The Design and Performance of Circular Disc, Helical and Layer Windings for Power Transformer Applications
The Design and Performance.pdf (Size: 2.42 MB / Downloads: 330)
Winding design and manufacturing practices for power and distribution transformers has focused
in the differences between rectangular core and coils and layer windings common in the
production of distribution transformers and disc and helical windings and circular core designs
common in power transformers. In the small power transformer market defined as 5-15 MVA,
ONAN, with primary winding voltages up through 69 kV class, both core and coil designs are
Rectangular core and coils and layer windings offer the advantage of lower costs for
manufacturing compared to power class circular core and coils with disc and helical windings.
Distribution transformers are applied at lower voltages, ratings stated in KVA, lower short circuit
and thermal duties serving residential and commercial loads. Power transformers are applied at
voltages from 15 kV to 765 kV and higher, ratings stated in MVA, short circuit duties are high,
thermal loading variable and in some cases severe. Generator Step-up transformers, unit
auxiliary transformers, multiple winding transformers, autotransformers, transmission and
distribution bulk power substation service with and without load tap changing all must be
designed to withstand the rigorous duty for these power class transformer applications.
Thermal design and operation considerations
Heating of the windings results from the load losses of the winding and are a sum of the
conductor resistance losses and stray and eddy losses. Resistance losses (I2R) are a function of
load current and the resistance of the winding conductor. Stray and eddy losses include the
circulating current losses between parallel connected conductors as well as the eddy current
losses caused by the leakage flux entering the conductor, which depend on the dimension of the
conductor and direction of the flux impinging upon it.
A second function for the transformer oil is to act as a cooling system to transfer the heat
produced in the transformer windings to the cooling system through the processes of conduction,
convection and radiation. For optimum winding cooling, it is desireable that the moving oil
contact with every winding conductor for maximum convection and conduction of the conductor
heat from the winding to the mineral oil.
Through Fault - Short Circuit withstand considerations
The windings are subject to both radial and axial forces related to the current and flux
interactions. Radial forces in the inner winding (normally the LV winding) are in compression
while the outer winding (normally the HV winding) forces are in tension. Design of the
windings and bracing must consider the magnitude of these forces and provide adequate strength
to withstand them without significant mechanical deformation which could result in a dielectric
failure. The picture below is an example of a free bucking mechanical failure of an inner
winding resulting from radial forces in compression on the winding. Note, even though there is
mechanical failure, there wasn’t a dielectric failure of this winding.