SE511363C2 - Dry power transformer / reactor - Google Patents

Abstract

The invention refers to a power transformer/reactor having a high voltage winding (2, 3) wound around a core (1). The transformer/reactor is a so-called dry type of transformer, i.e. air-cooled. According to the invention the high voltage winding is divided into a first section (2) and a second section (3) in series to each other. The winding is electrically connected (4, 5) to the core (1) between both these sections.

Description

511363 10 15 20 25 30 2 Examples of core constructions are described i.a. in DE 40414. The core may consist of conventional magnetizable materials such as the said oriented plate, of other magnetizable materials such as ferrites, amorphous materials, metal wires or bands. In the case of reactors, as is well known, the magnetizable core can be discontinued.

The above-mentioned windings are formed by one or more series-connected coils built up of a number of series-connected turns. The turns in a single coil are normally combined into a geometrically cohesive unit, physically separated from the other coils. The insulation system partly within a coil / winding and partly between coils / windings and other metal parts is normally designed as a solid cellulose or lacquer-based insulation closest to the individual conductor element and outside of solid cellulose and liquid, possibly also gaseous, insulation. Windings with insulation and any bracing parts in this way represent large volumes which will be exposed to high electric field strengths which occur in and around the active electromagnetic parts of the transformer. In order to be able to predetermine the dielectric stresses that occur and achieve a dimensioning with minimal risk of electrical breakdown, good knowledge of the properties of the insulation material is required. It is also important to create such an ambient environment that it does not change or reduce the insulation properties.

The currently prevailing external insulation system for high-voltage conventional power transformers / reactors consists of cellulosic material as the solid insulation and transformer oil as the liquid insulation. The transformer oil is based on so-called mineral oil.

Conventional insulation systems at the transformers referred to above require, in addition to a relatively complicated construction, also special manufacturing measures to utilize the good insulation properties of the insulation system.

Power transformers in the lower part of the above-mentioned power range are sometimes made with air cooling to remove the inevitable own losses in the form of heat. However, most power transformers are oil-cooled, usually with so-called pressure oil cooling. This is especially true of high power transformers. Oil-cooled transformers have a number of disadvantages which are well known. Among other things, they are large, awkward and heavy, which in particular causes major transport problems, and they make extensive demands in terms of safety and peripherals.

However, it has proved possible to largely replace oil-cooled power transformers with dry type transformers of a new type. This new dry transformer is equipped with a winding made with a high-voltage cable, ie a high-voltage insulated electrical conductor. Thus, dry transformers can be used at considerably higher powers than was previously possible. The term dry transformer or reactor thus refers to a transformer / reactor which is not oil-cooled but preferably air-cooled.

In the case of reactors, they comprise a core which is provided with only one winding and any control windings. In other respects, what has been said above regarding transformers is largely relevant also with regard to reactors. It should be noted in particular that even large reactors are oil-cooled.

With a dry transformer at higher power levels, the insulation around the windings becomes both space-consuming and costly to provide satisfactory insulation against the core. This in turn makes both the winding length and the core dimension large to make room for the required number of winding turns.

The present invention is directed to this aspect and its purpose is to seek to reduce the thickness of the winding insulation while maintaining sufficient insulation against the core.

SUMMARY OF THE INVENTION In accordance with the invention, this object has been achieved in that a transformer / reactor of the type specified in the preamble of claim 1 has the special features stated in the characterizing part of the claim.

Thanks to the fact that the high-voltage winding is electrically connected to the core at a point along the winding, the core will be connected to the potential that currently prevails at that point and have a corresponding potential in relation to earth. This reduces the maximum voltage that occurs between the conductor in the winding and the core. In a power transformer / reactor without such a connection, the maximum voltage Umax between line and core is equal to the supply voltage U, while the maximum voltage in the embodiment according to the invention is in the interval U> Umax> U / 2, where the value of Umax depends on where on the winding the contact with the core is established. This means that the required insulation around the conductor decreases correspondingly. A power transformer / reactor according to the invention can thus be designed with less insulation around the conductor in the winding.

This reduces the cost of wrapping. The fact that the winding conductor in this way has a smaller diameter also means that the winding length is reduced, since the external winding turns have a smaller winding diameter as the inside turns take up less space. The dimensions of the core also become smaller because due to the smaller dimension of the winding line, there is better space in the window so that it can be made correspondingly smaller. All in all, this leads to significant cost reductions compared to a conventional power transformer / reactor of the same power class and it is also much smaller.

The invention is primarily intended for use at high powers, in the range of 120 MVA and upwards. In such a case, it is advantageous to use a high-voltage cable in the windings, which makes it easier to reach the power level even with a dry transformer.

The high voltage cable comprises one or two conductors surrounded by a solid insulation consisting of at least two semiconductor layers with solid insulation between them.

In a preferred embodiment of the invention, the connection between the conductor in the winding and the core is made in the middle of the winding, i.e. so that the winding sections on each side of the connection have the same number of winding turns. This means an optimal utilization of the winding idea because the maximum voltage between line and core will then be at the lower limit of the above range, i.e. U / 2. Thus, with this embodiment, the greatest possibility is achieved of reducing the thickness of the insulating layer around the conductor in the winding. Another advantage of this embodiment is that the potential of the core will then be constant = U / 2.

Since the core of a power transformer / reactor according to the invention will have a potential ground, the core according to a preferred embodiment is supported by support insulators which insulate the core against ground. At least the outer semiconductor layer should constitute an equipotential surface, preferably with a potential corresponding to the potential of the core, half the supply voltage.

In transformer applications, according to a preferred embodiment, the low voltage winding is also electrically connected to the core at a point at preferably its center point.

The greatest advantage is then obtained with a transformer where the voltage ratio is 2: 1, for example 400 kV to 200 kV preferably or with a so-called spare transformer where the center socket is connected to the core. At voltage ratios other than 2: 1, an insulating layer is preferably arranged between the core and the low-voltage winding to insulate it against the core.

The transformer / reactor is primarily intended for single-phase application.

The insulated conductor or high voltage cable used in the present invention is flexible and flexible and of the type further described in WO 97/45919 and WO 97/45847. Further description of the insulated conductor or cable is given in WO 97/45918, WO 97/45930 and WO 97/45931.

The above and other preferred embodiments of the invented power transformer / reactor are set out in the subclaims dependent on claim 1.

The invention is explained in more detail by the following detailed description of a preferred embodiment thereof with reference to the accompanying drawings.

Brief description of the drawings Figure 1 is a schematic end view of a transformer according to the invention.

Figure 2 is a principle sketch in side view of the transformer in fl g. 1.

Figure 3 is a section through a high voltage cable used in the transformer in fi g. 1.

Figure 4 is a schematic diagram of a transformer according to an alternative embodiment of the invention.

Detailed description of the preferred embodiment of the invention Den i fi g. 1 has a toroid type core 1. Other core types are of course conceivable within the framework of the invention idea. The core, regardless of h "a" A "! ' "Li iimiiiä fl dliihniurßnh ..iii i. Ri illl * fiir 'ip-i -« i- i. I .. ii | ii iii -i iii .. iii i ~,' '. 511 363 6 its construction is of conventional type and may be constructed in a conventional manner, so that no more detailed description of this should be required.The transformer is a dry transformer, i.e. it does not have an oil-based cooling system.

High-voltage winding and low-voltage winding are arranged around the core, for the sake of clarity only the high-voltage winding is aligned. In the example shown, this can be considered to be the primary winding of the transformer. The transformer is supplied on the high voltage side with voltage U and operates in the power range 120 MVA or higher. The high voltage winding is divided into a first 2 and a second 3 section and where these meet the winding line is electrically connected to the core 1 at the contacts 4, 5. Each section 2, 3, has the same number of winding turns so that the contact points with the core share the winding mitt itu. The core 2 will then have a potential towards earth which is equal to half the supply voltage, i.e. U / 2. The potential for earth will be in principle constant at this value. The voltage difference between the conductor in the winding and the core will then amount to a maximum of U-U / 2, = U / 2, which voltage difference will appear at the ends of the winding connected to the mains. The voltage difference then decreases towards the contact points where it is equal to zero.

Since the transformer operates with high voltage and high power, great demands are placed on the insulation of the winding. Although the need for insulation is considerably reduced by the invention, the voltage levels are still relatively high. To meet these needs, the winding is designed as a high-voltage cable of the type shown in Fig. 3.

To insulate the core against earth 7, the core is supported by support insulators 6 as shown in fi g. Fig. 3 shows a cross-sectional view of a high-voltage cable 2, 3 according to the present invention. The high-voltage cable 2, 3 comprises a number of strands 31 with circular cross-sections of, for example, copper (Cu). These strands 31 are arranged in the middle of the high-voltage cable 2, 3. A first semiconducting layer 32 is arranged around the strands 31. An insulating layer 33 is arranged around the first semiconducting layer 32, e.g. PEX insulation. Arranged around the insulating layer 33 is a second semiconducting layer 34. Each semiconducting layer 32, 34 forms an equipotential surface when the cable is arranged in the transformer during operation. The two semiconducting layers 32, 34 have substantially the same coefficient of thermal expansion as the insulating layer 33.

Fig. 4 schematically shows how an insulating layer 10 is arranged between the low-voltage winding 8, 9 and the core 1 of a transformer according to an alternative embodiment of the invention.

Claims (13)

511 363 PATENT REQUIREMENTS Dry power transformer / reactor with a high voltage winding (2, 3) wound around a core (1), characterized in that the high voltage winding (2, 3) is divided into a first (2) and a second (3) section in series with each other and are electrically connected (4, 5) to the core (1) between the first (2) and the second section (3), which high-voltage winding (2, 3) consists of a high-voltage cable comprising one or more live conductors (31) and a solid insulation system comprising at least two semiconductor layers (32, 34) and fixed insulation (33) arranged between them. 10 A power transformer / reactor according to claim 1, wherein the first (2) and second (3) sections have substantially the same number of winding turns. A power transformer / reactor according to claim 1 or 2, in which the insulating means (bucket) means (6) are arranged to support the core (1) and are arranged to insulate it. mot jord (7). 'lllïllli ir A power transformer / reactor according to any one of claims 1 to 3, wherein the outer semiconductor layer (34) substantially constitutes an equipotential surface. 2 o A power transformer / reactor according to claim 4, wherein said equipotential surface has a potential to ground corresponding to half the voltage of the high voltage winding. 25 A power transformer / reactor according to any one of claims 1 to 5, wherein at least one of the semiconductor layers (3234) has substantially the same coefficient of thermal expansion as the solid insulation (33). A power transformer / reactor according to any one of claims 1-6, wherein the cable has a conductor area of between 50 and 3000 mm and has an outer diameter of between 20 and 250 mm. 10 15 20 511 363 9 Power transformer / reactor according to one of Claims 1 to 7, in which it is arranged for a rated power of at least 120 MVA and a voltage of at least 3 kV. Power transformer according to one of Claims 1 to 8, which also comprises a low-voltage line wound around the core (1), the low-voltage line also being divided into a first and a second section in series with one another and being electrically connected to the core between the first and second sections. Even the first and second sections have substantially the same number of winding turns. A power transformer according to claim 9, wherein the low voltage line 11. at which the voltage ratio is substantially 2: 1. Power transformer according to claims 1 - 7 or claim 9 or 10, A power transformer according to any one of claims 1-11, in which it is designed as a saving transformer. 13. the second layer is arranged between the low voltage winding and the core. A power transformer according to any one of claims 1-12, wherein an isolating