 2 As shown. The shield 9 includes reinforcements 13 located 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 adhesive bonding. The mechanical fasteners can be in the form of insulating supports. As an example, the winding supports can extend, thereby also securing the shield 9.

[0037] Figure 5 Transformer 1 is shown, which includes winding arrangement 2 and on-load tap changer 3. Winding arrangement 2 and on-load tap changer 3 are located in oil-filled tank 15.

[0038] Winding arrangement 2 and Figure 3 The winding arrangement 2 is the same, but shown from the opposite side. The position of the shield 9 is indicated by a dashed line. However, the shield 9 is positioned on the side of the winding core post facing away from the observer.

[0039] The on-load tap changer 3 is connected to the lead outlet 14 of the regulating winding (only some connections are depicted). Due to the limited angle range of the shield 9, the connection of the lead outlet 14 is unaffected.

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

[0041] The characteristic values ​​of transformers with lap resistors and transformers with shielded designs are compared below.

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

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

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

[0045] C1 = 1.776nF

[0046] C2 = 0.995nF.

[0047] By using a bridging resistor, the maximum AC recovery voltage is reduced to 16.8kV, which is below the permissible level.

[0048] For comparison, an external shield is used instead of a bridging resistor. This external shield is located on the neutral adjustment element and connected to the neutral terminal.

[0049] In this case, the following capacitance value is obtained:

[0050] C1 = 1.776nF

[0051] C2 = 3.126nF.

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

[0053] Overall, when using an external shielding design instead of a bridging resistor, the AC recovery voltage can be kept below permissible levels, while avoiding the additional costs and losses associated with bridging resistors. This results in a power transformer with improved environmental and efficiency indices. Furthermore, the shielding can be easily retrofitted to the winding arrangement without requiring significant additional space.

[0054] Figure Labels

[0055] 1. Power Transformer

[0056] 2. Winding Arrangement

[0057] 3 On-load tap changer

[0058] 4. Tank wall

[0059] 5 Iron core

[0060] 6 Outermost winding

[0061] 7 Inner winding

[0062] 8 Innermost winding

[0063] 9 Shielding components

[0064] 10 core pillars

[0065] 11 core pillars

[0066] 12 core pillars

[0067] 13 Reinforcing components

[0068] 14 Lead wire exit

[0069] 15 storage tanks

Claims

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

2. The power transformer (1) as described in claim 1. in, The outermost winding (6) is an adjusting winding, which includes several lead outlets (14) for changing the output voltage of the transformer.

3. The power transformer (1) as described in claim 1 or 2. in, The shielding element (9) is in the form of a material layer.

4. The power transformer (1) as described in claim 1 or 2. in, The shielding element (9) is in the form of a partial cylinder.

5. The power transformer (1) as described in claim 1 or 2. in, The shield (9) is connected to the ground potential or to the regulated neutral point or center point potential.

6. The power transformer (1) as described in claim 1 or 2. in, The shield (9) covers an angle range of at least 45° around the axis of the winding.

7. The power transformer (1) as described in claim 1 or 2. in, The shield (9) covers an angle range of less than 180° around the axis of the winding.

8. The power transformer (1) as described in claim 1 or 2. in, The shielding element (9) is formed of an insulating material layer with added conductive or semi-conductive particles.

9. The power transformer (1) as described in claim 1 or 2. in, The shielding element (9) is formed of carbonized paper.

10. The power transformer (1) as described in claim 1 or 2. in, The shielding element (9) includes a conductive material, wherein the conductive material is a metal.

11. The power transformer (1) as described in claim 1 or 2. in, The core (5) includes a plurality of winding core posts (10, 11, 12), each winding core post including a plurality of windings (6, 7, 8) and a shield (9) located outside the outermost winding (6) of the windings (6, 7, 8).

12. The power transformer (1) as described in claim 11. The power transformer is a three-phase power transformer, wherein... Each of the winding cores (10, 11, 12) is associated with a different phase.

13. The power transformer (1) as described in claim 1 or 2. The power transformer includes an on-load tap changer (3) electrically connected to the winding arrangement (2).

14. The power transformer (1) as described in claim 1 or 2. The power transformer includes an oil-filled tank (15), and the winding arrangement (2) is located in the oil-filled tank.

15. The power transformer (1) as described in claim 1 or 2, wherein the power transformer comprises a plurality of winding core posts (10, 11, 12), wherein, The shield (9) does not extend into the space between adjacent winding cores (10, 11, 12).

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