PROTECTION ARRANGEMENT FOR AN INDUCTIVE DEVICE

Abstract

 The invention is concerned with a protection arrangement for an inductive device (42, 44, 46, 48, 50, 52) as well as to an inductive apparatus comprising such a protection arrangement and inductive device. The protection arrangement comprises a first enclosure for housing the inductive device, where the first enclosure has at least one wall (12) comprising a first layer (24) of non-magnetic steel and a second layer (26) of bullet protecting material.

Description

FIELD OF INVENTION

[0001] The present invention generally relates to inductive devices. The invention is more particularly concerned with a protection arrangement for an inductive device as well as to an inductive apparatus comprising such a protection arrangement and inductive device.

BACKGROUND

[0002] Protection arrangements for inductive devices, such as for transformers, may comprise a tank with cooling and insulating fluid such as transformer oil in which the inductive device is immersed. The protection arrangement may in this case also comprise a cooling bank with cooling radiators for the transformer oil which cooling bank is enclosed in a protective enclosure and connected to the tank via ducts.

[0003] It is important to protect the transformer from various potential problems, such as vandalism and transformer tank rupture caused by transformer short circuiting. It may also be important to protect the inductive device from unnecessary losses due to magnetic leakage flux.

[0004] It is known to protect a transformer tank against vandalism by using a special polyurea coating, see US 2016/0118186.

[0005] With regard to preventing transformer tank rupture, there has been proposed the use of integral stiffeners for reinforcing the tank during overpressure conditions, see US 2016/0107795.

[0006] There has also been proposed to use high manganese non/magnetic steel for a transformer tank, see CN102747273.

[0007] There is in view of the above mentioned documents a need for an improved protection arrangement and especially one that addressees all three of the above-mentioned protective issues.

SUMMARY OF THE INVENTION

[0008] One object of the present invention is therefore to provide a protective arrangement for an inductive device that achieves protection against a wide variety of potential problems, such as vandalism, transformer tank rupture and unnecessary losses due to magnetic leakage flux.

[0009] This object is achieved through a protection arrangement for an inductive device comprising a first enclosure for housing the inductive device, where the first enclosure has at least one wall comprising a first layer of non-magnetic steel and a second layer of bullet protecting material.

[0010] The object is also achieved through an inductive apparatus comprising such a protection arrangement and inductive device.

[0011] The invention has a number of advantages. It achieves protection against a wide variety of potential problems, such as vandalism, transformer tank rupture and unnecessary losses due to magnetic leakage flux. Moreover, this is done in a compact structure at low costs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will in the following be described with reference being made to the accompanying drawings, where

fig. 1 shows a side view of a protection arrangement comprising a transformer tank and a cooling bank enclosure connected to the tank via two ducts,

fig. 2 schematically shows a plan view of walls of the tank,

fig. 3 schematically shows a transformer core together with examples of tank shunt elements, and

fig. 4 schematically shows a view from above of the tank with transformer windings around transformer core legs together with magnetic screens.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention concerns inductive devices, such as transformers like power transformers, which are protected by protection arrangements.

[0014] In fig. 1, there is shown a side view of an exemplifying protection arrangement that comprises a first enclosure for housing the inductive device. As the inductive device is a transformer, the first enclosure is a transformer tank 10. The transformer tank 10 has at least one wall 12, a lid 14 and a bottom 16. In the example given here the tank has a rectangular shape and therefore there are four walls. It should however be realized that other shapes can exist, such as cylindrical, in which case there may be only one wall. The tank 10 which is to house a transformer is also to be filled with cooling and insulation fluid, such as transformer oil. In order to cool this fluid, the protection arrangement may also comprise a second enclosure 22 housing a cooling bank comprising radiators for cooling the fluid. In the example given in fig. 1, the tank 10 is connected to the cooling bank radiators (not shown) in the enclosure 22 via a first and a second duct 18 and 20.

[0015] Fig. 2 shows a plan view of one realization of the tank walls 12. These walls 12 comprise two layers of material joined together, where a first inner layer 24 is a layer of non-magnetic steel, such as TWinning Induced Plasticity steel (TWIP), while a second outer layer is a layer 26 of bullet protecting material, which may be polyurea. Such a non-magnetic steel is typically a high manganese steel typically comprising 15-35wt%Mn, <3wt%Al, <3wt%Si and <1.5wt%C and sometimes Nitrogen and other micro-alloying elements such as Chromium, Boron, Niobium, Titanium, Vanadium. The remainder of the steel is made up of Iron and impurities. As an alternative to TWIP steel it is possible to use other grades of austenitic steel, such as stainless steel. From fig. 2 it can be seen that the whole wall has this layer combination. It should however be realized that it is possible with variations in the way that the first and second layers are used in the wall.

[0016] Fig. 3 schematically shows a core for the transformer comprising an upper yoke 28 and a lower yoke 30 joined together by three parallel core legs 32, 34 and 36. Transformer windings are supposed to be wound around each of the core legs 32, 34 and 36 and they may be fastened using at least one enclosure shunt element for the inductive device, e.g. using at least one tank shunt element, such as a flitch plate, tie rod or a core clamp. The at least one enclosure shunt element thus comprises at least one element in the group of flitch plate, tie rod and core clamp. In fig. 3 there is shown one such flitch plate 38 and tie rod 40, although more may exist. It should however be realized that the tank shunt could be completely eliminated in case the tank wall is made of TWIP steel.

[0017] Finally fig. 4 shows the tank 12 from above with the transformer core legs 32, 34 and 36 surrounded by primary 42, 44 and 46 and secondary 48, 50 and 52 windings. There is also at least one magnetic screen placed around at least a part of the at least one wall. In the example in fig. 4 each of the two long sides of the tank walls are for this reason magnetically shielded by a corresponding magnetic shield element 54 and 56 of soft magnetic material forming said magnetic screen, which shield elements may be glued laminated mild steel sheets, Grain oriented FeSi steel lamination or amorphous (AMF) magnetic steel tapes.

[0018] The tank for a fluid filled power transformer is traditionally made of engineering steel of certain dimension, such as 6 - 15 mm thick, and designed for a certain pressure such as for a 1.5 bar pressure. However, in case of arc fault inside a fluid filled transformer, there may be generated gas, which in turn may cause the creation of a pressure wave of approximately 8 to 12 bar, often causing serious damage to infrastructure, sometimes leading to fatal consequences, beside interruption of power supply.

[0019] During a transformer short-circuit, the electrical arc vaporizes oil and creates a dynamic pressure peak which travels at the speed of 1,200 meters per second (4,000 feet per second). This phenomenon occurs within a few milliseconds. Because of reflections in the tank 10, the pressure peak will generate pressure waves. The integration of all of the waves of pressure peaks creates static pressure. Eventually, the pressure is equalized throughout the entire transformer tank within 50 to 100 milliseconds after the electrical arc, and this may cause the transformer tank 10 to rupture causing interruption of the power supply and often large damage to the environment in which the transformer is placed, such as in a substation, and sometimes the rupture could lead to fatal accidents.

[0020] It can also be mentioned that concerns for these risks are rather high in case of the transformer being installed over an offshore platform. These transformers may be provided with a further enclosure around the tank for reducing the risk to the environment. However, such a further enclosure often gives rise to a poor cooling performance due to a lack of ventilation and to an overall larger foot print.

[0021] Moreover, the traditional transformer tank has losses due to an eddy current leakage flux cutting through the tank wall. This eddy current leakage flux induce losses and lead to lower efficiency of transformer and tank wall hot spots. These hotspots in the tank wall are not limited to the tank itself but if the hot spot temperature exceeds the vaporization temperature of the transformer oil, bubbles can be created that are detrimental to the dielectric integrity of the transformer or the oil might catch fire leading to catastrophic failure. It is therefore also of interest to lower such losses and the magnitude of the hottest spot.

[0022] Another risk is posed to power network by vandalism e.g. shot bullet on transformer tanks and hence they need to be designed to be bullet proof. One objective is to mitigate these problems by use of an advanced material and design solution.

[0023] The problem of providing an explosion proof tank is addressed through the use of the previously mentioned TWIP steel, which has inherent high energy absorption capability.

[0024] As TWIP steel is also non-magnetic, the losses due to the leakage flux cutting through the tank wall are minimized and use of tank shunt is minimized and hence the tank is compact and transformer will have lower losses.

[0025] In case of vandalism proof hardened transformer concept the soft bullet cannot be stopped by a polyurea coating along with a mild steel tank. A stainless steel tank material and coating can do the purpose. However, stainless steel is difficult to machine and expensive. Through the tank walls comprising TWIP steel along with a polyurea coating, effective vandalism protection is obtained. Additionally, the combination of TWIP steel and polyurea provides a higher strength than the combination of polyurea and stainless steel and thereby a higher resistance to internal explosions is also achieved.

[0026] Generally speaking, at room temperature, TWIP steel has a high tensile strength of above 800 MPa, a high Yield Strength of above 400 MPa, a hardness of about 300 Vickers Pyramid Number (HV), an electrical resistivity of about 0.71 µΩ.m., a thermal conductivity of about 15 W/mK and an elongation at break of 60% and more. It can thus be seen that TWIP steel is tough and has a high energy absorption capability.

[0027] Since TWIP steel has higher energy absorption capability, the tank wall will be stronger with regard to internal explosions as well as external factors, such as vandalism. TWIP steel is inexpensive compared to stainless steel. In case of a normal transformer when explosion and vandalism proof requirements are not necessary, then the TWIP steel tank can be made thinner and lighter. Further since the TWIP steel is non-magnetic, the eddy losses on the tank wall and hot spot problems are eliminated.

[0028] In the tank it is necessary that the tank walls have the above mentioned realization with TWIP steel and polyurea. The lid and bottom may also be realized in the same way. However, they may also be differently realized. At least one or both of the lid 14 and bottom 16 may therefore comprise the non-magnetic steel. Alternatively at least one of the lid 14 and bottom 16 may comprise magnetic steel.

[0029] It is possible that the bottom 16 is made of ordinary construction steel with or without polyurea, while the lid 14 may be made of stainless steel with polyurea. It is therefore possible that the lid 14 and only the lid of the lid 14 and bottom 16 comprises the bullet protecting material.

[0030] Moreover, it is possible that the cooling bank radiators do not need to be vandalism proof so these may therefore be realized using only the non-magnetic steel such as TWIP steel. However, the second enclosure enclosing radiators may comprise the bullet protecting material.

[0031] In order to reduce the eddy current losses even further, it is possible that some or all of the various tank shunt elements are realized using TWIP steel. At least one of the tank shunt elements may comprise the non-magnetic steel. It is for instance possible that the tie rod 40, flitch plate 38 and core clamp are made up of TWIP steel. However, as these elements are internal to the tank, they would normally not comprise any polyurea layer.

[0032] The magnetic fields caused by the eddy current leakage flux decay quite fast with distance from the windings. However, it may in some cases be necessary to limit the magnetic field strength outside of the transformer tank 10, for instance if the transformer is installed close to population or on an offshore platform. In this case it is possible to place a magnetic fence, for instance of very thin mild steel sheet, outside the tank. One example of this are the provision of the shields 54 and 56 outside of the long sides of the tank shown in fig. 4. These shields may be used when it is required that magnetic fields are extremely low outside the protection arrangement. It should here be realized that it is in this case possible also with shields along the short sides.

[0033] From the foregoing discussion it is evident that the present invention can be varied in a multitude of ways. It is for instance possible to protect other inductive devices than transformers, such as reactors or autotransformers.

[0034] It shall consequently be realized that the present invention is only to be limited by the following claims.

Claims

1. A protection arrangement for an inductive device (42, 44, 46, 48, 50, 52) comprising a first enclosure (10) for housing the inductive device, said first enclosure having at least one wall (12), said wall comprising a first layer (24) of non-magnetic steel and a second layer (26) of bullet protecting material.

2. The protection arrangement according to claim 1, wherein the first layer (24) of non-magnetic steel is a twinning inducted plasticity steel having a tensile strength of above 800 MPa.

3. The protection arrangement according to claim 1 or 2, wherein the second layer (26) of bullet protecting material is polyurea.

4. The protection arrangement according to any previous claim, further comprising at least one enclosure shunt element (38, 40) for the inductive device, said enclosure shunt element comprising said non-magnetic steel.

5. The protection arrangement according to any previous claim, wherein the at least one enclosure shunt element comprises at least one element in the group of flitch plate (38), tie rod (40) and core clamp.

6. The protection arrangement according to any previous claim, further comprising at least one magnetic screen (54, 56) around at least a part of said at least one wall (12).

7. The protection arrangement according to claim 6, wherein the at least one magnetic screen (54, 56) comprises a soft magnetic material.

8. The protection arrangement according to any previous claim, further comprising a second enclosure (22) enclosing radiators of a cooling bank for cooling fluid of the inductive device, where the second enclosure (22) comprises said bullet protecting material.

9. The protection arrangement according to claim 8, wherein the radiator elements comprise said non-magnetic steel.

10. The protection arrangement according to any previous claim, wherein the first enclosure comprises a lid (14) and a bottom (16).

11. The protection arrangement according to claim 10, wherein at least one of the lid (14) and bottom (16) comprises said non-magnetic steel.

12. The protection arrangement according to claim 10, wherein at least one of the lid (14) and bottom (16) comprises magnetic steel.

13. The protection arrangement according to any of claims 10 - 12, wherein the lid (14) and only the lid of the lid (14) and bottom (16) comprises said bullet protecting material.

14. The protection arrangement according to any previous claim, wherein the inductive device (42, 44, 46, 48, 50, 52) is a transformer and the first enclosure (10) is a transformer tank.

15. An inductive apparatus comprising an inductive device (42, 44, 46, 48, 50, 52) and a protection arrangement according to any previous claim.

Drawing