Transposition of Parallel conductors
If the transpositions of parallel conductors used in the winding are not made proper, it will create unequal length and thereby, resistance will vary conductor to conductor due to variation in length circulating current will flow which in turn shall increase the stray losses. The eddy current which is the main component of stray losses mainly due to higher cross section of conductors are used, so designer should try to use as minimum as possible and to meet the total cross sectional area as required, number of parallel conductors or bunch conductors should be selected. Details of transposition are shown in the figure 1.
Figure 1: Optimum transposition of parallel conductor
If there is any short between the parallel strands of the winding, it will result to circulating current flow and thus, will generate stray losses so that one should confirm time to time that there is no any short between any strands by continuity tester during winding periodically.
Continuous transposed cable/conductor (CTC – figure 2), the cable manufactured with more numbers of strands depending upon the current and sizes, then individual strands are insulated with super enamel varnish and finally with paper covering as per the design requirements. The enamel varnish sometime gets damaged during tension and setting of turns/ conductor during winding by even with a soft plastic mallet resulted inter strand shorts and finally increase the stray losses, detail shown in Figure 3a & 3b.
Figure 2: Continuous Transposed Cable
The stray losses / circulating current losses are the source of heat generation in the transformer and ultimately paper insulation aged or loses its insulation property or strength & failed during course of its service rendering the short of life expectancy.
Figure 3a: Inter strand shorts, increasing stray losses
Figure 3b: Inter strand shorts, increasing stray losses
Stray Losses due to Linkage of Magnetic Flux
Highest linkage of magnetic flux is on the magnetic conductive material such as Core Lamination, Frame Part and MS Tank body. The flux generated due to change of frequency starts linking with magnetic material but due to long path some of the lines of fluxes could not return back to its principal / originating field called leakage flux and increasing the losses which is a component of stray losses in the transformer.
To avoid / minimize the above losses two types of shields are generally used called (1) Yoke Shunt and (2) Tank Shield which are shown in figure 4 & 5.
Figure 4: Placement of Yoke Shunt & Earthing arrangement
Figure 5: Placement of wall shield & earthing arrangement
The advantages of above two shields are many (1) It reduces the stray losses of the transformer by 40-45 % (2) it reduces the overheating of core and coil and thus, reduce the overall temperature rise of the winding & oil by 7-10% (3) It provides the firm common base for all the three phases of winding and in turn avoid unbalancing effect of forces during short circuit short application.
All the magnetic field produced by the primary is intercepted by the secondary winding, an e.m.f induced in that winding, this is called back e.m.f and current is called eddy current (As per Lenz’s Law ) this eddy current is a cause of losses in the transformer winding and the losses so produced in the transformer winding due to eddy current are called stray losses. These losses are dependent on the frequency of alternating supply because skin effects involved in this case. These losses are directly proportional to the frequency during inrush current (harmonics) these losses increase because frequency increases. A portion of the leakage flux may induce eddy current within nearby conductive objects such as the transformer’s support, MS structure, and be converted to heat.
Hysteresis and Eddy current losses both occur in the transformer core. The former on the quality of core lamination and latter the thickness and resistance of the core used to construct the core. Stray losses are also called iron losses. Refer the photo of core assembly figure 6.
Figure 6: Stray Losses
Leakage field present in the transformer induce eddy currents in conductors, tanks, channels, bolts etc and these eddy currents give rise to stray losses. Refer photo of complete Tank with cover and Channels / Frame Parts figure- 6&7.
Figure 7: Complete Tank with cover and Channels /Frame Parts
Magnetic shielding of the tank from HV side for the optimal design of investigated transformer order to select the version that would protect the wall best, while keeping losses to a minimum. The vertical shunts were shortened and horizontal shunts were introduced in the regions where hotspots were predicted as shown in figure 8.
Figure: 8: Vertical shunts were shortened and horizontal shunts were introduced in the regions
The result obtained or the final design of the magnetic shielding is presented in Table 1. Total stray losses obtained for the original design were assumed to be 100%. Total stray losses were decreased by 11.3 %. The highest reduction is observed in the HV wall 52 %. The losses in the shunts themselves were also reduced by about 24 %. The design changes have a significant impact on the temperature obtained in the transformer Tank.
Table 1: Stray losses for Optimal Design of Investigated Transformer
As illustrated in figure 8, the highest temperatures are located near the vertical edges of the embossments near the top of the tank. Previously observed hotspots were eliminated.
Leakage inductance is by itself largely lossless since energy supplied to its magnetic fields next half cycle. However, any leakage flux that intercepts nearby conductive material such structure will give rise to eddy currents and be converted to heat. You can reduce leakage inductance by using a winding shape that maximizes the window breadth of the chosen transformer. You can also reduce leakage inductance by interleaving the windings.
Eddy losses can be reduced by providing non-magnetic steel like stainless steel in flitch plate or providing partial slots in its length. Refer figure no 6 for more details.
Electromagnetic simulations of power transformers have proven to be a very powerful tool applicable in the development and Design stage. Different alternative shielding solution could be compared using FEM software and appropriate numerical models. Stray losses were predicted accurately, well within the uncertainty of measurement.
The applied methodology of tank shunts as shown in figure 8. Optimization is practical, inexpensive and easy to follow. Magneto –thermal coupled analysis provides important information on the electromagnetic and thermal behavior of the transformer.
Total Stray Load Losses with Modified Shunt and Divided Edge Stack as Shown in Table 2.
To conclude, a design engineer would have probably never dared to use this shielding configuration without the insight obtained from 3-D simulations. Therefore, such an approach brings multiple benefits in terms of the opportunity to run simulation test of different solutions and result in improved designs with lower stray losses and grater efficient. The comparison of stray losses after modification in shunt and edge stack in shown in Table 3.
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