What is current chopping ?
Definition: it is the phenomenon of current interruption before the natural current zero is reached.Current chopping mainly occurs in air-blast circuit breakers because they retain the same extinguishing power irrespective of the magnitude of the current to be interrupted.
When breaking low current (e.g., transformer magnetising current) with such breakers, the powerful de-ionising effect of air blast cases the current to fall abruptly to zero will before the natural current zero is reached. This phenomenon is known as current chopping and results in the production of high voltage transient across the contacts of the circuit breaker as discussed below :
Fig.(i) suppose the arc current is i when it is chopped down to zero value as shown by point a.
Fig.(ii) As the chop occurs at current i, therefore, the energy stored in inductance is Li²/2. The energy will be transferred to the capacitance C, charging the latter to a prospective voltage e given by :
½Li² = Ce²/2
or,
e = i √L/C volts......(i)
What is Voltage surge or Transient voltage ?
The prospectve voltage e is very high as compared to the dielectric strength gained by the gap so that the breaker restrikes. As the de-ionising force is still in action, therefore, chop occurs again but the arc current this time is smaller than the previous case. This induces a lower prospective voltage to re-ignite the arc.
In fact,several chops may occur until a low enough current is interrupted which produces insufficient induced voltage to re-strike across the breaker gap. Consequently, the final interruption of current takes place.
Excessive voltage surges due to current chopping arc prevented by shunting the contacts of the breaker with a resistor (resistance switching) such that reignition is unlikely to occurs.
This is dummy text. It is not meant to be read. Accordingly, it is difficult to figure out when to end it. But then, this is dummy text. It is not meant to be read. Period.
Please do not enter any spam link in the comment box. ConversionConversion EmoticonEmoticon