High Voltage Circuit BreakerHV AC Switchgear

Small inductive current switching condition for switchgear

When a CB opens and closes a shunt reactor banks or unloaded power transformer, it switches small inductive currents of several ten amperes with 90 lagging to the voltage phase.
The current in fact is usually forced to a premature zero by a phenomenon known as current chopping. However, the resultant chopping overvoltages, and subsequent reignition overvoltage may generate severe consequence depending on the CB performance and the circuit conditions.
Fig shows typical voltage and current behavior when current chopping occurs in case of small inductive current interruption. The current chopping accompanies an expanding high-frequency current oscillation leading to sudden current zero, which is an arc instability phenomena caused by arc characteristic and circuit conditions.
Another phenomenon after small inductive current interruption is reignition. CB can easily interrupt small inductive current even at short arcing time with small contact gap. The dielectric withstand strength of a CB increases with the contact gap; therefore a small contact gap tends to generate voltage breakdown during TRV period when the TRV exceeds the dielectric withstand across the contact gap.

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