MOBİLE SUBSTATİONS AND TRANSFORMER LOAD LOSSES

MOBİLE SUBSTATİONS AND TRANSFORMER LOAD LOSSES;

LOAD LOSSES
The currents floating in the windings create losses due to the resistance in the windings. They are
equal to RI2 in each of the windings, where R is the direct current winding resistance and I the
current through the winding.
The eddy current losses come in addition. The winding conductors are situated in a time-varying
magnetic field due to the current floating in the conductors themselves. This magnetic field creates
eddy currents in the winding conductors. The consequence of the eddy currents is that the resulting
current in the conductors is unevenly distributed in the cross section of the conductor, which in turn
increases the losses.
The magnetic flux density varies also across the radial width of the winding. In a simple winding
configuration of a two-winding transformer, where the windings are situated as two concentric
shells, the flux density is highest at conductors nearest the duct between the shells. Consequently
the losses and also the temperature will highest in these conductors.
To avoid high eddy current losses the conductor dimension perpendicular to the magnetic flux lines
must be kept small. Further tall and slim windings have lower eddy current losses than low and
wide windings. However, in practise the design of a transformer is often a compromise between
different considerations, like for example short-circuit impedance and space limitations.
Because of the magnetic leakage field additional losses arise in the tank and in metal details
keeping the core and the windings firmly together.
The RI2 losses increase with the temperature while the additional losses decrease with the
temperature. The temperature that guaranteed losses shall be referred to is stated in the
standards.

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