Why CT secondary should not be kept open?
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Answer Posted / achint rajput
A current transformer should never be open-circuited while
main current is passing through the primary winding. If the
load is removed from the secondary winding while the main
circuit current is flowing, most of the primary winding
current becomes magnetizing current, but the vector angles
change in such a way as to keep the total current in the
primary the same as before. Because the main circuit is now
mostly magnetizing current, the flux in the core shoots up
to a high level and a very high voltage appears across the
secondary. Due to the high turn ratio usually found in these
transformers, the voltage in this condition can reach a
dangerously high level, which can break down the insulation.
It also becomes a hazard to personnel. The high flux can
saturate the core and result in strong residual magnetism
left in the core, thereby increasing magnetization current
and introducing error in the transformation ratio. One has
to put a short on the secondary winding before removing the
connections.
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