From failure experience on power transformers, it
was very often suspected that inrush currents, occurring when energizing
unloaded transformers, were reason for damage. In this
paper, it was investigated how mechanical forces within the transformer
coils build up under inrush compared to those occurring
at short circuit. Two-dimensional and three-dimensional computer
modeling for a real 268 MVA, 525/17.75 kV three-legged step up
transformer was employed. The results show that inrush current
peaks of 70% of the rated short circuit current cause local forces
in the same order of magnitude as those at short circuit. The resulting
force summed up over the high voltage coil is even three
times higher. Although inrush currents normally are smaller, the
forces can have similar amplitudes as those at short circuit however
with longer exposure time. Therefore, care has to be taken to
avoid such high inrush currents. Today controlled switching offers
an elegant and practical solution.
power transformers in electric systems are one of the most
cost effective apparatuses. Breakdown causes high costs
for repair or replacement and may often lead to very high costs
due to temporary loss of the power delivery capability. Therefore,
power transformers are highly protected against various
causalities of failure. One major reason for internal faults is attrition
of the winding and conductor insulation due to vibrations
initiated by the electromechanical forces at service current and
over currents. Accordingly, the coils of transformers are mechanically
supported and prepressed by bandages, wedges, and heavy bolts.