Power transformers for on-load tap changer applications

Abstract

The power transformer (1) for on-load tap changer applications comprises a winding arrangement (2) having a core (5), several windings (6, 7, 8) wound around the core (5), and a shield (9) disposed on the outside of the outermost one of the windings (6, 7, 8), the shield (9) comprising or consisting of a conductive or semi-conductive material.

Description

[Technical Field]

[0001] explanation The present 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 of a power transformer is connected and disconnected from the regulating winding. This can subject the tap changer to high stress due to high recovery voltages. To keep the recovery voltage below the maximum level allowed for a particular OLTC design, so-called "tie-in resistors" can be installed. Tie-in resistors are external additional devices for resistive control of the recovery voltage and are also described in the international standard for tap changers, IEC / IEEE 60214-2. Additionally, "tie-in switch" devices can be used to disconnect the resistor during continuous operation, avoiding additional losses during operation.

[0003] However, the use of tie-in resistors increases the size and cost of power transformers. The impact of tie-in resistors is often greater in smaller units and tap-changer models because tie-in resistors and tie-in switches require additional space that is often only available in the selectors of large tap changers. 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. If tie-in resistors are used without switches, losses increase, especially no-load losses. Furthermore, tie-in resistors can affect the performance of the connected transformer, such as its Peak Efficiency Index (PEI).

[0004] U.S. Patent Application Publication No. 2021 / 057147 discloses a tap changer assembly in which a semiconductive coating and a conductive shield are applied to the cylindrical outer portion of the coil. German Patent Application Publication No. 3534843 discloses a winding configuration for a transformer in which the regulating winding is covered by shield rings at the top and bottom of its end faces. Japanese Patent Publication No. 59-126615 discloses a winding configuration for an on-load tap-changing transformer in which an insulated shield conductor is wound around an insulating cylinder and both ends of the shield conductor are connected to end electrostatic shields of the tap winding.

[0005] "On-Load Tap-Changers for Power Transformers," a September 1, 2013 publication by Dieter Dohnal (retrieved from the internet on December 7, 2017, https: / / / / www.reinhausen.com / XparoDownload.ashx?raid=58092), discloses a winding configuration for on-load tap changers that uses a screen between windings to reduce recovery voltage. Austrian Patent No. 260352 discloses a transformer winding configuration that includes a capacitor formed by a partial cylinder for surge protection. Summary of the Invention [Means for solving the problem]

[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 includes a winding configuration having a core, several windings wound around the core, and a shield including a conductive or semiconductive material, the shield being positioned outside one of the outermost windings, the shield covering an angular range around the winding axis of the winding configuration of up to 270°.

[0008] The shield allows the recovery voltage level to be reduced without the need for tie-in resistors. The shield requires less space and reduces no-load losses compared to using tie-in resistors.

[0009] As an example, the outermost winding may be a regulating winding with several lead outlets for varying the transformer output voltage. Furthermore, the winding configuration 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 wound on top of each other. The transformer may be a three-phase transformer. As an example, the core may have three winding limbs, each limb belonging to one phase.

[0010] The shield may be in the form of a layer of conductive or semi-conductive material. As one example, the shield may be in the form of a metal sheet. As a further example, the shield may be in the form of a layer of insulating material to which conductive or semi-conductive particles have been added to provide sufficient conductivity for electrical screening.

[0011] The shield may have openings. By way of example, the shield may have a mesh structure. The geometry of the shield may be adapted to the geometry of the outermost winding. By way of 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 potential. The shield may circumferentially surround the outermost winding, except for gaps, to prevent current circulation. The shield may cover an angular range of at least 45°.

[0013] In some embodiments, the shield may be located on only one side of the winding configuration. 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 limbs. In this case, the core dimensions and the distance between the core limbs do not need to be increased.

[0014] The shield may have an opening, which may be provided for a lead exit, through which the lead exit may lead to the tap changer contact.

[0015] The shield may completely or substantially completely cover the outermost winding along the winding axis, and by way of example, the shield may extend along at least 90% of the extension of the outermost winding along the winding axis.

[0016] The power transformer may include a tank in which the winding arrangement is disposed. 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 disposed within the tank.

[0017] Further features, improvements, and advantages will become apparent from the following description of exemplary embodiments taken in conjunction with the drawings, in which elements of the same structure and / or function may be referred to by the same reference numerals. It should be understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale. [Brief explanation of the drawings]

[0018] [Figure 1] 1 shows a schematic diagram of a power transformer winding configuration. [Figure 2] 1 shows a cross-sectional view of a power transformer winding configuration. [Figure 3] 1 shows a perspective view of a winding configuration of a power transformer. [Figure 4] One possible structure that can be used for the shield is shown in top view. [Figure 5]1 shows a schematic diagram of a power transformer winding configuration in an oil-filled tank. DETAILED DESCRIPTION OF THE INVENTION

[0019] 1 shows a schematic diagram of a power transformer 1 with a winding arrangement 2 for on-load tap changing. The winding arrangement 2 is connected to an on-load tap changer 3 and is disposed in an oil-filled tank with tank walls 4.

[0020] The winding configuration 2 includes a core 5 around which several overlapping windings 6, 7, and 8 are wound. The outermost winding 6 is a regulating winding for varying the transformer output voltage. The regulating winding is connected to a 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. Various arrangements of the high-voltage, low-voltage, and regulating windings are possible.

[0021] During switching operations, the regulating winding is disconnected from and reconnected to the high voltage winding by the tap changer 3. When breaking contacts during switching operations, the tap changer may be subjected to high stresses due to the high recovery voltage. The regulation may be connected, for example, by a coarse / fine or plus / minus changeover selector.

[0022] The main factor in the recovery voltage level is the ratio of the internal capacitance C1 between the outermost winding 6 and the nearest innermost winding 7 to the internal capacitance C2 between the outermost winding 6 and the tank wall 4. As a general rule, the smaller the ratio C2 / C1, the higher the recovery voltage generated across the changeover selector.

[0023] In Figure 1, V1 is the potential that the geometric midpoint of the nearest inner winding 7 rises to during operation, which is zero for the core limb. V3 is the potential that the geometric midpoint of the nearest outer winding rises to during operation, which is zero for the tank wall 4.

[0024] Figure 2 shows a schematic cross-section of one embodiment of winding configuration 1. Winding configuration 2 is shown in Figure 1, but with an additional shield 9 placed outside the outermost winding 6. The shield 9 may consist of a conductive or semiconductive material, or may be made primarily of this material, for example, apart from the edge protection. The shield 9 may also comprise an insulating material to which one or more conductive or semiconductive materials have been added in order to obtain conductive or semiconductive properties and thereby a screening effect.

[0025] The shield 9 may be made of a conductive material such as aluminum. The shield 9 may also be made of a semiconductive material. For example, carbon may be used as the semiconductive material. The shield 9 may be made of insulating paper to which conductive or semiconductive particles have been added. The shield 9 may also be carbonized paper.

[0026] The shield 9 may be composed of a conductive or semiconductive material, or may be made primarily of this material, for example, apart from the edge protection. The shield 9 is located outside the windings 6, 7, and 8 wound on the core 5, i.e., it is not surrounded by any additional windings wound on the respective core sections. The shield 9 is a component provided in addition to the windings 6, 7, and 8, particularly in addition to the electrodes of the windings 6, 7, and 8. The shield 9 may be in the form of a thin layer of conductive material. The shield 9's geometry conforms to the outer surface of the outermost winding 6. The shield 9 may also be in the form of an open cylinder. According to an unclaimed example, the cylinder may also be substantially closed except for a small gap to prevent circular current flow.

[0027] The shield 9 is connected to ground potential or the neutral or center potential of the adjustment. The center potential can be, for example, the potential in a three-phase voltage system arranged in an equivalent star connection. If the shield has the same potential as the adjustment, is located at the neutral end, or is directly grounded, the potential difference between the shield and the adjustment is very low, allowing for a closer distance between the shield and the winding. This reduces voltage reflections or oscillations in the adjustment itself during impulse distribution, allowing for a more compact and safer overall solution. The shield 9 acts as the outer "tank wall," as shown in the schematic diagram in Figure 1. The shield 9 allows for a significant increase in capacitance C2, reducing the recovery voltage. When using the shield 9, additional tie-in resistors to reduce the recovery voltage of the switch selector are not required. The shield 9 provides a cost-effective and space-saving alternative to tie-in resistors.

[0028] The shield 9 may cover only a portion of the outermost winding 6. The shield 9 may be disposed on only one side of the winding configuration 1. As an example, the shield 9 may cover an angular range α of less than 180° around the circumference of the winding configuration 1. In other embodiments, the shield may cover 180° or more around the circumference. The shield 9 may cover an angular range of at least 45°. The shape of the shield 9 may be such that it is avoided from being in close proximity to the adjustment lead outlet.

[0029] The shield 9 may be covered on both sides by an insulating material such as pressboard or a paper layer. Additionally, the shield may have additional edge protection at the top and bottom near the winding ends.

[0030] FIG. 3 shows a winding configuration 2 with a core 5 having three winding limbs 10, 11, and 12. Each of the winding limbs 10, 11, and 12 is associated with a different phase. Each of the winding limbs 10, 11, and 12 can have a winding configuration 12 similar to that shown in FIG. 2. In either case, a shield 9 is placed on the outermost winding. The shield 9 covers an angular range of less than 180° so that it does not extend into the gaps between the winding limbs 10, 11, and 12. This has the advantage that no extra space is required for the shield 9 between the winding limbs 10, 11, and 12, and the dimensions of the core 5 do not need to be increased. This avoids increasing the core limb pitch, which would otherwise increase the no-load losses of the transformer.

[0031] FIG. 4 shows a further embodiment of a shield 9 for the winding configuration 2. In this example, the shield 9 is in the form of a conductive net. The shield 9 may be wrapped around the outermost winding 6, as shown in FIG. 2. The shield 9 includes reinforcements 13 at the edges and corners. The shield 9 may be fixed to the outer surface of the winding configuration 2, for example, by mechanical fasteners or adhesive bonding. The mechanical fasteners may be in the form of insulating supports. As an example, supports for the windings may be extended so that the fixation of the shield 9 is also achieved.

[0032] 5 shows a transformer 1 with a winding arrangement 2 and an on-load tap changer 3. The winding arrangement 2 and the on-load tap changer 3 are arranged in an oil-filled tank 15.

[0033] Winding configuration 2 is the same as winding configuration 2 in Figure 3, but shown from the opposite side. The position of shield 9 is shown in dotted lines. However, shield 9 is positioned on the side of the winding limb facing away from the viewer.

[0034] The on-load tap changer 3 is connected to the lead outlet 14 of the regulating winding (only part of the connection is shown). Due to the limited angular range of the shield 9, the connection of the lead outlet 14 is not affected.

[0035] The shield 9 may also have an opening for the lead outlet 14. In this case, according to a non-claimed example, the shield 9 may extend around almost the entire circumference of the winding limbs 10, 11, 12, except for a small gap to prevent circular current flow. The gap may extend along the entire length of the shield 9 in the winding axis direction. According to a non-claimed example, the shield 9 may cover an angular range of almost 360°, for example, 340° or more. In addition to the gap, the shield 9 may have an opening for the lead outlet 14. It is also possible for the gap to provide the opening for the lead outlet 14.

[0036] Below we compare the performance values ​​of a transformer with a tie-in resistor and a transformer with a shielded design.

[0037] In both cases the tap changer has plus and minus adjustment and graduated neutral level. The connection is a three-phase star point connection.

[0038] For the transformer design without tie-in resistors and shielding, the maximum AC recovery voltage was 57.1 kV, which exceeded the maximum allowable level of 35 kV.

[0039] When using tie-in resistors, an additional no-load loss of about 3.1% was added. For capacitance, the following values ​​were obtained:

[0040] C1=1.776nF C2=0.995nF.

[0041] The tie-in resistors reduced the maximum AC recovery voltage to 16.8 kV, below the acceptable level. For comparison, an outer shield was used instead of the tie-in resistor, which was placed in the neutral adjustment and connected to the neutral end.

[0042] In this case, the following values ​​for capacitance were obtained: C1=1.776nF C2=3.126nF.

[0043] Therefore, the use of an external shield significantly increases C2. Increasing C2 reduces the maximum AC recovery voltage. In the example, the maximum AC recovery voltage is calculated to be 32.5 kV, which is below the maximum allowable level.

[0044] Overall, using an external shield design instead of tie-in resistors avoids the additional cost and loss of tie-in resistors while keeping AC recovery voltages below acceptable levels. This results in a power transformer with improved environmental and efficiency metrics. Furthermore, the shield can be easily retrofitted into the winding configuration without requiring significant additional space. [Explanation of symbols]

[0045] Reference sign 1. Power transformer Two-winding configuration 3 On-load tap changer 4 Tank wall 5 cores 6 Outermost winding 7 Inner Winding 8 Innermost winding 9 Shield 10 Limbs 11 Limbs 12 Limbs 13 Reinforcement 14 Lead Exit 15 Tank

Claims

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

2. The outermost winding (6) is a regulating winding with several lead outlets (14) for varying the transformer output voltage. A power transformer (1) according to claim 1.

3. The shield (9) is in the form of a layer of material. A power transformer (1) according to claim 1 or 2.

4. The shield (9) is in the form of a partial cylinder. A power transformer (1) according to claim 1 or 2.

5. The shield (9) is connected to ground potential or to a regulated neutral or center potential. A power transformer (1) according to claim 1 or 2.

6. The shield (9) covers an angular range of at least 45° around the winding axis. A power transformer (1) according to claim 1 or 2.

7. The shield (9) covers an angular range of less than 180° around the winding axis. A power transformer (1) according to claim 1 or 2.

8. the shield (9) is formed by a layer of insulating material doped with conductive or semiconductive particles; A power transformer (1) according to claim 1 or 2.

9. The shield (9) is made of carbonized paper. A power transformer (1) according to claim 1 or 2.

10. The shield (9) comprises a conductive material, the conductive material being a metal. A power transformer (1) according to claim 1 or 2.

11. The core (5) comprises a plurality of winding limbs (10, 11, 12), each of which comprises a plurality of windings (6, 7, 8) and a shield (9) located outside the outermost winding (6) of the windings (6, 7, 8). A power transformer (1) according to claim 1 or 2.

12. a three-phase power transformer, each of said winding limbs (10, 11, 12) being associated with a different phase; A power transformer (1) according to claim 11.

13. an on-load tap changer (3) electrically connected to the winding arrangement (2); A power transformer (1) according to claim 1 or 2.

14. an oil-filled tank (15) in which the winding arrangement (2) is placed; A power transformer (1) according to claim 1 or 2.

15. 3. A power transformer (1) according to claim 1 or 2, comprising several winding limbs (10, 11, 12), wherein the shield (9) does not extend into spaces between adjacent winding limbs (10, 11, 12).

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