# Transformer winding designs

## Basic types of windings

A conductor that covers the core of the magnetic circuit once and in which an EMF is induced under the influence of the magnetic field of the transformer is called a coil. The coil is the main element of the winding and consists of one or more parallel wires. The set of turns that form an electrical circuit in which the EMF induced in individual turns are summed up is called the transformer winding. The winding consists of conductors and insulating parts that protect the turns from electrical breakdown, prevent them from displacement under the influence of electromagnetic forces and create channels for cooling.
The transformer windings differ in their relative position on the rod, the direction and method of winding, the number of turns, the voltage class, the connection scheme of the ends of the windings to each other.

The beginnings and ends of the LV (low voltage) windings of three-phase transformers are designated by the letters a, b, c (beginning) and x. y, z (ends), HV windings (high voltage) - respectively A, B, C and X, Y, Z. According to their relative position on the rod, the windings are divided into concentric and alternating.

Concentric windings are made in the form of cylinders arranged concentrically (one inside the other) on the core of the magnetic circuit (Fig. 1). The alternating high and low voltage windings of the transformer alternate in the axial direction on the core of the magnetic circuit (Fig. 2). An alternating winding is usually subdivided into symmetrical groups, each of which consists of one or more parts of the HV winding and parts of the LV winding located on either side of them. Parallel circuits can be easily formed from individual groups at high currents. Alternating windings are used only in special transformers (for example, electric furnaces, test ones). The most common are concentric windings. Usually, the LV winding is placed first on the rod, but other options are also possible, when the medium voltage, regulating or even high voltage winding is placed first.
By design and winding method, cylindrical (single or multi-layer), coil and screw windings are distinguished. There are also one- or two-turn leaf and busbar windings used in special transformers with high secondary currents. The general requirements for transformer windings can be divided into operational and production.
The main operational requirements are electrical and mechanical strength and heat resistance of both windings and other parts and the entire transformer as a whole. The insulation of windings and other parts of the transformer must withstand without damage the switching and atmospheric overvoltages that may occur in the network where the transformer will operate. The mechanical strength of the windings must guarantee them against mechanical deformation and damage at short-circuit currents that are many times higher than the rated operating current of the transformer. Heating of windings and other parts from losses arising in the transformer during normal operation and short circuit of limited duration should not lead to insulation of windings and other parts, as well as transformer oil to thermal wear or destruction in terms shorter than the normal life of the transformer (20 - 25 years). The general operational requirements for transformers and their windings are regulated by the relevant standards for general-purpose power transformers, for various special transformers, for electrical tests of transformer insulation, etc. materials and advanced insulation processing technology. The requirement for the mechanical strength of the winding is met by careful calculation of the stray field, i.e., the correct choice of the type and design of the winding and the location of its turns and coils so that the mechanical forces arising in this winding are as low as possible and the mechanical resistance is as large as possible.
To achieve the required heat resistance, it is necessary to provide free heat transfer to the environment of all the heat released in the windings when the winding temperature exceeds the ambient temperature permissible for a given class of insulation heat resistance, i.e. to provide a sufficiently large contact surface of the winding with the cooling medium - oil or air. The general production requirements are reduced to the manufacture of the transformer with the least cost of materials and labor, i.e. the simplest in design, ensuring compliance with all operational requirements. These requirements for the transformer as a whole fully apply to the windings. The task of the designer is to reasonably balance the interests of operation and production. This problem is solved to a large extent when choosing one or another type of winding. Therefore, special attention should be paid to the choice of the type of winding that most fully meets the requirements of operation and at the same time is simple and cheap to manufacture.
When calculating the winding after choosing its type, it is necessary to achieve the greatest compactness in its placement, distribution of turns and coils in order to obtain the best filling of the transformer window. At the same time, one should strive to obtain a sufficiently developed cooling surface of the winding and a sufficient number and sizes of oil (air in a dry transformer) cooling channels in the windings while providing the least resistance for the movement of the cooling medium in them, which makes it possible to reduce the internal temperature difference in the windings and, as a consequence, , slightly reduce the cooled surface of the transformer tank.

### Types of transformer windings

 Types Advantages disadvantages Cylindrical one-two-layer rectangular wire Simple manufacturing technology, good cooling Low mechanical strength Cylindrical multilayer rectangular wire Good filling of the magnetic system window, simple manufacturing technology Reduction of the cooled surface compared to windings with radial channels Cylindrical multilayer round wire Simple manufacturing technology Deterioration of heat transfer and a decrease in mechanical strength with increasing power Screw one-two- and multi-pass from rectangular wire High mechanical strength, reliable insulation, good cooling Higher cost compared to cylindrical winding Continuous reel of rectangular wire High electrical and mechanical strength, good cooling The need to shift half of the coils when winding Cylindrical multilayer and reel aluminum foil High mechanical strength, good filling of the magnetic system window Sophisticated technology for manufacturing high voltage windings

Cylindrical winding designs:

• simple,
• multilayer,
• multilayer foil

A series of turns wound on a cylindrical surface is called a winding layer. In one layer there can be from one to several tens of turns, and in a turn up to six to eight or more parallel wires. A winding consisting of a layer of turns located on a cylindrical surface without intervals, i.e. close to each other, is called cylindrical (Fig. .3), and consisting of two (or more concentrically located layers - two-layer (multilayer) cylindrical (Fig. 4).

The turns of double and multilayer windings have the same unfolded length and position with respect to the leakage field of the transformer. The transition from layer to layer is performed without breaking the wire at the end of each layer, while the direction of winding the layers changes. Double-layer winding is usually wound from

fig. 3 1-turn; 2,4-equalizing rings, 3-insulating washers.
fig. 4 /. 4 - viitki, 2, 5 - distance slats; 3-leveling ring; 6-paper-bakelite cylinder; 7-layer insulation; 8-channel. 9 - rail. 10-insulating ring; 11- bakelite cylinder; X1, X2, X3 - adjusting branches
rectangular wire flat, but you can also on the edge. To align the screw surface, cut paper-bakelite rings (in the form of a "wedge") are attached to the extreme turns, which give the winding the shape of a cylinder. The rings protect the turns from mechanical damage and create a supporting surface for the winding. Between the layers of the two-layer winding, paper (electric cardboard) insulation is installed or several strips (gaskets) are evenly placed around the circumference, forming a vertical cooling channel (Fig. 4, a).

One- and two-layer cylindrical windings are used as low voltage windings up to 690 V in transformers with a capacity of less than 630 kVA. A multilayer cylindrical winding is wound, as a rule, from a wire of round cross-section.
the windings are tightly stacked to each other with transitions from layer to layer. The first layer is wound on a paper-bakelite cylinder. Cable paper is placed between subsequent layers. To improve cooling, an axial channel is made between some layers of the winding using spacers made of electric cardboard or beech. Such multilayer cylindrical windings are used as high voltage windings for oil transformers with a capacity of up to 400 kVA at a voltage of up to 35 kV (Fig. 4, b). In the direction of winding, like the thread of a screw, left and right windings are distinguished. This applies to cylindrical, coil and helical windings. In multilayer layered windings, the direction of the entire winding is considered in the direction of its first inner layer (Fig. 5).
A fundamentally new modification of the cylindrical winding are windings wound from non-insulated aluminum foil, which are used in transformers with a capacity of 25 to 630 kVA. The roll foil tape has a width equal to the height of the coil, and for windings with an operating voltage of up to 1 kV - the height of the winding. The insulation between the turns is a strip (or several strips folded together) of capacitor, telephone or cable paper. The width of the paper strip is taken 6-8 mm more than the width of the tape. The foil tape together with the paper strip (s) is wound on a cylindrical mandrel with a diameter equal to the inner diameter of the winding. After winding, the winding is removed from the mandrel, the paper protruding from the ends of the winding by 3-4 mm is impregnated with epoxy resin, baked and crimped, forming a monolithic insulating layer on the end surfaces of the winding (coil).
Aluminum foil windings are easy to wind, withstand mechanical stress during short-circuit of the transformer and have a high thermal conductivity in the axial and radial directions, which leads to a more uniform temperature distribution along the height and width of the winding and to a decrease in the temperature of the hottest point in comparison with windings wound from insulated wire.
The main disadvantages of aluminum foil windings are: high price of foil, which exceeds the price of insulated aluminum wire by about 40%; the complexity of the manufacture of high-voltage windings of voltage classes 10 and 35 kV with the obligatory division of these windings into coils connected by soldering, and the difficulty of attaching the taps to the foil windings with a thickness of less than 0.1 mm due to the low mechanical strength of this foil. The last (and first) turn of the foil winding with a thickness of 0.1-0.2 mm can be completed with an aluminum bus, which is attached to the foil by spot welding. The complexity of manufacturing a high voltage winding leads to the fact that in some cases it is preferred to make a low voltage winding from foil, and a high voltage winding from a wire.
Screw winding designs: single-pass, multi-pass, from transposed wire
Screw windings can be one-way (Fig. 6 a) and two-way (multi-way) (Fig. 6 b). A one-way helical winding consists of a series of turns that follow one after the other along a helical line with channels between them. Each turn includes one or more parallel wires, stacked in one row close to each other in the radial direction (Fig. 6, a, c).
A two-way (multi-way) helical winding consists of two (or more) one-way windings, wound one into the other during the manufacturing process. Each such "stroke" can include up to 40 parallel wires. The vertical channel along the inner surface of the winding and the channels between its turns are formed by rails and spacers (Fig. 6, d).
The turns of a helical winding consist, as a rule, of a large number of parallel wires located concentrically and at different distances from its axis, therefore the wires located closer to the axis will be shorter, and more distant ones longer. The difference in the length and position of the wires in the stray field causes the inequality of their electrical and inductive resistances. Different resistances lead to an uneven distribution of the current between them, that is, to an overcurrent and an increase in losses in some and underload in other conductors.
To equalize the distribution of the current and, consequently, to reduce the additional losses in the helical windings, various types of transpositions (permutations) are performed. In a one-way winding (usually with the number of wires per turn up to 12), a combination of transposition is used (Fig. 7a-c)
two group, when the wires in a loop are divided into two groups and both groups are swapped, and common, when the relative position of all parallel wires changes. If there are 12, 16 or more parallel wires in a single-pass winding, then the Byud transposition is used, which makes it possible to further reduce the additional losses.
In a two-way screw winding, a uniformly distributed Hobart transposition is used, during which all the wires of the winding are equally located with respect to the longitudinal (axial) stray field (the length of the wires is also almost the same) (Fig. 8).

The helical winding has a significant end surface, ensuring its resistance to axial forces during short circuit, good mechanical strength and sufficient cooling surface. Her

widely used for low voltage windings with a relatively small number of turns and significant secondary currents in transformers with a capacity of 1000 kVA and more.

A screw winding with any number of strokes can be wound, also from a transposed wire. This eliminates the need for additional transposition of parallel conductors, in addition to that made in the wire itself.

Continuous coil winding designs: simple interlaced, interlaced coils, subdivided wire
A group of series-connected turns, wound in the form of a flat spiral and separated from other similar groups, is called a coil, and a winding consisting of a number of coils located in the axial direction is called a coil.
Coil windings can be disc and continuous. The disk winding is recruited from separately wound coils, which are then connected to each other by electric soldering or in another way (Fig. 9 a). Coils are considered left if the wire from the upper outer end is laid counterclockwise, and right if the wire is laid clockwise. A continuous winding (Fig. 9, b) is wound without breaks, that is, the transition from one coil 1 to another 6 (Fig. 9, e) is made without rations. To do this, when winding, the turns of each odd coil are shifted so that one transition (from coil to coil) is outside the winding, and the other inside. Continuous winding coils are wound on
rails 3, forming a vertical channel along the inner surface of the winding. Spacers 5 are fixed on the rails, creating horizontal channels between the coils. Sometimes the rails are also placed along the outer surface of the winding.
In the turns of the winding there can be several, from one to six parallel wires (Fig. 9, c). With two or more wires, it is necessary to align their lengths and position in the stray magnetic field, for which the wires are interchanged, that is, they are transposed (Fig. 9, d, e). The transposition of parallel wires in a continuous winding is performed during the winding process at each transition from coil to coil. As a rule, in one span between two adjacent gaskets (in one "field"), the transition is made with one parallel wire 2.
In places of transition, the wire bends to an edge, and its insulation in this place is often damaged. After bending, it is obligatory to be restored, and the wire itself is reliably isolated from adjacent coils (Fig. 9d). Continuous windings can be taps for voltage regulation. Typically, the branches are made from the outer turns, so that turns corresponding to one control stage are enclosed between two adjacent branches. The advantage of a continuous coil winding (besides the absence of breaks during winding) is its large supporting surface and, therefore, significant resistance to axial forces during short circuit. Another advantage is the relatively free oil passage both along the surface and across (into the horizontal channels between the coils). Good cooling allows you to increase the power of the winding without fear of thermal destruction of its insulation. Due to these advantages, continuous windings are widely used in transformers of various capacities and voltages. In recent years, protection of windings against impulse overvoltages at voltage classes of 220 kV and above is carried out by combining capacitive rings using intertwined
coil windings, i.e. windings in which the order of the serial connection of the turns differs from the sequence of their arrangement in the coils. One of the interlaced winding schemes is shown in Fig. ten. Each coil is wound with two parallel wires, and then these wires are connected according to the scheme. Other weaving methods are possible.
turns of the winding. Winding of any type of interlaced winding is more complex and time consuming than winding a conventional continuous coil winding. In this case, an increase in the dielectric strength of the insulation of the turns and an increase in the density of its overlap are required, however, this complication of the technology and an increase in the cost of the winding pays off with an almost linear initial distribution of the impulse voltage and good lightning protection of the winding. In the interlaced winding, there is no need for shielding turns, but capacitive rings are used. The use of interlaced windings is currently, apparently, the best method of surge protection for windings of voltage classes from 220 to 750 kV.
The disc winding (Fig. 9, a) consists of a number of separately wound single or double (paired) coils, each of which has several turns, wound one on top of the other in a spiral. Depending on coil voltage
disk winding can have additional insulation common for all turns, made of tapes of cable or crepe paper. The thickness of the additional insulation is selected depending on the winding voltage; in different coils of the same winding, it can also be different, - gradually decreasing from the input into the winding to its main part.