TRANSPOSITION PRESENTED BY PROF VG PATEL TRANSFORMER ENCYCLOPAEDIA

TRANSPOSITION PRESENTED BY PROF. VG PATEL

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Wednesday, March 3, 2021 VG PATEL 2

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION In large transformers, a turn in winding consists of large number of conductors. In such an arrangement, unequal voltages are induced in different conductors due to varying leakage field in radial direction and variation in length of each conductor. The unequal voltages in para-llel strands (conductors) give rise to circulating currents. The losses due to circulating current within a few stands of turn may not reflect much in overall losses, but still can cause overheating of strands. Wednesday, March 3, 2021 VG PATEL 3

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION It is worth mentioning here that large unbalance in ampere-turns may lead to very high stray loss. To eliminate the circulating current between parallel strands (conductors) of a turn, transposition is essential. This leads to positioning of strands of a turn, such that flux linkage is same, thus equalizing the induced emf in each strand. The use of transposed cables for high-current windings results in considerable reduction in stray losses. In high-voltage winding with moderate current, requiring two or three conductors in parallel, bunched conductors can be used to improve winding space factor, and to provide subdivision of conductors. Wednesday, March 3, 2021 VG PATEL 4

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION In transformer, why transposition is required? Here potential difference is zero. When potential difference is not zero, circulating current will flow. Wednesday, March 3, 2021 VG PATEL 5

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION In a D C phenomena, for parallel operation v voltage is same v resistance is same v potential difference is zero Transposition is required when 1. conductor is more than one (parallel conductors) 2. polarity is same To eliminate circulating current, transposition is required. Here, No load current =Im*N/l l= length of magnetic path The purpose of transposition is to establish same potential in each parallel conductor to avoid circulating current. Wednesday, March 3, 2021 VG PATEL 6

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION v. Inside conductor will be having lower emf due to leakage flux. v. Due to main flux, same emf will be induced in all parallel conductors. Wednesday, March 3, 2021 VG PATEL 7

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Transposition 1 2 3 unit no. 6 5 4 conductor no. +1 +2 +3 =total unit 7 7 7 4 5 6 3 2 1 +4 +5 +6 7 7 7 Here, conductor no. 1 is at 6 unit and transposition at 1, so total unit is 7. Wednesday, March 3, 2021 VG PATEL 8

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION v By transposition, practically occupy all space in winding, which results in equal length of all conductors and equal resistance of all conductors. v Due to passing current, magnetic field is set up. Circulating current must be minimized and that is why transposition is required v Make equal impedance for establishing equal potential. To know the amount of potential in it, consider no-load current first, then set up magnetic field and load gradually increased. When main flux developed in core, connect load. Secondary load current I 2 increases and attains a value of rated current, magnetic field sets up around the conductor. Wednesday, March 3, 2021 VG PATEL 9

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Wednesday, March 3, 2021 VG PATEL 10

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION In the winding transposition is done to avoid circulating currents. v The winding constructed with CTC have been found beneficial for high voltage high power transformer. v Continuously transposed conductor used for the transformer winding has a high thermo-mechanical strength, excellent insulation characteristics and low stray load loss. v. The stray losses in large transformers are tends to be very high. v. Stray loss has two components (i) Eddy current loss (ii) Circulating current loss v Eddy current loss within each strand can be reduced by using thinner insulated conductor while the circulating current loss between strands can be reduced through frequent transposition of conductors v The transposed conductor consists of odd number of copper strips connected in parallel. v. Each strip is insulated with enamel. Wednesday, March 3, 2021 VG PATEL 11

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Wednesday, March 3, 2021 VG PATEL 12

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Wednesday, March 3, 2021 VG PATEL 13

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Advantages of CTC v. The winding time reduced due to use of assembled transposed conductors in place of parallel strips. v. Improved mechanical strength of winding due to composite construction of transposed conductor. v. The use of CTC reduces the overall size of transformer on account of strands and consequent reduction in the weight of core, winding, tank and oil. v. The individual strips use thin enamel insulation, therefore cooling of the conductor is improved on account of increased heat dissipation. Wednesday, March 3, 2021 VG PATEL 14

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION It has already been explained that the winding conductor of an LV winding having a large copper cross-section is subdivided into a number of sub-conductors, or strands, to reduce eddy-current loss and transposing these throughout the length of the winding. Transposition is necessary because of the difference in the magnitude of the leakage flux throughout the radial depth of the winding. If the strands were not transposed, those experiencing the higher leakage flux would be subjected to higher induced voltages and these voltages would cause circulating currents to flow via the ends of the winding where strands are of necessity common to make the external connections. Wednesday, March 3, 2021 VG PATEL 15

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Transposition ensures that as nearly as possible each strand experiences the same overall leakage flux. There are various methods of forming conductor Transpositions, but typically these might be arranged as shown in fig. If the winding conductor is subdivided into, say, eight sub-conductors in the radial dimension, then eight transpositions equally spaced axially are needed over the winding length. Each of these is carried out by moving the inner conductor sideways from below the other seven, which then each move radially inwards by an amount equal to their thickness, and finally the displaced inner conductor would be bent outwards to the outer radial level and then moved to the outside of the stack. Wednesday, March 3, 2021 VG PATEL 16

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Wednesday, March 3, 2021 VG PATEL 17

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Even with an arrangement of transpositions of the type described above and using many sub-conductors, eddy currents in very high-current windings (perhaps of 2000 A or greater) cannot be easily limited in magnitude to, say, 25% of the resistance losses as suggested above. Wednesday, March 3, 2021 VG PATEL 18

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION In addition, transpositions of the type described above, take up a significant amount of space within the winding. As a result, in the early 1950 s, manufacturers introduced a type of continuously transposed conductor. This enables a far greater number of transpositions to be carried out. In fact, as the name suggests, these occur almost continuously in the conductor itself before it is formed into the winding. Although the ‘continuous’ transpositions result in some loss of space within the conductor group, this amounts to less space within the winding than that required for conventional transpositions, so that there is a net improvement in space factor as well as improved uniformity of ampere-conductor distribution. Wednesday, March 3, 2021 19 VG PATEL

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Fig. shows how the continuously transposed conductor is made up. It has an odd number of strands in flat formation insulated from each other by enamel only and these are in two stacks side by side axially on the finished winding. Transpositions are effected by the top strip of one stack moving over to the adjacent stack as the bottom strip moves over in the opposite direction. The conductor is moved sideways approximately every 50 mm along its length. In addition to the enamel covering on the individual strands, there is a single vertical paper separator placed between the stacks and the completed conductor is wrapped overall with at least two helical layers of paper in the same manner as a rectangular section conductor. Wednesday, March 3, 2021 VG PATEL 20

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Manufacture of the continuously transposed conductor involves considerable mechanical manipulation of the strands in order to form the transpositions and was made possible by the development of enamels which are sufficiently tough and resilient to withstand this. The introduction of continuously transposed strip has been particularly beneficial to the design of large transformers, which must be capable of carrying large currents, but its use is not without some disadvantages of which the following are most significant: Wednesday, March 3, 2021 VG PATEL 21

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION A single continuously transposed conductor stack which might be up to, say, 12 strands high, and two stacks wide wrapped overall with paper, tends to behave something like a cart spring in that it becomes very difficult to wind round the cylindrical former. This problem can be limited by the use of such strip only for large-diameter windings. It is usual to restrict its use to those windings which have a minimum radius of about 30 times the overall radial depth of the covered conductor. Wednesday, March 3, 2021 VG PATEL 22

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION When the covered conductor, which has significant depth in the radial dimension, is bent into a circle, the paper covering tends to wrinkle and bulge. This feature has been termed ‘bagging’. The bagging, or bulging, paper covering can restrict oil flow in the cooling ducts. The problem can be controlled by restricting the bending radius, as described above, and also by the use of an outer layer of paper covering which has a degree of ‘stretch’ which will contain the bagging such as the highly extensible paper. Alternatively some allowance can be made by slightly increasing the size of the ducts. Wednesday, March 3, 2021 VG PATEL 23

TRANSFORMER ENCYCLOPAEDIA TRANSPOSITION Joints in continuously transposed strip become very cumbersome because of the large number of strands involved. Most responsible manufacturers (and their customers) will insist that a winding is made from one length of conductor without any joints. This does not, however, eliminate the requirement for joints to the external connections. It is often found that these can best be made using crimped connectors but these have limitations and very careful control is necessary in making the individual crimps. A high degree of quality control of the manufacture is necessary to ensure that defects in the enamel insulation of the individual strands or metallic particle inclusions do not cause strand-to-strand faults. Wednesday, March 3, 2021 VG PATEL 24

TRANSFORMER ENCYCLOPAEDIA OPEN FORUM Wednesday, March 3, 2021 25

TRANSFORMER ENCYCLOPAEDIA THANQ Wednesday, March 3, 2021 26