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Abstract
Starting medium voltage motors via autotransformers can
result in high overvoltages stresses on the autotransformer
winding. These overvoltages are owing to opening the circuit
breakers during the motor acceleration before getting a significant
speed, and the small inductive current chopping.
Small inductive current chopping can lead to repeated
reignitions and clearings when the breakers are opening. This leads
to successive trapping of magnetic stored energy in the
autotransformer winding and its subsequent release to the
autotransformer and cables capacitances. The energy release drives
the capacitances voltages to dangerous levels to the
autotransformer winding. This sequence is frequently referred to as
“multiple reignitions”. Using vacuum circuit breakers in such
motor – autotransformer starting arrangements increase these
overvoltages probability and severity due to vacuum breakers
inherent excellent interrupting capability ofhigh frequency
currents.