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Transformer Windings

A complete electrical circuit of a transformer is formed by windings. Transformer winding must be electrically and mechanically strong to withstand over-voltages under transient surges. Transformer winding construction should ensure safety under normal and faulty conditions.

Winding Types

There are seven major types of transformer windings are available. They are

  • Distributed Cross-over windings
  • Spiral winding
  • Helical winding
  • Continuous Disc winding
  • Transposition
  • Interleaved Disc winding
  • Shielded Layer winding

Distributed Cross-over windings

  • When several coils are made circular cross-section and are placed in series,then Distributed Cross-over windings are made.
  • Distributed Cross-over windings are suitable for current not exceeding about 20 Amps.
  • Distributed Cross-over windings are mostly used in high voltage windings in samll transformers in the distribution range.
  • Each section of coils are seperated from other section of coil by insulating material to provide insulation as well as duct for cooling.

Spiral Winding

Single Layer

  • When several strip conductors are wounded tightly on a pressboard cylinder or bakelite cylinder in axial direction, then Spiral windings are made.
  • Spiral windings are normally used up to 33kv and lower current rating.
  • Thickness of the conductor used in spiral windings should be sufficient compared to its width, so that the winding remain twist-free.
  • Spiral windings are used in planar transformer.

Multi Layer

  • Multi layer spiral windings are formed by joining two single layer spiral windings by oil duct
  • So that, starting and ending leads lie at one end of the coil, which is more advantageous for making the terminal gear.
  • leakage flux are almost identical in Multi layer spiral windings.

Helical Winding

  • When several condutors are placed one over the other to form a single turn and are wound in a helix along the axial direction with each turn is seperated from the other turn by duct, then helical einding are made.
  • Conductors used to form a single turn might be different in length.
  • Due to different length of conductors, there is a formation of unequal impedance,resulting in eddy current losses due to the circulating current between the conductors.
  • To minimize the eddy current losses,the helical windings are provided with transposition of the conductiors which equalise the impedances of the parallel conductors.
  • Helical windings are used for low-voltage and high-current ratings.
  • Helical winding may be single-layer or double layer or multilayer.

Continuous Disc Winding

  • When four to five conductors of different sizes are connecting in series and are placed in the axial direction, then Continuous Disc Winding are made
  • Here, ducts are placed between each conductors to reduce eddy losses (heating losses).
  • Continuous Disc Winding are used for voltage between 33kv and 132kv and medium current rating.
  • Four to five conductors are joined to form a single turn for high mechanical strength and to withstand of high axial forces.

Transposition

  • The word transposition refered to a thing that has been produced by inversing something.
  • In helical windings, usually 3 transpositions are provided as shown below.
  • Middle winding (1) is provided with complete transposition.
  • Left side winding (2) is provided with 25% of transposition.
  • Right side winding (3)is provided with 75% of transposition.
  • In middle winding, each conductor position is varied symmetrically, relative to the middle point.
  • In left and right side winding , two halves of parallel conductors are interchanged in the positions, ie) upper half becomes lower half and vice versa.
  • This type of transposition needs extra space in the height of the coil.

Interleaved Disc Winding

  • A disadvantage with the continuous disc winding is that their strength against impulse voltage is not adequate for voltage above 140kv
  • This drawback is overcome by Interleaved Disc Winding where the turns are interleaved in such a fashion that two adjacent conductors belong to two different turns.
  • It is noticable from the below diagram, a winding in which interleaving has been done in each pair of discs.
  • Conductors of turns 8 and 9 are joined by brazing.
  • A cross-over is given at the bottom of the disc.

Shielded Layer Windings

  • Shielded Layer Windings are generally used for -connected transformer
  • Shielded Layer Windings are used for voltage greater than 132kv.
  • The winding consists of a number of concentric spiral coils placed one above the other called layer.
  • These layers are arranged between two concentric cylinderical shields, connected one to each end of the winding.
  • The length of this layers are longest near neutral shield and shortest near line shield.
  • Each layers are seperated by oil ducts for cooling purposes.
  • Both ends of shielded layer windings are bent over at 90o to provide a better insulation system to ground.
  • Uniform distribution of surge voltage throughout the winding is maintained as windings layer and shield form a series capacitors.
  • In shielded layer windings, Transposed conductors are only used for large current rating.

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