To make a conductor easier to bend and for reducing eddy cur- rents, a conductor of power transformer winding typically consists of several parallel flat copper or aluminium strand conductors instead of big rectangular one.

Each strand conductor have an insulation layer on its surface al- lowing the potential difference between adjacent strand conductors and so prohibiting the eddy current from one conductor to another.

Still there are serious problems. The length of parallel strand con- ductors becomes different from one to another as the average distance from the transformer core is different. Therefore the induced voltage between strand conductors becomes different from each other and being galvanically connected of both ends, circulating currents exist heating up the winding. Also the load current does not divide equally to each strand conductor decreasing the total load capacity. In the prior art there are two main solutions to avoid these problems.

The very conventional winding structure is to have a mirror-image type transposing cross-over between each two adjacent disc, wherein each individual strand conductor on each cross-over is bent separately.

Due to the unhomogeneity of magnetic flux, many kind of specific transposing cross-over arrangements are to be used, some of them being in- troduced in JP 1246807. A use of four strand conductors is presented wherein the winding being divided in two sections, the middle cross-over being made by two sets of two conductors, all other cross-overs in both section being typi- cal mirror-image type transposing cross-overs.

The problem on these solutions is a big amount of separate strand conductors to be bent on cross-overs to be manually done and carefully insu- lated. The strand conductor is typically wound by additional insulation strip and a piece of insulation sheet is arranged to be installed between two conductors to avoid short-circuit. The optimised location of cross-overs should be calcu- lated by a special software. Overall the labour cost is the main problem, but also long pass-through time.

Another solution is using a Continuous Transposed Cable. The prefabricated cable consists of parallel strand conductors which are trans- posed and wound as a cable. The disc winding is wound of this type of cable and the cross-overs could be done by bending the cable by e. g. a pneumatic pressing tool. This method reduces the working time for winding because there is no need for making the transposing cross-overs.

The problem of the Continuous Transposed Cable is the high pur- chase price and a long delivery time because there are only few major suppli- ers of it and the cable should be tailor-made for each different type of trans- former. Also the joining of taps is more difficult than with normal separate strand conductors.

Typically for large power transformers the strand conductor dimen- sions could not be standardised but case by case economically optimised by the material costs and long term energy losses.

BRIEF DESCRIPTION OF INVENTION The object of the invention is to provide a disc winding such that the above-mentioned problems can be solved. This is achieved by winding which is characterized in what is disclosed in the independent claim 1. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on a disc winding axially shared in plurality of sections, each cross-over in a section being a type where all strand con- ductors are bent essentially parallel. A transposing cross-over between each

two adjacent sections brings the current and voltage balance between the strand conductors. Preferably the winding is shared to as many axial sections as there are parallel copper or aluminium strand conductors in a conductor to achieve the good balance but also small total amount of transposing cross- overs. Alternatively the winding could be shared to a multiple of the amount of parallel strand conductors, which gives good balance even for three parallel strand conductors. The biggest savings in the working time could be achieved by sharing the winding just in two sections and only have one transposing cross-over between the sections. Two section solution could be used if the amount of parallel strand conductors is not too big, preferably two or three, and the voltage and current balance requirements are not important criteria for winding. Making the parallel cross-overs by bending the parallel conductors simultaneously instead of bending each individual strand conductor separately the working time is reduced dramatically. The amount of separate cross-over bendings e. g. for six parallel strand conductors could be reduced even more than 80% compared to the conventional winding arrangements.

Instead of measuring and placing each strand conductor as in a conventional mirror-type transposing cross-over on each disc, only one cross- over needs to be measured and placed in a parallel cross-over. All the parallel conductors are bent essentially parallel in one stage using e. g. a pneumatic, hydraulic or electrically driven hand tool.

The parallel cross-over is alternately on the outer or on the inner track of the winding, but the transposing cross-overs could be either on the outer or on the inner track depending the even or odd number of discs in a section.

There are evidently several different ways to make these current and voltage balancing transposing cross-overs between two adjacent section.

In a case of even number of sections and therefore odd number of transposing cross-overs between sections, a mirror-image transposing cross-

over is used in the middle of winding making the cross-over location optimisa- tion calculation procedure needless.

The mechanical stresses between conductors are minimal at the parallel cross-overs and the scissors effect that is inevitable with the mirror- image type cross-over is omitted. Thus the conductors do not need to be rein- sulated separately against each other because a common re-wrapping of the whole bunch of conductors would be enough.

Pieces of insulation sheets are not needed between each parallel strand conductors as they are needed in mirror-image type cross-overs. Only one piece of insulation sheet for whole bunch is needed against the previous turn. Of course additional paper strip can be placed between parallel conduc- tors for increasing dielectric strength. If the bending is smooth enough and so the conductor insulation remains undamaged in bending, any additional insu- lation is normally not needed.

As the scissors effect is avoided between parallel conductors the risk for a mechanical damage causing a short circuit is reduced remarkably.

Cooling of the winding is also improved, because of reduced number of cross- overs causing less blocking up the coolant flow in radial ducts.

The idea of the invention is anyhow to make the most of the cross- overs more simple to manufacture due to parallel bending and reducing the number of transposing cross-overs and thus make savings in manufacturing time.

BRIEF DESCRIPTION OF FIGURES In the following the invention will be described in greater detail in connection with preferred embodiments, with reference to the attached draw- ings, wherein

Figure 1 is a schematic view of a winding with a parallel cross-over where each strand conductors being bent parallel ; Figure 2 shows a schematic view of the parallel cross-over with ad- ditional insulation means; Figure 3 shows a principle drawing of the parallel cross-over; Figure 4 shows a principle drawing of mirror-image type transposing cross-over where each six parallel strand conductors being bent separately ; Figure 5 shows a principle drawing of transposing cross-over where six parallel strand conductors are bent in two sets; Figure 6 shows a disc winding cross-over diagram for six parallel strand conductors with mirror-image type transposing cross-over in the middle of the winding; Figure 7 shows a disc winding cross-over diagram for six parallel strand conductors with half and half twin type transposing cross-over in the middle of the winding; Figure 8 shows a principle drawing of transposing cross-over where six parallel strand conductors are bent in two sets of three strand conductors; DETAILED DESCRIPTION OF THE INVENTION Fig. 1 shows a partial schematic view of a disc winding of a power transformer according to the invention. A disc winding consists of a conductor (1) wound around an insulating cylinder (2). The sticks (3) ensure the axial flow of the coolant but also are arranged to align the insulating spacers (4).

One disc (5) typically consists of several radial turns of conductor (1) and is axially separated from previous disc and following disc by insulating spacers (4). According to the invention in one section the conductor (1) is transferred from a disc to another by bending it as a parallel cross-over (6).

Fig. 2 shows a detailed schematic view of the parallel cross-over (6). One conductor (1) consists of plurality of essentially parallel strand con-

ductors (1a... lez In a parallel cross-over (6) all of the essentially parallel strand conductors (1a,.. 1f) are being bent from one disc (5) to another be- tween two adjacent spacers (4).

The bending could advantageously be done e. g. by a pneumatic, hydraulic or electrically driven pressing hand tool in one stage, but could also be automated.

To ensure a sufficient insulation level, the conductor (1) in a parallel cross-over (6) could be wrapped by an additional insulation strip (7) as a whole bunch of strand conductors (1a,... 1f). To avoid the scissors effect wherein conductor (1) insulation could be damaged leading to a short circuit, an addi- tional insulating sheet (8) could be arranged between two turns of the con- ductor (1).

Fig. 3 principally shows how a continuous conductor (1) is trans- ferred from a disc (5) to the next in a parallel cross-over (6). All of the parallel strand conductors (1a,... 1f) are being smoothly bent from disc to another in one spacers (4) pitch. Because of the top view, only one strand conductor (1a) could be seen in the drawing. The parallel cross-over (6) is located alter- nately on outer or inner radial track of the disc (5).

The winding according to the invention also comprises at least one type of transposing cross-overs shown in Fig. 4, Fig. 5 or Fig. 8.

Fig. 4 principally shows a conventional mirror-image type transpos- ing cross-over (9) where each single strand conductor (1a,... 1 is being bent separately one after each other, typically in every spacer (4) pitch. In a winding according to the invention, this type of transposing cross-over is used as a middle transposing cross-over in a case wherein an odd number of transpos- ing cross-overs exists.

Fig. 5 shows a twin type transposing cross-over (10) where the strand conductors are being bent in two sets. In Fig. 5 six exemplary strand conductors (1a,... 1f) are shared in sets of two (1a, 1b) and four strand con- ductors (1c,... 1f).

According to the invention, the share of these two sets of strand conductors could be varying depending the total amount of strand conductors.

Typically in a power transformer there are one to eight but could be even more strand conductors in one conductor.

One strand conductor is out of the invention. Two strand conductors could be shared only in one way, one and one strand conductor in a set. Three strand conductors could be shared in one and two and vice versa, four strand conductors in one and three, in two and two, five strand conductors in one and four, in two and three, six strand conductors in one and five, in two and four.

Seven strand conductors in one and six, in two and five, in three and four, eight strand conductors in one and seven, in two and six and in three and five.

Even a combination of different type of twin transposing cross-overs could be used for one winding.

Fig. 6 shows a disc winding cross-over diagram exemplary for six parallel strand conductors (1a... 1f). The winding comprises six sections wherein each section comprises several discs (5), which are not all shown in the diagram. The parallel cross-overs (6) in a section remain the relative order of different strand conductors (1a,... 1fi.

Corresponding to six sections there are five transposing cross- overs. The middle transposing cross-over (9) is a mirror-image type wherein each strand conductor is bent separately and the order of strand conductors e. g. 1a, 1b, 1c, 1d, 1e, 1f is transposed to 1f, 1e, 1d, 1c, 1b, 1a. The rest of transposing cross-overs are twin transposing cross-overs (10) type wherein the strand conductors are being bent in two sets.

The best mode for these two sets, specially for automatization of the bending is that the first set always has two innermost strand conductors and another set all the rest of strand conductors, or correspondingly the first set always has two outermost strand conductors and another all the rest of strand conductors. Thus in a case of six strand conductors (1,... 1f) the first set has two strand conductors and another set four. In a case of four parallel

strand conductors (1a,... 1d) both set have two strand conductors. In a case of eight parallel strand conductors (1a,... 1h) the first set has two and another six strand conductors.

Fig. 6 exemplary shows how the set of two innermost strand con- ductors in each twin transposing cross-over (10) is transposed to the outer- most and another set of four strand conductors is transposed as innermost, but the radial order of couples of strand conductors (1a, 1b), (1c, 1d) and (1e, 1f) remains.

The middle cross-over is a type of mirror-image type cross-over (9).

The benefit of using this kind of arrangement is that the amount of discs of each section could be arranged equal and there is no need for current bal- ancing optimisation calculation.

The amount of discs in a winding depends of the electrical require- ments. In an exemplary case of total 96 discs and six parallel strand conduc- tors, each six section is comprised of 16 discs (5) when the amount of sections is selected to be equal to the number of strand conductors.

Fig. 7 shows another embodiment for disc winding cross-over dia- gram for six parallel strand conductors (1a... 1fol. In this embodiment the winding comprises six sections as well. Corresponding to six sections there are five transposing cross-overs. The middle transposing cross-over (11) is a half and half type twin transposing cross-over shown in Fig. 8, wherein the strand conductors are bent in two sets, the three innermost strand conductors e. g. (1c, 1d, 1e) and another set of three of strand conductors (1f, 1a, 1b) are transposed with each other. The rest of transposing cross-overs are twin transposing cross-overs (10) type wherein the strand conductors are being bent in two sets. This embodiment requires the calculation for determining of the position of transposition cross-overs, but the bending could be easier automated because only two bending heads are needed for each type of cross-overs (6,10, 11) in this type of winding.

In a case of even number of parallel conductors, as four, six or eight, it could be possible also to have only one strand conductor in a first set and respectively three, five or seven strand conductors in another set. In this embodiment every transposing cross-overs are equal, the innermost first set of one strand conductor is transposed to the outermost, or vice versa, in each transposing cross-over. There is no mirror-image type transposing cross-over in a winding at all. Using this embodiment, a current balance optimisation cal- culations are needed defining the location of the transposing cross-overs.

When automated two different type of bending heads would be needed, one bending head for one strand conductor could be a lighter type, but another should be a heavy type to take care of the rest of strand conductors.

An advantageous embodiment for eight strand conductor, specially for automatization of the bending is that the first set always has three inner- most strand conductors and another set all the rest five of the strand conduc- tors, or correspondingly the first set always has three outermost strand con- ductors and another all the rest five of strand conductors. Each seven trans- posing cross-over could be the same type. The calculations for determining the position of transposings are needed.

In an embodiment of an odd number of parallel strand conductors, as three, five or seven, an even number of twin transposing cross-overs (10) exist. In a case of three parallel strand conductor the width of conductor is quite narrow and the problems of current balance is minor, so quite good re- sults could be achieved without any current balance optimisation calculations.

The balancing problems become more remarkable for bigger amount of paral- lel strand conductors.

In a case wherein a twin cable is used, the middle mirror-imaging transposing cross-over (9) should be made by just twisting the cable 180 de- grees.

In distribution transformers typically the insulating tube is not used, but the winding is being wound on a mandrel.

Typically a transformer and its winding are on a position wherein the coolant horizontally flows between two winding discs within a space limited by two spacers (4), therefore bending all strand conductors parallel the most of the ducts are left open for the coolant to flow freely.

The transposition diagrams and the location of the transposing cross-overs can be standardised for different number of parallel conductors and it has found that verification calculations and measurements are only needed in the standardisation stage of the application.

Voltage stresses between conductors at LI-test are minor due to the high capacitive connection between conductors. High capacitance is filtering the high frequency voltages between the cables and the fact that cable lengths are equal only at the end of the winding does not cause travelling wave stresses between conductors.

According to the invention the number of separate cross-overs is decreased to a fraction compared to the conventional winding manufacturing procedure. The work related to cross-overs decreases correspondingly. This results to a shortened manufacturing time and cost savings. The invention al- lows to increase the use of single strand conductors instead of more expen- sive continuously transposed cable in a disc winding leading to remarkable economic cost savings on material costs.

Winding manufactured according to the invention has equivalent technical properties as the winding manufactured according to the conven- tional procedure.

Although only one embodiment of the present invention has been described, it should be understood that the present invention may be embod- ied in many other specific forms without departing from the scope of protection defined in the attached claims.