Abstract:
The MV Distribution Network of CEB is frequently exposed to lightning and it is the
main suspected reason for transformer failures. Appreciable number of transformer
failures occurs due to insulation failures notably at points near line ends. The short
rise time of a surge prompted by a lightning impulse can cause deterioration in the
insulation and ultimately lead to a dielectric breakdown. The voltage distribution
along transformer HV winding becomes non-linear under surge conditions due to the
capacitive currents. This research focuses on the possibility of neutralizing this effect
with the aid of an electrostatic shield inserted in to the HV winding.
The comparison of the behavior of the winding under surge conditions with and
without electrostatic shield requires appropriate transient models suitable for
simulations. The necessary calculations required for this purpose were carried out
and the proposed models are presented. The simulations were carried out with
PSCAD software and the results are included in this report. The oscillatory nature
due to the combined effect of capacitors and inductors is observed in the wavetail
region and some of the peaks created have potentials exceeding the peak potential of
the surge waveform as well. The simulations revealed the reduction of stresses when
a shield was used.
The proposed transformer model have a definite positive effect compared with the
available model as the results obtained by simulations clearly reveal the reduction of
the stresses on the transformer HV windings when a shield is used.
